Grazing impact of different heterotrophic nanoflagellates on eukaryotic ( Ostreococcus tauri ) and prokaryotic picoautotrophs ( Prochlorococcus and Synechococcus )

Summary Autotrophic picoplankton (<3 µm) composed of both prokaryotes and eukaryotes are the most abundant primary producers on Earth. In this study we examined the ingestion of the picoeukaryote Ostreococcus tauri by different marine heterotrophic nanoflagellates (HNF) with various morphologies, swimming and feeding behaviours. Cultures of specific bacterivorous nanoflagellates ( Rhynchomonas nasuta , Jakoba libera , and a culture of Cafeteria sp./ Monosiga sp.) and natural nanoflagellate populations were used as grazers. For comparison with Ostreococcus, we used similar‐sized prokaryotes as prey, Prochlorococcus and Synechococcus . We observed large species‐specific differences in terms of: use  of picoautotrophs among nanoflagellates, time lag between   prey   addition   and   prey   consumption (0–196 h), grazing rate (0–0.12 h −1 ), growth rate (0–0.3 h −1 ) and maximum abundance of HNF reached in experimental bottles (e.g. from 10 4 to 10 5 cells ml −1 , for a natural coastal population and a Cafeteria sp./ Monosiga sp. culture feeding Ostreococcus respectively). Overall, this study shows that the nanoflagellate community composition is conclusive for picoautotrophic community structure and, vice versa , the picoautotrophic community structure favours or inhibits the growth of some nanoflagellate groups.