Viruses have a profound impact on shaping the composition of marine prokaryotic communities and biogeochemical cycles. Virus-mediated cell lysis influences the microbial mortality and diversity as well as the organic carbon pool through the transformation of the living biomass towards the dissolved organic matter realm. Labile organic compounds and virus progeny that are released into the environment due to viral activity can be taken up by surrounding microbial populations (Dell’Anno et al. 2015). This short cut in the marine food web, the viral shunt (Wilhelm & Suttle 1999), represents an important supply of substrate for microorganisms, especially in the nutrient depleted deep-sea. At the seafloor, an estimated 0.37 and 0.63 Gt of carbon per year is globally released by viral killing of microorganisms sustaining 35% of the total prokaryotic metabolism (Danovaro et al. 2008). Viral lysis can thus stimulate prokaryotic production and nutrient regeneration, but can also decrease the efficiency of carbon and element transfer to higher trophic levels (Fuhrman 1999). Furthermore, viruses are reported to have divergent influences on the diversity of aquatic prokaryotes (e.g. Thingstad & Lignell 1997; Hewson & Fuhrman 2006; Bouvier & Del Giorgio 2007). A study of Danovaro et al. (2016) found that deep-sea archaea suffered from higher virus-induced mortality than the numerically superior bacteria implying that certain microbial populations are more susceptible to viral infection. As most studies on the viral shunt were performed on water samples, we are only beginning to understand the role of viruses for the functioning of deep-sea ecosystems.

In my PhD project, we will systematically investigate the effect of viruses on benthic prokaryotic community structures and the dissolved organic matter composition in deep-sea sediments. We hypothesize that both compartments are linked via viral lysis. To test this hypothesis and identify general responses to viral lysis, we incubated deep-sea sediments from various biogeographical provinces of the Pacific Ocean and induced prophages using mitomycin C. 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 via the viral shunt to heterotrophs. The analysis of the DOM composition will furthermore give insights into the impact of viral lysis on the benthic DOM fingerprint.

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The ecology of molecules