| Part of a series of overviews on |
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Marine microorganisms are defined by their habitat asmicroorganisms living in amarine environment, that is, in thesaltwater of a sea or ocean or thebrackish water of a coastalestuary. A microorganism (ormicrobe) is anymicroscopiclivingorganism orvirus, which is invisibly small to theunaided human eye withoutmagnification. Microorganisms are very diverse. They can besingle-celled[1] ormulticellular and includebacteria,archaea, viruses, and mostprotozoa, as well as somefungi,algae, and animals, such asrotifers andcopepods. Manymacroscopic animals andplants have microscopicjuvenile stages. Some microbiologists also classifyviruses as microorganisms, but others consider these as non-living.[2][3]
Marine microorganisms have been variously estimated to make up about 70%,[4] or about 90%,[5][6] of thebiomass in the ocean. Taken together they form themarine microbiome. Over billions of years this microbiome has evolved many life styles and adaptations and come to participate in theglobal cycling of almost all chemical elements.[7] Microorganisms are crucial to nutrient recycling inecosystems as they act asdecomposers. They are also responsible for nearly allphotosynthesis that occurs in the ocean, as well as the cycling ofcarbon,nitrogen,phosphorus and othernutrients and trace elements.[8] Marine microorganisms sequester large amounts of carbon and produce much of the world's oxygen.
A small proportion of marine microorganisms arepathogenic, causing disease and even death in marine plants and animals.[9] However marine microorganismsrecycle the majorchemical elements, both producing and consuming about half of all organic matter generated on the planet every year. As inhabitants of the largest environment on Earth, microbial marine systems drive changes in every global system.
In July 2016, scientists reported identifying a set of 355genes from thelast universal common ancestor (LUCA) of alllife on the planet, including the marine microorganisms.[10] Despite its diversity, microscopic life in the oceans is still poorly understood. For example, the role ofviruses in marine ecosystems has barely been explored even in the beginning of the 21st century.[11]
Microorganisms make up about 70% of themarine biomass.[4] Amicroorganism, or microbe, is amicroscopicorganism too small to be recognised adequately with the naked eye. In practice, that includes organisms smaller than about 0.1 mm.[12]: 13
Such organisms can besingle-celled[1] ormulticellular. Microorganisms are diverse and include allbacteria andarchaea, mostprotists includingalgae,protozoa andfungal-like protists, as well as certain microscopic animals such asrotifers. Manymacroscopic animals andplants have microscopicjuvenile stages. Some microbiologists also classifyviruses as microorganisms, but others consider these as non-living.[2][3]
Microorganisms are crucial to nutrient recycling inecosystems as they act asdecomposers. Some microorganisms arepathogenic, causing disease and even death in plants and animals.[9] As inhabitants of the largest environment on Earth, microbial marine systems drive changes in every global system. Microbes are responsible for virtually all thephotosynthesis that occurs in the ocean, as well as the cycling ofcarbon,nitrogen,phosphorus and othernutrients and trace elements.[8]
| Marine microorganisms | |
While recent technological developments and scientific discoveries have been substantial, we still lack a major understanding at all levels of the basic ecological questions in relation to the microorganisms in our seas and oceans. These fundamental questions are:
1. What is out there? Which microorganisms are present in our seas and oceans and in what numbersdo they occur?
2. What are they doing? What functions do each of these microorganisms perform in the marine environment and how do they contribute to the global cycles of energy and matter?
3. What are the factors that determine the presence or absence of a microorganism and how do they influence biodiversity and function and vice versa?
Microscopic life undersea is diverse and still poorly understood, such as for the role ofviruses in marine ecosystems.[13] Most marine viruses arebacteriophages, which are harmless to plants and animals, but are essential to the regulation of saltwater and freshwater ecosystems.[14] They infect and destroy bacteria in aquatic microbial communities, and are the most important mechanism ofrecycling carbon in the marine environment. The organic molecules released from the dead bacterial cells stimulate fresh bacterial and algal growth.[15] Viral activity may also contribute to thebiological pump, the process wherebycarbon issequestered in the deep ocean.[16]
A stream of airborne microorganisms circles the planet above weather systems but below commercial air lanes.[17] Some peripatetic microorganisms are swept up from terrestrial dust storms, but most originate from marine microorganisms insea spray. In 2018, scientists reported that hundreds of millions of viruses and tens of millions of bacteria are deposited daily on every square meter around the planet.[18][19]
Microscopic organisms live throughout thebiosphere. The mass ofprokaryote microorganisms — which includes bacteria and archaea, but not the nucleatedeukaryote microorganisms — may be as much as 0.8 trillion tons of carbon (of the total biospheremass, estimated at between 1 and 4 trillion tons).[20] Single-celledbarophilic marine microbes have been found at a depth of 10,900 m (35,800 ft) in theMariana Trench, the deepest spot in the Earth's oceans.[21][22] Microorganisms live inside rocks 580 m (1,900 ft) below the sea floor under 2,590 m (8,500 ft) of ocean off the coast of the northwesternUnited States,[21][23] as well as 2,400 m (7,900 ft; 1.5 mi) beneath the seabed off Japan.[24] The greatest known temperature at which microbial life can exist is 122 °C (252 °F) (Methanopyrus kandleri).[25] In 2014, scientists confirmed the existence of microorganisms living 800 m (2,600 ft) below the ice ofAntarctica.[26][27] According to one researcher, "You can find microbes everywhere — they're extremely adaptable to conditions, and survive wherever they are."[21] Marine microorganisms serve as "the foundation of all marine food webs, recycling major elements and producing and consuming about half the organic matter generated on Earth each year".[28][29]
Avirus is a smallinfectious agent thatreplicates only inside the livingcells of otherorganisms. Viruses can infect all types oflife forms, fromanimals andplants tomicroorganisms, includingbacteria andarchaea.[31]
When not inside an infected cell or in the process of infecting a cell, viruses exist in the form of independent particles. These viral particles, also known asvirions, consist of two or three parts: (i) thegenetic material (genome) made from eitherDNA orRNA, longmolecules that carry genetic information; (ii) aprotein coat called thecapsid, which surrounds and protects the genetic material; and in some cases (iii) anenvelope oflipids that surrounds the protein coat when they are outside a cell. The shapes of these virus particles range from simplehelical andicosahedral forms for some virus species to more complex structures for others. Most virus species have virions that are too small to be seen with anoptical microscope. The average virion is about one one-hundredth the size of the averagebacterium.
The origins of viruses in theevolutionary history of life are unclear: some may haveevolved fromplasmids—pieces of DNA that can move between cells—while others may have evolved from bacteria. In evolution, viruses are an important means ofhorizontal gene transfer, which increasesgenetic diversity.[32] Viruses are considered by some to be a life form, because they carry genetic material, reproduce, and evolve throughnatural selection. However, they lack key characteristics (such as cell structure) that are generally considered necessary to count as life. Because they possess some but not all such qualities, viruses have been described as "organisms at the edge of life"[33] and as replicators.[34]
Viruses are found wherever there is life and have probably existed since living cells first evolved.[35] The origin of viruses is unclear because they do not form fossils, somolecular techniques have been used to compare the DNA or RNA of viruses and are a useful means of investigating how they arose.[36]
Viruses are now recognised as ancient and as having origins that pre-date the divergence of life into thethree domains.[37]
Opinions differ on whether viruses are a form oflife or organic structures that interact with living organisms.[34] They are considered by some to be a life form, because they carry genetic material, reproduce by creating multiple copies of themselves through self-assembly, and evolve throughnatural selection. However they lack key characteristics such as a cellular structure generally considered necessary to count as life. Because they possess some but not all such qualities, viruses have been described as replicators[34] and as "organisms at the edge of life".[33]
Bacteriophages, often just calledphages, are viruses thatparasite bacteria and archaea.Marine phages parasite marine bacteria and archaea, such ascyanobacteria.[38] They are a common and diverse group of viruses and are the most abundant biological entity in marine environments, because their hosts, bacteria, are typically the numerically dominant cellular life in the sea. Generally there are about 1 million to 10 million viruses in each mL of seawater, or about ten times more double-stranded DNA viruses than there are cellular organisms,[39][40] although estimates of viral abundance in seawater can vary over a wide range.[41][42] For a long time,tailed phages of the orderCaudovirales seemed to dominate marine ecosystems in number and diversity of organisms.[38]However, as a result of more recent research, non-tailed viruses appear to be dominant in multiple depths and oceanic regions, followed by theCaudovirales families of myoviruses, podoviruses, and siphoviruses.[43]Phages belonging to the families:Corticoviridae,[44]Inoviridae,[45]Microviridae,[46] andAutolykiviridae[47][48][49][50]are also known to infect diverse marine bacteria.
