Microbial metabolic structure in a sulfidic cave hot spring: Potential mechanisms of biospeleogenesis
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Glenwood Hot Springs, Colorado, is a sulfidic hot-spring that issues from numerous sites. These waters are partially responsible for speleogenesis of the nearby Fairy Cave system, through hypogenic sulfuric-acid dissolution. To examine whether there may have been microbial involvement in the dissolution of this cave system we examined the present-day microbial flora of a cave created by the hot spring. Using molecular phylogenetic analysis of the 16S small subunit ribosomal RNA gene and scanning electron microscopy, we examined the microbial community structure within the spring. The microbial community displayed a high level of microbial diversity, with 25 unique phylotypes representing nine divisions of the Bacteria and a division of the Archaea previously not identified under the conditions of temperature and pH found in the spring. By determining a putative metabolic network for the microbial species found in the spring, it appears that the community is carrying out both sulfate reduction and sulfide oxidation. Significantly, the sulfate reduction in the spring appears to be generating numerous organic acids as well as reactive sulfur species, such as sulfite. Even in the absence of oxygen, this sulfite can interact with water directly to produce sulfuric acid. Consequently, such metabolic activity may represent a mechanism by which biospeleogenesis can lead to passage enlargement through sulfuric acid production without the influx of oxygen or oxygen-rich waters. Such activity may lead to higher levels of sulfuric acid production than could be accounted for by inorganic hydrogen sulfide oxidation. Therefore, rather than generating localized pockets of speleogenesis within cave systems, such biogenic sulfuric acid production may have a regional impact on water chemistry and subsequent speleogenesis of large cave systems.