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Virophage Found in Antarctica

By Stephen Luntz

The discovery of a third virus that parasites other viruses has been made in a hypersaline Antarctic lake near Davis Station.

Virophages reproduce by infecting cells that have already been taken over by other viruses. The virophage inserts its genes into the primary virus so that, instead of the cell being forced to make copies of the original virus, it now produces copies of the virophage.

The first virophage, named Sputnik, was found in 2008. The discovery led to the announcement in March 2011 of a second virophage species. Now Prof Rick Cavicchioli of the University of New South Wales has published the details of the Organic Lake Virophage (OLV) in the Proceedings of the National Academy of Sciences.

OLV was discovered as a result of two Australian Antarctic Division expeditions to study lakes in the Vestfold Hills, including Organic Lake. Cavicchioli says the lake is unusually rich in viruses, including algae-infecting phycodna­viruses. OLV has phycodnavirus genes in its genome, providing confirmation of the relationship.

Phycodnaviruses reduces algal growth and cause dramatic population crashes. “Modelling shows that the virophage stimulates secondary production through the microbial loop by reducing overall mortality of the host algal cell after a bloom, and by increasing the frequency of blooms during the summer light periods,” Cavicchioli says. In this way OLV helps the ecosystem adapt to an environment where photosynthetic species can only occur in summer.

OLV is the most common virophage in Organic Lake, but there is evidence that related species exist there and in at least one nearby lake. Comparisons with DNA samples taken from other environments show similar sequences from the Galapagos, Panama and New Jersey.

All three known virophages prey on very large viruses, with Sputnik targeting the largest known viral genome. However, Cavicchioli doesn’t know whether this is a requirement for virophage success or if it is simply easier to find the virophages associated with large genome viruses.

“There is a lot of discussion on blogs about whether virophages can be used to control diseases,” Cavicchioli says. He’s reluctant to comment on the practicality of applying them to “such a different system”, but believes it is essential to learn more about virophages and their interactions with viruses to discover if the potential exists.