Phytoplankton obtain their energy throughphotosynthesis, as trees and other plants do on land. This means phytoplankton must have light from the sun, so they live in the well-lit surface layers (euphotic zone) of oceans and lakes. In comparison with terrestrial plants, phytoplankton are distributed over a larger surface area, are exposed to less seasonal variation and have markedly faster turnover rates than trees (days versus decades). As a result, phytoplankton respond rapidly on a global scale to climate variations.[citation needed]
Phytoplankton form the base of marine and freshwater food webs[4][5][6] and are key players in the globalcarbon cycle.[7][8][9] They account for about half of global photosynthetic activity[10][11][7] and at least half of the oxygen production,[12][13][7][8] despite amounting to only about 1% of the global plant biomass.[12][13][14][15]
Phytoplankton are very diverse, comprising photosynthesizing bacteria (cyanobacteria) and various unicellularprotist groups (notably thediatoms).
Most phytoplankton are too small to be individually seen with theunaided eye. However, when present in high enough numbers, some varieties may be noticeable as colored patches on the water surface due to the presence ofchlorophyll within their cells and accessory pigments (such asphycobiliproteins orxanthophylls) in some species.
Phytoplankton arephotosynthesizing microscopic protists and bacteria that inhabit the upper sunlit layer of marine and fresh water bodies of water on Earth. Paralleling plants on land, phytoplankton undertakeprimary production in water,[2] creatingorganic compounds fromcarbon dioxide dissolved in the water. Phytoplankton form the base of — and sustain — the aquaticfood web,[16] and are crucial players in the Earth'scarbon cycle.[17]
Diatoms are one of the most common types of phytoplankton
Diatoms have asilica shell (frustule) with radial (centric) or bilateral (pennate) symmetry
Phytoplankton are very diverse, comprising photosynthesizing bacteria (cyanobacteria) and various unicellularprotist groups (notably thediatoms). Many other organism groups formally named as phytoplankton, includingcoccolithophores anddinoflagellates, are now no longer included as they are not onlyphototrophic but can also eat other organisms.[18] These organisms are now more correctly termed mixoplankton.[19] This recognition has important consequences for how the functioning of the planktonic food web is viewed.[20]
This visualization shows a model simulation of the dominant phytoplankton types averaged over the period 1994–1998. * Red =diatoms (big phytoplankton, which need silica) * Yellow =flagellates (other big phytoplankton) * Green =prochlorococcus (small phytoplankton that cannot use nitrate) * Cyan =synechococcus (other small phytoplankton) Opacity indicates concentration of the carbon biomass. In particular, the role of the swirls and filaments (mesoscale features) appear important in maintaining high biodiversity in the ocean.[17][21]
Phytoplankton live in thephotic zone of the ocean, wherephotosynthesis is possible. During photosynthesis, they assimilate carbon dioxide and release oxygen. If solar radiation is too high, phytoplankton may fall victim tophotodegradation. Phytoplankton species feature a large variety of photosyntheticpigments which species-specifically enables them to absorb differentwavelengths of the variable underwater light.[26] This implies different species can use the wavelength of light different efficiently and the light is not a singleecological resource but a multitude of resources depending on its spectral composition.[27] By that it was found that changes in the spectrum of light alone can alter natural phytoplankton communities even if the sameintensity is available.[28] For growth, phytoplankton cells additionally depend on nutrients, which enter the ocean by rivers, continental weathering, and glacial ice meltwater on the poles. Phytoplankton releasedissolved organic carbon (DOC) into the ocean. Since phytoplankton are the basis ofmarine food webs, they serve as prey forzooplankton,fish larvae and otherheterotrophic organisms. They can also be degraded by bacteria or byviral lysis. Although some phytoplankton cells, such asdinoflagellates, are able to migrate vertically, they are still incapable of actively moving against currents, so they slowly sink and ultimately fertilize the seafloor with dead cells anddetritus.[29]
