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Sulfur is one of the 10 most abundant elements in the solar system. Its complex redox chemistry and tendency to polymerize have led to a comparably complex array of microbial metabolisms linking sulfur with carbon, nitrogen, and iron in biogeochemical cycles. The Frasassi Cave aquifer has emerged as a model ecosystem for studying sulfur-based chemosynthesis outside the oceans. Our previous work indicated that the chemosynthetic microbial community is directly dependent on reducing power from ancient organic matter in stratigraphically adjacent shales, as well as oxidant anions (sulfate, nitrate) in evaporites immediately underlying the shales. This juxtaposition of rock types (evaporite, shale, carbonate) is common in sedimentary rock sequences globally, and thus the biogeochemistry of the Frasassi deep aquifer may reveal subsurface processes, metabolisms, and microbial taxa that have global significance. Continuing exploration of the Frasassi aquifer by cave divers has recently revealed that there are conspicuous and widespread microbial biofilms throughout the system. Our preliminary study of one of these biofilms using genetic approaches showed that the microbial community contained a large number of novel and uncultivated taxa. The CoDL project is motivated by our new awareness of the large biomass and widespread extent of these enigmatic biofilms throughout the aquifer, which encompasses more than 30 kilometers of passages navigable by humans.
Deep caves in central Italy offer a unique opportunity to investigate the diversity of microoxic and anoxic microbial communities in sulfidic sedimentary aquifers 100-600 m below ground surface. We expect the proposed pyrotag-enabled study to reveal the spatial distributions and redox/temperature niches of enigmatic Epsilonproteobacteria previously detected in extremely microoxic aquifer zones. We also expect to extend our understanding of a diverse anoxic aquifer community dominated by Deltaproteobacteria, Chloroflexi, and a large number of taxa in unnamed and novel clades.