Organic Sulfur Gas Production in Sulfidic Caves

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

The University of Texas at Austin, Department of Geological Sciences, Austin, Texas, 78712; lstern@mail.utexas.edu

Lower Kane Cave, Big Horn Basin, WY, permits access to an environment where anaerobic sulfide-rich groundwater meets the aerobic vadose zone.  At this interface microorganisms thrive on diverse metabolic pathways including autotrophic sulfur oxidation, sulfate reduction, and aerobic heterotrophy.  Springs introduce groundwater rich in H₂S to the cave where it both degasses into the cave atmosphere and is used by chemautotrophic sulfur oxidizing bacteria in the cave spring and stream habitat.   The cave atmosphere in the immediate vicinity of the springs has elevated levels of CO₂, H₂S and methane, mirroring the higher concentration of H₂S and methane in the spring water.  The high CO₂ concentrations are attenuated toward the two main sources of fresh air, the cave entrance and breathing holes at the rear of the cave.  

Conventional toxic gas monitors permit estimations of H₂S concentrations, but they have severe cross sensitivity with other reduced sulfur gases, and thus are inadequate for characterization of sulfur cave gases.  However employment of a field-based GC revealed elevated concentrations of carbonyl sulfide in cave atmosphere.  Cultures of microorganisms collected from the cave optimized for enriching fermenters and autotrophic and heterophic SRB each produced carbonyl sulfide suggesting a biogenic in origin of the COS in addition to H₂S.  Enrichment cultures also produced methanethiol (methyl mercaptan) and an additional as yet undetermined volatile organic sulfur compound.  In culture, the organo-sulfur compounds were less abundant than H₂S, whereas in the cave atmosphere the organo-sulfur compounds were the dominant sulfur gases.  Thus, these organo-sulfur gases may prove to be important sources of both reduced sulfur and organic carbon to microorganisms living on the cave wall in a subaerial habitat.  Moreover groundwater has not yet been recognized as a source of sulfur gases to the atmosphere, but with the abundance of sulfidic groundwater, this environment may prove to be important to the global sulfur cycle and its influence of the global radiation budget.

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