There are also archaean viruses which replicate withinarchaea: these are double-stranded DNA viruses with unusual and sometimes unique shapes.[51][52] These viruses have been studied in most detail in thethermophilic archaea, particularly the ordersSulfolobales andThermoproteales.[53]
Microorganisms make up about 70% of the marine biomass.[4] It is estimated viruses kill 20% of this biomass each day and that there are 15 times as many viruses in the oceans as there are bacteria and archaea. Viruses are the main agents responsible for the rapid destruction of harmfulalgal blooms,[40] which often kill other marine life.[54]The number of viruses in the oceans decreases further offshore and deeper into the water, where there are fewer host organisms.[16]
Viruses are an important natural means oftransferring genes between different species, which increasesgenetic diversity and drives evolution.[32] It is thought that viruses played a central role in the early evolution, before the diversification of bacteria, archaea and eukaryotes, at the time of thelast universal common ancestor of life on Earth.[55] Viruses are still one of the largest reservoirs of unexplored genetic diversity on Earth.[16]
Viruses normally range in length from about 20 to 300 nanometers. This can be contrasted with the length of bacteria, which starts at about 400 nanometers. There are alsogiant viruses, often calledgiruses, typically about 1000 nanometers (one micron) in length. All giant viruses belongtophylumNucleocytoviricota (NCLDV), together withpoxviruses.The largest known of these isTupanvirus. This genus of giant virus was discovered in 2018 in the deep ocean as well as a soda lake, and can reach up to 2.3 microns in total length.[56]
The discovery and subsequent characterization of giant viruses has triggered some debate concerning their evolutionary origins.[57] The two main hypotheses for their origin are that either they evolved from small viruses, picking up DNA from host organisms, or that they evolved from very complicated organisms into the current form which is not self-sufficient for reproduction.[58] What sort of complicated organism giant viruses might have diverged from is also a topic of debate. One proposal is that the origin point actually represents a fourthdomain of life,[59][60] but this has been largely discounted.[61][62]
Bacteria constitute a largedomain ofprokaryoticmicroorganisms. Typically a fewmicrometres in length, bacteria have a number of shapes, ranging from spheres to rods and spirals. Bacteria were among the first life forms to appear onEarth, and are present in most of itshabitats. Bacteria inhabit soil, water,acidic hot springs,radioactive waste,[63] and the deep portions ofEarth's crust. Bacteria also live insymbiotic andparasitic relationships with plants and animals.
Once regarded asplants constituting the classSchizomycetes, bacteria are now classified asprokaryotes. Unlike cells of animals and othereukaryotes, bacterial cells do not contain anucleus and rarely harbourmembrane-boundorganelles. Although the termbacteria traditionally included all prokaryotes, thescientific classification changed after the discovery in the 1990s that prokaryotes consist of two very different groups of organisms thatevolved from an ancient common ancestor. Theseevolutionary domains are calledBacteria andArchaea.[64]
The ancestors of modern bacteria were unicellular microorganisms that were thefirst forms of life to appear on Earth, about 4 billion years ago. For about 3 billion years, most organisms were microscopic, and bacteria and archaea were the dominant forms of life.[65][66] Although bacterialfossils exist, such asstromatolites, their lack of distinctivemorphology prevents them from being used to examine the history of bacterial evolution, or to date the time of origin of a particular bacterial species. However, gene sequences can be used to reconstruct the bacterialphylogeny, and these studies indicate that bacteria diverged first from the archaeal/eukaryotic lineage.[67] Bacteria were also involved in the second great evolutionary divergence, that of the archaea and eukaryotes. Here, eukaryotes resulted from the entering of ancient bacteria intoendosymbiotic associations with the ancestors of eukaryotic cells, which were themselves possibly related to theArchaea.[68][69] This involved the engulfment by proto-eukaryotic cells ofalphaproteobacterial symbionts to form eithermitochondria orhydrogenosomes, which are still found in all known Eukarya. Later on, some eukaryotes that already contained mitochondria also engulfed cyanobacterial-like organisms. This led to the formation ofchloroplasts in algae and plants. There are also some algae that originated from even later endosymbiotic events. Here, eukaryotes engulfed a eukaryotic algae that developed into a "second-generation" plastid.[70][71] This is known assecondary endosymbiosis.
![The chloroplasts of glaucophytes have a peptidoglycan layer, evidence suggesting their endosymbiotic origin from cyanobacteria.[72]](/mt/?noimg=&dark=on&url=http%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aGlaucocystis_sp.jpg)
Pelagibacter ubique and its relatives may be the most abundant organisms in the ocean, and it has been claimed that they are possibly the most abundant bacteria in the world. They make up about 25% of all microbialplankton cells, and in the summer they may account for approximately half the cells present in temperate ocean surface water. The total abundance ofP. ubique and relatives is estimated to be about 2 × 1028 microbes.[73] However, it was reported inNature in February 2013 that thebacteriophageHTVC010P, which attacksP. ubique, has been discovered and "it probably really is the commonest organism on the planet".[74][75]
The largest known bacterium, the marineThiomargarita namibiensis, can be visible to the naked eye and sometimes attains 0.75 mm (750 μm).[76][77]
Thearchaea (Greek forancient[79]) constitute adomain andkingdom ofsingle-celledmicroorganisms. These microbes areprokaryotes, meaning they have nocell nucleus or any other membrane-boundorganelles in their cells.
Archaea were initially classified asbacteria, but this classification is outdated.[80] Archaeal cells have unique properties separating them from the other two domains of life,Bacteria andEukaryota. The Archaea are further divided into multiple recognizedphyla. Classification is difficult because the majority have not been isolated in the laboratory and have only been detected by analysis of theirnucleic acids in samples from their environment.
Archaea and bacteria are generally similar in size and shape, although a few archaea have very strange shapes, such as the flat and square-shaped cells ofHaloquadratum walsbyi.[81] Despite this morphological similarity to bacteria, archaea possessgenes and severalmetabolic pathways that are more closely related to those of eukaryotes, notably theenzymes involved intranscription andtranslation. Other aspects of archaeal biochemistry are unique, such as their reliance onether lipids in theircell membranes, such asarchaeols. Archaea use more energy sources than eukaryotes: these range fromorganic compounds, such as sugars, toammonia,metal ions or evenhydrogen gas. Salt-tolerant archaea (theHaloarchaea) use sunlight as an energy source, and other species of archaeafix carbon; however, unlike plants andcyanobacteria, no known species of archaea does both. Archaeareproduce asexually bybinary fission,fragmentation, orbudding; unlike bacteria and eukaryotes, no known species formsspores.
Archaea are particularly numerous in the oceans, and the archaea inplankton may be one of the most abundant groups of organisms on the planet. Archaea are a major part of Earth's life and may play roles in both thecarbon cycle and thenitrogen cycle.Thermoproteota (also known as eocytes or Crenarchaeota) are a phylum of archaea thought to be very abundant in marine environments and one of the main contributors to the fixation of carbon.[82]
Where Did Eukaryotic Cells Come From? –Journey to the MicrocosmosAll living organisms can be grouped as eitherprokaryotes oreukaryotes. Life originated assingle-celled prokaryotes and later evolved into the more complex eukaryotes. In contrast to prokaryotic cells, eukaryotic cells are highly organised. Prokaryotes are the bacteria and archaea, while eukaryotes are the other life forms —protists, plants, fungi and animals. Protists are usually single-celled, while plants, fungi and animals are usuallymulti-celled.
