grow in the solute-rich water sequestered between the triple junctions of ice crystals (Price 2000). These microbiota could be transferred deeper into the system through sampling and could potentially grow (e.g., in meltwaters produced during hot-water drilling). In the Lake Vostok core, different sections of the accretion ice contain materials frozen in from different parts of the lake, allowing contamination from one part of the lake to another. This latter effect is likely to be small; however, if hydrodynamic processes are operating across the lake, the effect could be magnified. Furthermore, the melting of overlying glacial ice is a continuous natural source of microbiota for Lake Vostok and probably other subglacial lakes.

POTENTIAL FOR TESTING AND ASSESSING CONTAMINATION: EXPERIENCES FROM DEEP-BIOSPHERE SAMPLING

The advent of molecular and genomic approaches to biological measurements has continued to broaden our concepts and understanding of microbial diversity. It is now commonplace to sample the presence and diversity of microbial communities from many environments throughout the biosphere. Of particular importance to these investigations, especially those occurring in areas where biomass is predicted to be low (such as the deep biosphere), is the prevention of contamination of the environment and subsequent samples with exogenous life or nucleic acids.

Most protocols examined to date have focused on preventing (or at least minimizing) contamination of samples extracted from the environment in question and, by extension, providing a measure of protection to the environment itself. These protocols center on (1) using advancements in drilling technology to obtain the necessary depths and (2) incorporating the use of sterile liners and other sterile devices for extracting samples from rock, sediment, or water.

Most protocols for sediment cores recommend further processing of the core under sterile conditions and removing interior portions for actual microbiological analysis. The inclusion of tracer studies to track possible contamination during any deep-biosphere sampling has also been viewed as an important approach to these studies. Interestingly, much less emphasis has been placed on determining the risks or effects of reverse contamination—where the surface environment or personnel operating drilling equipment or collecting samples are exposed to microorganisms from the deep environment under investigation.

Terrestrial Deep Subsurface Drilling

Investigation of microbial life in the deep biosphere represents an important undertaking in the search for the nature and diversity of microbial life. The majority of experience gained thus far in sampling has occurred in the terrestrial deep subsurface (>100-m depth) including groundwater, aquifer sediment, and rock matrices. The ability to conduct sampling in these environments has been aided by advances in specialized drilling technologies. In terrestrial settings, rotary drilling is a common method when drilling to depths >100 m, although cable tool drilling has also been used to obtain high-quality samples for microbial analysis from unconsolidated sediments from at least 200 m and has the advantage that no recirculating drill fluids are necessary. Push-type core barrels such as split-spoon core barrel devices are the most common samplers used in cable drilling to obtain core samples of formation materials for microbiological



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