Grazing impact of two small heterotrophic flagellates

Grazing impact of two small heterotrophic flagellates - Vol...

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AQUATIC MICROBIAL ECOLOGY Aquat Microb Ecol Vol. 26: 201–207, 2001 Published December 5 In most oligotrophic and mesotrophic areas of the world oceans, primary production is dominated by cells of <2 to 3 μm (picophytoplankton) (Li et al. 1983, Platt et al. 1983). Picophytoplankton is composed of the 2 cyanobacteria genera Prochlorococcus (Chisholm et al. 1992) and Synechococcus (Waterbury et al. 1979) and a poorly defined assemblage of eukaryotic algae (Johnson & Sieburth 1982, Andersen et al. 1996). Despite growth rates of the order of 1 division d 1 (Liu et al. 1995, Vaulot et al. 1995), the abundance of these populations remains at nearly constant levels over time scales ranging from days to years (Campbell et al. 1997). Prochlorococcus and Synechococcus cells are too small to be consumed directly by mesozooplankton, including small copepods and cladocerans. The carbon sequestered as a result of photosynthesis moves to higher trophic levels via inter- mediate small grazers, mainly identified as flagellates (Sherr et al. 1986, Hagström et al. 1988). Size fractiona- tion experiments have revealed that the first level of grazers is significantly smaller than 5 μm (typically be- low 2 to 3 μm in diameter) in coastal waters (Wikner & Hagström 1988), in the upper water column of the Arctic Ocean (Sherr et al. 1997), and in more oligotrophic ecosystems, such as the Mediterranean Sea (Zohary & Robarts 1992), the Sargasso Sea (Caron et al. 1999), and the Arabian Sea (Reckerman & Veldhuis 1997). Never- theless, most of these tiny oceanic planktonic predators remain unidentified. During an oceanographic cruise conducted in the equatorial Pacific Ocean (OLIPAC, November 1994), we made systematic isolations to investigate pico- planktonic diversity. Surprisingly, without any addition of organic matter, 2 different heterotrophic flagellates of very small size (<4 μm) were isolated from surface waters in the transition zone separating the oligo- trophic gyre from the equatorial upwelling (Guillou et al. 1999a). Because very little is known about the potential grazing activity of such small flagellates on picoplanktonic populations, we conducted laboratory experiments to examine the consumption by Pico- phagus flagellatus and Symbiomonas scintillans of the 2 widespread very small marine cyanobacteria Pro- chlorococcus (0.6 μm) and Synechococcus (1.0 μm). © Inter-Research 2001 · www.int-res.com Present addresses: * *Institut de Ciències del Mar, CISC Passeig Maritim de la Barceloneta 37–45, 08003 Barcelona, Spain. ** E-mail: [email protected] **Station INRA d’Hydrologie Lacustre, Laboratoire Dynam- ique et Evolution des Communautés Phytoplanctoniques, UMR CARRTEL, BP 511, 74203 Thonon Cedex, France NOTE Grazing impact of two small heterotrophic flagellates on Prochlorococcus and Synechococcus Laure Guillou 1, *, Stéphan Jacquet 1, **, Marie-Josèphe Chrétiennot-Dinet 2 , Daniel Vaulot 1 1 Station Biologique de Roscoff, CNRS, INSU and Université Pierre et Marie Curie, Place Georges Teissier, BP 74, 29682 Roscoff Cedex, France 2 Observatoire Océanologique de Banyuls, CNRS, INSU and Université Pierre et Marie Curie, BP 44, 66651 Banyuls-sur-mer, France
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