It seems very plausible that the root of the eukaryotes lie within archaea; the closest relatives nowadays known may be theHeimdallarchaeota phylum of the proposedAsgard superphylum. This theory is a modern version of a scenario originally proposed in 1984 asEocyte hypothesis, whenThermoproteota were the closest known archaeal relatives of eukaryotes then.A possibletransitional form of microorganism between a prokaryote and a eukaryote was discovered in 2012 by Japanese scientists.Parakaryon myojinensis is a unique microorganism larger than a typical prokaryote, but with nuclear material enclosed in a membrane as in a eukaryote, and the presence ofendosymbionts. This is seen to be the first plausible evolutionary form of microorganism, showing a stage of development from the prokaryote to the eukaryote.[83][84]
Protists are eukaryotes that cannot be classified as plants, fungi or animals. They are usually single-celled and microscopic. Life originated assingle-celled prokaryotes (bacteria and archaea) and later evolved intomore complex eukaryotes. Eukaryotes are the more developed life forms known as plants, animals, fungi and protists. The term protist came into use historically as a term of convenience for eukaryotes that cannot be strictly classified as plants, animals or fungi. They are not a part of modern cladistics, because they areparaphyletic (lacking a common ancestor).
Protists can be broadly divided into four groups depending on whether their nutrition is plant-like, animal-like, fungal-like,[85] or a mixture of these.[86]
Protists according to how they get food | |||||||
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| Type of protist | Description | Example | Some other examples | ||||
| Plant-like | Autotrophic protists that make their own food without needing to consume other organisms, usually by using photosynthesis |  | Green algae,Pyramimonas | Red andbrown algae,diatoms and somedinoflagellates. Plant-like protists are important components of phytoplanktondiscussed below. | |||
| Animal-like | Heterotrophic protists that get their food consuming other organisms (bacteria, archaea and small algae) |  | Radiolarian protist as drawn byHaeckel | Foraminiferans, and some marineamoebae,ciliates andflagellates. | |||
| Fungal-like | Saprotrophic protists that get their food from the remains of organisms that have broken down and decayed |  | Marineslime nets form labyrinthine networks of tubes in which amoeba without pseudopods can travel | Marine lichen | |||
| Mixotrops | Various | Mixotrophic andosmotrophic protists that get their food from a combination of the above | _(cropped).jpg) | Euglena mutabilis, a photosyntheticflagellate | Many marine mixotrops are found among protists, particularly among ciliates and dinoflagellates[87] | ||
Protists are highly diverse organisms currently organised into 18 phyla, but are not easy to classify.[89][90] Studies have shown high protist diversity exists in oceans, deep sea-vents and river sediments, suggesting a large number of eukaryotic microbial communities have yet to be discovered.[91][92] There has been little research onmixotrophic protists, but recent studies in marine environments found mixotrophic protests contribute a significant part of the protistbiomass.[87] Since protists are eukaryotes they possess within their cell at least onenucleus, as well asorganelles such asmitochondria andGolgi bodies. Protists are asexual but can reproduce rapidly throughmitosis or byfragmentation.
![Diatoms are a major algae group generating about 20% of world oxygen production.[93]](/mt/?noimg=&dark=on&url=http%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aDiatoms_through_the_microscope.jpg)
![Diatoms have glass like cell walls made of silica and called frustules.[94]](/mt/?noimg=&dark=on&url=http%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aDiatom_algae_Amphora_sp.jpg)
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| External videos | |
|---|---|
 How microscopic hunters get their lunch | |
| Euglenoids: Single-celled shapeshifters | |
| How do protozoans get around? |
In contrast to the cells of prokaryotes, the cells of eukaryotes are highly organised. Plants, animals and fungi are usuallymulti-celled and are typicallymacroscopic. Most protists are single-celled and microscopic. But there are exceptions. Some single-celled marine protists are macroscopic. Some marine slime molds have unique life cycles that involve switching between unicellular,colonial, and multicellular forms.[95] Other marine protist are neither single-celled nor microscopic, such asseaweed.
![Gromia sphaerica is a large spherical testate amoeba which makes mud trails. Its diameter is up to 3.8 cm.[96]](/mt/?noimg=&dark=on&url=http%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aGromia_in_situ_closeup.png)
![The xenophyophore, another single-celled foraminiferan, lives in abyssal zones. It has a giant shell up to 20 cm across.[97]](/mt/?noimg=&dark=on&url=http%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aXenophyophore.jpg)
Protists have been described as a taxonomic grab bag of misfits where anything that doesn't fit into one of the mainbiological kingdoms can be placed.[98] Some modern authors prefer to exclude multicellular organisms from the traditional definition of a protist, restricting protists to unicellular organisms.[99][100] This more constrained definition excludes manybrown, multicellularred andgreen algae, andslime molds.[101]
Another way of categorising protists is according to their mode of locomotion. Many unicellular protists, particularly protozoans, aremotile and cangenerate movement usingflagella,cilia orpseudopods. Cells which use flagella for movement are usually referred to asflagellates, cells which use cilia are usually referred to asciliates, and cells which use pseudopods are usually referred to asamoeba oramoeboids. Other protists arenot motile, and consequently have no movement mechanism.
Protists according to how they move | ||||||||
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| Type of protist | Movement mechanism | Description | Example | Other examples | ||||
| Motile | Flagellates |  | Aflagellum (Latin forwhip) is a lash-like appendage that protrudes from the cell body of some protists (as well as some bacteria). Flagellates use from one to several flagella for locomotion and sometimes as feeding and sensoryorganelle. |  | Cryptophytes | Alldinoflagellates andnanoflagellates (choanoflagellates,silicoflagellates, mostgreen algae)[102][103] (Other protists go through a phase asgametes when they have temporary flagellum – someradiolarians,foraminiferans andApicomplexa) | ||
| Ciliates |  | Acilium (Latin foreyelash) is a tiny flagellum. Ciliates use multiple cilia, which can number in many hundreds, to power themselves through the water. | .jpg) | Paramecium bursaria click to see cilia | Foraminiferans, and some marineamoebae,ciliates andflagellates. | |||
| Amoebas (amoeboids) |  | Amoeba have the ability to alter shape by extending and retractingpseudopods (Greek forfalse feet).[104] |  | Amoeba | Found in every major protistlineage. Amoeboid cells occur among theprotozoans, but also in thealgae and thefungi.[105][106] | |||
| Not motile | none |  | Diatom | Diatoms,coccolithophores, and non‐motile species ofPhaeocystis[103] Among protozoans the parasiticApicomplexa are non‐motile. | ||||
Flagellates include bacteria as well as protists. The rotary motor model used by bacteria uses the protons of anelectrochemical gradient in order to move their flagella.Torque in the flagella of bacteria is created by particles that conduct protons around the base of the flagellum. The direction of rotation of the flagella in bacteria comes from the occupancy of the proton channels along the perimeter of the flagellar motor.[107]
Ciliates generally have hundreds to thousands of cilia that are densely packed together in arrays. During movement, an individual cilium deforms using a high-friction power stroke followed by a low-friction recovery stroke. Since there are multiple cilia packed together on an individual organism, they display collective behavior in ametachronal rhythm. This means the deformation of one cilium is in phase with the deformation of its neighbor, causing deformation waves that propagate along the surface of the organism. These propagating waves of cilia are what allow the organism to use the cilia in a coordinated manner to move. A typical example of a ciliated microorganism is theParamecium, a one-celled, ciliated protozoan covered by thousands of cilia. The cilia beating together allow theParamecium to propel through the water at speeds of 500 micrometers per second.[108]
| External videos | |
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| Paramecium: The White Rat of Ciliates |
Over 1500 species offungi are known from marine environments.[109] These are parasitic onmarine algae or animals, or aresaprobes feeding on dead organic matter from algae, corals, protozoan cysts, sea grasses, and other substrata.[110] Spores of many species have special appendages which facilitate attachment to the substratum.[111] Marine fungi can also be found insea foam and aroundhydrothermal areas of the ocean.[112] A diverse range of unusual secondarymetabolites is produced by marine fungi.[113]
Mycoplankton aresaprotropic members of theplankton communities ofmarine andfreshwaterecosystems.[114][115] They are composed offilamentous free-livingfungi and yeasts associated with planktonic particles orphytoplankton.[116] Similar tobacterioplankton, these aquatic fungi play a significant role inheterotrophicmineralization andnutrient cycling.[117] While mostly microscopic, some mycoplankton can be up to 20 mm in diameter and over 50 mm in length.[118]
