Limestone Corrosion by Neutrophilic Sulfur-Oxidizing Bacteria: A Coupled Microbe-Mineral System

Subsurface karst aquifers receiving sulfidic water can host complex chemolithotrophic microbial communities that are capable of dissolving limestone, forming new karstic habitat. Neutrophilic sulfur-oxidizing bacteria use reduced sulfur compounds as energy rich substrate, potentially producing sulfuric acid as a geochemically reactive byproduct. The physicochemical relationship between a biofilm forming on a limestone surface and the extent of microbial influence on dissolution rate, however, are unknown. We investigated the rate of Madison limestone dissolution by sulfur-oxidizers both in the field at Lower Kane Cave, WY (LKC), and in the laboratory using continuous flow culture reactors and microbial mat collected from LKC. In the field, a microbial consortium rapidly colonized limestone chips forming a thick biofilm, with deep etching of mineral surfaces underneath. In the laboratory we found that a microbial biofilm oxidizing thiosulfate on the limestone surface accelerated dissolution rate up to 7 times faster than the abiotic baseline rate. In contrast, experiments done with H₂S or a mixture of H₂S and thiosulfate had no effect on dissolution rate. We hypothesize that the laboratory mat community dominated by Thiothrix sp. oxidizes thiosulfate to sulfate and H⁺, while H₂S is partially oxidized to S°. When all sulfur substrate is withheld, the community oxidizes stored intracellular sulfur, briefly accelerating limestone dissolution even in the absence of external supplied substrate. Accelerated corrosion occurs only in the reactive micro-environment under the biofilm, disconnected from the bulk reactor solution. When experiments are repeated where the microbial population is separated from the limestone by a dialysis membrane barrier, measured pH drop is greater, but there is only slight enhancement of rate. This work confirms our working hypothesis that neutrophilic sulfur-oxidizers colonize and rapidly dissolve limestone surfaces, possibly to buffer the production of excess acidity.