Viruses are the most common biological entities in the marine environment. Most marine viruses are phages (bacteriophages) that kill the heterotrophic and autotrophic microbes (both Bacteria and presumably Archaea) that dominate the world's oceans. Phages and the other major microbial predator guild, nanoflagellates, control the numbers of marine microbes to a concentration of about 5 x 105 cells per ml of surface seawater.
Figure 1: Sampling Sites
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Phages affect microbial evolution by inserting themselves into genomes as prophages. Prophages often account for most of the differences between strains of the same microbial species, and they can dramatically change the phenotype of the hosts via lysogenic conversion. For example, many nonpathogens and pathogens only differ by prophages that encode exotoxin genes. Phages also affect microbial evolution by moving genes from host to host. It has been hypothesized that most of the orphan open reading frames (ORFans) in microbial genomes are actually of phage origin. Phages may also affect microbial evolution by killing specific microbes. A type of Lotka-Volterra models, called "kill-the-winner," predict that as one microbial strain becomes dominant, its viral predator kills it and leaves open a niche that can be used by a related strain that is resistant to the phage. This model may explain the enormous microdiversity observed in microbial communities.
There has not been a global survey of marine viruses, and consequently, it is not known what types of viruses are in Earth's oceans or how they are distributed. Is the enormous viral diversity explained by a strong local selection, making every marine location a unique habitat? Or are marine currents and sea breezes responsible for a global spreading, homogenizing viral assemblages and supporting the observation of identical phages in different locations. Metagenomic analyses of viral samples from different provinces can help us answer these questions.<
184 viral assemblages collected over a decade and representing 68 sites in four major oceanic regions (Sargasso Sea, Coast of British Columbia, Gulf of Mexico and Arctic Ocean) showed that most of the viral sequences were not similar to those in the current databases. There was a distinct "marine-ness" quality to the viral assemblages. Global diversity was very high, presumably several hundred thousand of species, and regional richness varied on a North-South latitudinal gradient. The marine regions had different assemblages of viruses. Cyanophages and a newly discovered clade of single-stranded DNA phages dominated the Sargasso Sea sample, whereas prophage-like sequences were most common in the Arctic. However most viral species were found to be widespread. With a majority of shared species between oceanic regions, most of the differences between viral assemblages seemed to be explained by variation in the occurrence of the most common viral species and not by exclusion of different viral genomes. These results support the idea that viruses are widely dispersed and that local environmental conditions enrich for certain viral types through selective pressure.