A typical milliliter of seawater contains about 103 to 104 fungal cells.[119] This number is greater in coastal ecosystems andestuaries due to nutritional runoff from terrestrial communities. A higher diversity of mycoplankton is found around coasts and in surface waters down to 1000 metres, with avertical profile that depends on how abundantphytoplankton is.[120][121] This profile changes between seasons due to changes in nutrient availability.[122] Marine fungi survive in a constant oxygen deficient environment, and therefore depend on oxygen diffusion byturbulence and oxygen generated byphotosynthetic organisms.[123]
Marine fungi can be classified as:[123]
Lichens aremutualistic associations between a fungus, usually anascomycete, and an alga or acyanobacterium. Several lichens are found in marine environments.[124] Many more occur in thesplash zone, where they occupy different vertical zones depending on how tolerant they are to submersion.[125] Some lichens live a long time; one species has been dated at 8,600 years.[126] However their lifespan is difficult to measure because what defines the same lichen is not precise.[127] Lichens grow by vegetatively breaking off a piece, which may or may not be defined as the same lichen, and two lichens of different ages can merge, raising the issue of whether it is the same lichen.[127]
Thesea snailLittoraria irrorata damages plants ofSpartina in the sea marshes where it lives, which enables spores of intertidal ascomycetous fungi to colonise the plant. The snail then eats the fungal growth in preference to the grass itself.[128]
According to fossil records, fungi date back to the lateProterozoic era 900-570 million years ago. Fossil marine lichens 600 million years old have been discovered in China.[129] It has been hypothesized that mycoplankton evolved from terrestrial fungi, likely in thePaleozoic era (390 million years ago).[130]
| External videos | |
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| Copepods: The Diatom-Devouring King of Plankton | |
| Gastrotrichs: Four-day-old grandmothers | |
| Rotifers: Charmingly bizarre and often ignored | |
| Tardigrades: Chubby, misunderstood, and not immortal |
As juveniles, animals develop from microscopic stages, which can includespores,eggs andlarvae. At least one microscopic animal group, theparasiticcnidarianMyxozoa, is unicellular in its adult form, and includes marine species. Other adult marinemicroanimals are multicellular. Microscopic adultarthropods are more commonly found inland in freshwater, but there are marine species as well. Microscopic adult marinecrustaceans include somecopepods,cladocera andtardigrades (water bears). Some marinenematodes androtifers are also too small to be recognised with the naked eye, as are manyloricifera, including the recently discoveredanaerobic species that spend their lives in ananoxic environment.[131][132] Copepods contribute more to thesecondary productivity andcarbon sink of the world oceans than any other group of organisms.
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Primary producers are theautotroph organisms that make their own food instead of eating other organisms. This means primary producers become the starting point in thefood chain forheterotroph organisms that do eat other organisms. Some marine primary producers are specialised bacteria and archaea which arechemotrophs, making their own food by gathering aroundhydrothermal vents andcold seeps and usingchemosynthesis. However most marineprimary production comes from organisms which usephotosynthesis on the carbon dioxide dissolved in the water. This process uses energy from sunlight to convert water andcarbon dioxide[133]: 186–187 into sugars that can be used both as a source of chemical energy and of organic molecules that are used in the structural components of cells.[133]: 1242 Marine primary producers are important because they underpin almost all marine animal life by generating most of theoxygen and food that provide other organisms with the chemical energy they need to exist.
The principal marine primary producers arecyanobacteria,algae and marine plants. Theoxygen released as a by-product of photosynthesis is needed bynearly all living things to carry outcellular respiration. In addition, primary producers are influential in the globalcarbon andwater cycles. They stabilize coastal areas and can provide habitats for marine animals. The termdivision has been traditionally used instead ofphylum when discussing primary producers, but theInternational Code of Nomenclature for algae, fungi, and plants now accepts both terms as equivalents.[134]
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| External videos | |
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| How cyanobacteria took over the world |
Cyanobacteria were the first organisms to evolve an ability to turn sunlight into chemical energy. They form a phylum (division) of bacteria which range from unicellular tofilamentous and includecolonial species. They are found almost everywhere on earth: in damp soil, in both freshwater and marine environments, and even on Antarctic rocks.[136] In particular, some species occur as drifting cells floating in the ocean, and as such were amongst the first of thephytoplankton.
The first primary producers that used photosynthesis were oceaniccyanobacteria about 2.3 billion years ago.[137][138] The release of molecularoxygen bycyanobacteria as a by-product of photosynthesis induced global changes in the Earth's environment. Because oxygen was toxic to most life on Earth at the time, this led to the near-extinction ofoxygen-intolerant organisms, adramatic change which redirected the evolution of the major animal and plant species.[139]
The tiny (0.6μm) marine cyanobacteriumProchlorococcus, discovered in 1986, forms today an important part of the base of the oceanfood chain and accounts for much of the photosynthesis of the open ocean[140] and an estimated 20% of the oxygen in the Earth's atmosphere.[141] It is possibly the most plentiful genus on Earth: a single millilitre of surface seawater may contain 100,000 cells or more.[142]
Originally, biologists thoughtcyanobacteria was algae, and referred to it as "blue-green algae". The more recent view is that cyanobacteria are bacteria, and hence are not even in the sameKingdom as algae. Most authorities exclude allprokaryotes, and hence cyanobacteria from the definition of algae.[143][144]
Algae is an informal term for a widespread and diverse group of photosyntheticprotists which are not necessarily closely related and are thuspolyphyletic. Marine algae can be divided into six groups:green,red andbrown algae,euglenophytes,dinoflagellates anddiatoms.
Dinoflagellates and diatoms are important components of marine algae and have their own sections below.Euglenophytes are a phylum of unicellular flagellates with only a few marine members.
Not all algae are microscopic. Green, red and brown algae all have multicellular macroscopic forms that make up the familiarseaweeds.Green algae, an informal group, contains about 8,000 recognised species.[145] Many species live most of their lives as single cells or are filamentous, while others formcolonies made up from long chains of cells, or are highly differentiated macroscopic seaweeds.Red algae, a (disputed) phylum contains about 7,000 recognised species,[146] mostlymulticellular and including many notable seaweeds.[146][147]Brown algae form aclass containing about 2,000 recognised species,[148] mostlymulticellular and including many seaweeds such askelp.Unlike higher plants, algae lack roots, stems, or leaves. They can be classified by size asmicroalgae ormacroalgae.
Microalgae are the microscopic types of algae, not visible to the naked eye. They are mostlyunicellular species which exist as individuals or in chains or groups, though some aremulticellular. Microalgae are important components of the marine protistsdiscussed above, as well as the phytoplanktondiscussed below. They are verydiverse. It has been estimated there are 200,000-800,000 species of which about 50,000 species have been described.[149] Depending on the species, their sizes range from a few micrometers (μm) to a few hundred micrometers. They are specially adapted to an environment dominated by viscous forces.
Unicellular organisms are usually microscopic. There are exceptions.Mermaid's wineglass, a genus of subtropicalgreen algae, is single-celled but remarkably large and complex in form with a single large nucleus, making it a model organism for studyingcell biology.[150] Another single-celled algae,Caulerpa taxifolia, has the appearance of a vascular plant including "leaves" arranged neatly up stalks like a fern. Selective breeding in aquariums to produce hardier strains resulted in an accidental release into the Mediterranean where it has become aninvasive species known colloquially askiller algae.[151]
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![Chlorella vulgaris, a common green microalgae, in endosymbiosis with a ciliate[152]](/mt/?noimg=&dark=on&url=http%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3a%25D0%2598%25D0%25BD%25D1%2584%25D1%2583%25D0%25B7%25D0%25BE%25D1%2580%25D0%25B8%25D0%25B8_Ophridium_versatile.jpg)
Macroalgae are the larger,multicellular and more visible types of algae, commonly calledseaweeds. Seaweeds usually grow in shallow coastal waters where they are anchored to the seafloor by aholdfast. Like microalgae, macroalgae (seaweeds) can be regarded asmarine protists since they are not true plants. But they are not microorganisms, so they are not within the scope of this article.
Plankton (from Greek forwanderers) are a diverse group of organisms that live in thewater column of large bodies of water but cannot swim against a current. As a result, they wander or drift with the currents.[153] Plankton are defined by theirecological niche, not by anyphylogenetic ortaxonomic classification. They are a crucial source of food for many marine animals, fromforage fish towhales. Plankton can be divided into a plant-like component and an animal component.
