AME 26:201-207 (2001)  -  doi:10.3354/ame026201

Grazing impact of two small heterotrophic flagellates on Prochlorococcus and Synechococcus

Laure Guillou1,*, Stéphan Jacquet1,**, Marie-Josèphe Chrétiennot-Dinet2, Daniel Vaulot1

1Station Biologique de Roscoff, CNRS, INSU and Université Pierre et Marie Curie, Place Georges Teissier, BP 74, 29682 Roscoff Cedex, France
2Observatoire Océanologique de Banyuls, CNRS, INSU and Université Pierre et Marie Curie, BP 44, 66651 Banyuls-sur-mer, France
Present addresses: *Institut de Ciències del Mar, CISC Passeig Maritim de la Barceloneta 37-45, 08003 Barcelona, Spain. E-mail: **Station INRA d¹Hydrologie Lacustre, Laboratoire Dynamique et Evolution des Communautés Phytoplanctoniques, UMR CARRTEL, BP 511, 74203 Thonon Cedex, France

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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