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Bureau Seminar, October 2, 2009

Microbes, Microbial Geochemistry, and Sulfuric Acid Karst

Philip C. Bennett
Department of Geological Sciences, The University of Texas at Austin

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In Lower Kane Cave (LKC), Wyoming, sulfidic spring waters host complex chemoautotrophic microbial communities. Most caves are formed by conventional carbonic acid dissolution, with CO2 derived from photosynthetic organic carbon. Sulfide-rich springs discharge into some caves, however, and from observations of LKC in the 1970s, the original Sulfuric Acid Speleogenesis (SAS) model was proposed as an alternative cave enlargement process. The original SAS model was via abiotic chemical autooxidation, but H2S is a rich energy source for chemoautotrophic microorganisms, and a surprisingly complex consortium of microorganisms, dominated by sulfur-oxidizing bacteria, is found in the LKC springs and stream. The dominant microorganisms are neutrophilic sulfur-oxidizing bacteria that use reduced sulfur compounds for energy, with the potential to produce sulfuric acid as a geochemically reactive byproduct.

Several evolutionary lineages within the class Epsilonproteobacteria dominate the biovolume of subaqueous microbial mats, with abundant Gammaproteobacteria similar to Thiothrix unzii. These microbes support the cave ecosystem through chemoautotrophic carbon fixation, and a diverse community of aerobes and anaerobes are established in the thick mats of sulfide oxidizers. The anoxic interior of the mats is a habitat for anaerobic metabolic guilds, dominated by sulfate-reducing bacteria in the class Deltaproteobacteria, as well as fermenting bacteria. The sulfide generated by the SRB populations in turn feed the sulfide oxidizers, resulting in an extension of the mat coverage via a nutrient spiral. In this system neutrophilic Epsilonproteobacteria generate sulfuric acid as a metabolic by-product, rapidly dissolving the host limestone and enlarging the cave, while taking advantage of the buffering capacity of the carbonate rock to consume the acid. The microbial pathway explains >90% of the sulfide oxidation, and accounts for most of the cave enlargement process.

Experiments reveal that microbial biofilms grow rapidly on limestone surfaces and oxidize different reduced sulfur compounds. Thiothrix sp. oxidizes thiosulfate to SO4= and H+, resulting in aggressive limestone corrosion, while H2S is only partially oxidized to S0 - a reaction that does not produce excess acidity or enhanced corrosion. Epsilon-proteobacteria species, in contrast, apparently oxidize H2S all the way to sulfuric acid. When all aqueous S-substrate is exhausted the community oxidizes stored intracellular sulfur, generating additional acidity and dissolving carbonate rocks.

Enhanced corrosion occurs only in the highly reactive low pH microenvironment under a biofilm, and bulk chemical parameters measured in the field do not reflect conditions at the microbe-mineral interface or the geochemical reactions that occur there. Based on observations and experiments we believe that microbial sulfide oxidation is the primary cave-forming phenomenon here, and that different taxa as well as different sulfur substrates and metabolic pathways, result in vastly different rates of calcite weathering. In other words, the microbial ecology directly influences cave formation.


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