Phytoplankton

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Phytoplankton are the autotrophic component of the plankton that drift in the water column. The name comes from the Greek terms, phyton or "plant" and πλαγκτος ("planktos"), meaning "wanderer" or "drifter" (Thurman, 1997). Most phytoplankton are too small to be individually seen with the unaided eye. However, when present in high enough numbers, they may appear as a green discoloration of the water due to the presence of chlorophyll within their cells (although the actual color may vary with the species of phytoplankton present).

Phytoplankton, like plants, obtain energy through a process called photosynthesis, and so must live in the well-lit surface layer (termed the euphotic zone) of an ocean, sea, or lake. Through photosynthesis, phytoplankton (and terrestrial plants) are responsible for much of the oxygen present in the Earth's atmosphere. Their cumulative energy fixation in carbon compounds (primary production) is the basis for the vast majority of oceanic and some freshwater food chains (chemosynthesis is a notable exception). As a side note, one of the more remarkable food-chains in the ocean — remarkable because of the small number of links — is that of phytoplankton fed on by krill (a type of shrimp) fed on by baleen whales.

Phytoplankton is a term that encompasses the autotrophic sector of aquatic microorganisms. Phytoplankton serves as the base of the marine food chain, providing a critical ecological function for all aquatic life. Most phytoplankton species are not classified as plants, despite their photosynthetic capabilities. Rather, these creatures are classified as protozoan, a phylogenetically more primitive organism.¹

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Aside from light, phytoplankton are also crucially dependent on the availability of nutrients for growth. These are primarily macronutrients such as nitrate, phosphate or silicic acid, whose availability is governed by the balance between the so-called biological pump and upwelling of deep, nutrient-rich waters. However, across large regions of the World Ocean such as the Southern Ocean, phytoplankton are also limited by the availability of the micronutrient iron. This has led to some scientists advocating iron fertilization as a means to counteract the accumulation of anthropogenic carbon dioxide (CO₂) in the atmosphere.

While almost all phytoplankton species are obligate photoautotrophs, there are some that are mixotrophic and other, non-pigmented species that are actually heterotrophic (the latter are often viewed as zooplankton). Of these, the best known are dinoflagellate genera such as Noctiluca and Dinophysis, that obtain organic carbon by ingesting other organisms or detrital material.

In terms of numbers, the most important groups of phytoplankton include the diatoms, cyanobacteria and dinoflagellates, although many other groups of algae are represented. One group, the coccolithophorids, is responsible (in part) for the release of significant amounts of dimethyl sulfide (DMS) into the atmosphere. DMS is converted to sulfate and these sulfate molecules act as cloud condensation nuclei, increasing general cloud cover. In oligotrophic oceanic regions such as the Sargasso Sea or the South Pacific, phytoplankton is dominated by the small sized cells, called picoplankton, mostly composed of cyanobacteria (Prochlorococcus, Synechococcus) and picoeucaryotes such as Micromonas.