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Phytoplankton are photosynthetic microorganisms that live in the sunlit upper layer of oceans, seas, and freshwater bodies. The group includes cyanobacteria (which are bacteria), and eukaryotic algae such as diatoms, dinoflagellates, coccolithophores, green algae, and cryptophytes. Most are unicellular, although some form chains or colonies. The term describes an ecological role and not a taxonomic group, and it includes organisms from several unrelated lineages. Phytoplankton are the main primary producers of aquatic ecosystems, and some mixotrophic species also consume prey.

Phytoplankton account for about half of global net primary production, roughly 45 to 50 petagrams of carbon per year, even though they make up only about 1 percent of the photosynthetic biomass on Earth. Their turnover time is a matter of days, whereas that of land plants is years or decades. They produce a large share of the oxygen that is released by photosynthesis each year, but most of this oxygen is consumed again by respiration and decomposition, so the net contribution to atmospheric oxygen is small. The statement that phytoplankton generate half of the oxygen in the atmosphere refers to gross production and not to a net addition. They are central to the marine carbon cycle and to the biological pump, and they are studied in oceanography, ecology, and Earth system science.

Major groups

Phytoplankton are also grouped by size: picoplankton (0.2 to 2 μm), nanoplankton (2 to 20 μm), and microplankton (20 to 200 μm). Small cells dominate in nutrient-poor water, because their high surface-to-volume ratio favors the uptake of nutrients, and large cells dominate in nutrient-rich water.

Evolutionary history

Oxygenic photosynthesis evolved in cyanobacteria by about 2.4 to 3 billion years ago, and cyanobacterial photosynthesis led to the Great Oxidation Event. Eukaryotic algae arose through endosymbiosis. Primary endosymbiosis of a cyanobacterium produced the chloroplasts of green algae, red algae, and glaucophytes, and secondary endosymbiosis, in which a eukaryote took up a eukaryotic alga, produced the plastids of diatoms, dinoflagellates, and coccolithophores. The fossil record of phytoplankton includes the biomarkers and microfossils of the Mesozoic, when dinoflagellates, coccolithophores, and diatoms diversified, and the rise of these groups changed the composition of marine food webs and the burial of carbon and silica.

Phytoplankton use sunlight, absorbed by chlorophyll a and accessory pigments, to convert carbon dioxide and water into organic compounds and oxygen:

Photosynthesis is limited to the photic zone, the layer where light is sufficient, which is typically the upper 50 to 200 m of the open ocean and much less in turbid coastal waters. The intensity of light falls exponentially with depth according to the Beer-Lambert law:

where I₀ is the irradiance at the surface, K_d is the diffuse attenuation coefficient, and z is depth. The euphotic depth is conventionally the depth at which light falls to 1 percent of the surface value, z_eu ≈ ln(100) / K_d ≈ 4.6 / K_d. In clear open-ocean water, K_d is about 0.03 per meter, so z_eu is about 150 m, whereas in productive coastal water K_d can be 0.3 per meter or more, which gives about 15 m.

Light and growth

The relationship between photosynthesis P and irradiance E is described by a photosynthesis-irradiance curve, for example:

where α is the initial slope and P_max is the light-saturated rate. At high irradiance, photosynthesis saturates and can be inhibited by photodamage, and cells adjust their pigment content to the light that they receive (photoacclimation). Growth also depends on temperature. The specific growth rate μ of a population follows exponential growth, _N_(t) = N₀ e^μt, and a rate of μ = 0.7 per day means that the population doubles about once a day (ln 2 ≈ 0.69).

Nutrient limitation

Growth is limited by the supply of nutrients. The main macronutrients are nitrogen, phosphorus, and (for diatoms) silicon. Phytoplankton take up nitrogen and phosphorus in a ratio that is on average close to the Redfield ratio of 106 C : 16 N : 1 P, although this ratio varies among species and conditions. The dependence of growth on a limiting nutrient is often described by the Monod equation:

where S is the nutrient concentration and K_s is the half-saturation constant. Nitrogen limits production in most of the open ocean, and phosphorus can limit in some regions, such as parts of the subtropical Atlantic and Mediterranean. Iron is a limiting micronutrient in the large "high-nutrient, low-chlorophyll" (HNLC) regions: the Southern Ocean, the subarctic Pacific, and the equatorial Pacific. In these regions, macronutrients are abundant at the surface, but the supply of iron limits growth, as shown by iron fertilization experiments, in which the addition of iron caused blooms. Other trace elements, including zinc, cobalt, and vitamin B₁₂, also limit growth in some places.

