Significance and Diversity of Lake Bacteriophages

Abstract: Viruses has a relatively recently been discovered in high abundances in aquatic systems. Their possible importance has therefore been largely overlooked. In aquatic food webs there should be large differences in carbon and nutrient fluxes depending on whether the main cause of bacterial mortality is viral infection or grazing, where lysis following viral infection should result in a release of organic carbon and nutrients from the lysed bacteria and hence not reach higher trophic levels. Recent research on aquatic viruses has mainly focused on marine environments and the number of studies on freshwater viral ecology is limited. Hence, there is a need for more studies on the importance and functioning of viruses in freshwater systems.The aims of this thesis were to explore the functioning and diversity of viruses that infects bacteria (phages) in freshwater systems. To effectively address this I conducted two experiments and three field studies in 23 lakes in different parts of Sweden. The results show that viral infection and subsequent lysis of the host cell can partly explain the formation of non-nucleoid-containing bacteria and further that viruses can respond to increases in phosphorus concentrations without any net increase in bacterial abundance. Generally, a larger part of bacterial production in lakes were grazed by flagellates than lysed by viruses, but a larger fraction of the total bacterial mortality can be attributed to viruses in hypolimnion compared to in epilimnion. Further, the largest impact of phages on bacterial production may be in humic lakes, which have a relatively high frequency of visibly infected bacterial cells, but low flagellate abundance. Community composition of bacteria and viruses were only weakly coupled in the studied systems. The most important factors for predicting viral community composition were temperature and concentrations of dissolved organic carbon, total phosphorus and soluble reactive phosphorus. The viral community composition changed over the season and temperate phages could be detected by incubations with mitomycin C showing that a large fraction of the viruses detected appeared to be temperate phages. The most important environmental factor co-varying with viral community composition was again concentrations of total phosphorus.To summarize, bacteriophages, as a bacterial mortality factor, are important in freshwater microbial food webs and phosphorus supply has a potential central function in the regulation of the importance of bacteriophages and additionally for viral diversity.