Marine viruses are small infectious agents found in the ocean that require living host machinery for replication. They consist of a core of nucleic acids coated with protein, as they have the traditional virus assemblage. The dominant hosts for viruses in the ocean are marine microorganisms such as cyanobacteria. Viruses that that live as obligate parasitic agents in marine bacteria are known as marine bacteriophages or marine phages. The existence of viruses in the ocean was discovered through electron microscopy and epifluorescence microscopy of ecological water samples, and later through metagenomic sampling of uncultured viral samples. Marine viruses, although microscopic and essentially unnoticed by scientists until recently, are the most abundant and diverse biological entities in the ocean. Viruses have an estimated abundance of 1030 in the ocean, or between 1 and 100,000x106 per millilitre. Quantification of marine viruses was originally performed using transmission electron microscopy but has been replaced by epifluorescence or flow cytometry.
Viruses are highly host specific. Studies have shown that marine viruses are more likely to infect cooccurring organisms, those that live in the same region a virus exists in. Therefore, biogeography is an important factor in a virion’s ability to infect.
The knowledge of variation of viral populations across spatiotemporal and other environmental gradients is supported viral morphology, determined by transmission electron microscopy (TEM). Non-tailed viruses appear to be dominant in multiple depths and oceanic regions, followed by the Caudovirales myoviruses, podoviruses, and siphoviruses. However, viruses belonging to families Corticoviridae, Inoviridae and Microviridae are also known to infect diverse marine bacteria. Metagenomic evidence suggests that microviruses (icosahedral ssDNA phages) are particularly prevalent in marine habitats.
Metagenomic approaches to assess viral diversity are often limited by a lack of reference sequences, leaving many sequences unannotated. However, viral contigs are generated through direct sequencing of a viral fraction, typically generated after 0.02-um filtration of a marine water sample, or through bioinformatics approaches to identify viral contigs or viral genomes from a microbial metagenome. Novel tools to identify putative viral contigs, such as VirSorter and VirFinder, allow for the assessment of patterns of viral abundance, host range, and functional content of marine bacteriophage.
The virus-to-prokaryote ratio, VPR, is often used as an indicator of the relationship between viruses and hosts. Studies have used VPR to indirectly infer virus impact on marine microbial productivity, mortality, and biogeochemical cycling. However, in making these approximations, scientists assume a VPR of 10:1, the median observed VPR in the surface ocean. The actual VPR varies greatly depending on location, so VPR may not be the accurate proxy for viral activity or abundance as it has been treated.
Although marine viruses have only recently been studied extensively, they are already known to hold critical roles in many ecosystem functions and cycles. Marine bacteriophages and other viruses appear to influence biogeochemical cycles globally, provide and regulate microbial biodiversity, cycle carbon through marine food webs, and are essential in preventing bacterial population explosions. Scientists are exploring the potential of marine cyanophages to be used to prevent or reverse eutrophication.
In the water column
Marine viral activity presents a potential explanation of the Paradox of the Plankton, proposed by George Evelyn Hutchinson in 1961. The Paradox of the Plankton is that many plankton species have been identified in small regions in the ocean, where limited resources should create competitive exclusion, limiting the number of coexisting species. Marine viruses could play a role in this effect, as viral infection increases as potential contact with hosts increases. Viruses could therefore control the populations of plankton species that grow too abundant, allowing a wide diversity of species to coexist.
Marine bacteriophages play an important role in deep sea ecosystems. There are between 5x1012 and 1x1013 phages per square metre in deep sea sediments and their abundance closely correlates with the number of prokaryotes found in the sediments. They are responsible for the death of 80% of the prokaryotes found in the sediments, and almost all of these deaths are caused by cell lysis (bursting). This allows nitrogen, carbon, and phosphorus from the living cells to be converted into dissolved organic matter and detritus, contributing to the high rate of nutrient turnover in deep sea sediments. Because of the importance of deep sea sediments in biogeochemical cycles, marine bacteriophages influence the carbon, nitrogen and phosphorus cycles. More research needs to be done to more precisely elucidate these influences.
Marine viruses are thought to play an important role in nutrient cycles by increasing the efficiency of the biological pump. Viruses cause lysis of living cells, releasing compounds such as amino acids and nucleic acids, which tend to be recycled near the surface. Lysis also releases more indigestible carbon-rich material like that found in cell walls, which is likely exported to deeper waters. Thus, the material that is exported to deeper waters by the 'viral shunt' is probably more carbon rich than the material from which it was derived. This would increase the efficiency of the biological pump.
Marine bacteriophages often contain auxiliary metabolic genes, host-derived genes thought to sustain viral replication by supplementing host metabolism during viral infection. These genes can impact multiple biogeochemical cycles, including carbon, phosphorus, sulfur, and nitrogen.
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