Microbial influence on sulfur speciation in Lower Kane Cave, WY

A distinctive microbial community is being studied within Lower Kane Cave (LKC) of the Madison Limestone, near Lovell, WY, where the cave forming process is principally sulfuric acid replacement of limestone with gypsum. The aquatic microbial mat includes a consortium of both S-reducing and S-oxidizing bacterial communities, which cycle sulfur along the reach of the cave stream. Multiple techniques are being employed to characterize the speciation and distribution of sulfur within LKC in order to identify the individual metabolic pathways, and to what degree sulfur chemistry within the cave is controlled by microbial processes. Aqueous sulfide levels were determined immediately in the field using colorimetric methods and volatilization was directly measured by field GC. Dissolved sulfide levels generally decrease with distance from the stream source, ranging from 0.85 to 0.03 ppm. Volatilization increases over the microbial mats however due to local sulfide production by sulfate reducing bacteria. Cave water, sediment and microbial biomass were sampled from the cave and characterized for major element and sulfur chemistry. Laboratory HPLC determination of transient aqueous sulfoxy species was done to characterize intermediate species, and low concentrations of thiosulfate and trace polythionates were detected. Sediment samples were analyzed for total sulfur and operational sulfur fractions, including acid volatile sulfur (AVS), total reducible sulfur, pyrite and elemental S. Elemental analysis was used to determine the distribution of total S within sediment and biomass to identify potential sulfur storage within the system. Total S ranges from 0.35% dry weight in sediment to 51% dry weight in mats. Operational sulfur fractions were isolated using a modified Johnson-Nishita method, and AVS fractions range up to 0.2% (wt/wt). The presence of microbial mats appears to enhance volatilization of sulfur gases by mechanisms as yet unknown. Correlation of S distribution and speciation with the current microbial communities and stream morphology within LKC indicates influences from both biotic and abiotic processes. The results from this study, however, suggest that microbial consumption dominates over abiotic auto-oxidation and volatilization of dissolved sulfides, while the anaerobic community provides an additional source of reduced S.