Flagellum

Nucleoid
   region

Chromosome
      (DNA)

Ribosome

Cell
membrane

Cell wall

Capsule

Pili

Parts of a prokaryote

Cellular life forms can be divided into prokaryotes and eukaryotes. Prokaryotes are bacteria or archaea, and the diagram shows some (clickable) parts shared by both. But bacteria and archaea also have fundamental differences, as indicated by their placement in different domains.

Part of a series of overviews on
Marine life
Habitats Microorganisms Microbiomes Viruses Prokaryotes Protists Fungi Invertebrates Vertebrates Primary production Food web Carbon pump Biogeochemical cycles Human impact Conservation
Marine life portal
v t e

Marine prokaryotes are marinebacteria and marinearchaea. They are defined by their habitat asprokaryotes that live inmarine environments, that is, in thesaltwater of seas or oceans or thebrackish water of coastalestuaries. All cellularlife forms can be divided into prokaryotes and eukaryotes.Eukaryotes areorganisms whose cells have anucleus enclosed withinmembranes, whereas prokaryotes are the organisms that do not have a nucleus enclosed within a membrane.^[1][2][3] Thethree-domain system ofclassifying life adds another division: the prokaryotes are divided into two domains of life, the microscopic bacteria and the microscopic archaea, while everything else, the eukaryotes, become the third domain.^[4]

Prokaryotes play important roles inecosystems asdecomposers recycling nutrients. Some prokaryotes arepathogenic, causing disease and even death in plants and animals.^[5] Marine prokaryotes are responsible for significant levels of thephotosynthesis that occurs in the ocean, as well as significant cycling ofcarbon and othernutrients.^[6]

Prokaryotes live throughout thebiosphere. In 2018 it was estimated the totalbiomass of all prokaryotes on the planet was equivalent to 77 billiontonnes of carbon (77Gt C). This is made up of 7 Gt C for archaea and 70 Gt C for bacteria. These figures can be contrasted with the estimate for the total biomass for animals on the planet, which is about 2 Gt C, and the total biomass of humans, which is 0.06 Gt C.^[7] This means archaea collectively have over 100 times the collective biomass of humans, and bacteria over 1000 times.

There is no clear evidence of life on Earth during the first 600 million years of its existence. When life did arrive, it was dominated for 3,200 million years by the marine prokaryotes. More complex life, in the form ofcrown eukaryotes, did not appear until theCambrian explosion a mere 500 million years ago.^[8]

Timeline of prokaryote domination
−4500 — – −4000 — – −3500 — – −3000 — – −2500 — – −2000 — – −1500 — – −1000 — – −500 — – 0 —	no clear evidence of life marine prokaryotes crown eukaryotes	← Earth (−4540) ← earliest undisputed life ← earliest oxygen ← atmospheric oxygen ← oxygen crisis ← sexual reproduction ← earliest plants ← earliest animals ← Ediacara biota ← Cambrian explosion ← tetrapods ← earliest apes P h a n e r o z o i c P r o t e r o z o i c A r c h e a n H a d e a n Pongola Huronian Cryogenian
(million years ago) Marine prokaryotes have been the dominant life form for most of Earth's history, perhaps because water protected them fromionizing radiation[8]

TheEarth is about 4.54 billion years old.^[9][10][11] The earliest undisputed evidence oflife on Earth dates from at least 3.5 billion years ago,^[12][13] during theEoarchean Era after a geologicalcrust started to solidify following the earlier moltenHadean Eon.Microbial matfossils have been found in 3.48 billion-year-oldsandstone inWestern Australia.^[14][15]

Past species have also left records of their evolutionary history. Fossils, along with the comparative anatomy of present-day organisms, constitute the morphological, or anatomical, record.^[16] By comparing the anatomies of both modern and extinct species, paleontologists can infer the lineages of those species. However, this approach is most successful for organisms that had hard body parts, such as shells, bones or teeth. Further, as prokaryotes such as bacteria and archaea share a limited set of common morphologies, their fossils do not provide information on their ancestry.

