Viruses are the most abundant organisms in the sea, with approximately ten billion in every litre of sea water(1, 2). Viruses are very small at generally 20-200nm, and have a simple structure consisting of genetic material with a protein coat, and sometimes a lipid envelope(2). Due to their simplicity, viruses rely on exploiting living cells and using the host’s cellular machinery to replicate(3). Irrespective of their size, viruses have been found to have a significant influence on many ecological processes and biogeochemical cycles in marine ecosystems(2, 4). Studies suggest that viruses contribute to nutrient recycling through the infection and lysis of marine microorganisms, which in turn controls the composition and diversity of microbial communities in marine environments(1, 5). Genes involved in photosynthesis have also been found in viral genomes, leading researchers to believe that proteins involved in bacterial photosynthesis originate from bacteriophages, viruses which infect the bacteria(6, 7). Research on marine viruses is important because it allows us to gain a deeper understanding of ocean ecology and the driving forces behind marine and global ecosystems(8).
Every second, approximately 1023 viral infections occur in the ocean(8). These infections affect the mortality of marine organisms and are subsequently a major force behind global geochemical cycles and the structure of microbial populations and communities(4, 5, 8). Microorganisms constitute 90% of the living biomass in the sea, and it is estimated that 20% of this biomass is eliminated by viruses every day(8).Viruses influence the mortality of bacteria in the ocean and therefore regulate both bacterial populations and those in subsequent trophic levels(9, 10). Studies have found that the distribution of viruses in marine environments correlates to bacterial abundance(11). Phytoplankton abundance tends to be higher in surface and coastal waters due to light intensity and penetration, and viral abundance decreases further offshore and deeper in the water column(8, 11). This presence of viruses corresponding to bacterial abundance suggests that virus replication rates increase in conjunction with increases in host growth rates, and it is this that allows viruses to control bacterial abundance and community composition(12). Studies have shown that viruses contribute to the collapse and control of blooms of a single species by high levels of viral-mediated mortality(8). This ‘killing the winner’ strategy controls the dominant competitive species and can lead to greater biodiversity(8, 12). Reducing the abundance of viruses has also been found to effect community composition, as species previously susceptible to viral infection are able to thrive and dominate other species, demonstrating the vital role of viruses in marine ecosystems(13). Viral lysis of microorganisms in order to control the composition of marine communities also contributes to recycling...