Tullis Onstott (Princeton University)
Abstract. What is life like 4 km beneath the surface of the Earth? You could ask one of the Zuma Zumas, the illegal gold miners that live down in the ultradeep mines of South Africa’s Witwatersrand Basin, if you could find them. They are living proof of how far life will go if the rewards make you very rich. Beyond the tunnels of human activity exists a vast dark desert of rock where hot, high-pressured, gaseous fluid-filled fractures act as very narrow (< 1 cm) interconnected freeways for microbial transport. This network occupies only 0.01% of the volume with the remainder comprised of low porosity rock where the habitable living volume is only 0.005 to 0.0005%, but where the energy nutrients are very rich. For over ten years we have been exploring this subterranean desert down to depths of 5 km and finding within it oases inhabited by very tenacious life forms. At depths ranging from 0.6 to 1.5 km a surprisingly diverse community exists comprised of everything from viruses, to Archaea and Bacteria to bactivorous nematodes. At greater depths and temperatures up to 65°C the communities are dominated by deeply rooted and novel Firmicutes. A few, like candidatus D. audaxviator, are found at great depths though separated by 300 km. These latter species may represent subsurface bacterial “finches” whose genomes enable self-sustainability and mobility under a broad environmental range. But most bacteria are unique to their own fractures suggesting a “desert oases” biogeographical pattern that could encompass ~8,000 OTUs within a 1016 cm3 volume of the Witwatersrand Basin and containing ~1016-17 cells. Compared to soils, which typically contain 109 cells and ~8,000 OTUs per cm3, the diversity of deep subsurface biosphere is very low. New molecular tools, including single cell genomes, will enable us to find out who is really alive down there and how they have evolved over time. Whether or not life could have originated in the deep subsurface or whether similar inhabited oases exist beneath the surface of Mars remain questions for future generations.