Riparian wetland hyporheic zones, where oxic surface water and anoxic groundwater mix, exhibit spatiotemporally dynamic conditions that drive steep redox gradients and promote hotspots of diverse and fluctuating microbial activity. Anthropogenically-induced changes in climate, water quality, and water quantity can disturb hydrologic flow and biogeochemical processing in these environments. In freshwater ecosystems, increased sulfate loading due to industrial and mining activities may alter expected biogeochemical regimes, resulting in fast “cryptic” sulfur (S) cycling linked to iron (Fe) and carbon (C) cycling in wetland sediments.  This coupled abiotic-biotic process poorly understood, but could ultimately impact ecosystem health and global climate. <br/><br/>To gain a better understanding of how sulfate impacts freshwater biogeochemical processes, we examined a heavily-impacted (higher sulfate) and a lightly-impacted wetland stream system in northern Minnesota (USA).   We conducted microbiological and geochemical analyses of the stream waters, porewaters, and sediments and coupled these measurements with hydrological observations to gain insight into how wetland biogeochemistry is impacted by changes in hyporheic flux and sulfate loading.  Analytical results from this study suggested that sulfate reduction and S oxidation were important processes in these freshwater wetlands, whereby fast “cryptic” S cycling was linked to Fe and methane cycling.  For example, X-ray absorption spectroscopy showed that intermediate S species such as zero-valent S and thiosulfate as well as Fe(III) compounds were prevalent at depth in anoxic sediments.  Microbiome analysis revealed a high relative abundance of taxa capable of dissimilatory sulfate reduction and the presence of taxa capable of intermediate-S disproportionation.  Further, the concentrations of dissolved and solid S and Fe species were impacted by changes in the groundwater and surface water flux directions.  Future work will incorporate functional gene quantification of S and methane cycling genes to better estimate these microbial activities relative to the sediment and porewater geochemistry in these dynamic freshwater systems.

Coupled S-Fe-C biogeochemical cycling influenced by dynamic hyporheic fluxes in an environmentally-impacted wetland stream system

ACS Spring 2022

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