Diversity of aquatic viruses from genes to host interactions

Sammanfattning: Viruses are the most numerous entities on Earth, and can be found in all waters. In aquatic ecosystems, viruses are key players since they alter the way energy and nutrients are transferred by killing larger organisms. However, we still lack fundamental knowledge about aquatic viruses’ diversity, prevalence and in what ways and to what extant viruses influence their hosts. Therefore, I used laboratory and bioinformatic methods to investigate who these viruses are and in what ways they interact with their hosts.Previously unknown viruses within the deep biosphere as well as in Baltic Sea surface water were characterised. Bioinformatic approaches were used to describe the entire viral community in three different waters from the deep biosphere. These viral communities turned out to be highly novel compared to what has previously been seen. Water from the Baltic Sea was used to isolate viruses (bacteriophages, phages in short) infecting select bacterial hosts, and 93 previously unknown phages, encompassing 21 novel species in seven genera, were described. All these newly discovered viruses suggest that our knowledge of viral diversity is far from complete and that further studies are needed.By combining laboratory and bioinformatic methods, I disentangled who infected whom and was able to link this to abundance patterns in the wild. While the first part is important to recognise the potential of natural phage communities, the latter is key to understand what impact phages will have. Further, with a laboratory experiment I showed that phage infection could change the metabolism of the host, and that the changes were nutrient dependent. These studies highlight the importance of considering complex interactions to comprehend natural ecosystems.Taken together, this thesis increases our knowledge regarding aquatic viruses’ diversity, their functions and host interactions, which is of great relevance when trying to understand the world around us and how it will change in the future. Viruses are diverse and active both in the deep biosphere and surface waters, they persist over time and they alter host metabolism. These findings provide novel insights into microbial aquatic ecology and highlights the importance of viruses.