CRYPTIC SULFUR CYCLING IN IRON-RICH STREAM AND WETLAND HYPORHEIC ZONES

Hyporheic zones, where oxic surface water and anoxic groundwater mix, are characterized by dynamic redox gradients that promote hotspots and hot moments of biogeochemical processes. In freshwater wetland and stream hyporheic zones, carbon (C) turnover and fate is heavily influenced by the biogeochemical cycling of iron (Fe). “Hidden” or “cryptic” sulfur (S) redox processes may be further coupled to these Fe and C cycles. S biogeochemical cycling is not well constrained in freshwater systems but can include the production of reactive intermediate S species that promote further biotic and abiotic redox reactions (including those coupled with Fe reduction and methane oxidation), thus supporting higher rates of sulfur biogeochemical cycling than otherwise expected in these low sulfate environments. To better understand cryptic S processes and impacts in freshwater hyporheic zone environments, we examined the hydrobiogeochemistry of a dynamic, Fe-rich wetland and stream system using continuous surface- and ground-water level and flux measurements and seasonal sampling of water and sediments for metagenomic and geochemical analyses. Analytical results suggested that both sulfate reduction and S oxidation were important processes in these freshwater wetlands. For example, X-ray absorption spectroscopy showed that intermediate S species such as zero-valent S and thiosulfate were prevalent at depth in anoxic sediments. Quantitative PCR showed that both dissimilatory sulfite reduction (dsr) and adenylylsulfate reductase (apr) genes were enriched in gaining stream sites (i.e. upward hyporheic flux), indicating that sulfate reduction was an important process despite relatively low sulfate levels and below-detection porewater sulfide concentrations. Shotgun metagenome sequencing also revealed an abundance of sulfur oxidation and thiosulfate oxidation genes in anoxic sediment, particularly in areas experiencing predominantly upward hyporheic flux. It is suspected that high measured concentrations of aqueous or colloidal Fe(III), likely stabilized by organic matter, helps fuel a cryptic S cycle.