1. Department of Molecular, Cellular and Developmental Biology and the Center for Astrobiology, University of Colorado, Boulder, Colorado 80309-0347, USA

Correspondence to: Norman R. Pace¹ Correspondence and requests for materials should be addressed to N.R.P. (Email: nrpace@colorado.edu). The GenBank database accession numbers for rRNA clone sequences are AY911422 to AY911496.

Abstract

The endolithic environment, the pore space of rocks, is a ubiquitous habitat for microorganisms on the Earth¹ and is an important target of the search for life elsewhere in the Solar System². Photosynthetic, endolithic microbial communities commonly inhabit the outer millimetres to centimetres of all rocks exposed to the Earth's surface. In the most extreme terrestrial climates, such as hot and cold deserts, endolithic microorganisms are often the main form of life^{3, 4, 5}. The endolithic microhabitat gives protection from intense solar radiation and desiccation, and it provides mineral nutrients, rock moisture and growth surfaces^{4, 5}. Here we describe the discovery and identification of the constituents of an extremely acidic (pH 1) endolithic microbial community inhabiting the pore space of rocks in the geothermal environment of Yellowstone National Park, USA. Subjected to silica mineralization, such endolithic communities constitute biomarkers that can become fossilized and potentially preserved in the geological record. Remnants of these communities could serve as biosignatures and provide important clues about ancient life associated with geothermal environments on the Earth or elsewhere in the Solar System.

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