Phytoplankton are the plant-like components of the plankton community ("phyto" comes from the Greek forplant). They areautotrophic (self-feeding), meaning they generate their own food and do not need to consume other organisms.
Phytoplankton perform three crucial functions: they generate nearly half of the world atmospheric oxygen, they regulate ocean and atmospheric carbon dioxide levels, and they form the base of the marinefood web. When conditions are right,blooms of phytoplankton algae can occur in surface waters. Phytoplankton arer-strategists which grow rapidly and can double their population every day. The blooms can become toxic and deplete the water of oxygen. However, phytoplankton numbers are usually kept in check by the phytoplankton exhausting available nutrients and by grazing zooplankton.[156]
Phytoplankton consist mainly of microscopic photosyntheticeukaryotes which inhabit the upper sunlit layer in all oceans. They need sunlight so they can photosynthesize. Most phytoplankton are single-celled algae, but other phytoplankton are bacteria and some areprotists.[157] Phytoplankton includecyanobacteria (above),diatoms, various other types ofalgae (red, green, brown, and yellow-green),dinoflagellates,euglenoids,coccolithophorids,cryptomonads,chlorophytes,prasinophytes, andsilicoflagellates. They form the base of theprimary production that drives the oceanfood web, and account for half of the current global primary production, more than the terrestrial forests.[158]
Diatoms form a (disputed) phylum containing about 100,000 recognised species of mainly unicellular algae. Diatoms generate about 20 per cent of the oxygen produced on the planet each year,[93] take in over 6.7 billion metric tons ofsilicon each year from the waters in which they live,[159] and contribute nearly half of the organic material found in the oceans.
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Diatoms are enclosed in protective silica (glass) shells calledfrustules. Each frustule is made from two interlocking parts covered with tiny holes through which the diatom exchanges nutrients and wastes.[156] The frustules of dead diatoms drift to the ocean floor where, over millions of years, they can build up as much ashalf a mile deep.[160]
![Guinardia delicatula, a diatom responsible for algal blooms in the North Sea and the English Channel[161]](/mt/?noimg=&dark=on&url=http%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aFjouenne_sbrmvr012w_20070924163039_small.jpg)
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| Diatoms: Tiny factories you can see from space |
Coccolithophores are minute unicellular photosynthetic protists with two flagella for locomotion. Most of them are protected by a shell covered with ornate circular plates or scales calledcoccoliths. The coccoliths are made from calcium carbonate. The calcite shells are important to the marine carbon cycle.[163] The term coccolithophore derives from the Greek for aseed carrying stone, referring to their small size and the coccolith stones they carry. Under the right conditions they bloom, like other phytoplankton, and can turn the ocean milky white.[164]
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Phototrophic metabolism relies on one of three energy-converting pigments:chlorophyll,bacteriochlorophyll, andretinal. Retinal is thechromophore found inrhodopsins. The significance of chlorophyll in converting light energy has been written about for decades, but phototrophy based on retinal pigments is just beginning to be studied.[166]
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In 2000 a team of microbiologists led byEdward DeLong made a crucial discovery in the understanding of the marine carbon and energy cycles. They discovered a gene in several species of bacteria[168][169] responsible for production of the proteinrhodopsin, previously unheard of in bacteria. These proteins found in the cell membranes are capable of converting light energy to biochemical energy due to a change in configuration of the rhodopsin molecule as sunlight strikes it, causing the pumping of aproton from inside out and a subsequent inflow that generates the energy.[170] The archaeal-like rhodopsins have subsequently been found among different taxa, protists as well as in bacteria and archaea, though they are rare in complexmulticellular organisms.[171][172][173]
Research in 2019 shows these "sun-snatching bacteria" are more widespread than previously thought and could change how oceans are affected by global warming. "The findings break from the traditional interpretation of marine ecology found in textbooks, which states that nearly all sunlight in the ocean is captured by chlorophyll in algae. Instead, rhodopsin-equipped bacteria function like hybrid cars, powered by organic matter when available — as most bacteria are — and by sunlight when nutrients are scarce."[174][166]
There is anastrobiological conjecture called thePurple Earth hypothesis which surmises that original life forms on Earth were retinal-based rather than chlorophyll-based, which would have made the Earth appear purple instead of green.[175][176]
During the 1930sAlfred C. Redfield found similarities between the composition of elements in phytoplankton and the major dissolved nutrients in the deep ocean.[177] Redfield proposed that the ratio of carbon to nitrogen to phosphorus (106:16:1) in the ocean was controlled by the phytoplankton's requirements, as phytoplankton subsequently release nitrogen and phosphorus as they remineralize. This ratio has become known as theRedfield ratio, and is used as a fundamental principle in describing thestoichiometry of seawater and phytoplankton evolution.[178]
However, the Redfield ratio is not a universal value and can change with things like geographical latitude.[179] Based on allocation of resources, phytoplankton can be classified into three different growth strategies: survivalist, bloomer and generalist. Survivalist phytoplankton has a high N:P ratio (>30) and contains an abundance of resource-acquisition machinery to sustain growth under scarce resources. Bloomer phytoplankton has a low N:P ratio (<10), contains a high proportion of growth machinery and is adapted to exponential growth. Generalist phytoplankton has similar N:P to the Redfield ratio and contain relatively equal resource-acquisition and growth machinery.[178]
Thef-ratio is the fraction of totalprimary production fuelled bynitrate (as opposed to that fuelled by othernitrogencompounds such asammonium). The ratio was originally defined by Richard Eppley and Bruce Peterson in one of the firstpapers estimating global oceanic production.[180]
Zooplankton are the animal component of the planktonic community ("zoo" comes from the Greek foranimal). They areheterotrophic (other-feeding), meaning they cannot produce their own food and must consume instead other plants or animals as food. In particular, this means they eat phytoplankton.
Zooplankton are generally larger than phytoplankton, mostly still microscopic but some can be seen with the naked eye. Manyprotozoans (single-celledprotists that prey on other microscopic life) are zooplankton, includingzooflagellates,foraminiferans,radiolarians, somedinoflagellates andmarine microanimals. Macroscopic zooplankton (not generally covered in this article) include pelagiccnidarians,ctenophores,molluscs,arthropods andtunicates, as well as planktonicarrow worms andbristle worms.
Microzooplankton: major grazers of the plankton...
Many species ofprotozoa (eukaryotes) andbacteria (prokaryotes) prey on other microorganisms; the feeding mode is evidently ancient, and evolved many times in both groups.[181][182][183] Among freshwater and marinezooplankton, whether single-celled or multi-cellular, predatory grazing onphytoplankton and smaller zooplankton is common, and found in many species ofnanoflagellates,dinoflagellates,ciliates,rotifers, a diverse range ofmeroplankton animal larvae, and two groups of crustaceans, namelycopepods andcladocerans.[184]
Radiolarians are unicellular predatoryprotists encased in elaborate globular shells usually made of silica and pierced with holes. Their name comes from the Latin for "radius". They catch prey by extending parts of their body through the holes. As with the silica frustules of diatoms, radiolarian shells can sink to the ocean floor when radiolarians die and become preserved as part of theocean sediment. These remains, asmicrofossils, provide valuable information about past oceanic conditions.[185]
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| Radiolarian geometry | |
| Ernst Haeckel's radiolarian engravings |
Like radiolarians,foraminiferans (forams for short) are single-celled predatory protists, also protected with shells that have holes in them. Their name comes from the Latin for "hole bearers". Their shells, often calledtests, are chambered (forams add more chambers as they grow). The shells are usually made of calcite, but are sometimes made ofagglutinated sediment particles orchiton, and (rarely) of silica. Most forams are benthic, but about 40 species are planktic.[187] They are widely researched with well established fossil records which allow scientists to infer a lot about past environments and climates.[185]
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| foraminiferans | |
| Foraminiferal networks and growth |
![The Egyptian pyramids were constructed from limestone that contained nummulites.[188]](/mt/?noimg=&dark=on&url=http%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aAll_Gizah_Pyramids.jpg)
A number of forams aremixotrophic (see below). These have unicellularalgae asendosymbionts, from diverse lineages such as thegreen algae,red algae,golden algae,diatoms, anddinoflagellates.[187] Mixotrophic foraminifers are particularly common in nutrient-poor oceanic waters.[189] Some forams arekleptoplastic, retainingchloroplasts from ingested algae to conductphotosynthesis.[190]
Amoeba can be shelled (testate) or naked.