Prokaryotes inhabited the Earth from approximately 3–4 billion years ago.^[17][18] No obvious changes inmorphology or cellular organisation occurred in these organisms over the next few billion years.^[19] The eukaryotic cells emerged between 1.6 and 2.7 billion years ago. The next major change in cell structure came when bacteria were engulfed by eukaryotic cells, in a cooperative association calledendosymbiosis.^[20][21] The engulfed bacteria and the host cell then underwent coevolution, with the bacteria evolving into either mitochondria orhydrogenosomes.^[22] Another engulfment ofcyanobacterial-like organisms led to the formation of chloroplasts in algae and plants.^[23]

The history of life was that of theunicellular prokaryotes and eukaryotes until about 610 million years ago when multicellular organisms began to appear in the oceans in theEdiacaran period.^[17][26] Theevolution of multicellularity occurred in multiple independent events, in organisms as diverse assponges,brown algae,cyanobacteria,slime moulds andmyxobacteria.^[27] In 2016 scientists reported that, about 800 million years ago, a minor genetic change in a single molecule called GK-PID may have allowed organisms to go from a single cell organism to one of many cells.^[28]

Soon after the emergence of these first multicellular organisms, a remarkable amount of biological diversity appeared over a span of about 10 million years, in an event called theCambrian explosion. Here, the majority oftypes of modern animals appeared in the fossil record, as well as unique lineages that subsequently became extinct.^[29] Various triggers for the Cambrian explosion have been proposed, including the accumulation ofoxygen in theatmosphere from photosynthesis.^[30]

The words prokaryote and eukaryote come from the Greek wherepro means "before",eu means "well" or "true", andkaryon means "nut", "kernel" or "nucleus".^[31][32][33] So etymologically, prokaryote means "before nucleus" and eukaryote means "true nucleus".

The division of life forms between prokaryotes and eukaryotes was firmly established by the microbiologistsRoger Stanier andC. B. van Niel in their 1962 paper,The concept of a bacterium.^[34] One reason for this classification was so what was then often calledblue-green algae (now calledcyanobacteria) would cease to be classified as plants but grouped with bacteria.

In 1990Carl Woeseet al. introduced thethree-domain system.^[35][36] The prokaryotes were split into twodomains, the archaea and the bacteria, while the eukaryotes become a domain in their own right. The key difference from earlier classifications is the splitting of archaea from bacteria.

microbial mats

Microbial mats are the earliest form of life on Earth for which there is goodfossil evidence. The image shows acyanobacterial-algal mat.

Stromatolites are formed from microbial mats as microbes slowly move upwards to avoid being smothered by sediment.

External videos
How bad was the Great Oxidation Event?

The earliest evidence for life on earth comes frombiogeniccarbon signatures andstromatolite fossils discovered in 3.7 billion-year-old rocks.^[37][38] In 2015, possible "remains ofbiotic life" were found in 4.1 billion-year-old rocks.^[39][40] In 2017 putative evidence of possibly the oldest forms of life on Earth was reported in the form of fossilizedmicroorganisms discovered inhydrothermal vent precipitates that may have lived as early as 4.28 billion years ago, not long after theoceans formed 4.4 billion years ago, and not long after theformation of the Earth 4.54 billion years ago.^[41][42]

Microbial mats of coexistingbacteria andarchaea were the dominant form of life in the earlyArchean Eon and many of the major steps in early evolution are thought to have taken place in this environment.^[43][44] The evolution ofphotosynthesis around 3.5 Ga resulted in a buildup of its waste productoxygen in the atmosphere, leading to thegreat oxygenation event beginning around 2.4 Ga.^[45]

The earliest evidence of eukaryotes dates from 1.85 Ga,^[46][47] and while they may have been present earlier, their diversification accelerated when they started using oxygen in theirmetabolism. Later, around 1.7 Ga,multicellular organisms began to appear, withdifferentiated cells performing specialised functions.^[48]

A stream of airborne microorganisms, including prokaryotes, circles the planet above weather systems but below commercial air lanes.^[52] 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.^[53][54]

Microscopic life undersea is diverse and still poorly understood, such as for the role ofviruses in marine ecosystems.^[55] Most marine viruses arebacteriophages, which are harmless to plants and animals, but are essential to the regulation of saltwater and freshwater ecosystems.^[56]: 5 They infect and destroy bacteria and archaea 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.^[56]: 593 Viral activity may also contribute to thebiological pump, the process wherebycarbon issequestered in the deep ocean.^[57]

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,^[61] 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.^[36]

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.^[62][63] 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.^[64] 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.^[21][65] 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.^[66][67] This is known assecondary endosymbiosis.

