In-situ microbial calcite dissolution by sulfur-oxidizing bacteria in a limestone cave

Libby A. Stern, Annette Summers Engel, Philip C. Bennett

University of Texas, Austin, TX 78705, USA

Sulfide in anaerobic groundwater enters Lower Kane Cave, Wyoming, where it oxidizes to sulfuric acid via abiotic and microbial metabolic pathways. This redox reaction drives a chemsynthetically-based ecosystem supporting diverse aquatic microbial communities.  The host limestone is replaced by gypsum, which is washed out by the cave stream, leaving a residuum of chert as the cave floor. Limestone clasts from recent breakdown are found on the chert pebbles in the cave stream with thick colonization by filamentous bacteria.  The limestone is deeply etched despite near calcite saturation of the circum-neutral stream water, and authigenic gypsum crystals have formed.

To test how the abundant microbial mats influence the rates and mechanisms of limestone dissolution in the cave stream, we deployed sterile and live in-situ microcosms containing Iceland spar calcite for 4.5 months.  Plastic mesh retains the calcite granules inside the live microcosms, whereas the sterile capsules have 0.2-micron filters to exclude microorganisms while permitting chemical exchange with the ambient water.  Environmental-SEM imaging of the calcite from the live capsules shows abundant etching of the entire mineral surface, coating by biofilms, attachment of bacterial filaments, some containing intracellular sulfur crystals, and localized calcite dissolution beneath these filaments.  The presence of intracellular sulfur indicates sulfide oxidation, and potential biogenic acid.  In contrast, the calcite in sterile capsules shows only slight, diffuse dissolution.  This experiment demonstrates that microorganisms directly enhance subaqueous sulfuric acid speleogenesis.

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