Viruses in a coloured electron micrograph. The first global survey of marine viruses has found that bacteriophages are the most common viruses in the sea.
Credit: Marc Adrian
SYDNEY: Viruses are the most abundant 'life form' on Earth according to U.S. researchers, who have found the oceans are full of the not-quite-alive entities.
A team lead by Forest Rohwer of San Diego State University in California conducted the first global survey of marine viruses, finding a tremendous variety of viruses with more than 90 per cent of their DNA sequences not present in existing genetic databases.
"From an earlier perspective of viruses as purveyors of disease and tools of genetic engineering we have realised that viruses are the most abundant 'life forms' on Earth, are crucial cogs in the biosphere and likely harbour its greatest genetic diversity," author Curtis Suttle said.
Viruses are scraps of DNA covered in protein that have the ability to commandeer a cell's functions in order to replicate themselves. Unless they are activated within a cell, they exist in a state of suspended animation and are thus often not considered to be alive in the true sense.
In the study, which is published online in the open access journal PloS Biology, the authors analysed water samples collected from 68 sites over 10 years from four oceanic regions: the Sargasso Sea, the Gulf of Mexico, British Columbia coastal waters, and the Arctic Ocean.
In what is known as a 'metagenomic' study, the team analysed the total pool of DNA from all the viruses mixed together. This technique makes identifying individual viruses more difficult, but provides a snapshot of the biodiversity and functioning of specific ecosystems.
The most common viruses in the sea and elsewhere are known as phages, and target bacteria. According to the researchers, the relationship between bacteria and phages is an ancient one. "Life on our world originally consisted of prokaryotes and their phage predators," Suttle said.
Phages are thought to play an important role in bacterial communities, but not just through predation and parasitism. According to the researchers, "Phages often account for most of the difference between strains of the same microbial species." This can include the difference between harmless and harmful bacteria.
Rohwer and colleagues compared the distribution of marine viruses from each oceanic region, finding "clear evidence that the composition of viral assemblages varies in different geographic regions - probably reflecting selective pressure".
They also found that differences in the 'virome' - the viral genomes - between locations were mostly explained by the relative amounts of viral species, rather than the existence of entirely different viruses. In microbiology, this trend has been summarised as 'everything is everywhere, but the environment selects'.
Samples from the British Columbia coast were the most genetically diverse. "The coast of British Columbia is in an upwelling area... [and is] also enclosed and fed by many rivers," the authors said. These conditions are thought to provide more nutrients than other marine areas, allowing for greater bacterial diversity which in turn supports viral diversity.
The other three oceanic regions showed greater diversity the closer they were to the equator - a trend that mirrors findings from land-based ecosystems.
The researchers also discovered an entirely new group of phage viruses with only a single-strand of DNA. Overall, the authors predict that the world's oceans hold a few hundred thousand broadly distributed viral species.
"These results support the idea that viruses are widely dispersed and that local environmental conditions enrich for certain viral types through selective pressure," the authors said.
The cloning methods used in the study detected viruses with DNA, but not their cousins, the RNA viruses. While other methods have not detected RNA viruses in large numbers, the authors said that they "are still believed to be important components of the marine virome that need additional study".
The size and diversity of the virome, combined with the fact that throughout evolutionary history viruses have targeted all known life forms, has major implications. As Suttle puts it, "I suspect that viruses may be an archive of all genetic information on Earth."
