Virioplankton in a river-floodplain system

Viruses are the most abundant "life forms" in aquatic systems, are thought to affect phyto- and bacterioplankton and potentially influence the cycling of organic carbon and nutrients. Viruses can be either lytic or lysogenic, however, there are contrasting reports about the relative importance of these two strategies. Work on marine systems indicates that viral lysis can profoundly affect the carbon cycle by converting cells into dissolved organic matter. Such "rapidly cycling carbon" could be of particular relevance in river systems with a usually high proportion of aged and recalcitrant carbon. Surprisingly, the role of biotic interactions between viral and bacterial potamoplankton is poorly studied, and almost nothing is known about the effects of viral lysis on the carbon cycle in riverine systems. Natural floodplains are known to increase the heterogeneity of a river system. Here, the driving force for lateral exchange processes is hydrological connectivity, which profoundly influences biological processes within the system. Almost all large rivers in Europe are greatly affected by pollution, alterations in the catchment, damming and regulation, some large rivers even being among the world`s most degraded ecosystems. The proposed study aims to contribute to our understanding of the structure and function of riverine microbial communities and of carbon cycling in human-impacted river-floodplains by tackling the following issues: - impact of variable hydrology (e.g. flooding) on virally-induced mortality of prokaryotes - impact of variable hydrology (e.g. flooding) on the percentage of lysogenized prokaryotes - consequences of viral lysis of prokaryotes on the microbial carbon cycle in different floodplain subsystems under variable hydrological situations - influence of varying hydrology on the bacterial and viral community structures The planned project attempts to combine important issues of microbial and river ecology and is therefore thought to contribute significantly to the field of aquatic ecology.

Viruses are the most abundant "life forms" in aquatic systems, are thought to affect phyto- and bacterioplankton and potentially influence the cycling of organic carbon and nutrients. Viruses can be either lytic (killing their microbial host immediately) or lysogenic (latent infection), however, there are contrasting reports about the relative importance of these two strategies. Further, viral lysis can profoundly affect the carbon cycle by converting host cells into dissolved organic matter. In our study we support the view that such "rapidly cycling carbon" is of particular relevance in river systems, which typically harbor a high proportion of aged and more stable carbon. The study revealed for the first time both typical infection frequencies and seasonality of virus-mediated processes in such a river-floodplain system. There is evidence that during flooding, the performance of prokaryotes is largely physically controlled (temperature, discharge), whereas between floods and at lower water levels, a biological control, also mediated by viral infections, becomes more important. At high discharge and water levels, conspicuous low levels of viral mortality could be detected. However, the ratio of lysogenic to lytically infected cells apparently was highly variable. Furthermore, increased hydrological connectivity of floodplains with the main river has the potential to change the microbial carbon cycle by increasing prokaryotic production and, as a consequence, viral infection, diversity and lysis. This may cause the bacterial compartment to shift from a link- function to higher trophic levels towards a sink, with more carbon respired at the base of the food chain thus leaving the ecosystem as emitted CO 2 . In this context suspended particles were found to be also of importance. As those particles with significant organic constituents increase in abundance, more viruses are attached and fewer are freely suspended. Therefore, such particles apparently remove viruses from the water column. Many of our results are breaking new scientific ground in river and general inland water ecology. We are confident that this also contributes to our understanding of microbially mediated processes, carbon fluxes and the ecology of large, human-impacted river systems.