ABSTRACT
Phytoplankton comprises prokaryotic and eukaryotic microalgae responsible for primary production in aquatic environments; their collective movements and position in the water column are largely determined by water motion. While marine phytoplankton are responsible for 40 % of the global primary production yr-1, they only account for 1 % of total plant biomass. Their small size responds to the evolutionary strategy of maximizing surface area:volume as a means of optimizing nutrient uptake; colony formation is common and beneficial against predation and disease. Individual phytoplankton cells are routinely considered ‘particles’ but differ in (i) having pigments that determine their bio-optical properties; (ii) possessing a distinctive cell buildup that differs from an idealized spherical particle; and (iii) having flagella or behaviors to modulate their position in suspension. Most phytoplankton areosmotrophs relying on the uptake of dissolved nutrients. Besides photoautotrophy other forms of nutrition include heterotrophy, mixotrophy, kleptochloroplastidy, and N2 fixation in prokaryotes. Phytoplankton present a fairly constant average atomic molar ratio of 106 C : 16N : 1P (Redfield ratio). Physical factors determine phytoplankton distribution. Their abundance is determined by bottom-up and top-down controls. The four dominant phytoplankton groups in modern oceans are prokaryotic cyanobacteria, and the eukaryotic dinoflagellates, coccolithophores and diatoms.