Bacteria grow to a fixed size and then reproduce throughbinary fission, a form ofasexual reproduction.^[68] Under optimal conditions, bacteria can grow and divide extremely rapidly, and bacterial populations can double as quickly as every 9.8 minutes.^[69]

Pelagibacter ubique and its relatives may be the most abundant microorganisms 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.^[70] However, it was reported inNature in February 2013 that thebacteriophageHTVC010P, which attacksP. ubique, has been discovered and is probably the most common organism on the planet.^[71][72]

Roseobacter is also one of the most abundant and versatile microorganisms in the ocean. They are diversified across different types of marine habitats, from coastal to open oceans and from sea ice to sea floor, and make up about 25% of coastal marine bacteria. Members of theRoseobacter genus play important roles inmarine biogeochemical cycles and climate change, processing a significant portion of the total carbon in the marine environment. They form symbiotic relationships which allow them to degrade aromatic compounds and uptake trace metals. They are widely used in aquaculture and quorum sensing. During algal blooms, 20–30% of the prokaryotic community are Roseobacter.^[73][74]

The largest known bacterium, the marineThiomargarita namibiensis, can be visible to the naked eye and sometimes attains 0.75 mm (750 μm).^[75][76]

Marine bacteria

The marineThiomargarita namibiensis, largest known bacterium

The bacteriumMarinomonas arctica grows inside Arctic sea ice at subzero temperatures.

Cyanobacteria

Cyanobacteria from amicrobial mat. Cyanobacteria were the first organisms to release oxygen via photosynthesis.

The tiny cyanobacteriumProchlorococcus is a major contributor to atmospheric oxygen.

NASA image of a largebloom ofNodularia cyanobacteria swirling in the Baltic Sea^[77]

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.^[78] 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.^[79][80] 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.^[81]

• 
  Bloom of thefilamentous cyanobacteriaTrichodesmium
• 
  Cyanobacteriablooms can contain lethalcyanotoxins.

• 
  Synechococcus, a widespread marine cyanobacterium
• 
  Carboxysomes appearing as polyhedral dark structures within a species ofSynechococcus

The tiny (0.6 μm) 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^[84] and an estimated 20% of the oxygen in the Earth's atmosphere.^[85] It is possibly the most plentiful genus on Earth: a single millilitre of surface seawater may contain 100,000 cells or more.^[86]

Originally, biologists classifiedcyanobacteria as an 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.^[87][88]

External videos
How cyanobacteria took over the world

Other marine bacteria, apart from cyanobacteria, are ubiquitous or can play important roles in the ocean. These include the opportunisticcopiotroph,Alteromonas macleodii.^[89][90]

Thearchaea (Greek forancient^[92]) 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.^[93] 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.

Bacteria and archaea 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.^[94] 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 called Crenarchaeota or eocytes) are a phylum of archaea thought to be very abundant in marine environments and one of the main contributors to the fixation of carbon.^[95]

• 
  Eocytes may be the most abundant of marine archaea
• 
  Halobacteria, found in water near saturated with salt, are now recognised as archaea.
• 
  Flat, square-shaped cells of the archaeaHaloquadratum walsbyi
• 
  Methanosarcina barkeri, a marine archaea that producesmethane
• 
  Thermophiles, such asPyrolobus fumarii, survive well over 100 °C (212 °F).

