Microbial Community Controls on Sulfide Oxidation Rates and Cave Formation in a Subsurface Biogeochemical System

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This project aims to discover rules governing microbial community assembly and diversity, and the effects of these on geochemical process rates. These questions are difficult to address in most ecosystems, where microbial diversity is very high and spatial heterogeneities are large. The Frasassi cave ecosystem is distant from surface sources of fixed carbon and nitrogen and is fueled only by sulfur oxidation, so that microbial activity can be directly related to sulfuric acid production. The cave hosts both neutral and pH 0-1 sulfur-oxidizing communities, and these community types will be compared to test hypothesis about microbial diversity and community function. For example, we hypothesize that microbial diversity is strongly controlled by the number of available electron donors and electron acceptors, rather than by limitations posed by environmental extremes such as low pH. In this case, both pH 0-1 and neutral Frasassi microbial communities will have lower diversity than a previously studied pH 0-1 acid mine drainage system where both iron and sulfur electron donors are available. The project also aims to identify biological and geological controls on sulfuric acid production (cave dissolution). Molecular biological methods will be used to quantify microbial populations, geochemical methods to document and monitor cave habitats and microbial cultures, and lipid and isotope-labeling methods to quantify biomass and identify autotrophic microorganisms in the cave communities.The project will involve 6 undergraduate thesis students and the development of an educational module to be incorporated into the highly successful Carleton College Geology Program in Italy. The work will generate young scientists with interdisciplinary training in geomicrobiology and publications that identify factors that control the structure and function of subsurface microbial communities responsible for sulfuric acid production and cave formation. More generally, understanding how microbial communities are assembled, and how these patterns of assembly affect the use of available resources, is a critical component of understanding the biogeochemistry of both ancient and present-day environments.