Phytoplankton are crucially dependent on a number ofnutrients. These are primarilymacronutrients such asnitrate,phosphate orsilicic acid, which are required in relatively large quantities for growth. Their availability in the surface ocean is governed by the balance between the so-calledbiological pump andupwelling of deep, nutrient-rich waters. Thestoichiometric nutrient composition of phytoplankton drives — and is driven by — theRedfield ratio of macronutrients generally available throughout the surface oceans. Phytoplankton also rely on trace metals such as iron (Fe), manganese (Mn), zinc (Zn), cobalt (Co), cadmium (Cd) and copper (Cu) as essential micronutrients, influencing their growth and community composition.[30] Limitations in these metals can lead to co-limitations and shifts in phytoplankton community structure.[31][32] Across large areas of the oceans such as theSouthern Ocean, phytoplankton are often limited by the lack of themicronutrientiron.[33] This has led to some scientists advocatingiron fertilization as a means to counteract the accumulation ofhuman-produced carbon dioxide (CO2) in theatmosphere.[34] Large-scale experiments have added iron (usually as salts such asferrous sulfate) to the oceans to promote phytoplankton growth and drawatmospheric CO2 into the ocean. Controversy about manipulating the ecosystem and the efficiency of iron fertilization has slowed such experiments.[35][36] The ocean science community still has a divided attitude toward the study of iron fertilization as a potential marine Carbon Dioxide Removal (mCDR) approach.[37][38]
Phytoplankton depend onB vitamins for survival. Areas in the ocean have been identified as having a major lack of some B Vitamins, and correspondingly, phytoplankton.[39]
The effects ofanthropogenic warming on the global population of phytoplankton is an area of active research. Changes in the vertical stratification of the water column, the rate of temperature-dependent biological reactions, and the atmospheric supply of nutrients are expected to have important effects on future phytoplankton productivity.[40][41]
Bioluminescence in phytoplankton triggered by the agitation of waves crashing on a beach
The effects of anthropogenic ocean acidification on phytoplankton growth and community structure has also received considerable attention. The cells of coccolithophore phytoplankton are typically covered in a calcium carbonate shell called acoccosphere that is sensitive to ocean acidification. Because of their short generation times, evidence suggests some phytoplankton can adapt to changes in pH induced by increased carbon dioxide on rapid time-scales (months to years).[42][43]
Phytoplankton serve as the base of the aquatic food web, providing an essential ecological function for all aquatic life. Under future conditions of anthropogenic warming and ocean acidification, changes in phytoplankton mortality due to changes in rates ofzooplankton grazing may be significant.[44] One of the manyfood chains in the ocean – remarkable due to the small number of links – is that of phytoplankton sustainingkrill (acrustacean similar to a tiny shrimp), which in turn sustainbaleen whales.
The El Niño-Southern Oscillation (ENSO) cycles in the Equatorial Pacific area can affect phytoplankton.[45] Biochemical and physical changes during ENSO cycles modify the phytoplankton community structure.[45] Also, changes in the structure of the phytoplankton, such as a significant reduction in biomass and phytoplankton density, particularly during El Nino phases can occur.[46] The sensitivity of phytoplankton to environmental changes is why they are often used as indicators of estuarine and coastal ecological condition and health.[47] To study these events satellite ocean color observations are used to observe these changes. Satellite images help to have a better view of their global distribution.[45]
When two currents collide (here theOyashio andKuroshio currents) they createeddies. Phytoplankton concentrates along the boundaries of the eddies, tracing the motion of the water.
In the early twentieth century,Alfred C. Redfield found the similarity of the phytoplankton's elemental composition to the major dissolved nutrients in the deep ocean.[53] 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 are remineralized. This so-called "Redfield ratio" in describingstoichiometry of phytoplankton and seawater has become a fundamental principle to understand marine ecology, biogeochemistry and phytoplankton evolution.[54] However, the Redfield ratio is not a universal value and it may diverge due to the changes in exogenous nutrient delivery[55] and microbial metabolisms in the ocean, such asnitrogen fixation,denitrification andanammox.
The dynamic stoichiometry shown in unicellular algae reflects their capability to store nutrients in an internal pool, shift between enzymes with various nutrient requirements and alter osmolyte composition.[56][57] Different cellular components have their own unique stoichiometry characteristics,[54] for instance, resource (light or nutrients) acquisition machinery such as proteins and chlorophyll contain a high concentration of nitrogen but low in phosphorus. Meanwhile, growth machinery such as ribosomal RNA contains high nitrogen and phosphorus concentrations.
Based on allocation of resources, phytoplankton is classified into three different growth strategies, namely survivalist, bloomer[58] and generalist. Survivalist phytoplankton has a high ratio of N:P (>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.
