Microbial ecology of acidic, biogenic gypsum: community structure and distribution of extremophiles on freshly formed and relict sulfate deposits in a hydrogen sulfide-rich cave.

Sulfate minerals are abundant on the Martian surface, and many of these evaporite deposits are thought to have precipitated from acidic fluids. On Earth, gypsum (CaSO IMPORTANCE: Gypsum and other sulfate salts are common on Mars, and many of these deposits are thought to have formed from acidic fluids early in the planet's history. Understanding the life that survives and thrives in similar environments on Earth is therefore crucial for evaluating whether these Martian sulfates are or ever were habitable. One such environment where acidic gypsum occurs is in sulfuric acid caves, where extremophilic microorganisms drive the precipitation of sulfate minerals by oxidizing hydrogen sulfide gas from the cave atmosphere. Here, we characterized the communities of microorganisms on freshly formed and ancient gypsum in the Frasassi Caves and found that the gypsum deposits hosted microbial communities that changed based on chemical energy availability and the age of the gypsum. Our findings underscore the importance of chemical and microbiological interactions in shaping habitable niches and provide context for searching for past or present life in acidic Martian sulfates.