Summary
Sediments in the hyper-arid core of the Atacama Desert represent one of the closest terrestrial analogues to Mars’ regolith. Understanding the distribution and drivers of life in the sediment may thus give critical clues on how to search for biosignatures on Mars. Here, we identify the spatial distribution of highly specialised bacterial communities in previously unexplored depth horizons of subsurface sediments and their correlation with moisture and geochemical variables. We deployed an autonomous rover in a mission-relevant Martian drilling scenario with manual sample validation to recover and analyse sediments in two Mars-like terrains, a gravel desert pavement and an evaporite rich playa. Subsurface communities were distinct from surface-associated communities and were delineated by depth related to sediment moisture. Geochemical analysis indicated soluble salts and minerology that influence water bio-availability, particularly in deeper sediments. Colonisation was also patchy and uncolonised sediment was associated with indicators of extreme osmotic challenge. Bacterial diversity reflected strong selection for halotolerant and desiccation tolerant taxa throughout depth horizons. The most diverse communities occurred within the zone of highest moisture, whereas deeper sediments were colonized by methylotrophic taxa with limited diversity. The study provides important baseline microbial ecological data on the linkage between biocomplexity, moisture and geochemistry in Mars-like sediments at the limit of habitability and demonstrates feasibility of the rover-mounted drill for future Mars sample recovery.