Nanoarchaeum equitans is a species of marine archaea discovered in 2002 in ahydrothermal vent. It is athermophile that grows in temperatures at about 80 °C (176 °F).Nanoarchaeum appears to be anobligate symbiont on thearchaeonIgnicoccus. It must stay in contact with the host organism to survive sinceNanoarchaeum equitans cannot synthesize lipids but obtains them from its host. Its cells are only 400nm in diameter, making it one of the smallest known cellular organisms, and the smallest known archaeon.^[96][97]

Marine archaea have been classified as follows:^{[98][99][100][101][102]}

• Marine Group I (MG-I or MGI): marineNitrososphaerota with subgroups Ia (aka I.a) up to Id
• Marine Group II (MG-II): marineEuryarchaeota, orderPoseidoniales^[103] with subgroups IIa up to IId (IIa resemblingPoseidoniaceae, IIb resemblingThalassarchaceae)
  Viruses parasiting MGII are classified asmagroviruses
• Marine Group III (MG-III): also marine Euryarchaeota, Marine Benthic Group D^[104]
• Marine Group IV (MG-IV): also marine Euryarchaeota^[105]

Prokaryote metabolism is classified intonutritional groups on the basis of three major criteria: the source ofenergy, theelectron donors used, and the source ofcarbon used for growth.^[106][107]

Marine prokaryotes have diversified greatly throughout their long existence. The metabolism of prokaryotes is far more varied than that of eukaryotes, leading to many highly distinct prokaryotic types. For example, in addition to usingphotosynthesis ororganic compounds for energy, as eukaryotes do, marine prokaryotes may obtain energy frominorganic compounds such ashydrogen sulfide. This enables marine prokaryotes to thrive asextremophiles in harsh environments as cold as the ice surface of Antarctica, studied incryobiology, as hot as underseahydrothermal vents, or in high saline conditions as (halophiles).^[108] Some marine prokaryotes livesymbiotically in or on the bodies of other marine organisms.

• Phototrophy is a particularly significant marker that should always play a primary role in bacterial classification.^[109]
• Aerobic anoxygenic phototrophic bacteria (AAPBs) are widely distributed marineplankton that may constitute over 10% of the open ocean microbial community. Marine AAPBs are classified in two marine (Erythrobacter andRoseobacter) genera. They can be particularly abundant inoligotrophic conditions where they were found to be 24% of the community.^[110] These areheterotrophic organisms that use light to produce energy, but are unable to utilise carbon dioxide as their primary carbon source. Most are obligatelyaerobic, meaning they require oxygen to grow. Current data suggests thatmarine bacteria have generation times of several days, whereas new evidence exists that shows AAPB to have a much shorter generation time.^[111] Coastal/shelf waters often have greater amounts of AAPBs, some as high as 13.51% AAPB%. Phytoplankton also affect AAPB%, but little research has been performed in this area.^[112] They can also be abundant in variousoligotrophic conditions, including the most oligotrophic regime of the world ocean.^[113] They are globally distributed in theeuphotic zone and represent a hitherto unrecognized component of the marine microbial community that appears to be critical to the cycling of both organic and inorganic carbon in the ocean.^[114]
• Purple bacteria:
• Zetaproteobacteria: areiron-oxidizingneutrophilicchemolithoautotrophs, distributed worldwide in estuaries and marine habitats.
• Hydrogen oxidizing bacteria are facultative autotrophs that can be divided into aerobes and anaerobes. The former usehydrogen as anelectron donor and oxygen as an acceptor while the latter use sulphate or nitrogen dioxide aselectron acceptors.^[115]

Motility is the ability of anorganism to move independently, using metabolic energy.

Model of anarchaellum ^[116]

Prokaryotic flagella run in a rotary movement, while eukaryotic flagella run in a bending movement

Prokaryotes, both bacteria and archaea, primarily useflagella for locomotion.

• Bacterial flagella are helical filaments, each with arotary motor at its base which can turn clockwise or counterclockwise.^{[117][118][119]} They provide two of several kinds of bacterial motility.^[120][121]
• Archaeal flagella are calledarchaella, and function in much the same way as bacterial flagella. Structurally the archaellum is superficially similar to a bacterial flagellum, but it differs in many details and is considered non-homologous.^[122][116]

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.^[123]

Some eukaryotic cells also use flagella—and they can be found in some protists and plants as well as animal cells. Eukaryotic flagella are complex cellular projections that lash back and forth, rather than in a circular motion. Prokaryotic flagella use a rotary motor, and the eukaryotic flagella use a complex sliding filament system. Eukaryotic flagella areATP-driven, while prokaryotic flagella can be ATP-driven (archaea) orproton-driven (bacteria).^[124]

External videos
Why do bacteria move like vibrating chaos snakes?

