SRA

Microbially mediated sulfate reduction in aquatic sediments mineralizes organic carbon, generates hydrogen sulfide, and mediates the geochemical cycles of other elements, such as iron, phosphorus, and mercury. While organic matter contains a number of sulfur compounds, little is known of the fate of this organic sulfur pool during mineralization and more importantly its contribution to the inorganic sulfur cycle that fuels sulfate reduction. The current paradigm of sulfate reduction involves diffusion of sulfur from overlying water into the sediments where sulfur-reducing microorganisms are present. Contrary to the paradigm, modeling and preliminary results demonstrate that under low-sulfate conditions organic sulfur buried in sediment may be the dominant source of sulfur for sulfate reduction, and once mobilized, via microbial biotransformation, may be exported to the overlying water column. Contributions from organic sulfur may be pervasive in environments such as oligotrophic freshwater lakes or the oceans of the geologic past. By characterizing the organic sulfur transformations in sediments across a range of sulfate and organic carbon levels in Lake Superior and its largest American tributary, investigators will address the following questions under a range of environmental conditions: A. To what extent does organic sulfur contribute to the pool of sulfur that fuels sulfate reduction? B. Does organic sulfur undergo cryptic, microbially-mediated biogeochemical transformations, and what microorganisms are responsible for these transformations? They have assembled a multidisciplinary research team that combines expertise in sediment geochemistry, organic geochemistry, and geomicrobiology to address these objectives using sediment characterizations, rate measurements, molecular characterizations of microbial communities, and modeling. The results will quantify an important part of the diagenetic sulfur cycle that has received little attention despite its potential significance in environments such as freshwater lakes, deep subsurface, and the low sulfate oceans of the geological past. Verifying the proposed hypotheses may lead to reevaluation of the geochemical cycles of sulfur and associated elements such as iron and nitrogen, including cryptic reactions in the sulfate-methane transition zone; reinterpretation of the origins of the isotopic signatures of sulfur preserved in both modern aquatic sediments and ancient sedimentary rocks; and conservation and management practices in sulfide affected water bodies. The project will generate novel microbial and geochemical data that will be publicly available.