Disentangling the effect of viruses and nanoflagellates on prokaryotes in bathypelagic waters of the Mediterranean Sea

ABSTRACT: Bathypelagic ecosystems depend on prokaryotic heterotrophic biomass fuelled by vertical particulate organic matter (POM) fluxes, but very little information is available on the interactions among viruses, prokaryotes and nanoflagellates in deep waters. We simultaneously investigated the relative importance of the viral and heterotrophic nanoflagellate (HNF) grazing-induced prokaryotic mortality in bathypelagic waters by means of dilution experiments performed on samples collected at 1500 m depth from the Atlantic Ocean to the Eastern Mediterranean Sea. Prokaryotic abundance (range: 1.4 to 8.9 7 104 cells ml\u20131), although different from one station to another, was on average not significantly different among biogeographic regions. The potential predators of prokaryotes (viruses, HNF and microzooplankton) followed a similar spatial pattern. Viruses were responsible for an important fraction of prokaryotic mortality (on average 13.4% d\u20131). Dilution experiments carried out to estimate the potential predation of HNF suggested a high effect on prokaryotic abundance. However, since the latter experiments also include the effect of viruses on prokaryotes, when this factor was disentangled from the overall mortality, the potential rates of HNF predation on prokaryotes (on average 49.5%) were ca. 4 times higher than the effect due to viral infections. Conversely to patterns of distribution, the relative importance of virus-mediated mortality vs. HNF predation changed significantly among different regions. Results should be treated with caution due to the intrinsic difficulty in reproducing experimentally natural deep-sea conditions, but they permit disentangling of the relative effect of viruses and HNF on prokaryotes and compare the potential predatory control in different biogeographic regions

Structure and interactions within the pelagic microbial food web (from viruses to microplankton) across environmental gradients in the Mediterranean Sea

We have investigated here the structure of the pelagic microbial food web and quantified the carbon fluxes from viruses to microplankton along trophic gradients in the Mediterranean Sea. To explore the complex trophic pathways of the pelagic food web, we conducted independent and replicated experiments to measure (i) predation on prokaryotes by microzooplankton, (ii) predation on prokaryotes by heterotrophic nanoflagellates, (iii) virus-induced prokaryotic mortality, and (iv) microzooplankton grazing on nanoplankton and microphytoplankton. Our study covered more than 5000 km, from the Atlantic Ocean to the Levantine basin, and from conditions of high primary production and nutrient availability to ultraoligotrophic and phosphate-limited waters. Microphytoplankton abundance and biomass were typically scarce across the entire Mediterranean basin, with almost negligible levels in the eastern part. Also, nanoplankton biomass was typically low. Conversely, prokaryotes, and particularly the heterotrophic components, were abundant and represented the only significant food source for both nanoplankton and microplankton grazers. Viral infections were not the primary agents of prokaryotic mortality, but in some areas, such as the Ligurian Sea, they had a key role in prokaryotic dynamics. The scenario depicted in this study in summer reveals the pivotal role of microzooplankton in the pelagic food web of the Mediterranean Sea, with a key role in the potential transfer of biomass to higher trophic levels. We also show that converse to theoretical expectations, the microbial food web was relatively complex under the mesotrophic conditions (Atlantic and western Mediterranean) and was much more simplified in the ultraoligotrophic conditions of the eastern Mediterranean