Virus-killer lurks in Antarctic lake

A virus that attacks other viruses has been discovered in a hypersaline lake near Davis station in Antarctica.

The virus is only the third ‘virophage’ discovered. The first one, called Sputnik, was discovered in 2008 and the second one, Mavirus, was discovered earlier this year.

The new virophage, called Organic Lake Virophage or OLV, after the lake in which it was found, was discovered by a team of scientists led by Professor Rick Cavicchioli of the University of New South Wales.

In 2006 and 2008 the team travelled to Antarctica’s Vestfold Hills with the Australian Antarctic Division to collect water samples from a number of hypersaline lakes that dot the landscape. The samples were filtered through different sized filters (3.0 μm, 0.8 μm and 0.1 μm; where 1 μm = 0.001 mm) to separate different microbial components, including viruses, bacteria and algae.

Back in their laboratory the team used ‘metagenomics’ and ‘metaproteomics’ to identify the DNA sequences and protein components, respectively, in the entire microbial communities captured in each filtered sample.

Transmission electron micrographs of virus-like particles from Organic Lake resembling (A) phycodnavirus or PV, (B) Sputnik virophage, (C) bacteriophage – a virus that infects bacteria. Photo: Rick Cavicchioli

In the 0.1 μm sample the team found a previously unknown DNA sequence which they were eventually able to reconstruct to reveal a complete circular genome with similarities to the Sputnik genome. Using transmission electron microscopy, they identified virus particles in their sample which also resembled Sputnik in size and appearance.

Viruses reproduce by infecting host cells and using the cell’s molecular machinery to make multiple copies of their own genome and to package these genomes into protein shells. These new viruses then break out of the cell and are free to infect many more cells.

A virophage is different in that it targets a host cell that is already infected by a ‘regular’ virus. As the regular virus goes about replicating itself, the virophage hijacks this process by inserting its own genome into the virus. It can then make copies of itself at the expense of the regular virus, whose own replication is significantly reduced.

In each of the three new virophage discoveries, the virophages are associated with giant viruses. Mavirus targets the CroV virus, which infects a plankton species called Cafeteria roenbergensis. Sputnik targets a giant virus known as ‘mamavirus’ which has the world’s largest viral genome, and infects an amoeba. Professor Cavicchioli’s team found OLV associated with a group of giant ‘phycodnaviruses’, or PVs, that infect algae and consequently help control algal blooms.  Like Sputnik and Mavirus, OLV’s genome includes genes that it collected from the Organic Lake phycodnaviruses, confirming the predator-prey relationship.

Professor Rick Cavicchioli takes samples from Organic Lake in 2006. Photo: Rick Cavicchioli

The discovery of a virophage in Organic Lake adds new complexity to the dynamics of the microbial community in the Antarctic system. Professor Cavicchioli’s team modelled the impact of OLV as a predator in the marine system.

‘By reducing the number of PVs in the community, OLV shortens the time it takes for the host algae population to recover,’ he says.

‘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.

‘Antarctic lake systems have evolved mechanisms to cope with long light-dark cycles and a limited food web. In Organic Lake and similar systems, a decrease in PV activity may be instrumental in maintaining the stability of the microbial food web.’

Organic Lake from the air as it is beginning to melt in December 2008. Photo: Rick Cavicchioli

While OLV was the dominant virophage in Organic Lake, the teams’ analyses suggest that there are other related virophages present. The team has also found genome sequences that match OLV from nearby Ace Lake. And there may be many more virophages waiting to be discovered. Professor Cavicchioli’s team has found OLV-type sequences in a saline lagoon in the Galapagos, an estuary in New Jersey and a freshwater lake in Panama.

The research is published in the Proceedings of the National Academies of Science in March: Yau S., Lauro F., DeMaere M., et al. Virophage control of antarctic algal host–virus dynamics. PNAS

Related links:

Sequencing secrets of whole microbial communities (Australian Antarctic Magazine 14: 8, 2008)