Microbial ecology of subsurface granitic bedrock: a humid-arid site comparison in Chile

Subsurface microorganisms face extreme challenges such as anoxic, xeric, and oligotrophic conditions. In igneous systems, nutrient limitation is critical, as biomass input relies on surface-derived fluids via tectonic fractures. Despite growing interest in subsurface habitats, little is known about ecosystems beneath arid landscapes, where surface water input is limited by the low annual precipitation. This study compares granitic subsurface environments beneath arid and humid surface ecosystems, highlighting the link between surface climate and subsurface biodiversity. DNA was extracted from granitic subsurface rocks recovered from two endmember sites along a north-south climate gradient in Chile's Coastal Cordillera. Microbial communities inhabiting down to 55 m deep subsurface rocks were characterized using 16S rRNA amplicon and shotgun metagenomic sequencing. We identified an abundant and potentially active subsurface community below both climates dominated by heterotrophic bacteria, including Pseudarthrobacter, Janthinobacterium, and Pseudomonas. However, rare taxa affiliated with common chemolithoautrophs, e.g. Thiobacillus, Sulfuriferula, and Sulfuricurvum, were only observed in the arid subsurface, indicating increased oligotrophic conditions and reliance on inorganic electron donors in the deeper subsurface of the desert. Functional analysis revealed sulphur, hydrogen, and carbon monoxide as potential inorganic electron donors. These findings expand the current understanding of microbial life in the subsurface of granite rocks showing the influence of surface climate on nutrient conditions in the deeper subsurface, providing new insights into the extent and functional capacity of terrestrial subsurface habitats and their role in global biogeochemical processes.