Marine phytoplankton diversity and ecology

PAN, JERÓNIMO

Marine Biology: a functional approach to the oceans and their organisms

CRC Press/Taylor & Francis Group

Lugar: Boca Ratón, FL; Año: 2022; p. 43 - 71

1. Phytoplankton comprises planktonic prokaryotic and eukaryotic microalgae that are responsible for primary production in aquatic environments; their collective movement and position in the water column are largely determined by water motion. Marine phytoplankton are responsible for 40% of the global primary production yr?1; yet they only account for 1% of total plant biomass.2. Phytoplankton are typically small, responding to the evolutionary strategy of maximizing surface area:volume, as a means of optimizing nutrient uptake (governed by Michaelis-Menten kinetics). As small cells in a fluid medium, they live at low Reynolds numbers influenced by viscous forces.3. Colony formation is not uncommon, with apparent benefits against predation, viruses and parasites, and modulation of pigment properties through the ?package effect?. The latter is notable in deep chlorophyll maximum (DCM) communities.4. Individual phytoplankton cells are routinely considered ?particles?; yet, they differ from non-living/inert particles in that (i) phytoplankton have distinct pigments that determine their bio-optical properties; (ii) they possess a distinctive cell ?architecture? that differs from an idealized spherical particle (?form resistance?); (iii) some phytoplankton have flagella and mechanisms to modulate their position in suspension.5. Phytoplankton have traditionally been regarded as osmotrophs, basing their nutrition on the uptake of dissolved nutrients (inorganic forms of N, P, Si). Some species uptake organic forms of nutrients (aminoacids, urea). Macronutrients are found in seawater at μM concentrations, while micronutrients (Fe, trace metals and vitamins) exist at nM concentrations.6. Besides photoautotrophy, other forms of nutrition include heterotrophy, mixotrophy, kleptochloroplastidy, as well as N2 fixation (exclusive of prokaryotes). Phytoplankton present a fairly constant average atomic molar ratio of 106 C:16 N:1 P (known as Redfield ratio), although deviations from the ratio have been documented.7. Phytoplankton abundance and distribution is jointly determined by bottom-up (nutrients, light) and top-down (predation, disease) controls. From a population ecology perspective, this balance is expressed by the logistic equation. Physical factors play a preeminent role in determining phytoplankton distribution.8. The four dominant phytoplankton groups in the modern oceans are: (i) the prokaryotic Cyanobacteria which invented oxygenic photosynthesis; and three eukaryotic clades which originated from secondary and/or tertiary endosymbioses in the Mesozoic, after an initial primary endosymbiosis with a cyanobacterium plastid. These are (ii) the dinoflagellates?a primitive polyphyletic group; (iii) the coccolithophores within the Haptophytes; and (iv) the diatoms, which evolved and radiated in the Cenozoic.9. Picoplanktonic cyanobacteria (Prochlorococcus, Synechococcus) are the most numerically-abundant phytoplankton with ecotypes that occupy segregated niches in the pelagic environment. The colonial cyanobacterium Trichodesmium forms blooms responsible for pelagic N2 fixation.10. Dinoflagellates have a complex cell cover (the amphiesma) and plastids containing chlorophyll a and peridinin, and a suite of accessory pigments reflecting their complex phylogeny. Most forms exhibit facultative heterotrophy or mixotrophy and are known for other symbioses (e.g., with corals), and the formation of toxic harmful algal blooms (HABs).11. Coccolithophore cells have a protective exoskeleton (the coccosphere) made up calcite platelets (coccoliths). Their biomineralizations are of stratigraphic importance in paleoceanographic reconstructions, and currently play a major role in controlling ocean alkalinity. Coccolithophores are one of the main phytoplankton responsible for the production of dimethyl sulfide (DMS), a volatile compound implied in the formation of aerosols and cloud condensation, with global impacts on climate.12. Biogeochemically, diatoms are responsible for half of the global ocean net primary production and promote vertical carbon flux through transparent exopolymer (TEP) exudates and the formation of large particle aggregates; they are also the main Si exporters to the deep sea. The single most distinctive feature of diatoms is their siliceous cell wall (the frustule) which follows species-specific geometries and patterns. Their life history alternates between several cycles of vegetative divisions with sexual reproduction. Diatoms play a paramount role in pelagic food webs in terms of their nutritional quality, while some species produce neurotoxins (domoic acid). It is inferred that diatoms accelerated the ?biological carbon pump? with their peak in abundance from the mid-Miocene onwards.