Viruses can influence the genetic diversity of prokaryotes in various ways. They can affect the community composition of prokaryotes by `killing the winner' and keeping in check competitive dominants. This may sustain species richness and the amount of information encoded in genomes. Viruses can also transfer (viral and host) genes between species. Such mechanisms have probably influenced the speciation of prokaryotes. Whole-genome sequencing has clearly revealed the importance of (virus-mediated) gene transfer. However, its significance for the ecological performance of aquatic microbial communities is only poorly studied, although the few available reports indicate a large potential. Here, we present data supporting the hypothesis that viral genes and viral activity generate genetic variability of prokaryotes and are a driving force for ecological functioning and evolutionary change.