Linked Iron, Sulfur, and Carbon Biogeochemical Cycling in Hydrologically Dynamic, Riparian Wetland Sediments

Abstract:

Wetland ecosystems are vital for environmental health – interactions between microbiological, physicochemical, and hydrological components greatly influence the global carbon cycle as well as local water quality by transforming nutrients, organic compounds, and contaminants. Dynamic hydrologic conditions in riparian wetland sediments drive steep, fluctuating redox gradients that can critically influence the extent of iron (Fe), sulfur (S), and carbon (C) biogeochemical cycling, although these processes remain poorly resolved.

We are examining Fe-S-C cycling within two hydrologically dynamic, freshwater riparian wetland ecosystems – an organic-and sulfate- rich site in northeastern Minnesota, and a wetland-stream system in South Carolina (part of the Argonne Hydrobiogeochemistry Scientific Focus Area) with relatively lower and more spatially-variable amounts of organic matter and sulfate. Field examinations of geochemistry (aqueous and sediment composition), microbiology (microbiome composition and distribution, metagenomics, and functional gene abundance), and hydrogeologic fluxes will be combined with laboratory experiments and reactive transport modeling. Initial results suggest that a cryptic S cycle contributes to Fe redox cycling and organic matter oxidation in these environments. Sulfur X-ray absorption near edge structure (XANES) of wetland sediments show an abundance of S intermediates (e.g., thiosulfate and S(0)). Interestingly, the organic-rich sediments host an unexpectedly large amount of S(0), which calls into question the roll of natural organic matter in stabilization of these phases. We are also interrogating metagenomic data from these environments to examine the relative abundance of genes involved in specific metabolic functions, such as sulfur cycling (e.g., production and consumption of sulfate, sulfide, and intermediate S species) and the production of organic compounds (e.g., acetate, formate, etc.), as well as the dominant pathways by which these processes occur and under which geochemical conditions. Metagenomic data can also help us determine which microbes may be involved in sulfur and carbon cycling, helping us better resolve the microbial mechanics of nutrient and metal cycling in these environments.