MICROBIAL COLONIZATION AND BIOSIGNATURE PRESERVATION POTENTIAL IN ACIDIC GYPSUM DEPOSITS FROM SULFIDIC CAVES

Sulfidic caves form in carbonate rock where rising hydrogen sulfide (H₂S)-rich ground waters interact with oxygen from meteoric waters or cave air in a process called sulfuric acid speleogenesis (SAS). When these caves are actively forming, they are hotspots for microbial chemosynthesis. Cave streams are filled with sulfide-oxidizing bacterial mats, and extremely acidic biofilms form on the walls and ceilings where H₂S(g) degasses into the oxygen-rich cave atmosphere. These wall communities generate sulfuric acid that corrodes cave limestone and leaves secondary deposits of acidic (pH <2) microcrystalline gypsum. These gypsum deposits can build up over time on walls and floors and may remain for thousands to millions of years, well after sulfidic waters recede from the area. However, little is known about the microbial community associated with freshly-formed gypsum, and we don’t know how long these deposits continue to serve as microbial habitats in ancient sulfidic caves. Here, we used high-throughput 16S rRNA “amplicon” sequencing and metagenomics to characterize microbial communities in freshly formed SAS gypsum, and we will present preliminary data on communities in “relict” gypsum deposits from the now-inactive portions of SAS caves. We also explored whether these deposits contain isotopic, lipid, and amino acid biosignatures, to determine if cave gypsum can trap and preserve evidence of microbial life over long time-scales. We found that the acidophiles that dominate the active SAS environment produce diagnostic biosignatures including hopanes and fatty acids extracted from highly acidic gypsum. In contrast to freshly formed sediments, relict gypsum collected from inactive SAS regions had much lower microbial biomass, with few of the microbial taxa observed in freshly formed gypsum. We will discuss the implications for long-term biosignature preservation in acidic gypsum, and whether chemically analogous gypsum on the Martian surface could be a potential repository for the search for evidence of past or present Martian life.