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![Mesodinium rubrum produce deep red blooms using enslaved chloroplasts from their algal prey [191]](/mt/?noimg=&dark=on&url=http%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aMesodinium_rubrum.jpg)
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Amixotroph is an organism that can use a mix of differentsources of energy and carbon, instead of having a single trophic mode on the continuum from completeautotrophy at one end toheterotrophy at the other. It is estimated that mixotrophs comprise more than half of all microscopic plankton.[192] There are two types of eukaryotic mixotrophs: those with their ownchloroplasts, and those withendosymbionts—and others that acquire them throughkleptoplasty or by enslaving the entire phototrophic cell.[193]
The distinction between plants and animals often breaks down in very small organisms. Possible combinations arephoto- andchemotrophy,litho- andorganotrophy,auto- andheterotrophy or other combinations of these. Mixotrophs can be eithereukaryotic orprokaryotic.[194] They can take advantage of different environmental conditions.[195]
Recent studies of marine microzooplankton found 30–45% of the ciliate abundance was mixotrophic, and up to 65% of the amoeboid, foram and radiolarianbiomass was mixotrophic.[87]
Phaeocystis is an important algal genus found as part of the marinephytoplankton around the world. It has apolymorphic life cycle, ranging from free-living cells to large colonies.[196] It has the ability to form floating colonies, where hundreds of cells are embedded in a gel matrix, which can increase massively in size duringblooms.[197] As a result,Phaeocystis is an important contributor to the marinecarbon[198] andsulfur cycles.[199]Phaeocystis species are endosymbionts toacantharian radiolarians.[200][201]
Mixotrophic plankton that combine phototrophy and heterotrophy – table based on Stoecker et al., 2017[202] | |||||||
|---|---|---|---|---|---|---|---|
| General types | Description | Example | Further examples | ||||
| Bacterioplankton | Photoheterotrophicbacterioplankton |  | Vibrio cholerae | Roseobacter spp. Erythrobacter spp. Gammaproteobacterial clade OM60 Widespread among bacteria and archaea | |||
| Phytoplankton | Calledconstitutive mixotrophs by Mitra et al., 2016.[203] Phytoplankton that eat: photosynthetic protists with inheritedplastids and the capacity to ingest prey. |  | Ochromonas species | Ochromonas spp. Prymnesium parvum Dinoflagellate examples:Fragilidium subglobosum,Heterocapsa triquetra,Karlodinium veneficum,Neoceratium furca,Prorocentrum minimum | |||
| Zooplankton | Callednonconstitutive mixotrophs by Mitra et al., 2016.[203] Zooplankton that are photosynthetic: microzooplankton or metazoan zooplankton that acquire phototrophy through chloroplast retentiona or maintenance of algal endosymbionts. | ||||||
| Generalists | Protists that retain chloroplasts and rarely other organelles from many algal taxa |  | Mostoligotrich ciliates that retain plastidsa | ||||
| Specialists | 1. Protists that retain chloroplasts and sometimes other organelles from one algal species or very closely related algal species |  | Dinophysis acuminata | Dinophysis spp. Myrionecta rubra | |||
| 2. Protists or zooplankton with algal endosymbionts of only one algal species or very closely related algal species |  | Noctiluca scintillans | Metazooplankton with algalendosymbionts Most mixotrophicRhizaria (Acantharea,Polycystinea, andForaminifera) GreenNoctiluca scintillans | ||||
| aChloroplast (or plastid) retention = sequestration = enslavement. Some plastid-retaining species also retain other organelles and prey cytoplasm. | |||||||
Dinoflagellates are part of thealgae group, and form a phylum of unicellular flagellates with about 2,000 marine species.[204] The name comes from the Greek "dinos" meaningwhirling and the Latin "flagellum" meaning awhip orlash. This refers to the two whip-like attachments (flagella) used for forward movement. Most dinoflagellates are protected with red-brown, cellulose armour. Like other phytoplankton, dinoflagellates arer-strategists which under right conditions canbloom and createred tides.Excavates may be the most basal flagellate lineage.[102]
By trophic orientation dinoflagellates cannot be uniformly categorized. Some dinoflagellates are known to bephotosynthetic, but a large fraction of these are in factmixotrophic, combining photosynthesis with ingestion of prey (phagotrophy).[205] Some species areendosymbionts of marine animals and other protists, and play an important part in the biology ofcoral reefs. Others predate other protozoa, and a few forms are parasitic. Many dinoflagellates aremixotrophic and could also be classified as phytoplankton.
The toxic dinoflagellateDinophysis acuta acquire chloroplasts from its prey. "It cannot catch the cryptophytes by itself, and instead relies on ingesting ciliates such as the redMyrionecta rubra, which sequester their chloroplasts from a specific cryptophyte clade (Geminigera/Plagioselmis/Teleaulax)".[202]
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Dinoflagellates often live insymbiosis with other organisms. Manynassellarian radiolarians housedinoflagellatesymbionts within their tests.[207] The nassellarian providesammonium andcarbon dioxide for the dinoflagellate, while the dinoflagellate provides the nassellarian with a mucous membrane useful for hunting and protection against harmful invaders.[208] There is evidence fromDNA analysis that dinoflagellate symbiosis with radiolarians evolved independently from other dinoflagellate symbioses, such as withforaminifera.[209]
Some dinoflagellates arebioluminescent. At night, ocean water can light up internally andsparkle with blue light because of these dinoflagellates.[210][211] Bioluminescent dinoflagellates possessscintillons, individualcytoplasmic bodies which containdinoflagellate luciferase, the main enzyme involved in the luminescence. The luminescence, sometimes calledthe phosphorescence of the sea, occurs as brief (0.1 sec) blue flashes or sparks when individual scintillons are stimulated, usually by mechanical disturbances from, for example, a boat or a swimmer or surf.[212]
![Oodinium, a genus of parasitic dinoflagellates, causes velvet disease in fish[213]](/mt/?noimg=&dark=on&url=http%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aArchives_de_zoologie_exp%25C3%25A9rimentale_et_g%25C3%25A9n%25C3%25A9rale_(1920)_(20299351186).jpg)
![Karenia brevis produces red tides highly toxic to humans[214]](/mt/?noimg=&dark=on&url=http%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aKarenia_brevis.jpg)
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![Noctiluca scintillans, a bioluminescent dinoflagellate[215]](/mt/?noimg=&dark=on&url=http%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aNoctiluca_scintillans_unica.jpg)
Sediments at the bottom of the ocean have two main origins, terrigenous and biogenous.
Terrigenous sediments account for about 45% of the total marine sediment, and originate in the erosion ofrocks on land, transported by rivers and land runoff, windborne dust, volcanoes, or grinding by glaciers.
Biogenous sediments account for the other 55% of the total sediment, and originate in the skeletal remains ofmarine protists (single-celled plankton and benthos microorganisms). Much smaller amounts of precipitated minerals and meteoric dust can also be present.Ooze, in the context of a marine sediment, does not refer to the consistency of the sediment but to its biological origin. The term ooze was originally used byJohn Murray, the "father of modern oceanography", who proposed the termradiolarian ooze for the silica deposits of radiolarian shells brought to the surface during theChallenger expedition.[217] Abiogenic ooze is apelagic sediment containing at least 30 per cent from the skeletal remains of marine organisms.
