Mineralogic and Geochemical Consequences of Microbial Habitat Modification in Karst Systems: Examples from the Kane Caves, Wyoming

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

The University of Texas at Austin, Department of Geological Sciences, Austin, Texas, 78712

Characterization of active cave and karst systems, with nearly constant physicochemical conditions and an assortment of energy and nutrient sources, can enhance our understanding of how subterranean microbial communities interact with their geological surroundings. Several caves along the Bighorn River, in north-central Wyoming, have sulfidic springs that discharge into cave passages, resulting in a diverse array of geochemical zones and microbial habitats in aqueous and subaerial settings. Sulfide concentrations vary between 0.05 and 1.2 ppm depending on proximity to spring discharge, and most cave waters have low oxygen concentrations. Subaerial surfaces are covered with gypsum, either as gypsum paste, needles, or short blades, so that virtually no carbonate surfaces are exposed to the cave atmosphere. Mounds of gypsum, up to 4 m high, fill the bottoms of large cave passages, especially between and near spring orifices. Sediment in the cave streams consists of mostly chert fragments and clays, with rare carbonate clasts. Preliminary evidence suggests that there is a direct correlation between water chemistry and the type of microbial mats observed in the cave, and that colonization of certain areas may be driven by metabolic substrate concentration and abundance. At least three distinct types of microbial mats exist in flooded cave passages: 1) white filamentous mats near sulfidic springs associated with flowing water and high sulfide concentrations; 2) thin gelatinous black coatings on stream sediments in stagnant areas; and 3) thick red and orange mats that are suspended in slow-moving water or that partially float on the water surface, and some mats are associated with areas undergoing desiccation. Mucous-like biofilms, associated with gypsum and elemental sulfur mineralization, have a pH of 0 to 1 and form droplets suspended from cave-walls and ceilings. Presumably, sulfuric acid is being produced by sulfur-oxidizers within the wall biofilms, as well as by populations in the cave streams, as evidenced by laboratory enrichments and screening for acid production by cultured strains. Therefore, we propose that carbonate destruction and secondary mineral formation within these karst systems may be due to microbial activity within these various habitats.

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