Growth and grazing losses of prokaryotes in the central Atlantic Ocean

The trophic relation between prokaryotes and heterotrophic nanoflagellates was studied during two latitudinal cruises in the central Atlantic Ocean. The losses to predation on prokaryotes were determined in 12 locations covering a wide range of trophic situations, from ultraoligotrophic [<0.05 mg chlorophyll a (Chl a) m–3] to moderately eutrophic waters (>1 mg Chl a m–3). In these locations, the abundance of prokaryotes (P) covaries with that of heterotrophic nanoflagellates, thus suggesting that resources controlled the abundance of heterotrophic nanoflagellates (HNF). Besides, the losses to predation were positively related to prokaryotic and heterotrophic nanoflagellate biomass, which points toward higher consumption rates associated with larger concentrations of preys and predators. Conversely, declining trends between prokaryotic production (PP) and the fraction of this production lost to predation revealed higher relative losses in the environments with lower productions. Our study shows for the central Atlantic that 35% of prokaryotic biomass (BP), equating to between 40 and 83% of PP can be ingested daily and that 55% of the variability observed in the rate of prokaryotic loss to predation was related with the HNF. As predators graze on many prey types, in an oligotrophic system containing many prey species but little numeric loading, there will still be prey for predators but not enough hosts for viruses. In this sense, our study confirms the importance of the prey–predator relationship between prokaryotes and heterotrophic nanoflagellates in the flow of carbon of the less productive regions of the ocean.