Sulfur disproportionating microbial communities in a dynamic, microoxic-sulfidic karst system

Abstract

Biogeochemical sulfur cycling in sulfidic karst systems is largely driven by abiotic and biological sulfide oxidation, but the fate of elemental sulfur (S0) that accumulates in these systems is not well understood. The Frasassi Cave system (Italy) is intersected by a sulfidic aquifer that mixes with small quantities of oxygen-rich meteoric water, creating Proterozoic-like conditions and supporting a prolific ecosystem driven by sulfur-based chemolithoautotrophy. To better understand the cycling of S0in this environment, we examined the geochemistry and microbiology of sediments underlying widespread sulfide-oxidizing mats dominated byBeggiatoa. Sediment populations were dominated by uncultivated relatives of sulfur cycling chemolithoautotrophs related toSulfurovum,Halothiobacillus,Thiofaba,Thiovirga,Thiobacillus, andDesulfocapsa, as well as diverse uncultivated anaerobic heterotrophs affiliated with Bacteroidota, Anaerolineaceae, Lentimicrobiaceae, and Prolixibacteraceae.DesulfocapsaandSulfurovumpopulations accounted for 12-26% of sediment 16S rRNA amplicon sequences and were closely related to isolates which carry out autotrophic S0disproportionation in pure culture. Gibbs energy (ΔGr) calculations revealed that S0disproportionation under in situ conditions is energy yielding. Microsensor profiles through the mat-sediment interface showed thatBeggiatoamats consume dissolved sulfide and oxygen, but a net increase in acidity was only observed in the sediments below. Together, these findings suggest that disproportionation is an important sink for S0generated by microbial sulfide oxidation in this oxygen-limited system and may contribute to the weathering of carbonate rocks and sediments in sulfur-rich environments.