Twitching motility is a form of crawling bacterial motility used to move over surfaces. Twitching is mediated by the activity of hair-like filaments called type IVpili which extend from the cell's exterior, bind to surrounding solid substrates and retract, pulling the cell forwards in a manner similar to the action of agrappling hook.^{[125][126][127]} The nametwitching motility is derived from the characteristic jerky and irregular motions of individual cells when viewed under the microscope.^[128]

Gliding motility is a type of translocation that is independent of propulsive structures such asflagella orpili.^[129] Gliding allows microorganisms to travel along the surface of low aqueous films. The mechanisms of this motility are only partially known. The speed of gliding varies between organisms, and the reversal of direction is seemingly regulated by some sort of internal clock.^[130] For example, theapicomplexans are able to travel at fast rates between 1–10 μm/s. In contrastMyxococcus xanthus bacteria glide at a rate of 5 μm/min.^[131][132]

Swarming motility is a rapid (2–10 μm/s) and coordinated translocation of a bacterial population across solid or semi-solid surfaces,^[133] and is an example of bacterial multicellularity andswarm behaviour. Swarming motility was first reported in 1972 by Jorgen Henrichsen.^[128]

Non-motile species lack the ability and structures that would allow them to propel themselves, under their own power, through their environment. When non-motile bacteria are cultured in a stab tube, they only grow along the stab line. If the bacteria are mobile, the line will appear diffuse and extend into the medium.^[134]

Magnetosome chain with octahedral habits modelled lower right^[135]

Magnetotactic bacteria orient themselves along themagnetic field lines ofEarth's magnetic field.^[136] This alignment is believed to aid these organisms in reaching regions of optimal oxygen concentration.^[137] To perform this task, these bacteria havebiomineralisedorganelles calledmagnetosomes that contain magneticcrystals. The biological phenomenon of microorganisms tending to move in response to the environment's magnetic characteristics is known asmagnetotaxis. However, this term is misleading in that every other application of the termtaxis involves a stimulus-response mechanism. In contrast to themagnetoreception of animals, the bacteria contain fixed magnets that force the bacteria into alignment—even dead cells are dragged into alignment, just like a compass needle.^[137]

Marine environments are generally characterized by low concentrations of nutrients kept in steady or intermittent motion by currents and turbulence. Marine bacteria have developed strategies, such as swimming and using directional sensing–response systems, to migrate towards favorable places in the nutrient gradients. Magnetotactic bacteria utilize Earth's magnetic field to facilitate downward swimming into the oxic–anoxic interface, which is the most favorable place for their persistence and proliferation, in chemically stratified sediments or water columns.^[138]

Depending on their latitude and whether the bacteria are north or south of the equator, the Earth's magnetic field has one of the two possible polarities, and a direction that points with varying angles into the ocean depths, and away from the generally more oxygen rich surface.Aerotaxis is the response by which bacteria migrate to an optimal oxygen concentration in an oxygen gradient. Various experiments have clearly shown thatmagnetotaxis and aerotaxis work in conjunction in magnetotactic bacteria. It has been shown that, in water droplets, one-way swimming magnetotactic bacteria can reverse their swimming direction and swim backwards underreducing conditions (less than optimaloxygen concentration), as opposed to oxic conditions (greater than optimal oxygen concentration).

Regardless of their morphology, all magnetotactic bacteria studied so far are motile by means of flagella.^[139] Marine magnetotactic bacteria in particular tend to possess an elaborate flagellar apparatus which can involve up to tens of thousands of flagella. However, despite extensive research in recent years, it has yet to be established whether magnetotactic bacteria steer their flagellar motors in response to their alignment in magnetic fields.^[138]Symbiosis with magnetotactic bacteria has been proposed as the explanation formagnetoreception in somemarine protists.^[140] Research is underway on whether a similar relationship may underlie magnetoreception invertebrates as well.^[141] The oldest unambiguous magnetofossils come from theCretaceous chalk beds of southern England,^[142] though less certain reports of magnetofossils extend to 1.9 billion years oldGunflint Chert.^[143]

