Abstract EANA2024-22

Mars is one of top destinations to search for signs of ancient or current microorganisms. Martian caves, in particular, have been considered high-priority potential targets for the search for life or biosignatures. Yet, if primordial life forms had originated on Mars, they might be able to adapt and colonize these underground habitats, similarly to caves on Earth, finding a refuge from increasingly harsh and hostile conditions of Martian surface. Besides, if life may have not survived to date, the evidence of biomarkers produced by past life could persist, at least, in rocky substrate, sediments, speleothems and mineral assemblages due to the low-erosive pressure of these spatially confined environments. In this context, to unearth, for the first time, the microscopic life-forms that survive underneath, in the dark and isolated subterranean environment on our planet and to further support future extraterrestrial caves exploration to identify traces of extant or extinct colonization, we analyzed over 1,050 microbiomes, including fungi and bacteria, from 86 caves worldwide. The selection included multiple sample types (i.e. Soil sediment, Rock, Speleothem, Biofilm, Microbial mat and Air) from different climatic conditions from humid to arid regions, both in karst and volcanic landscapes, across 5 continents and 16 Countries. Through an amplicon sequencing approach, we revealed that albeit caves are inhabited by a myriad of microorganisms (7,411 fungal and 11,891 bacterial phylotypes), they are dominated by 50 fungal and 52 bacterial taxa, regardless of climatic, geographic and lithological context. We reported a contrasting effect of wet and dry conditions, in promoting the reduction or growth of numbers and variability in species for the caves microbial cores, with the arid climate leading to a reduction in total cave microbial diversity. Despite that, highly adapted fungal and bacterial phylotypes were selected from increasing conditions of water depletion, revealing a striking prevalence of fungal core under arid climate. These dominant taxa are commonly recognized as resistant to oligotrophy and drought, as well as actively involved in biomineralization and bioweathering processes of rocks and minerals.  Finally, biofilm and soil-sediment samples represented the largest diversity reservoirs, while bacterial core showed similar coverage between volcanic and karst caves, the latter representing the preferential lithological background for fungal-dominant taxa. Insights from this research and continuing to study adaptation, physiological and ecological traits of microorganism inhabiting terrestrial caves could help Martian rovers pinpoint promising sites to sample for signs of past or present life.