that have been selected for over long periods of time by this unusual combination of properties. However, Lake Vostok and other subglacial aquatic systems are liquid-water environments with a number of features that are conducive to microbial growth, and the growth of a small subset of contaminating cells cannot be precluded. Growth may be especially favored for those microbes that are introduced from viable populations in glacial ice higher in the borehole (Chapter 3) and within drilling fluids that contain both organic substrates and living cells (Chapter 4).

Persistent liquid water, stable conditions, and extended time for reproduction are features of subglacial aquatic environments that make them suitable habitats for microbial growth. In this respect they contrast with many polar environments that are subject to intense freeze-thaw cycles (e.g., shallow lakes and ponds, ice shelf ecosystems), physical disruption (e.g., sea ice), and extreme aridity (e.g., polar desert soils, rock surfaces).

POTENTIAL IMPEDIMENTS TO LIFE IN SUBGLACIAL AQUATIC ENVIRONMENTS

Adverse conditions in the subglacial environment that could inhibit or preclude growth include low temperature, elevated pressure, and any reactive oxygen species favored by high oxygen tensions. Despite their extremely cold environments, many microbial species in the polar environment are psychrotolerant (able to grow at low temperatures) rather than psychrophilic (optimal growth at low temperatures). All of these microbes grow slowly under extremely cold conditions, yet they can sometimes achieve large standing stocks because losses are low (Vincent 2000). These includes well-developed benthic microbial mats in polar lakes with ultra-oligotrophic water columns (Bonilla et al. 2005) and microbial communities in the anoxic sediments of Antarctic saline lakes (Mancuso et al. 1990). It is also well known that microbial communities grow in polar oceans at temperatures down to −1.8°C, and can be sustained at much lower temperatures (−20°C) in cold saline environments such as the brine channels in sea ice (Junge et al. 2004).

The deep ocean habitat is similar to that of subglacial aquatic environments, being both high pressure and cold. Despite these two adverse conditions, microbes can live as deep as 10,000 m in the ocean (Parkes et al. 1994; Kormas et al. 2003), well above the pressures found in subglacial aquatic environments where they are less than or equal to 400 atmospheres, equivalent to 4000 m in the ocean. Supersaturated oxygen habitats are also known to harbor active microbial communities, including the perennially ice-covered lakes of the McMurdo Dry Valleys (Wharton et al. 1986). The upper waters of Lake Vostok, however, are predicted to contain extreme oxygen levels more than an order of magnitude higher. McKay et al. (2003) calculated that in situ gas concentrations in Lake Vostok would be up to 2.5 L per liter of lake water (at standard temperature and pressure), equivalent to 750 mg O2 L−1. Even higher values (up to 1300 mg O2 L−1) have been estimated by Lipenkov and Istomin (2001).

These oxygen tensions could be a severe constraint on growth if accompanied by the formation of reactive oxygen species (see below), particularly for microbial contaminants that would largely comprise taxa that are adapted to less severe conditions. Resistant cells and endospores attached to particles might find more favorable oxygen conditions for proliferation as they sink deeper in the water column.



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