Part of a series on
Plankton
Trophic mode Phytoplankton Zooplankton Mixoplankton Mycoplankton Bacterioplankton Virioplankton
By size Heterotrophic picoplankton Microalgae Microzooplankton Nanophytoplankton calcareous Photosynthetic picoplankton Picoeukaryote Picoplankton Marine microplankton
By taxonomy Algae diatoms coccolithophores dinoflagellates Protozoans radiolarians foraminiferans amoebae ciliates Bacteria cyanobacteria Archaea Viruses
By habitat Aeroplankton Geoplankton Marine plankton prokaryotes protists Freshwater plankton
Other types Gelatinous zooplankton Holoplankton Ichthyoplankton Meroplankton Pseudoplankton Tychoplankton
Blooms Algal bloom Critical depth Cyanobacterial bloom Harmful algal bloom Spring bloom Eutrophication Great Atlantic Sargassum Belt Great Calcite Belt Milky seas effect
Related topics Algaculture CLAW hypothesis CPR Diel vertical migration f-ratio Marine primary production Ocean fertilization iron Paradox of the plankton Planktivore Planktology Thin layers NAAMES
Category
v t e

Some marine prokaryotes possessgas vacuoles. Gas vacuole arenanocompartments freely permeable to gas which allow marine bacteria and archaea to control theirbuoyancy. They take the form of spindle-shaped membrane-boundvesicles, and are found in someplankton prokaryotes, including someCyanobacteria.^[144] Positive buoyancy is needed to keep the cells in the upper reaches of the water column, so that they can continue to performphotosynthesis. Gas vacuoles are made up of a shell of protein that has a highlyhydrophobic inner surface, making it impermeable to water (and stopping water vapour from condensing inside) but permeable to mostgases. Because the gas vesicle is a hollow cylinder, it is liable to collapse when the surroundingpressure increases. Natural selection has fine tuned the structure of the gas vesicle to maximise its resistance tobuckling, including an external strengthening protein, GvpC, rather like the green thread in a braided hosepipe. There is a simple relationship between the diameter of the gas vesicle and pressure at which it will collapse—the wider the gas vesicle the weaker it becomes. However, wider gas vesicles are more efficient, providing more buoyancy per unit of protein than narrow gas vesicles. Different species produce gas vesicle of different diameter, allowing them to colonise different depths of the water column (fast growing, highly competitive species with wide gas vesicles in the top most layers; slow growing, dark-adapted, species with strong narrow gas vesicles in the deeper layers).

The cell achieves its height in the water column by synthesising gas vesicles. As the cell rises up, it is able to increase itscarbohydrate load through increased photosynthesis. Too high and the cell will suffer photobleaching and possible death, however, the carbohydrate produced during photosynthesis increases the cell's density, causing it to sink. The daily cycle of carbohydrate build-up from photosynthesis and carbohydratecatabolism during dark hours is enough to fine-tune the cell's position in the water column, bring it up toward the surface when its carbohydrate levels are low and it needs to photosynthesis, and allowing it to sink away from the harmfulUV radiation when the cell's carbohydrate levels have been replenished. An extreme excess of carbohydrate causes a significant change in the internal pressure of the cell, which causes the gas vesicles to buckle and collapse and the cell to sink out.

Large vacuoles are found in threegenera of filamentoussulfur bacteria, theThioploca,Beggiatoa andThiomargarita. The cytosol is extremely reduced in these genera and the vacuole can occupy between 40 and 98% of the cell.^[145] The vacuole contains high concentrations ofnitrate ions and is therefore thought to be a storageorganelle.^[146]

Hawaiian bobtail squid live in symbiosis with bioluminescent bacteria.

