Nanoflagellate predation on auto- and heterotrophic picoplankton in the oligotrophic Mediterranean Sea

Dynamics of autotrophic and heterotrophic prokaryotes and their consumption by nanoflagellates were studied in the euphotic zone at nine stations located from the Levantine Basin (34 degreesE) to the Balearic sea (5 degreesE) in June 1999. Bacterial biomass constituted the largest proportion of living biomass at all stations. Integrated bacterial production at the furthest east station, was sixfold lower than integrated bacterial production at the furthest west (13 and 75 mg C m(-2) d(-1) respectively). Estimated heterotrophic nanoflagellate bacterivory accounted for 45-87% of bacterial production. Small protists (<3 m) dominated the bacterivore assemblage and accounted for more than 90% of the heterotrophic bacterial consumption. Our results indicated that there was no negative selection against Synechococcus and that both picoplankton groups were grazed according to their standing stocks. An estimated consumption of Synechococcus derived from food vacuole content analysis of nanoflagellates revealed that they consumed from 0.5 to 45% (mean 13%) of Synechococcus stock per day. These data are among the first documenting the relative grazing impact of heterotrophic nanoflagellates on bacteria and Synechococcus in situ. Assuming that there was no selection for or against Prochlorococcus, heterotrophic nanoflagellates could ingest from 1.4 to 21% (mean 6%) of Prochlorococcus stock per day. The amount of organic carbon obtained by heterotrophic nanoflagellates from photosynthetic prokaryotes represented 27% of the total amount of carbon obtained from total prokaryotes.