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^3–5. 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.

Lush and unusual photosynthetic communities inhabit silica rocks in Yellowstone’s Norris Geyser Basin. These rocks are primarily chalcedonic sinters and are warmed to ~35°C by subsurface geothermal activity. The stark, weathered surfaces of these exposed rocks show no evidence of the rich life hidden beneath the surface (Fig. 1). Fractured rock samples show clear signs of photosynthetic endolithic communities, which inhabit a distinct green band from 1 to 15mm thick and 2–10mm beneath the surface exposed to light. Photosynthetic pigments, primarily chlorophyll of red algae (see below), impart a green colour. Although endolithic growth of red algae is known in other volcanic areas6, this is the first comprehensive molecular analysis of the microbial diversity and composition of these unique communities and their potential mineralization and fossilization. Pore waters extracted from the rock had a pH of ~1 and contained high concentrations of...