ABSTRACT: In open oceanic waters, phytoplankton biomass is dominated by organisms below 2 to 3 µm in size (pico- and small nanophytoplankton). The cell concentration of these populations is very stable in time and space as a consequence of nutrient limitation and strong grazing pressure. Although the identity of the organisms that directly graze on picoplankton is largely unknown, they are thought to be very small, i.e. <3 to 5 µm. Here, we analyze the grazing impact of 2 small flagellates, Symbiomonas scintillans and Picophagus flagellatus, upon 2 oceanic cyanobacteria, Prochlorococcus and Synechococcus. S. scintillans does not feed on the 2 cyanobacteria. In contrast, P. flagellatus appears as an active predator capable of drastically reducing prey concentrations. The flagellate displays a substantial division rate of the order of 2 doublings d^-1 when fed on Prochlorococcus cells, but no significant growth is recorded when Synechococcus is used as prey. As the majority (>80%) of P. flagellatus cells can pass throughout a 2 µm filter, the impact of such tiny predators should be taken into consideration during field experiments that rely on size fractionation to separate grazers from prey.

KEY WORDS: Heterotrophic flagellates · Stramenopiles · Picoplankton · Picophagus flagellatus · Symbiomonas scintillans · Prochlorococcus · Synechococcus · Grazing

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Published in AME Vol. 26, No. 2 (2001) on December 5
Print ISSN: 0948-3055; Online ISSN: 1616-1564. Copyright © Inter-Research, Oldendorf/Luhe, 2001