Viral abundance and size-fractionated viral mediated bacterial mortality in the NW Mediterranean

Abstract

Suspended organic aggregates are hotspots for bacterial growth but do not offer a refuge from protistan grazing. Concerning another mortality agent, i.e. viral lysis, published results are ambiguous: High viral abundance on transparent exopolymer particles, different stages of marine snow and mucilage and riverine aggregates suggest high viral infection of attached bacteria. However, inactivation of viruses attached to particles has also been reported. From freshwater studies, there is some speculation of decreased viral lysis of attached bacteria inferred from lower viral infectivity in freshwater benthic habitats compared to the upper water column. However, to our best knowledge viral production associated to particles has never been assessed directly. In the present study viral abundance as well as lytic and lysogenic production was determined along with the viral induced mortality of attached bacteria and compared to viral infection of the free-living bacteria through 0.8 μm and 3.0 μm fractionation of surface, deep chlorophyll maximum (DCM) and deep water samples from the NW Mediterranean. Samples were taken during two cruises, labeled NEMO-1 and NEMO-2 in May and June 2012, respectively, from a transect perpendicular to the coast between Blanes and Majorca. Profiles of viral abundance showed a pronounced peak at the DCM and a sharp decrease by one order of magnitude at 200m. Viral production, the fraction of infected cells and viral mediated mortality of bacteria was significantly higher in the total than in the size fraction in the DCM and deep water samples. The latter were found to originate from a moderate benthic nepheloid layer. These preliminary data suggest that (1) viral mediated mortality of bacteria is important in deep waters and (2) the attached fraction of bacteria was found especially prone to viral lysis