Biogeochemical features of lipids in endolithic microbial communities in the Ross Desert (McMurdo Dry Valleys), Antarctica.

Endolithic microbial communities inhabiting porous rocks in the cold, dry mountainous regions of Antarctica have been studied extensively as examples of life's adaptations to extreme environments. Here, we examine hydrocarbons and fatty acids occurring in these communities in order to clarify their biogeochemical features with respect to source organisms, microbial activity, fossilization processes and the influence of Gondwanaland sediments. Unusually, long-chain (>C19) n-alkanes and anteiso-alkanes were often the major hydrocarbons in the samples. A suite of n-alkanoic acids (n-C9-n-C32) and long-chain anteiso-alkanoic acids (a-C20-a-C30) were found, along with short-chain iso- and anteiso-alkanoic acids, and n-alkenoic acids. The relationship between long-chain n-alkanoic acids (n-C20-n-C32) and long-chain anteiso-alkanoic acids suggests that these compounds probably originated from the same group of microorganisms, such as bacteria or endolithic lichens, under moderate pH conditions (pH 3-5). Relatively high trans/cis-C16:1 alkenoic acid ratios suggest the presence of unfavorable environmental conditions in the endolithic microbial habitat. Normal-alkenoic/alkanoic acid ratios may be a useful marker for the fossilization of endolithic microbial communities. Thermally matured triterpanes and steranes from fossilized associations on Mount Fleming strongly suggest the presence of Gondwanaland sediments formed during Devonian and Jurassic (400-180 million years ago).