Microbial influence on the kinetics of karstification
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The traditional model of karst and cave formation is that of carbonic acid limestone dissolution, where biologically-produced CO₂ in meteoric water reacts with and dissolves limestone. However, an alternative model has been proposed for several karst sysems where sulfide is abundant, known as sulfuric acid speleogenesis (SAS). Here, acid produced by chemoautotrophic sulfur-oxidizing bacteria (SOB) corrodes limestone while producing dissolved calcium and sulfate. Little is known about the rate of limestone dissolution due to SOB activity, or the nature of the microbe-limestone attachment and interaction. The field site for this study is Lower Kane Cave, WY, an active SAS-formed cave where rapid steam H₂S oxidation is associated with sulfur-oxidizing microbial mats. In this study, the rate of limestone dissolution due to microbial oxidation of reduced sulfur compounds was investigated using laboratory and field microcosms. Laboratory chemostat chamber experiments were designed to mimic the cave environment with and without SOB (native Kane Cave bacteria and Paracoccus versutus), and using different energy sources (thiosulfate, sulfide, and elemental sulfur stored in bacterial filaments). Limestone dissolution rates of abiotic chemostat experiments from this study are comparable to those in previous literature. However, dissolution rates from the experiments with bacteria are 3-4 times faster than the abiotic control rates, a result which is consistent across duplicate experiments and between experiments using different types of SOB. This rate increase represents a complex chemical system influenced by the bacteria on the mineral surface. SEM images confirm that the limestone chips both in the cave and in the biotic chemostat chambers are uniformly covered in biofilm, and that the mineral surface beneath the biofilm is much more etched and corroded than the surface of limestone chips dissolving without bacteria. The results from the lab experiments and the cave microcosms suggest that a biofilm on limestone chips will physically and chemically separate the mineral surface from the bulk solution. Because the bacteria are generating acid directly on the mineral surface, a microenvironment develops beneath the biofilm with low-pH and low saturation state with respect to calcite. The neutrophilic sulfur oxidizing bacteria found in the cave and used in the experiments benefit from attachment to limestone (high buffering capacity), and create a microenvironment that triggers limestone corrosion at a rate several times faster than the abiotic rate.