Viral dynamics and impacts on the permanently ice-covered McMurdo Dry Valley lakes

The perennially ice-covered lakes in the McMurdo Dry Valleys are microbially dominated systems lacking a complex food-web. This feature, together with the physically stable water column provides a unique system to study microbial and viral processes. Viruses are now recognized as an important component in marine and freshwater planktonic dynamics and can play a significant role in the regulation of microbial community structure, food-web interactions, biogeochemical transformations, and microbial genetic diversity. Limited top-down control as well as the high number of viruses (10⁶ viruses mL^-1) imply that the relative role of viral particles in these systems may be greater than in temperate and tropical lakes. Virally mediated mortality could be a major factor in regulating bacterial biomass in the dry valley lakes and might also play an important role in the regeneration of nutrients and organic carbon. The high number of lysogenic bacteria (63% of the total population) previously reported in these lakes indicates high potential for introduction of new genetic information into bacterioplankton or progeny viruses, driving the evolution of host and viral assemblages. It has been shown that viral infection of the bacterial population varies on both daily and seasonal time scales. Based on this information, and the fact that these polar desert environments are particularly sensitive to discrete climate events, changes in bacterial diversity due to viral infections could be directly influenced by changing climatic conditions. Our data, in concert with past results, address the importance of virally mediated biogeochemical cycling, food-web interactions and modeling of microbial genetic diversity using a combination of microscopy and flow cytometry, together with burst size experiments, induction assays and genomic analyses.