Vibrio harveyi incubated in seawater at 30 °C for 3 days^[147]

Bioluminescent bacteria arelight-producingbacteria that are predominantly present in sea water, marine sediments, the surface of decomposing fish and in the gut of marine animals. While not as common, bacterial bioluminescence is also found in terrestrial and freshwater bacteria.^[125] These bacteria may be free living (such asVibrio harveyi) or in symbiosis with animals such as theHawaiian bobtail squid (Aliivibrio fischeri) or terrestrialnematodes (Photorhabdus luminescens). The host organisms provide these bacteria a safe home and sufficient nutrition. In exchange, the hosts use the light produced by the bacteria for camouflage, prey and/or mate attraction. Bioluminescent bacteria have evolved symbiotic relationships with other organisms in which both participants benefit close to equally.^[148] Another possible reason bacteria use luminescence reaction is forquorum sensing, an ability to regulate gene expression in response to bacterial cell density.^[149]

The Hawaiian bobtail squid lives in symbiosis with the bioluminescent bacteriaAliivibrio fischeri which inhabits a special light organ in the squid's mantle. The bacteria are fed sugar andamino acid by the squid and in return hide the squid's silhouette when viewed from below,counter-illuminating it by matching the amount of light hitting the top of the mantle.^[150] The squid serves as a model organism for animal-bacterial symbiosis and its relationship with the bacteria has been widely studied.

Vibrio harveyi is a rod-shaped, motile (via polarflagella) bioluminescent bacterium which grows optimally between 30 and 35 °C (86 and 95 °F). It can be found free-swimming in tropical marine waters,commensally in the gut microflora ofmarine animals, and as both a primary andopportunistic pathogen of a number of marine animals.^[151] It is thought to be the cause of themilky seas effect, where a uniform blue glow is emitted from seawater during the night. Some glows can cover nearly 6,000 sq mi (16,000 km²).

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.^[153]

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^[155][156] 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.^[157] 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.^{[155][158][159]}

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."^[160][153]

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.^[161][162]

Some marine organisms have asymbiosis with bacteria or archaea. Pompeii worms live at great depths by hydrothermal vents at temperatures up to 80 °C (176 °F). They have what appear to be hairy backs, but these "hairs" are actually colonies of bacteria such asNautilia profundicola, which are thought to afford the worm some degree of insulation. Glands on the worm's back secrete a mucus on which the bacteria feed, a form ofsymbiosis.

• 
  The "hairy" backs ofPompeii worms are colonies of symbiotic bacteria.
• 
  Hesiocaeca methanicola lives at great depths onmethane ice and appear to survive in symbiosis with bacteria whichmetabolize theclathrate.^[163]
• 
  Olavius algarvensis depends on five different species of symbiotic bacteria for its nutrition.
• 
  EpiphyticCalothrixcyanobacteria (arrows) in symbiosis with aChaetoceros diatom. Scale bar 50 μm.

Endosymbiont bacteria are bacteria that live within the body or cells of another organism. Some types of cyanobacteria areendosymbiont and cyanobacteria have been found to possess genes that enable them to undergo nitrogen fixation.^[165]

Organisms typically establish a symbiotic relationship due to their limited availability of resources in their habitat or due to a limitation of their food source. Symbiotic, chemosynthetic bacteria that have been discovered associated with mussels (Bathymodiolus) located near hydrothermal vents have a gene that enables them to utilize hydrogen as a source of energy, in preference to sulphur or methane as their energy source for production of energy.^[166]

Olavius algarvensis is a worm which lives in coastal sediments in the Mediterranean and depends on symbiotic bacteria for its nutrition. It lives with five different species of bacteria located under its cuticle: twosulfide-oxidizing, twosulfate-reducing and onespirochaete. The symbiotic bacteria also allow the worm to usehydrogen andcarbon monoxide as energy sources, and to metabolise organic compounds likemalate andacetate.^[167][168]

Astrangia poculata, the northern star coral, is a temperatestony coral, widely documented along the eastern coast of the United States. The coral can live with and withoutzooxanthellae (algal symbionts), making it an idealmodel organism to study microbial community interactions associated with symbiotic state. However, the ability to developprimers andprobes to more specifically target key microbial groups has been hindered by the lack of full length16S rRNA sequences, since sequences produced by the Illumina platform are of insufficient length (approximately 250 base pairs) for the design of primers and probes.^[169] In 2019, Goldsmith et al. demonstratedSanger sequencing was capable of reproducing the biologically-relevant diversity detected by deepernext-generation sequencing, while also producing longer sequences useful to the research community for probe and primer design (see diagram on right).^[170]

