"Killing the winner" only? The virome of grassland soils under different land use intensity as the driver for microbial community structure and function (KiWion)

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Soils, although intensively studied with respect to their microbiology, are lagging virus ecology research compared to marine and freshwater systems. Indeed, soils harbour high numbers of viruses and virus - to - bacteria ratios (VBR) in soil seem to vary much more than in aquatic environments, frequently exceeding these by 1 to 2 orders of magnitude. Variations in virus populations are partly explained by differences in soil pH, water and organic content, hinting at land use differences posing a significant influence on virus diversity and viral - host interactions. With up to 68% of soil bacteria potentially containing inducible prophages, there is indication that the heterogeneous soil environment and the patchy distribution of soil microbial communities select for lysogenic rather than lytic reproduction in soil viruses, with the consequence, that prevalent lysogenic relationships increase the probability of transferring genetic information between phage and prokaryotic hosts by transduction. And indeed, the viral metagenomes analysed, show that up to 50% consists of transduced bacterial and archaeal genes. Although we assume that viruses in soil significantly contribute to host dynamics (Killing the Winner Hypothesis) and adaptation (Red Queen Hypothesis), ecosystem functioning, and biogeochemical processes similar to aquatic systems, the type and frequency of interactions are poorly understood and empirical data are missing. We hypothesize that transduction plays a very important role for resilience in soils under strong land use intensity, since in these soils i) diversity of soil microbes is lower and thus the ecosystem might be more sensitive to changing conditions and ii) the activity of soil microbes is particularly higher which may increase transduction rates, as the replication of the virus in the host requires metabolically active cells. To investigate the guiding hypothesis we will analyse the 150 plots within the Biodiversity Exploratories with a specific focus on grassland ecosystems under different land use intensity. We will investigate the relation between land use intensity, vegetation period, site specific conditions and the number of viruses. We will further describe the composition of the soil viromes as well as major functional traits of prokaryotic origin carried by viruses. Host - virus interaction studies and infection networks derived from CRISPR like structures in the prokaryotic hosts will provide information on community ecology. Finally, we will isolate viruses infecting dominating bacterial groups common to the different grassland soils (e.g. Pseudomonadaceae) for studying their cross - infection potentials.