Upwelling Regimes and Spring Bloom Dynamics

Phytoplankton are not distributed evenly. Concentrations are highest where nutrients are supplied to the surface:

• Upwelling zones on the eastern margins of ocean basins, such as the Humboldt (Peru-Chile), California, Canary, and Benguela systems, and the equatorial upwelling, where wind-driven upwelling brings cold, nutrient-rich water to the surface. These regions cover a small part of the ocean but support a large share of fisheries.
• High latitudes and temperate seas, where deep winter mixing replenishes the nutrients, and the increase in light and the stabilization of the water column in spring cause the spring bloom. The classical explanation, in which a bloom begins when the mixed layer becomes shallower than a critical depth (the critical depth hypothesis of Sverdrup), has been refined, and other mechanisms, such as the reduced mixing and the release from grazing, are also proposed.
• Coastal waters, where rivers and mixing provide nutrients.

Oligotrophic Gyres and Satellite Ocean Color Radiometry

Productivity is low in the centers of the subtropical gyres, where the surface layer is warm and stratified. The thermocline and the density difference between warm surface water and cold deep water act as a barrier to vertical mixing, and the supply of nutrients to the surface is low. These regions have low chlorophyll concentrations and are sometimes called ocean deserts, though because they are so large, they contribute a significant part of the total ocean production.

Phytoplankton biomass is estimated from space by ocean color radiometry, in which the concentration of chlorophyll a is derived from the color of the water. Satellite records since 1997 (SeaWiFS, MODIS, and later sensors) show the seasonal cycles and the large-scale patterns.

A bloom is a rapid increase in phytoplankton biomass. Blooms occur when light and nutrients allow growth to exceed losses by grazing, sinking, and mortality. They can be seen from space and can cover thousands of square kilometers. Most blooms are natural and beneficial, but some are harmful:

• Some species produce toxins that accumulate in shellfish and cause paralytic, diarrhetic, or amnesic shellfish poisoning, or that kill fish and marine mammals. Examples include dinoflagellates of the genera Alexandrium and Karenia, and the diatom genus Pseudo-nitzschia (domoic acid).
• Cyanobacterial blooms in fresh water (for example Microcystis) produce toxins that affect drinking water.
• The decay of large blooms uses up oxygen and can cause hypoxia and "dead zones", as in the Gulf of Mexico, where the supply of nutrients from agriculture in the Mississippi basin is a major driver of eutrophication.

The frequency of harmful blooms is influenced by nutrient pollution, warming, and changes in stratification and rainfall, though the evidence for a global increase is mixed, and monitoring has also improved.

Fixation, Aggregation, and Mesopelagic Remineralization

Phytoplankton take up dissolved inorganic carbon at the surface, and the biological pump transports part of the carbon to depth. The process has several stages:

1. Fixation. Phytoplankton fix CO₂ into organic matter. This lowers the partial pressure of CO₂ at the surface and permits more uptake from the atmosphere.
2. Transfer and sinking. Part of the organic matter is eaten by zooplankton, which produce fecal pellets, and part of it aggregates into larger particles ("marine snow"). Diatom and coccolithophore particles, which are ballasted by silica and carbonate minerals, sink faster than unballasted particles. Zooplankton that migrate vertically each day also carry carbon down.
3. Remineralization and burial. Most of the sinking organic matter is consumed and respired by bacteria and animals in the upper few hundred meters, which releases CO₂ at depth. Only a small fraction reaches the deep sea floor, and a smaller fraction is buried in sediments.

Martin Curve Attenuation and the Carbonate Counter-Pump

The amount of carbon that sinks past a given depth is described by an empirical power law, the Martin curve:

where F is the flux, z₀ is a reference depth (often 100 m), and b is an exponent that is typically about 0.86. For b = 0.86, about 2^−0.86 ≈ 55 percent of the flux at 100 m remains at 200 m, and about 17 percent remains at 1,000 m. A larger b means faster attenuation and shallower remineralization, and a smaller one means that more carbon reaches the deep sea. Estimates of the export of organic carbon from the surface ocean are about 5 to 12 Pg C per year, and a smaller amount is sequestered below 1,000 m. Carbon that is remineralized at depth is isolated from the atmosphere for as long as the water takes to return to the surface, which ranges from decades to centuries for intermediate water and a thousand years or more for deep water. Without the biological pump, the atmospheric CO₂ concentration would be much higher, by estimates of the order of 200 ppm.

The process of building calcium carbonate shells by coccolithophores has a more complex effect, because the formation of calcium carbonate releases CO₂ in surface water (the "carbonate counter pump"), and the sinking of the shells ballasts the organic matter. Phytoplankton also release dissolved organic carbon, which supports bacteria in the microbial loop, and part of it is persistent and stored for a long time. The oceans also release sulfur compounds, such as dimethyl sulfide from DMSP, which influence cloud formation, but the size of the climate effect is uncertain.

