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The climate of the South Pole is about as different from the warm, subtropical climate of St. Petersburg, FL, as a researcher can imagine. The South Pole, however, is where John Lisle, a microbial ecologist at the U.S. Geological Survey (USGS)'s St. Petersburg Science Center in St. Petersburg, worked for three Antarctic summers in a row, from 2000 to 2002. He was part of a team investigating how bacteria and their viruses, or bacteriophage, interact and how these interactions influence carbon and nutrient cycling in aquatic systems.
Before joining the USGS, John was part of a multidisciplinary-research group working in the National Science Foundation (NSF)'s Long Term Ecological Research (LTER) site in the McMurdo Dry Valleys, Antarctica (for more information about the site, visit the McMurdo Dry Valleys Web page.) The Dry Valleys compose the coldest and driest desert on Earth, making this region one of the most extreme environments on the planet. Within the ice-free valleys are lakes that have been perennially covered with 3 to 6 m of ice for hundreds of years. The lakes were formed when glaciers receded. Owing to the absence of wind-driven mixing, all the lakes are highly stratified with regard to temperature, salinity, nutrients, and primary productivity, and all are extremely stable. An additional factor contributing to the extreme conditions is a 6-month period of darkness during which there is no sunlight to drive photosynthesis and primary productivity.
The question John and his colleagues were addressing was "How are the microbial components of these lakes surviving during the cold, dark months of winter?" The first step in answering this question was to determine the abundances of bacteria and bacteriophage in each lake at different depths. The next step was to assess whether bacteria-bacteriophage interactions and cycles, which have been documented in more temperate climates, also occur in these extreme environments. Examples are lysis (the rupture of a bacterial cell by bacteriophage that have replicated within the cell, resulting in bacterial death and the release of thousands of new bacteriophage) and lysogeny (a state in which bacteriophage lie dormant within a bacterium, awaiting favorable conditions for replication).
Bacterial abundances in the lakes were found to range from 3.04×104 to 3.48×107 per milliliter, and bacteriophage abundances from 1.66×105 to 4.64×107 per milliliter. Both abundances are similar to those found in temperate climates. When samples were treated with a lysogenic stimulant, as much as 62.5 percent of the bacterial population were found to be lysogenic (bacteria that are hosts to dormant bacteriophage until more favorable conditions for bacteriophage replication exist outside the cell). This percentage is relatively high compared with lysogeny rates in more temperate and productive waters, suggesting that lysogeny may be the preferred "lifestyle" for bacteriophage in extreme environments.
These observations are also significant with regard to a fundamental understanding of how dissolved and particulate organic carbon and nutrients contained with the bacterial biomass are cycled through the microbial loop and made available to the photosynthetic algae that are the sole source of primary production in these lakes. The importance of the microbial loop to cycling of carbon and other nutrients associated with the lysed bacterial biomass is heightened because there are no grazers in these ecosystems. Therefore, most of the dissolved organic carbon from the bacteriophage-lysed bacteria is recycled back through the bacterial populations.
An additional observation during these studies was the presence of lake aggregates at all depths in all the lakes. When stained with fluorescent labels, the aggregates were found to be composed of proteins and carbohydrates, with bacteria and bacteriophage embedded within the aggregates' matrix. We feel that these aggregates serve as nutritional "oases" within these extreme environments, in which the bacteria can survive during the dark months of winter by using extracellular enzymes to slowly digest the aggregate matrix. In addition to direct use of the aggregate for nutrients, the lysis of bacteria by bacteriophage within these aggregates also provides a source of carbon.
It may appear that the study of bacteria-bacteriophage interactions in the McMurdo Dry Valleys is not applicable to studies of temperate coastal water and sediment systems; however, this is not the case for the following reasons: (1) Transition zones, such as oxic-anoxic interfaces, are expanded to meter thicknesses in the Dry Valley lakes in comparison with the same types of zone in more temperate and mixed aquatic systems, where the zone may be only a few millimeters thick; and (2) the Dry Valley lakes are not mixed and do not support trophic levels above the phytoplankton. Together, these characteristics make the perennially frozen lakes of the McMurdo Dry Valleys excellent model systems for studying microbial interactions, because most of the confounding variables that exist in other aquatic systems are absent. The data from these studies will be published in the international journal Microbial Ecology in the coming months.
in this issue: Microbial Life in Antarctic Lakes
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