Most of the volume of the world ocean is in darkness. The processes occurring within the thin illuminated surface layer (thephotic layer from the surface down to between 50 and 170 metres) are of major significance to the global biosphere. For example, the visible region of the solar spectrum (the so-calledphotosynthetically available radiation or PAR) reaching this sunlit layer fuels about half of theprimary productivity of the planet, and is responsible for about half of the atmospheric oxygen necessary for most life on Earth.^[172][173]

Heterotrophicbacterioplankton are main consumers ofdissolved organic matter (DOM) inpelagic marine food webs, including the sunlit upper layers of the ocean. Their sensitivity to ultraviolet radiation (UVR), together with some recently discovered mechanisms bacteria have evolved to benefit from photosynthetically available radiation (PAR), suggest that natural sunlight plays a relevant, yet difficult to predict role in modulating bacterial biogeochemical functions in the oceans.^[171]

Ocean surface habitats sit at the interface between the atmosphere and the ocean. Thebiofilm-like habitat at the surface of the ocean harbours surface-dwelling microorganisms, commonly referred to asneuston. This vast air–water interface sits at the intersection of major air–water exchange processes spanning more than 70% of the global surface area . Bacteria in the surface microlayer of the ocean, calledbacterioneuston, are of interest due to practical applications such as air-sea gas exchange of greenhouse gases, production of climate-active marine aerosols, and remote sensing of the ocean.^[174] Of specific interest is the production and degradation ofsurfactants (surface active materials) via microbial biochemical processes. Major sources of surfactants in the open ocean include phytoplankton,^[175] terrestrial runoff, and deposition from the atmosphere.^[174]

Unlike coloured algal blooms, surfactant-associated bacteria may not be visible in ocean colour imagery. Having the ability to detect these "invisible" surfactant-associated bacteria usingsynthetic aperture radar has immense benefits in all-weather conditions, regardless of cloud, fog, or daylight.^[174] This is particularly important in very high winds, because these are the conditions when the most intense air-sea gas exchanges and marine aerosol production take place. Therefore, in addition to colour satellite imagery, SAR satellite imagery may provide additional insights into a global picture of biophysical processes at the boundary between the ocean and atmosphere, air-sea greenhouse gas exchanges and production of climate-active marine aerosols.^[174]

The diagram on the right shows links among the ocean'sbiological pump and the pelagic food web and the ability to sample these components remotely from ships, satellites, and autonomous vehicles. Light blue waters are theeuphotic zone, while the darker blue waters represent thetwilight zone.^[176]

Archaea recycle elements such ascarbon,nitrogen, andsulfur through their various habitats.^[177] Archaea carry out many steps in thenitrogen cycle. This includes both reactions that remove nitrogen from ecosystems (such asnitrate-based respiration anddenitrification) as well as processes that introduce nitrogen (such as nitrate assimilation andnitrogen fixation).^[178][179]

Researchers recently discovered archaeal involvement inammonia oxidation reactions. These reactions are particularly important in the oceans.^[180][181] In thesulfur cycle, archaea that grow by oxidizingsulfur compounds release this element from rocks, making it available to other organisms, but the archaea that do this, such asSulfolobus, producesulfuric acid as a waste product, and the growth of these organisms in abandoned mines can contribute toacid mine drainage and other environmental damage.^[182] In thecarbon cycle, methanogen archaea remove hydrogen and play an important role in the decay of organic matter by the populations of microorganisms that act asdecomposers in anaerobic ecosystems, such as sediments and marshes.^[183]

• Bacterioplankton counting methods
• Bioluminescent bacteria
• Iron-oxidizing bacteria
• Pelagibacterales – model organisms in streamlining theory
• Streamlining theory

 This article incorporatestext available under theCC BY 3.0 license. This article incorporatestext available under theCC BY 4.0 license.

• Microorganisms
• Marine organisms
• Planktology
• Biological oceanography
• Marine biology

• Webarchive template wayback links
• CS1: long volume value
• Articles with short description
• Short description matches Wikidata
• Use British English from August 2021
• Use dmy dates from August 2021
• Articles with imported Creative Commons Attribution 3.0 text
• Articles with imported Creative Commons Attribution 4.0 text