The endolithic bacterial diversity of shallow bedrock ecosystems

Terrestrial subsurface microbial communities are not restricted to the fluid-filled void system commonly targeted during groundwater sampling but are able to inhabit and dwell in rocks. However, compared to the exploration of the deep biosphere, endolithic niches in shallow sedimentary bedrock have received little interest so far. Despite the potential contribution of rock matrix dwellers to matter cycling and groundwater resource quality, their identity and diversity patterns are largely unknown. Here, we investigated the bacterial diversity in twenty-two rock cores in common limestone-mudstone alternations that differed in rock permeabilities and other geostructural and petrological factors. 16S rRNA gene analysis showed the existence of a unique rock matrix microbiome compared to surrounding groundwater. Typically, shallow weathered limestones contained bacterial groups most likely originating from soil habitats. In low-permeable mudstones, we found similar communities of oligotrophic heterotrophs, and thiosulfate-oxidizing autotrophs, without relation to depth, rock type and bulk rock permeability. In fractured limestone, the bacterial communities of fracture surfaces were distinct from their matrix counterparts and ranged from organic matter decomposers in outcrop areas to autotrophs in downdip positions that receive limited surface input. Contrastingly, rock matrices from lithologically corresponding, but highly isolated environments, were dominated by spore-forming bacteria, oligotrophic heterotrophs and hydrogen-oxidizing autotrophs. Neither depth, matrix permeability nor major mineralogy dominantly controlled the endolithic bacterial diversity. Instead, a combination of subsurface factors drives the supply of niches by fluids, matter and energy as well as the (re)dispersal conditions that likely shape bacterial diversity.