Does organic sulfur make a significant and overlooked contribution to sediment sulfate reduction in low-sulfate environments?

Sulfur is an ever-present component of living matter. As aquatic organisms grow, die, and decay, sulfur is exchanged between the organisms and their environment, cycling between its organic forms in living cells and inorganic forms in ambient water. An important process in the geochemistry of sulfur is sulfate reduction. This process, carried out in the environment by microorganisms in the absence of oxygen, converts sulfate, the oxidized and commonly available form of inorganic sulfur, into hydrogen sulfide, which is highly reactive and generally toxic to other organisms. This process has multiple environmental significances: it regulates the fluxes of important nutrients such as phosphorus and pollutants such as mercury; it is a pathway by which significant amounts of organic carbon are converted into carbon dioxide and low molecular weight carboxylic acids; and it affects the life cycles of commercially important aquatic plants such as wild rice. Over geological time scales, it is responsible for the formation of iron sulfides, which are preserved in sedimentary rocks and contain the record of environmental conditions dating back to the earliest stages of Earth's history. Present understanding of sulfate reduction, however, has been largely shaped by studies in marine settings where sulfate is abundant and easily available to organisms from seawater. The situation is different in freshwater lakes, rivers, and other low-sulfate environments, which include the oceans of distant geologic past and sediments deep below seafloor. Organic sulfur appears to be a much more important source of sulfur for sulfate reduction in these low-sulfate environments, however the pathways by which it circulates and the magnitude of its effects in different conditions are unknown. A team of geochemists, organic chemists, and geomicrobiologists from the University of Minnesota Duluth will address these questions by studying the sulfur transformations and relevant microorganisms, focusing on Lake Superior and its largest American tributary as the study area. They will collect sediments, analyze them for geochemically relevant sulfur species, measure reaction rates between these different species, and perform genetic analyses to identify key involved microbes. If the initial hypotheses are confirmed, the results are likely to transform the current paradigm of sulfur chemistry in such low-sulfate environments, influencing several scientific disciplines and providing a foundation for better environmental management practices. The project will support two beginning investigators and will train two graduate and several undergraduate students. Research cruises will provide no-cost support for several collaborative efforts on Lake Superior. Results, models, and methods will be incorporated into an innovative Limnology curriculum being developed by the PIs with NSF support. Findings will be communicated to public through a series of talks, K-12 teacher education events including a teacher education cruise on Lake Superior aboard the R/V Blue Heron broadcasted on YouTube, and exhibitions at Duluth Freshwater Aquarium.

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.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.