environment given that rates of evolution tend to be slow under conditions of low temperature (Gillooly et al. 2005) and low productivity (Horner-Devine et al. 2003).

BIOLOGY—CONCLUSIONS

Summary

The subglacial aquatic environments of the Antarctic lie beneath kilometers of ice. Microbes of a number of types are found in low numbers throughout all depths of the ice sheet, carried to the Antarctic by atmospheric winds and deposited at the surface. Some of these survive their extremely slow downward travel through the ice and are still capable of respiration and growth when sampled in cores from thousands of meters below the surface. It is, therefore, highly likely that the subglacial aquatic environments contain microbes capable of growth and are constantly being inoculated with microbes from the ice sheet. The important question then centers on the probability that these environments support growing and evolving microbial populations. After considering the requirements for microbial growth and the chemical and physical conditions predicted for subglacial environments, the committee concluded that the hypothesis of microbial life and growth cannot be rejected. Therefore, future exploration of these environments must assume an aquatic ecosystem containing growing microbial populations. This assumption affects issues of preservation of habitats and minimization of contamination during exploration. The uncertainty can be resolved only by direct sampling of the lakes and flowing waters beneath the ice sheet.

Requirements for Life

The proliferation of microorganisms requires, at a minimum an inoculum of cells; water; electron donors and acceptors (e.g., reduced iron, sulfides, organic matter, oxygen) for biological energy supply; a source of nutrients (e.g., C, N, P, S, Fe, and other elemental constituents of biomolecules); sufficient time for reproduction; and the physical, chemical, or biological conditions that prevent cell destruction and promote growth. Subglacial aquatic environments have not been studied directly; therefore no unequivocal evidence exists to confirm the presence or absence of life in these ecosystems.

Subglacial Aquatic Environments and Potential Microbial Communities

Two types of evidence have been used to predict what will be found in waters beneath the ice sheet. First, microbes exist in all extreme environments on Earth where there is water, from a depth of 10,000 m in the ocean to surface saline lakes and from subzero temperatures in the brine channels of ocean ice to hot springs. Subsurface microbial communities of land and ocean are driven by abiotically produced materials from the Earth’s interior such as sulfides, hydrogen, and carbon dioxide. Based on this, it appears likely that actively growing microbes will be present in subglacial waters. Second, although Lake Vostok has not been studied directly, there are samples of the chemistry and microbiology of the ice that has been derived from the lake (accreted ice); these indicate that there are no environmental conditions that would rule out microbial life within the lake.



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