1. Virus-like particle (VLP) abundances in nine freshwater to saline lakes in the Vestfold Hills, Eastern Antarctica (68° S) were determined in December 1999. In the ultra-oligotrophic to oligotrophic freshwater lakes, VLP abundances ranged from 1.01 to 3.28 × 106 mL–1 in the top 6 m of the water column. In the saline lakes the range was between 6.76 and 36.5 × 106 mL–1. The lowest value was found in meromictic Ace Lake and the highest value in hypersaline Lake Williams. Virus to bacteria ratios (VBR) were lowest in the freshwater lakes and highest in the saline lakes, with a maximum of 23.4 in the former and 50.3 in the latter.
2. A range of morphologies among VLP was observed, including phages with short (Podoviridae) and long tails, icosahedric viruses of up to 300 nm and star-like particles of about 80 nm diameter.
3. In these microbially dominated ecosystems there was no correlation between VLP and either bacterial numbers or chlorophyll a. There was a significant correlation between VLP abundances and dissolved organic carbon concentration (r=0.845, P < 0.01).
4. The data suggested that viruses probably attack a spectrum of bacteria and protozoan species. Virus-like particle numbers in the freshwater lakes were lower than values reported for lower latitude systems. Those in the saline lakes were comparable with abundances reported from other Antarctic lakes, and were higher than most values published for lower latitude lakes and many marine systems. Across the salinity spectrum from freshwater through brackish to hypersaline, VLP concentrations increased roughly in relation to increasing trophy.
5. Given that Antarctic lakes have a plankton almost entirely made up of bacteria and protists, and that VLP abundances are high, it is likely that viruses play a pivotal role in carbon cycling in these extreme ecosystems.