Phytoplankton are eaten by zooplankton, by larvae of fish and invertebrates, and by filter feeders such as mussels, and they support the food web that leads to forage fish, such as anchoveta and sardines, and to larger predators, such as tuna, seabirds, and whales. Some large whales, such as blue whales, feed on krill, which eat phytoplankton. A large part of the production, especially from small cells, is consumed by microzooplankton and recycled in the microbial loop, and a smaller part goes to larger animals. The efficiency of energy transfer between trophic levels is about 10 percent in a simple estimate, with a wide range of values, so that the supply of phytoplankton limits the production of fish. Changes in the timing of blooms can cause mismatches with the breeding of consumers. The statement that without phytoplankton the marine ecosystem would collapse is correct in the sense that almost all marine food webs depend on photosynthetic production, although chemosynthetic production at hydrothermal vents and seeps supports local communities.

The ocean has taken up about a quarter to 30 percent of the anthropogenic emissions of CO₂ and more than 90 percent of the excess heat in the climate system. The changes that result affect phytoplankton in several ways, and the direction and the size of the effects differ among regions and are uncertain.

• Warming and stratification. The surface ocean is warming, which strengthens stratification. A more stable surface layer reduces the supply of nutrients from below. Models generally predict a decline in global net primary production and in the export of carbon over the century, with the largest declines in the low-latitude oceans, and an increase in some polar regions where melting ice provides more light, although the models differ in the size of the change. Satellite records are too short, and the natural variability (for example, from the El Niño-Southern Oscillation) is too large, to separate a long-term trend with confidence, and published estimates of recent trends in chlorophyll vary.
• Changes in community. Warmer, nutrient-poor conditions favor small phytoplankton, such as picocyanobacteria, over diatoms, which would reduce the efficiency of export. Ranges of species are shifting toward the poles.
• Ocean acidification. The dissolution of CO₂ in seawater forms carbonic acid, lowers the pH (by about 0.1 units since the pre-industrial period) and reduces the carbonate ion concentration, which decreases the saturation state of calcium carbonate. This could harm calcifying organisms, but the responses of coccolithophores vary among species and strains, and some experiments show reduced calcification, whereas others show little change or an increase. Higher CO₂ may also benefit some non-calcifying phytoplankton.
• Deoxygenation, light, and nutrients. Loss of oxygen in the ocean, changes in sea ice, in mixed layer depth, and in the supply of dust and iron, and changes in nutrients from rivers and from the atmosphere, also affect phytoplankton.
• Pollution. Nutrient pollution causes eutrophication, and plastics, metals, and other contaminants can affect growth.

Phytoplankton can also respond by adaptation, because of their large populations and short generation times. Experimental evolution studies show that some can adapt to warming and to higher CO₂ over hundreds of generations.

Methods include chlorophyll measurement by fluorescence and by high-performance liquid chromatography, flow cytometry and microscopy for cell counts, satellite ocean color, autonomous floats (such as the biogeochemical Argo array) that measure chlorophyll, nitrate, and oxygen, and molecular methods such as sequencing of marker genes and metagenomics, which have revealed a large diversity of picoplankton. Primary production is measured by the uptake of ¹⁴C-labeled bicarbonate or by oxygen change in bottles and estimated by models that use chlorophyll, light, and temperature. Large-scale expeditions, such as Tara Oceans, have sampled plankton worldwide. Iron fertilization has been proposed as a method for carbon dioxide removal, but experiments found that the amount of carbon exported to depth was small and variable, and there are concerns about side effects, so it is not an established method.

How can phytoplankton generate 50% of global primary production with only 1% of global photosynthetic biomass?

Unlike terrestrial trees, which invest the majority of their fixed carbon into metabolically inert structural woody tissues (cellulose and lignin) with turnover times of decades, unicellular phytoplankton consist almost entirely of photosynthetically active cytoplasm. With population doubling times of roughly 1 to 3 days (μ ≈ 0.3 to 1.0 day^-1), their entire global standing stock (~1 Pg C) is grazed and replaced roughly 45 to 50 times per year.

Why are the Southern Ocean and Subarctic Pacific classified as High-Nutrient, Low-Chlorophyll (HNLC) regions?

In HNLC oceans, vigorous upwelling and deep mixing supply abundant dissolved nitrate and phosphate to the euphotic zone, yet phytoplankton standing stocks remain low because these remote basins receive minimal atmospheric mineral dust deposition and are starved of dissolved bioavailable iron (Fe < 0.1 nM). Because iron is an obligate cofactor for Photosystem I, ferredoxin, and nitrate/nitrite reductases, iron scarcity limits nitrate assimilation and diatom bloom formation.

Why does coccolithophore calcification act as a "carbonate counter-pump" in the surface ocean?

When coccolithophores precipitate calcium carbonate plates (coccoliths) via the reaction Ca²⁺ + 2HCO₃^- → CaCO₃ + CO₂ + H₂O, they consume two moles of bicarbonate alkalinity for every mole of carbon fixed into mineral shell, shifting the seawater carbonate buffer equilibrium to release dissolved CO₂ gas in the surface mixed layer while simultaneously providing dense mineral ballast that accelerates organic carbon sinking to the abyss.

• Plankton
• Zooplankton
• Primary production
• Biological pump
• Ocean acidification
• Algal bloom
• Marine food web