Geochemical and microbial characterization of a sulfidic cave in Santa Cesarea Terme, Italy

Fetida Cave belongs to an active sulfuric acid system located along the Adriatic coastline (Santa Cesarea Terme, southern Apulia). It is characterized by a rectilinear and sub-horizontal branch following regional lineaments, and opening at the present sea level. Fetida Cave has a natural entrance along the cliff, opened by marine erosion, and an artificial access, used in the past to collect sulfuric mud for human treatments. In this cave, seawater mixes and interacts with warm rising sulfide-rich groundwater, generating an extremely acidic environment causing the formation of features and deposits with peculiar geomorphology (ceiling cupolas, megascallops, rising channels) and mineralogy (gypsum, sulfur, and jarosite). Moving from the entrance to the inner part, a decrease of marine influence, together with an increase of the acidic effect due to the upwelling waters, can be observed. The amount of sulfuric acid is strongly influenced by tidal fluctuations, controlling the H2S degassing. Concentration of HS in the cave atmosphere and water is highly variable and can reach 15 ppm. Thanks to the oxidation of H2S, providing a rich energy source, Fetida Cave hosts abundant microbial communities related to different types of deposits, visible in its inner part: i) white filamentous streamers floating on the water surface or deposited on the pool floor, ii) white creamy moonmilk deposits visible as wall crusts, and iii) worm-like vermiculations of different colors, copiously covering the cave walls and ceilings. Here, we present the first characterization of the microbial communities within these types of deposits. Illumina sequencing approach targeting the 16S rRNA gene was combined with geochemical and microscopy (FESEM) analyses to describe the microbial diversity and biomineralization processes contributing to their formation.