Ghost-rocks' microbiota: metagenomic insights into their influence on the biogeochemistry of karstic cave and groundwater.

Microbial communities in the critical zone drive key geochemical processes, but many subsurface habitats remain poorly characterized. Ghost-rock karst systems in particular represent unexplored microbial niches. Here, we provide the first genome-resolved metagenomic comparison of ghost-rock and groundwater microbial communities from the Sterkfontein karst system (South Africa). Ghost-rock and groundwater communities host distinct taxonomic and metabolic assemblages. Groundwater communities are dominated by chemolithotrophs capable of oxidizing sulfur- and nitrogen-bearing compounds, and by heterotrophs degrading refractory, plant-derived organic matter. In contrast, primary producers in ghost-rocks likely rely on atmospheric chemosynthesis via trace gas oxidation, while glycogen metabolism and necromass recycling point to adaptations to oligotrophic and fluctuating hydrological conditions. Groundwater taxa with metal-interacting pathways may initiate bedrock colonization via metal oxidation, whereas ghost-rock communities include potential metal reducers that could drive iron and manganese oxide dissolution and influence trace element mobility. Together, these results underscore ghost-rocks as active microbial and geochemical hot spots within karst systems that may play a non-negligible role on biomineralization/bioweathering processes and on shaping (sub)terrestrial landscapes and global biogeochemical cycles.