Phytoplankton (/ˌfaɪtoʊˈplæŋktən/) are the
autotrophic (self-feeding) components of the
plankton community and a key part of ocean and freshwater
ecosystems. The name comes from the
Greek words φυτόν (phyton), meaning '
plant', and (planktos), meaning 'wanderer' or 'drifter'.[1][2][3]
Phytoplankton obtain their energy through
photosynthesis, 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.
Phytoplankton form the base of marine and freshwater food webs and are key players in the global
carbon cycle. They account for about half of global photosynthetic activity and at least half of the oxygen production, despite amounting to only about 1% of the global plant biomass. Phytoplankton are very diverse, varying from photosynthesizing bacteria to plant-like
algae to armour-plated
coccolithophores. Important groups of phytoplankton include the
diatoms,
cyanobacteria and
dinoflagellates, although many other groups are represented.[2]
Most phytoplankton are too small to be individually seen with the
unaided eye. However, when present in high enough numbers, some varieties may be noticeable as colored patches on the water surface due to the presence of
chlorophyll within their cells and accessory pigments (such as
phycobiliproteins or
xanthophylls) in some species.
Types
Phytoplankton are
photosynthesizing 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 undertake
primary production in water,[2] creating
organic compounds from
carbon dioxide dissolved in the water. Phytoplankton form the base of — and sustain — the aquatic
food web,[4] and are crucial players in the Earth's
carbon cycle.[5]
"Marine photosynthesis is dominated by
microalgae, which together with cyanobacteria, are collectively called phytoplankton."[6] Phytoplankton are extremely diverse, varying from photosynthesizing bacteria (cyanobacteria), to plant-like diatoms, to armour-plated coccolithophores.[7][2]
Diatoms are one of the most common types of phytoplankton
Phytoplankton live in the
photic zone of the ocean, where
photosynthesis is possible. During photosynthesis, they assimilate carbon dioxide and release oxygen. If solar radiation is too high, phytoplankton may fall victim to
photodegradation. Phytoplankton species feature a large variety of photosynthetic
pigments which species-specifically enables them to absorb different
wavelengths of the variable underwater light.[13] This implies different species can use the wavelength of light different efficiently and the light is not a single
ecological resource but a multitude of resources depending on its spectral composition.[14] By that it was found that changes in the spectrum of light alone can alter natural phytoplankton communities even if the same
intensity is available.[15] For growth, phytoplankton cells additionally depend on nutrients, which enter the ocean by rivers, continental weathering, and glacial ice meltwater on the poles. Phytoplankton release
dissolved organic carbon (DOC) into the ocean. Since phytoplankton are the basis of
marine food webs, they serve as prey for
zooplankton,
fish larvae and other
heterotrophic organisms. They can also be degraded by bacteria or by
viral lysis. Although some phytoplankton cells, such as
dinoflagellates, 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 and
detritus.[16]
Phytoplankton are crucially dependent on a number of
nutrients. These are primarily
macronutrients such as
nitrate,
phosphate or
silicic acid, which are required in relatively large quantities for growth. Their availability in the surface ocean is governed by the balance between the so-called
biological pump and
upwelling of deep, nutrient-rich waters. The
stoichiometric nutrient composition of phytoplankton drives — and is driven by — the
Redfield ratio of macronutrients generally available throughout the surface oceans. However, across large areas of the oceans such as the
Southern Ocean, phytoplankton are limited by the lack of the
micronutrientiron. This has led to some scientists advocating
iron fertilization as a means to counteract the accumulation of
human-produced carbon dioxide (CO2) in the
atmosphere.[17] Large-scale experiments have added iron (usually as salts such as
ferrous sulfate) to the oceans to promote phytoplankton growth and draw
atmospheric CO2 into the ocean. Controversy about manipulating the ecosystem and the efficiency of iron fertilization has slowed such experiments.[18]
Phytoplankton depend on
B vitamins for survival. Areas in the ocean have been identified as having a major lack of some B Vitamins, and correspondingly, phytoplankton.[19]
The effects of
anthropogenic 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.[20][21]
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 a
coccosphere 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).[22][23]
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 of
zooplankton grazing may be significant.[24] One of the many
food chains in the ocean – remarkable due to the small number of links – is that of phytoplankton sustaining
krill (a
crustacean similar to a tiny shrimp), which in turn sustain
baleen whales.