TheNAAMES study was a five-year scientific research program conducted between 2015 and 2019 by scientists fromOregon State University andNASA to investigated aspects of phytoplankton dynamics in ocean ecosystems, and how such dynamics influenceatmospheric aerosols, clouds, and climate (NAAMES stands for the North Atlantic Aerosols and Marine Ecosystems Study). The study focused on the sub-arctic region of the North Atlantic Ocean, which is the site of one of Earth's largest recurring phytoplankton blooms. The long history of research in this location, as well as relative ease of accessibility, made the North Atlantic an ideal location to test prevailing scientific hypotheses[59] in an effort to better understand the role of phytoplankton aerosol emissions on Earth's energy budget.[60]
NAAMES was designed to target specific phases of the annual phytoplankton cycle: minimum, climax and the intermediary decreasing and increasing biomass, in order to resolve debates on the timing of bloom formations and the patterns driving annual bloom re-creation.[60] The NAAMES project also investigated the quantity, size, and composition of aerosols generated byprimary production in order to understand how phytoplankton bloom cycles affect cloud formations and climate.[61]
Competing hypothesis of plankton variability[59] Figure adapted from Behrenfeld & Boss 2014.[62] Courtesy of NAAMES, Langley Research Center, NASA[63]
World concentrations of surface ocean chlorophyll as viewed by satellite during the northern spring, averaged from 1998 to 2004. Chlorophyll is a marker for the distribution and abundance of phytoplankton.
Global patterns of monthly phytoplankton species richness and species turnover
(A) Annual mean of monthly species richness and (B) month-to-month species turnover projected by SDMs. Latitudinal gradients of (C) richness and (D) turnover. Colored lines (regressions with local polynomial fitting) indicate the means per degree latitude from three different SDM algorithms used (red shading denotes ±1 SD from 1000 Monte Carlo runs that used varying predictors for GAM). Poleward of the thin horizontal lines shown in (C) and (D), the model results cover only <12 or <9 months, respectively.[64]
Environmental factors that affect phytoplankton productivity [65][66]
Phytoplankton are the key mediators of thebiological pump. Understanding the response of phytoplankton to changing environmental conditions is a prerequisite to predict future atmospheric concentrations of CO2. Temperature, irradiance and nutrient concentrations, along with CO2 are the chief environmental factors that influence the physiology andstoichiometry of phytoplankton.[67] The stoichiometry or elemental composition of phytoplankton is of utmost importance to secondary producers such as copepods, fish and shrimp, because it determines the nutritional quality and influences energy flow through themarine food chains.[68]Climate change may greatly restructure phytoplankton communities leading tocascading consequences formarine food webs, thereby altering the amount of carbon transported to the ocean interior.[69][65]
The figure gives an overview of the various environmental factors that together affectphytoplankton productivity. All of these factors are expected to undergo significant changes in the future ocean due to global change.[70] Global warming simulations predict oceanic temperature increase; dramatic changes inoceanic stratification, circulation and changes in cloud cover and sea ice, resulting in an increased light supply to the ocean surface. Also, reduced nutrient supply is predicted to co-occur with ocean acidification and warming, due to increased stratification of the water column and reduced mixing of nutrients from the deep water to the surface.[71][65]
Role of phytoplankton on various compartments of the marine environment [72]
The compartments influenced by phytoplankton include the atmospheric gas composition, inorganic nutrients, and trace element fluxes as well as the transfer and cycling of organic matter via biological processes (see figure). The photosynthetically fixed carbon is rapidly recycled and reused in the surface ocean, while a certain fraction of this biomass is exported as sinking particles to the deep ocean, where it is subject to ongoing transformation processes, e.g., remineralization.[72]
Phytoplankton contribute to not only a basic pelagic marine food web but also to the microbial loop. Phytoplankton are the base of the marine food web and because they do not rely on other organisms for food, they make up the first trophic level. Organisms such as zooplankton feed on these phytoplankton which are in turn fed on by other organisms and so forth until the fourth trophic level is reached with apex predators. Approximately 90% of total carbon is lost between trophic levels due to respiration, detritus, and dissolved organic matter. This makes the remineralization process and nutrient cycling performed by phytoplankton and bacteria important in maintaining efficiency.[73]
Phytoplankton blooms in which a species increases rapidly under conditions favorable to growth can produceharmful algal blooms (HABs).
