Viruses as mortality agents of picophytoplankton in the deep chlorophyll maximum layer during IRONAGES III

We report viral-induced mortality rates of the picoeukaryotic (two size classes) and prokaryotic (cyanobacteria Prochlorococcus and Synechococcus) phytoplankton during a cruise in the oligotrophic subtropical northeastern Atlantic (October 2002). A dilution assay, simultaneously estimating viral lysis and microzooplankton grazing, was applied around the deep chlorophyll maximum (DCM) at six stations. For the smallest picoeukaryotes (group I), viral lysis was responsible for 50-100% of the total cell losses, with rates ranging from 0.1 to 0.8 d(-1). Viral lysis rates were positively linked to the abundance and contribution of large genome-sized (180-225 kb) putative algal viruses. In contrast, the prokaryotic picophytoplankton did not seem to be controlled by viral lysis. For Synechococcus, microzooplankton grazing dominated, with rates between 0.1 and 0.25 d(-1) (comparable to those for the eukaryotic algae). For Prochlorococcus, both viral lysis and microzooplankton grazing rates were very low (0-0.1 d(-1)). Overall, the total carbon production by the picophytoplankton community was balanced by the combined losses due to viral lysis and microzooplankton grazing. Viral lysis released 0.1-0.3 mu g of picophytoplanktonic C L-1 d(-1), which comprised 21% of the total carbon production by picophytoplankton.