The El Niño-Southern Oscillation (ENSO) cycles in the Equatorial Pacific area can affect phytoplankton.[25] Biochemical and physical changes during ENSO cycles modify the phytoplankton community structure.[25] 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.[26] The sensitivity of phytoplankton to environmental changes is why they are often used as indicators of estuarine and coastal ecological condition and health.[27] 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.[25]
Diversity
When two currents collide (here the
Oyashio and
Kuroshio currents) they create
eddies. 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.[33] 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 describing
stoichiometry of phytoplankton and seawater has become a fundamental principle to understand marine ecology, biogeochemistry and phytoplankton evolution.[34] However, the Redfield ratio is not a universal value and it may diverge due to the changes in exogenous nutrient delivery[35] and microbial metabolisms in the ocean, such as
nitrogen fixation,
denitrification and
anammox.
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.[36][37] Different cellular components have their own unique stoichiometry characteristics,[34] 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[38] 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.
Factors affecting abundance
The
NAAMES study was a five-year scientific research program conducted between 2015 and 2019 by scientists from
Oregon State University and
NASA to investigated aspects of phytoplankton dynamics in ocean ecosystems, and how such dynamics influence
atmospheric 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[39] in an effort to better understand the role of phytoplankton aerosol emissions on Earth's energy budget.[40]
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.[40] The NAAMES project also investigated the quantity, size, and composition of aerosols generated by
primary production in order to understand how phytoplankton bloom cycles affect cloud formations and climate.[41]
Factors affecting productivity
Phytoplankton are the key mediators of the
biological 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 and
stoichiometry of phytoplankton.[47] 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 the
marine food chains.[48]Climate change may greatly restructure phytoplankton communities leading to
cascading consequences for
marine food webs, thereby altering the amount of carbon transported to the ocean interior.[49][45]
The diagram on the right gives an overview of the various environmental factors that together affect
phytoplankton productivity. All of these factors are expected to undergo significant changes in the future ocean due to global change.[50] Global warming simulations predict oceanic temperature increase; dramatic changes in
oceanic 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.[51][45]
Role of phytoplankton
In the diagram on the right, 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. 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.[52]
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 of these phytoplankton which are 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.[53]
Phytoplankton blooms in which a species increases rapidly under conditions favorable to growth can produce harmful algal blooms (HABs).
Phytoplankton are a key food item in both
aquaculture and
mariculture. 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 of
rotifers,[54] which are in turn used to feed other organisms. Phytoplankton is also used to feed many varieties of aquacultured
molluscs, including
pearloysters and
giant 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,[55] and found the calorific value of phytoplankton to vary considerably across different oceanic regions and between different time of the year.[55][56]
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,[54] a nutritional supplement for captive
invertebrates in
aquaria. Culture sizes range from small-scale
laboratory cultures of less than 1L to several tens of thousands of litres for commercial aquaculture.[54] Regardless of the size of the culture, certain conditions must be provided for efficient growth of plankton. The majority of cultured plankton is marine, and
seawater of a
specific gravity of 1.010 to 1.026 may be used as a culture medium. This water must be
sterilized, usually by either high temperatures in an
autoclave or by exposure to
ultraviolet radiation, to prevent
biologicalcontamination of the culture. Various
fertilizers 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 dissolved
carbon dioxide for
photosynthesis. 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. The
colour 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.[54]
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.[57] 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).[57] 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 of
climate 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).[58][57][59][60][61][62] Increases in solar radiation, temperature and freshwater inputs to surface waters strengthen
ocean stratification and consequently reduce transport of nutrients from deep water to surface waters, which reduces primary productivity.[57][62][63] Conversely, rising CO2 levels can increase phytoplankton primary production, but only when nutrients are not limiting.[64][65][66][24]
Some studies indicate that overall global oceanic phytoplankton density has decreased in the past century,[67] 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.[68][69][70][71] Moreover, other studies suggest a global increase in oceanic phytoplankton production[72] and changes in specific regions or specific phytoplankton groups.[73][74] The global Sea Ice Index is declining,[75] leading to higher light penetration and potentially more primary production;[76] however, there are conflicting predictions for the effects of variable mixing patterns and changes in nutrient supply and for productivity trends in polar zones.[62][24]
The effect of human-caused
climate 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 in
species 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.[77]
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