The first global survey of marine viral genomes has shown that the oceans are awash with viruses. Calculations of viral diversity indicate there might be as many as several hundred thousand distinct marine viral species. Most viral species are widely dispersed, but local environmental conditions dictate which species are most common in a particular oceanic region.

The group of Forest Rohwer and their collaborators collected 184 samples from 68 different marine sites located in four main regions: the Sargasso Sea, the Arctic Ocean, the Gulf of Mexico and the coastal waters of British Columbia. They extracted and amplified DNA from uncultured viral particles and used pyrophosphate sequencing to determine the DNA sequences that were present. Fewer than 10% of the sequences in this viral assemblage were significantly related to sequences in current genomic and metagenomic databanks. The sequences were also phylogenetically distinct from those of known phage genomes, and indicated that marine phage share distinctive characteristics.

The richness of viral species varied along a latitudinal gradient. Diversity was highest in regions nearest the equator and lower towards the poles, in common with other marine biota. The marine virome from British Columbia, a region that is affected by seasonal upwelling and the outflows of many rivers, was exceptionally genotype-rich. The authors speculate that the richness of such regions could be boosted to levels approaching global diversity by the inward migration of viral species from other regions.

The prevalence of viruses differed among the four oceanic regions. The Arctic Ocean metagenome had the most prophage-like sequences. Cyanophages and a new single-stranded DNA microphage dominated the Sargasso Sea sample. As most viral species are widespread and shared between oceanic regions, the observed geographical distribution is largely the result of differences in the abundance of viral species rather than the exclusion of particular species.

Metagenomic analysis has provided important information on the abundance, distribution and dynamics of marine viruses. A more comprehensive characterization of marine viral diversity and, as a result, a better understanding of marine microbial ecosystems, can be anticipated with further improvements in the power of DNA sequencing technology.