Viruses in the plankton of freshwater and saline Antarctic lakes

1. Virus-like particle (VLP) abundances in nine freshwater to saline lakes in the Vestfold Hills, Eastern Antarctica (68 degrees S) were determined in December 1999. In the ultra-oligotrophic to oligotrophic freshwater lakes, VLP abundances ranged from 1.01 to 3.28 x 10(6) mL(-1) in the top 6 m of t he water column. In the saline lakes the range was between 6.76 and 36.5 x 10(6) mL(-1). The lowest value was found in meromictic Ace Lake and the hig hest value in hypersaline Lake Williams. Virus to bacteria ratios (VBR) wer e lowest in the freshwater lakes and highest in the saline lakes, with a ma ximum of 23.4 in the former and 50.3 in the latter. 2. A range of morphologies among VLP was observed, including phages with sh ort (Podoviridae) and long tails, icosahedric viruses of up to 300 nm and s tar-like particles of about 80 nm diameter. 3. In these microbially dominated ecosystems there was no correlation betwe en VLP and either bacterial numbers or chlorophyll a. There was a significa nt correlation between VLP abundances and dissolved organic carbon concentr ation (r = 0.845, P < 0.01). 4. The data suggested that viruses probably attack a spectrum of bacteria a nd protozoan species. Virus-like particle numbers in the freshwater lakes w ere lower than values reported for lower latitude systems. Those in the sal ine takes were comparable with abundances reported from other Antarctic lak es, 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 t o increasing trophy. 5. Given that Antarctic lakes have a plankton almost entirely made up of ba cteria and protists, and that VLP abundances are high, it is likely that vi ruses play a pivotal role in carbon cycling in these extreme ecosystems.