Geomicrobiology of sulfuric acid speleogenesis: microbial diversity, nutrient cycling, and controls on cave formation

Much of the terrestrial subsurface is inaccessible for study, but caves represent distinctive shallow subsurface habitats where biogeochemical processes can be easily examined. Previously defined speleogenesis models are almost entirely based on abiotic chemical and hydrologic controls, as biological controls on cave formation have not been considered significant. Hydrogen sulfide-rich groundwater discharges from springs into Lower Kane Cave, Wyoming, and the sulfuric acid speleogenesis model was introduced in the early 1970s as a cave enlargement process resulting primarily from hydrogen sulfide autoxidation to sulfuric acid and replacement of carbonate by gypsum on subaerially exposed surfaces. The reduced sulfur compounds serve as rich energy sources for microorganisms that colonize the cave in both subaqueous and subaerial environments. Several evolutionary lineages of the class “Epsilonproteobacteria” dominate the microbial diversity of subaqueous mats, and these microbes support the cave ecosystem through sulfur cycling and chemolithoautotrophic carbon fixation. The “Epsilonproteobacteria” occupy microbial mats in additional sulfidic cave and spring habitats, expanding the evolutionary and ecological diversity of these previously unknown organisms. The interior of the Lower Kane Cave microbial mats is devoid of oxygen and this provides habitat for anaerobic metabolic guilds, dominated by sulfate-reducing and fermenting bacteria. These anaerobic groups are responsible for autochthonous hydrogen sulfide and volatile organosulfur gas production. Cycling of carbon and sulfur compounds by the subaqueous microbial communities affects sulfuric acid speleogenesis. Compared to the total flux of sulfide into the cave, little hydrogen sulfide volatilizes into the cave atmosphere or oxidizes abiotically. Instead, the primary loss mechanism is from subaqueous microbial sulfur oxidation. Consequently, despite the cave waters being slightly supersaturated with respect to calcite, the “Epsilonproteobacteria” generate sulfuric acid as a byproduct of their metabolism, locally depress pH, and focus carbonate dissolution. The hydrogen sulfide that volatilizes into the cave air is oxidized at the cave walls where interactions between cave-wall biological and physicochemical factors influence subaerial speleogenesis and low temperature authigenic quartz precipitation. The recognition of the geomicrobiological contributions to subaqueous and subaerial carbonate dissolution fundamentally changes the model for sulfuric acid speleogenesis and the mechanisms for subsurface porosity development.