ABSTRACT

EAR-0617160
BENNETT
The Earth's subsurface contains innumerable habitats occupied by microorganisms that manipulate chemical forms of energy and alter the geologic surroundings to scavenge nutrients. These chemolithotrophic communities utilize energy from redox transformation of inorganic substrates, without an influx of reduced carbon or allochthonous macronutrients. For the last four years we have been characterizing the geochemistry and microbiology of Lower Kane Cave (LKC), a sulfidic carbonate cave ecosystem, as an accessible analog for deep subsurface environments. Here chemolithotrophic sulfur (S) oxidation forms the base of a complex microbial ecosystem, and these microbes appear to accelerate the oxidation of sulfide to sulfate, generating proton and potentially modifying their habitat by dissolving limestone. Our preliminary results suggest a much broader significance of terrestrial S-based microbial communities, raising new questions about subsurface ecosystems, S metabolism, the coupling of S and C cycles, and the geological consequences.
Scientific Merit: Our previous work has established that a subsurface chemoautotrophic microbial population thrives in the sulfidic waters of LKC, oxidizing sulfide. Mass balance calculations show that the standard model of sulfuric acid speleogenesis (SAS), the volatilization of H2S and autoxidation on the cave wall, is insignificant, and almost all sulfide is oxidized in the aquatic system. Unanswered, however, is the question of whether the microbes accelerate sulfide oxidation and speleogenesis, directly participating in the geologic process of speleogenesis. We propose to use the natural laboratory of LKC, combined with controlled laboratory microcosm experiments, to examine the importance of microbial SAS as a mechanism of karstification. This proposal will address the following questions:
1- How does the aquatic microbial community accelerate limestone weathering?
2- Do the different SOB populations, with diverse S-oxidation mechanisms, influence limestone weathering to different degrees? Is community composition important?
3- What is the importance of mineralogy to microbial habitat?
Batch reactors and flow-through chemostat reactors will be used to examine selected monoculture and environmental mixed population community influence on calcite and dolomite dissolution rate. We will use the tools and techniques developed from our previous work at LKC to characterize the cave geochemistry, and the microbial communities in both the field system and the laboratory reactors using culture-independent methods. The rate measurements will then be combined with the detailed molecular microbiological characterization using SSU Rdna phylogenetics, RFLP, FISH and PLFA analysis to characterize the influence of community on karstification. Both the role of microbes in carbonate weathering, and the role of mineral chemistry in defining the habitat for these neutrophilic microbial populations will be examined.
Broader Impacts: Sulfuric acid speleogenesis is potentially a widespread phenomenon that could be responsible for accelerated karstification and development of aquifer networks for regionally critical source of water, including the Edwards Aquifer of central Texas and its endemic stygobites and stygophiles. This project has implications for our understanding of the development of karst porosity for both aquifer and petroleum reservoirs, and our knowledge of subsurface microbial ecology. In particular this project will enhance our understanding of the role of mineralogy in microbial ecology, and allow a better understanding of the mechanisms for microbial survival and growth in the subsurface, with implications for pathogen transport in karst aquifers. This project will directly support one PhD, one MS, and one undergraduate student at the University of Texas, and introduce them to the developing field of microbial geochemistry and geomicrobiology. The LKC site has become a type locality for directly examining the theory of SAS in karstification, and the results from our previous research have been presented on PBS-NOVA, and rebroadcast by the BBC throughout Europe. The results from our work have also become a central theme in two graduate classes at UT, and the well characterized archived samples are used to teach molecular techniques to new graduate students in the geomicrobiology program

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