Main types of biogenic ooze | ||||||||
|---|---|---|---|---|---|---|---|---|
| type | mineral forms | protist involved | name of skeleton | typical size (mm) | ||||
| Siliceous ooze | SiO2 silica quartz glass opal chert | diatom |  | frustule | 0.002 to 0.2[218] |  | diatommicrofossil from 40 million years ago | |
| radiolarian | .jpg) | test or shell | 0.1 to 0.2 | %2c_Haeckel_(28187768550).jpg) | elaborate silica shell of a radiolarian | |||
| Calcareous ooze | CaCO3 calcite aragonite limestone marble chalk | foraminiferan | | test or shell | < 1 |  | Calcifiedtest of a planktic foraminiferan. There are about 10,000 living species of foraminiferans[219] | |
| coccolithophore |  | coccoliths | < 0.1[220] | .jpg) | Coccolithophores are the largest global source of biogenic calcium carbonate, and significantly contribute to the global carbon cycle.[221] They are the main constituent of chalk deposits such as thewhite cliffs of Dover. | |||
![Marble can contain protist microfossils of foraminiferans, coccolithophores, calcareous nannoplankton and algae, ostracodes, pteropods, calpionellids and bryozoa[222]](/mt/?noimg=&dark=on&url=http%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aMarmoCipollino_FustoBasMassenzioRoma.jpg)
![Opal can contain protist microfossils of diatoms, radiolarians, silicoflagellates and ebridians[222]](/mt/?noimg=&dark=on&url=http%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aCoober_Pedy_Opal_Doublet.jpg)
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Marine microbenthos are microorganisms that live in thebenthic zone of the ocean – that live near or on the seafloor, or within or on surface seafloor sediments. The wordbenthos comes from Greek, meaning "depth of the sea". Microbenthos are found everywhere on or about the seafloor of continental shelves, as well as in deeper waters, with greater diversity in or on seafloor sediments. In shallow waters,seagrass meadows, coral reefs and kelp forests provide particularly rich habitats. Inphotic zones benthic diatoms dominate as photosynthetic organisms. Inintertidal zones changingtides strongly control opportunities for microbenthos.
Both foraminifera and diatoms haveplanktonic andbenthic forms, that is, they can drift in thewater column or live on sediment at the bottom of the ocean. Either way, their shells end up on the seafloor after they die. These shells are widely used asclimate proxies. The chemical composition of the shells are a consequence of the chemical composition of the ocean at the time the shells were formed. Past water temperatures can be also be inferred from the ratios of stableoxygen isotopes in the shells, since lighter isotopes evaporate more readily in warmer water leaving the heavier isotopes in the shells. Information about past climates can be inferred further from the abundance of forams and diatoms, since they tend to be more abundant in warm water.[223]
The suddenextinction event which killed the dinosaurs 66 million years ago also rendered extinct three-quarters of all other animal and plant species. However, deep-sea benthic forams flourished in the aftermath. In 2020 it was reported that researchers have examined the chemical composition of thousands of samples of these benthic forams and used their findings to build the most detailed climate record of Earth ever.[224][225]
Someendoliths have extremely long lives. In 2013 researchers reported evidence of endoliths in the ocean floor, perhaps millions of years old, with a generation time of 10,000 years.[226] These are slowly metabolizing and not in a dormant state. SomeActinomycetota found inSiberia are estimated to be half a million years old.[227][228][229]
The concept of the holobiont was initially defined byDr. Lynn Margulis in her 1991 bookSymbiosis as a Source of Evolutionary Innovation as an assemblage of ahost and the many other species living in or around it, which together form a discreteecological unit.[231] The components of a holobiont are individual species orbionts, while the combinedgenome of all bionts is thehologenome.[232]
The concept has subsequently evolved since this original definition,[233] with the focus moving to the microbial species associated with the host. Thus the holobiont includes the host,virome,microbiome, and other members, all of which contribute in some way to the function of the whole.[234][235] A holobiont typically includes aeukaryotehost and all of thesymbioticviruses,bacteria,fungi, etc. that live on or inside it.[236]
However, there iscontroversy over whether holobionts can be viewed as single evolutionary units.[237]
Reef-building corals are well-studied holobionts that include the coral itself (a eukaryoticinvertebrate within classAnthozoa),photosyntheticdinoflagellates calledzooxanthellae (Symbiodinium), and associated bacteria and viruses.[238] Co-evolutionary patterns exist for coral microbial communities and coral phylogeny.[239]
![Coral holobiont[240]](/mt/?noimg=&dark=on&url=http%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aTrophic_connections_of_the_coral_holobiont_in_the_planktonic_food_web.jpg)
![Sponge holobiont[241]](/mt/?noimg=&dark=on&url=http%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aThe_sponge_holobiont.webp)
![Seagrass holobiont[242]](/mt/?noimg=&dark=on&url=http%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aProcesses_within_the_seagrass_holobiont.webp)
![Climate change and the rhodolith holobiont[243]](/mt/?noimg=&dark=on&url=http%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aClimate_change_stressors_and_rhodolith_holobiont_fitness.webp)
Marine microorganisms play central roles in themarine food web.
Theviral shunt pathway is a mechanism that prevents marine microbialparticulate organic matter (POM) from migrating uptrophic levels by recycling them intodissolved organic matter (DOM), which can be readily taken up by microorganisms.[244] Viral shunting helps maintain diversity within the microbial ecosystem by preventing a single species of marine microbe from dominating the micro-environment.[245] The DOM recycled by the viral shunt pathway is comparable to the amount generated by the other main sources of marine DOM.[246]
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| The secret Life of plankton |
![Marine snow is the shower of organic particles that falls from upper waters to the deep ocean[248] It is a major exporter of carbon.](/mt/?noimg=&dark=on&url=http%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aMarine_snow.jpg)
Sea ice microbial communities (SIMCO) refer to groups ofmicroorganisms living within and at the interfaces ofsea ice at the poles. The ice matrix they inhabit has strong vertical gradients of salinity, light, temperature and nutrients. Sea ice chemistry is most influenced by the salinity of the brine which affects thepH and the concentration of dissolved nutrients and gases. Thebrine formed during the melting sea ice creates pores and channels in the sea ice in which these microbes can live. As a result of these gradients and dynamic conditions, a higher abundance of microbes are found in the lower layer of the ice, although some are found in the middle and upper layers.[251]
Hydrothermal vents are located where thetectonic plates are moving apart and spreading. This allows water from the ocean to enter into the crust of the earth where it is heated by the magma. The increasing pressure and temperature forces the water back out of these openings, on the way out, the water accumulates dissolved minerals and chemicals from the rocks that it encounters. Vents can be characterized by temperature and chemical composition asdiffuse vents which release clear relatively cool water usually below 30 °C, aswhite smokers which emit milky coloured water at warmer temperatures, about 200-330 °C, and asblack smokers which emit water darkened by accumulated precipitates of sulfide at hot temperatures, about 300-400 °C.[252]
Hydrothermal vent microbial communities are microscopic unicellular organisms that live and reproduce in the chemically distinct area around hydrothermal vents. These include organisms inmicrobial mats, free floating cells, and bacteria inendosymbiotic relationships with animals. Because there is no sunlight at these depths, energy is provided bychemosynthesis where symbiotic bacteria and archaea form the bottom of the food chain and are able to support a variety of organisms such asgiant tube worms andPompeii worms. These organisms utilize this symbiotic relationship in order to utilize and obtain the chemical energy that is released at these hydrothermal vent areas.[253]Chemolithoautotrophic bacteria can derive nutrients and energy from the geological activity at a hydrothermal vent to fix carbon into organic forms.[254]
Viruses are also a part of the hydrothermal vent microbial community and their influence on the microbial ecology in these ecosystems is a burgeoning field of research.[255] Viruses are the most abundant life in the ocean, harboring the greatest reservoir of genetic diversity.[256] As their infections are often fatal, they constitute a significant source of mortality and thus have widespread influence on biological oceanographic processes,evolution andbiogeochemical cycling within the ocean.[257] Evidence has been found however to indicate that viruses found in vent habitats have adopted a moremutualistic thanparasitic evolutionary strategy in order to survive the extreme and volatile environment they exist in.[258] Deep-sea hydrothermal vents were found to have high numbers of viruses indicating high viral production.[259] Like in other marine environments,deep-sea hydrothermal viruses affect abundance and diversity ofprokaryotes and therefore impact microbial biogeochemical cycling bylysing their hosts to replicate.[260] However, in contrast to their role as a source of mortality and population control, viruses have also been postulated to enhance survival of prokaryotes in extreme environments, acting as reservoirs of genetic information. The interactions of the virosphere with microorganisms under environmental stresses is therefore thought to aide microorganism survival through dispersal of host genes throughhorizontal gene transfer.[261]
Thedeep biosphere is that part of thebiosphere that resides below the first few meters of the surface. It extends at least 5 kilometers below the continental surface and 10.5 kilometers below the sea surface, with temperatures that may exceed 100 °C.
