Structure and function of the mesopelagic microbial loop in the NW Mediterranean Sea

The mesopelagic layer is where the majority of the particulate organic carbon exported from the epipelagic layer is remineralized to CO(2). Recent studies at the DYFAMED time-series station in the NW Mediterranean Sea have shown that dominance of prokaryotes (Bacteria and Archaea) within the microbial community represented by biomass data increases with depth. The studies have also shown that the depth-integrated biomass of protists (heterotrophic flagellates and ciliates) in the mesopelagic layer is as great as that in the epipelagic layer, and that biomass of mesopelagic prokaryotes is controlled by both substrate availability (bottom-up control) and predation and viral infection (top-down control). Data on prokaryotic growth efficiency (derived from the prokaryotic heterotrophic production and the loss of organic carbon flux in the mesopelagic layer on an annual scale) suggest that mesopelagic prokaryotes are not simply remineralizers of organic carbon, but also play an important role in supporting the production of organisms belonging to higher trophic levels. Analysis using a simple food chain model suggests that heterotrophic nanoflagellates may be important remineralizers of organic carbon produced by prokaryotes, and that the viral loop may enhance remineralization of organic carbon in the mesopelagic layer. There are, however, uncertain elements in measuring organic carbon flux and biomass and activity of the microbial components in the mesopelagic layer. By comparing the results obtained at the DYFAMED station with those obtained from other environments, this study summarizes our current knowledge on the structure and function of the mesopelagic microbial loop in the NW Mediterranean Sea as well as in other oceanic regions.