Phytoplankton are a key food item in bothaquaculture andmariculture. Both utilize phytoplankton as food for the animals being farmed. In mariculture, the phytoplankton is naturally occurring and is introduced into enclosures with the normal circulation of seawater. In aquaculture, phytoplankton must be obtained and introduced directly. The plankton can either be collected from a body of water or cultured, though the former method is seldom used. Phytoplankton is used as a foodstock for the production ofrotifers,[74] which are in turn used to feed other organisms. Phytoplankton is also used to feed many varieties of aquaculturedmolluscs, includingpearloysters andgiant clams. A 2018 study estimated the nutritional value of natural phytoplankton in terms of carbohydrate, protein and lipid across the world ocean using ocean-colour data from satellites,[75] and found the calorific value of phytoplankton to vary considerably across different oceanic regions and between different time of the year.[75][76]
The production of phytoplankton under artificial conditions is itself a form of aquaculture. Phytoplankton is cultured for a variety of purposes, including foodstock for other aquacultured organisms,[74] a nutritional supplement for captiveinvertebrates inaquaria. Culture sizes range from small-scalelaboratory cultures of less than 1L to several tens of thousands of litres for commercial aquaculture.[74] Regardless of the size of the culture, certain conditions must be provided for efficient growth of plankton. The majority of cultured plankton is marine, andseawater of aspecific gravity of 1.010 to 1.026 may be used as a culture medium. This water must besterilized, usually by either high temperatures in anautoclave or by exposure toultraviolet radiation, to preventbiologicalcontamination of the culture. Variousfertilizers are added to the culture medium to facilitate the growth of plankton. A culture must be aerated or agitated in some way to keep plankton suspended, as well as to provide dissolvedcarbon dioxide forphotosynthesis. In addition to constant aeration, most cultures are manually mixed or stirred on a regular basis. Light must be provided for the growth of phytoplankton. Thecolour temperature of illumination should be approximately 6,500 K, but values from 4,000 K to upwards of 20,000 K have been used successfully. The duration of light exposure should be approximately 16 hours daily; this is the most efficient artificial day length.[74]
Marine phytoplankton perform half of the global photosynthetic CO2 fixation (net global primary production of ~50 Pg C per year) and half of the oxygen production despite amounting to only ~1% of global plant biomass.[77] In comparison with terrestrial plants, marine phytoplankton are distributed over a larger surface area, are exposed to less seasonal variation and have markedly faster turnover rates than trees (days versus decades).[77] Therefore, phytoplankton respond rapidly on a global scale to climate variations. These characteristics are important when one is evaluating the contributions of phytoplankton to carbon fixation and forecasting how this production may change in response to perturbations. Predicting the effects ofclimate change on primary productivity is complicated by phytoplankton bloom cycles that are affected by both bottom-up control (for example, availability of essential nutrients and vertical mixing) and top-down control (for example, grazing and viruses).[78][77][79][80][81][82] Increases in solar radiation, temperature and freshwater inputs to surface waters strengthenocean stratification and consequently reduce transport of nutrients from deep water to surface waters, which reduces primary productivity.[77][82][83] Conversely, rising CO2 levels can increase phytoplankton primary production, but only when nutrients are not limiting.[84][85][86][44]
Some studies indicate that overall global oceanic phytoplankton density has decreased in the past century,[87] but these conclusions have been questioned because of the limited availability of long-term phytoplankton data, methodological differences in data generation and the large annual and decadal variability in phytoplankton production.[88][89][90][91] Moreover, other studies suggest a global increase in oceanic phytoplankton production[92] and changes in specific regions or specific phytoplankton groups.[93][94] The global Sea Ice Index is declining,[95] leading to higher light penetration and potentially more primary production;[96] however, there are conflicting predictions for the effects of variable mixing patterns and changes in nutrient supply and for productivity trends in polar zones.[82][44]
The effect of human-causedclimate change on phytoplankton biodiversity is not well understood. Should greenhouse gas emissions continue rising to high levels by 2100, some phytoplankton models predict an increase inspecies richness, or the number of different species within a given area. This increase in plankton diversity is traced to warming ocean temperatures. In addition to species richness changes, the locations where phytoplankton are distributed are expected to shift towards the Earth's poles. Such movement may disrupt ecosystems, because phytoplankton are consumed by zooplankton, which in turn sustain fisheries. This shift in phytoplankton location may also diminish the ability of phytoplankton to store carbon that was emitted by human activities. Human (anthropogenic) changes to phytoplankton impact both natural and economic processes.[97]
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