The terrestrial subsurface is a well recognized habitat for microbes. Increasing temperature with depth in the Earth's crust results in an overlap of subsurface habitats and sites of low-T hydrothermal activity. Microbial influences on hydrothermal precipitates are potentially common, therefore. Here we present new data of well-preserved filaments of probable microbial origin from hydrothermally influenced subsurface environments. Filaments were investigated in thin section by optical microscopy and with the SEM using HF- and HCl-etched chips. Our samples are from hydrothermal environments situated in oceanic and continental volcanics and range in age from Proterozoic to Tertiary. Filaments have a very constant diameter of 1-2 micron and are typically preserved in a permineralized form as iron oxyhydroxides. The question of biogenicity has been addressed by morphometric analysis of filaments from such environments, and comparison with microbial filaments and nonbiogenic fibrous structures. A first qualitative comparison clearly shows a close similarity of fossil filamentous fabrics from hydrothermal environments with life microbial filaments. Besides simple filaments, twisted filaments/stalks, some of them very closely resembling the iron oxidizing bacterium Gallionella, were found at sites in Iceland and in fractured Indian Ocean basalt. Filamentous fabrics are overgrown by minerals typical of low-T hydrothermal environments (chalcedony-quartz, calcite, clay minerals, zeolites). Filamentous fabrics made up of microscopic filaments can reach macroscopic dimensions due to alignment of filament orientations by gravity or flow. The common occurrence of these probable microbial filaments in ancient low-T mineral assemblages indicates that such systems may be frequently colonized by microbes.

Low-T hydrothermal environments similar to those hosting the described filaments on Earth are also likely to be present on Mars. Volcanics and impact melt rocks, both known to commonly contain hydrothermal alteration assemblages on Earth, are also present on Mars. Thus conditions appear favorable for the formation of similar features on Mars, provided life was present.