Another complication for predicting the present conditions in the subglacial waters is that rock particles from glacial scour and sediments from preglacial lakes might be present. These materials could be sources for biota as well as for electron donors and acceptors for chemotrophic and heterotrophic organisms.

No matter what their origin, the substrates required for microbial growth appear to be present in the water column of subglacial aquatic systems at low concentrations, although a richer microbial habitat may be provided in the underlying sediments. The supply rate of these substrates is one of the key unknowns; however, the concentrations of growth substrates are considered to be low, suggesting that these environments are oligotrophic. As a consequence, microbes adapted to these environments by necessity will most likely have highly proficient metabolic processes capable of taking advantage of extremely oligotrophic conditions including efficient recycling strategies. Depending on the level of carbon and nutrients available to the microbial community, metabolic activity and growth could be very low in some aquatic environments and some fraction of the community may represent a viable but metabolically inactive state. It is likely that microbial communities, if present and active, will only be able to grow extremely slowly.

The growth of accretion ice in Lake Vostok may have created an environment where oxygen concentrations are 50 times higher than in normal lake waters. This situation is likely to occur because enclathrated hydrates of atmospheric air contained in glacial ice are released into the lake when this ice melts. Accretion ice excludes gases as it grows; the result is supersaturation of the upper levels of the lake water with gases. Unless the gases are removed by mixing into the deep waters or by transport out of the lake, high concentrations of oxygen may result in the production of superoxide radicals and other reactive oxygen species that could be detrimental to life. Alternatively, if the subglacial lakes are stratified there may be low oxygenic levels and anoxic bottom waters and sediments where microbes could exist.

Microbes in Glacial Ice and Subglacial Waters

The microbes in glacial and accretion ice have been examined by several methods. Microscopic and culture analysis of the Vostok ice core revealed bacteria, yeasts, fungi, and microalgae in glacial ice greater than 240,000 years old. In the oldest ice, only bacterial spores were found. Microbiological and molecular-based studies of the accretion ice found low but detectable amounts of bacterial cells and DNA. Molecular identification of microbes show that they are closely related to microbiota from the surface of the Earth: Proteobacteria, Firmicutes, Actinobacteria, and Bacteroidetes. Microscopic examination revealed bacteria, pollen, diatoms, and coccolithophorids. Of these, only the bacteria would be able to grow in the lake water.

Bacteria and yeasts from the accretion ice, thus presumably from Lake Vostok, respired glucose and grew on liquid and agar media. The total numbers of microorganisms detected in the accretion ice ranged from 100 to 900 cells mL−1.

Actual growth rates are unknown for bacteria in subglacial ecosystems, but are expected to be slow and possibly negligible in some situations. Analogous systems include the deep subseafloor biosphere, where one report estimated that the bacterial turnover time is up to 22 years (Schippers et al. 2005). These slow rates of metabolism and growth also have implications for the extent of genetic adaptation to the subglacial



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