Over two decades of research have indicated that viruses play crucial roles in aquatic food webs as active constituents of the microbial loop and in the population ecology of both prokaryotic and eukaryotic microorganisms. Over the past 5 years, there has been a sharp increase in reported aquatic virus research, notably in the areas of freshwater viral ecology, viruses of eukaryotic microorganisms and viral genetic diversity. Recent studies of the interactions between viral infection, bacterivory and grazing have demonstrated the complex dynamics of viral infection within aquatic ecosystems. These reports have helped solidify our understanding of the environmental controls on viral abundance, impacts of viral infection upon host community structure and have elucidated new roles of viruses in biogeochemical cycles – such as photosystem gene expression. Previously unrecognised groups of viruses (ribonucleic acid viruses and single-stranded deoxyribonucleic acid viruses) have also been revealed as diverse and active components of marine virioplankton assemblages.

Key Concepts:

• Viruses are the most numerically dominant organisms on earth, and their abundance varies between habitats, often according to local primary productivity.
  
• Both viral activity and bacterivory contribute significantly to bacterial mortality in aquatic ecosystems, which in turn influence global-scale biogeochemical cycles.
  
• Methodological advances, such as metagenomics and genomics, have greatly facilitated studies of marine viruses and resulted in key discoveries on viral diversity, viral–host gene transfer and viral influence on host cell physiology.
  
• Photosystem genes are common in cyanophage and are hypothesised to enhance viral production rates.
  
• Viral-induced mortality may be a key factor in controlling or terminating algal blooms.