"The impact of viruses on the composition of microbial communities and organic molecules in marine sediments"

Broader background of the proposed research project

Viruses play a critical role in shaping microbial community compositions and the marine or-ganic carbon pool: (i) lysis of prokaryotes controls microbial biomass and diversity (ii) non-infected populations take up labile organic compounds that were released into the environment. Most studies on this short-cut in the marine food web, the viral shunt, were performed on water samples (Wilhelm & Suttle 1999). At the seafloor, the viral shunt accounts for 35% of the total prokaryotic metabolism (Danovaro et al. 2008). Most recently, it was shown that the decomposition of virus particles also contribute substantially to benthic carbon cycling (Dell´Anno et al. 2015). The organic carbon which is released from degraded virus particles (up to 50 megatons Corg/year) is equivalent to 3% of all photosynthetically produced Corg. In our recent studies on deep sediment layers, we estimated that virus-bound carbon represents up to 30% of the total biomass at continental margins and up to 50-80% in the ultra-oligotrophic South Pacific Gyre (Engelhardt et al. 2014). As virus particles are composed to 90% of capsid proteins, they become a well-defined food source. Both, the viral shunt and the degradation of virus particles directly link the available organic carbon and the microbial diversity in marine sediments.

Outline for the proposed PhD research project

In the proposed project, we will test the hypothesis that benthic microbial community struc-tures and DOM compositions are linked by viral lysis. Next generation sequencing of ribosomal RNA will be applied to a selection of sediment samples from approved joint research cruises of RV Sonne in 2016 and 2017 with WPs 4 and 8. Targeting the RNA instead of DNA will identify community members that actively response to changes induced by viral lysis. The results will be correlated to molecular DOM data also collected during both cruises by Dittmar (WP 4). Community members that utilize virus particles as a food source will be identified by feeding experiments with 13C-labelled viruses. We hypothesize that the viral shunt and the decomposition of viruses will stimulate growth of distinct microbial community members and shape the remaining DOM composition. Furthermore, we will test if heterotrophic microorganisms can thrive on cell components that derive from viral lysis of autotrophs. For this, we will induce prophages from an autotrophic culture (Fichtel et al. 2012) and feed the lysate to a heterotrophic strain isolated from the same sediment horizon. Identifying the DOM composition in the course of the experiment, will answer the question if there is a shuttle of organic matter from autotrophs that build up biomass from CO2 via the viral shunt to heterotrophs.