Signatures of habitats and life in Earth's high-altitude lakes: clues to Noachian aqueous environments on Mars

14.1 IntroductionA series of astrobiological high-altitude expeditions to the South AmericanAndean Mountains were initiated in 2002 to explore the highest perenniallakes on Earth, including several volcanic crater lakes at or above 6000 min elevation. During the next five years, they will provide the first integratedlong-term astrobiological characterization and monitoring of lacustrineenvironments and their biology at such an altitude. These extreme lakesare natural laboratories that provide the field data, currently missingabove 4000 m, to complete our understanding of terrestrial lakes and biota.Research is being performed on the effects of UV in low-altitude lakesand models of UV flux over time have been developed (Cockell, 2000). Thelakes showing a high content of dissolved organic material (DOM) shieldorganisms from UV effects (McKenzie et al., 1999; Rae et al., 2000). DOMacts as a natural sunscreen by influencing water transparency, and thereforeis a determinant of photic zone depth (Reche et al., 2000). In sparselyvegetated alpine areas, lakes tend to be clearer and offer less protectionfrom UV to organisms living in the water. Transparent water, combinedwith high UV irradiance may maximize the penetration and effect of UVradiation as shown for organisms in alpine lakes (e.g., Vincent et al., 1984;Vinebrook and Leavitt, 1996). Shallow-water benthic communities in theselakes are particularly sensitive to UV radiation. Periphyton, which definescommunities of microorganisms in bodies of water, can live on varioussusbtrates. While on rocks, they include immobile species that cannot seeklow UV refuges unlike sediment-dwelling periphyton (Happey-Wood, 1988;Vincent et al., 1993) or alpine phytoflagellates (Rott, 1988) which bothundergo vertical migration. Inhibition of algal photosynthesis by UV radia-tion has been documented in the laboratory (Ha ¬der, 1993) and it has beenshown that phytoplankton production is reduced by formation of nucleic acidlesions (Karentz et al., 1991) or production of peroxides and free oxygenradicals (Cooper et al., 1989). Most of the experiments that have demon-strated in situ suppression of algal growth by UV radiation have eitherused artificially enhanced UV irradiance (Worrest et al., 1978) or shallowsystems (