Above the surface living organisms consume organic matter and oxygen. Lower down, these are not available, so they make use of "edibles" (electron donors) such as hydrogen released from rocks by various chemical processes, methane, reduced sulfur compounds and ammonium. They "breathe"electron acceptors such as nitrates and nitrites, manganese and iron oxides, oxidized sulfur compounds and carbon dioxide.
There is very little energy at greater depths, and metabolism can be up to a million times slower than at the surface. Cells may live for thousands of years before dividing and there is no known limit to their age. The subsurface accounts for about 90% of thebiomass in bacteria and archaea, and 15% of the total biomass for the biosphere. Eukaryotes are also found, mostly microscopic, but including some multicellular life. Viruses are also present and infect the microbes.
In 2018, researchers from theDeep Carbon Observatory announced thatlife forms, including 70% of the bacteria and archaea on Earth, totaling a biomass of 23 billion tonnescarbon, live up to4.8 km (3.0 mi) deep underground, including2.5 km (1.6 mi) below the seabed.[262][263][264] In 2019 microbial organisms were discovered living 7,900 feet (2,400 m) below the surface,breathing sulfur and eating rocks such aspyrite as their regular food source.[265][266][267] This discovery occurred in the oldest known water on Earth.[268]
In 2020 researchers reported they had found what could be the longest-living life forms ever:aerobic microorganisms which had been inquasi-suspended animation for up to 101.5 million years. The microorganisms were found inorganically poor sediments 68.9 metres (226 feet) below theseafloor in theSouth Pacific Gyre (SPG), "the deadest spot in the ocean".[269][270]
To date biologists have been unable toculture in the laboratory the vast majority of microorganisms. This applies particularly to bacteria and archaea, and is due to a lack of knowledge or ability to supply the required growth conditions.[271][272] The termmicrobial dark matter has come to be used to describe microorganisms scientists know are there but have been unable to culture, and whose properties therefore remain elusive.[271] Microbial dark matter is unrelated to thedark matter of physics and cosmology, but is so-called for the difficulty in effectively studying it. It is hard to estimate its relative magnitude, but the accepted gross estimate is that less than one per cent of microbial species in a givenecological niche is culturable. In recent years effort is being put to decipher more of the microbial dark matter by means of learning theirgenomeDNA sequence from environmental samples[273] and then by gaining insights to their metabolism from their sequenced genome, promoting the knowledge required for their cultivation.
![Estimates of microbial species counts in the three domains of life Bacteria are the oldest and most biodiverse group, followed by Archaea and Fungi (the most recent groups). In 1998, before awareness of the extent of microbial life had gotten underway, Robert M. May[274] estimated there were 3 million species of living organisms on the planet. But in 2016, Locey and Lennon[275] estimated the number of microorganism species could be as high as 1 trillion.[276]](/mt/?noimg=&dark=on&url=http%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aMicrobial_species_present_in_the_three_domains_of_life.png)
![Microbial diversity Comparative representation of the known and estimated (small box) and the yet unknown (large box) microbial diversity, which applies to both marine and terrestrial microorganisms. The text boxes refer to factors that adversely affect the knowledge of the microbial diversity that exists on the planet.[276]](/mt/?noimg=&dark=on&url=http%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aKnown%2c_estimated_and_unknown_microbial_diversity.webp)
Traditionally, thephylogeny of microorganisms was inferred and theirtaxonomy was established based on studies ofmorphology. However, developments inmolecular phylogenetics have allowed evolutionary relationship of species to be established by analyzing deeper characteristics, such as theirDNA andprotein sequences, for exampleribosomal DNA.[278] The lack of easily accessible morphological features, such as those present inanimals andplants, particularly hampered early efforts at classifying bacteria and archaea. This resulted in erroneous, distorted and confused classification, an example of which, notedCarl Woese, isPseudomonas whose etymology ironically matched its taxonomy, namely "false unit".[279] Many bacterial taxa have been reclassified or redefined using molecular phylogenetics.
| External videos | |
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| Microbes don't actually look like anything –Meet the Microcosmos | |
| How to Identify Microbes –Meet the Microcosmos | |
| Differential interference contrast (DIC) –Meet the Microcosmos |
Methods of identifying microorganisms[282] | |||
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| Chromogenic media  | Microscopy techniques  | Biochemical techniques  | Molecular techniques  |
Traditional media | Bright field Dark field SEM TEM CLSM ATM Inverted microscopy | Spectrometry –FTIR –Raman spectrometry Mass spectrometry –GC –LC –MALDI-TOF –ESI Electrokinetic separation Microfluidic chip Propriety methods – Wickerham card –API – BBL Crystal – Vitek – Biolog | PCR Real-time qPCR Rapid PCR PCR sequencing RFLP PFGE Ribotyping WGS MALDI-TOF MS |
Recent developments inmolecular sequencing have allowed for the recovery ofgenomesin situ, directly from environmental samples and avoiding the need for culturing. This has led for example, to a rapid expansion in knowledge of the diversity ofbacterial phyla. These techniques are genome-resolvedmetagenomics andsingle-cell genomics.
The new sequencing technologies and the accumulation of sequence data have resulted in a paradigm shift, highlighted both the ubiquity of microbial communities in association within higher organisms and the critical roles of microbes in ecosystem health.[283] These new possibilities have revolutionizedmicrobial ecology, because the analysis of genomes and metagenomes in a high-throughput manner provides efficient methods for addressing the functional potential of individual microorganisms as well as of whole communities in their natural habitats.[284][285][286]
Omics is a term used informally to refer to branches ofbiology whose names end in the suffix-omics, such asgenomics,proteomics,metabolomics, andglycomics.Marine Omics has recently emerged as a field of research of its own.[288] Omics aims at collectively characterising and quantifying pools of biological molecules that translate into the structure, function, and dynamics of an organism or organisms. For example,functional genomics aims at identifying the functions of as many genes as possible of a given organism. It combines different-omics techniques such as transcriptomics and proteomics with saturatedmutant collections.[289][290]
Many omes beyond the originalgenome have become useful and have been widely adopted in recent years by research scientists. The suffix-omics can provide an easy shorthand to encapsulate a field; for example, aninteractomics study is reasonably recognisable as relating to large-scale analyses of gene-gene, protein-protein, or protein-ligand interactions, whileproteomics has become established as a term for studyingproteins on a large scale.
Any given omics technique, used just by itself, cannot adequately disentangle the intricacies of a hostmicrobiome. Multi-omics approaches are needed to satisfactorily unravel the complexities of the host-microbiome interactions.[291] For instance,metagenomics,metatranscriptomics,metaproteomics andmetabolomics methods are all used to provide information on themetagenome.[292]
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In marine environments, microbialprimary production contributes substantially toCO2 sequestration. Marine microorganisms also recycle nutrients for use in themarine food web and in the process release CO2 to the atmosphere. Microbial biomass and other organic matter (remnants of plants and animals) are converted to fossil fuels over millions of years. By contrast, burning offossil fuels liberates greenhouse gases in a small fraction of that time. As a result, thecarbon cycle is out of balance, and atmospheric CO2 levels will continue to rise as long as fossil fuels continue to be burnt.[6]
Microorganisms have key roles in carbon and nutrient cycling, animal (including human) and plant health, agriculture and the global food web. Microorganisms live in all environments on Earth that are occupied by macroscopic organisms, and they are the sole life forms in other environments, such as the deep subsurface and ‘extreme’ environments. Microorganisms date back to the origin of life on Earth at least 3.8 billion years ago, and they will likely exist well beyond any future extinction events... Unless we appreciate the importance of microbial processes, we fundamentally limit our understanding of Earth's biosphere and response to climate change and thus jeopardize efforts to create an environmentally sustainable future.[6]
Marine microorganisms known as cyanobacteria first emerged in the oceans during thePrecambrian era roughly 2 billion years ago. Over eons, the photosynthesis of marine microorganisms generated by oxygen has helped shape the chemical environment in the evolution of plants, animals and many other life forms. Marine microorganisms were first observed in 1675 by Dutch lensmakerAntonie van Leeuwenhoek.
Modified text was copied from this source, which is available under aCreative Commons Attribution 4.0 International License.Algae is the crop of the future, according to researchers in Geel
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