Protist grazing and viral lysis as prokaryotic mortality factors at Baltic Sea oxic-anoxic interfaces

Interface zones between oxic and anoxic water masses (pelagic redoxclines) host highly active prokaryotic communities, mediating important biogeochemical transformations. However, for marine pelagic redoxclines almost no knowledge exists on the magnitude of the loss processes affecting these prokaryotic communities. We assessed the importance of protist grazing and viral lysis as prokaryotic mortality factors for 2 central Baltic Sea redoxclines using a combination of microscopy and experimental community manipulation techniques. Our results demonstrate that protist grazing dominated prokaryotic mortality at suboxic (<30 mu mol l(-1) oxygen) and oxygen-hydrogen sulphide interface depths, with 50 to 100% of prokaryotic standing stocks grazed daily, compared to 2 to 20% of virally infected cells. Grazing was mediated by 2 different protist associations, viz. dinoflagellates and Strombidium-like ciliates in the suboxic zone, and larger ciliates (cf. Mesodinium, Metacystis spp., cf. Coleps and un-identified morphotypes) at the oxygen-hydrogen sulphide interface. In contrast, heterotrophic nanoflagellates (HNF) played a minor role, generally grazing <5% daily of prokaryotic standing stocks. Thus, these redox zones show major differences in microbial food web structure when compared to surface waters, with ciliates and dinoflagellates constituting the major bacterivores instead of HNF. At sulphidic depths, grazing was below the detection limit, and the frequency of virally infected cells decreased, leaving the identity of the major pro karyotic mortality factor at these depths unresolved.