Microbial and Reactive Transport Modeling Evidence for Hyporheic Flux‐Driven Cryptic Sulfur Cycling and Anaerobic Methane Oxidation in a Sulfate‐Impacted Wetland‐Stream System

Abstract

This study reexamines the common expectations that in freshwater systems, sulfur plays a minor role in carbon cycling, and aerobic processes dominate methane oxidation. In anoxic sediments of a sulfate‐impacted wetland‐stream system in Minnesota (USA), a reactive transport model calibrated to geochemical observations predicted sulfate reduction to be the major terminal electron accepting process, and it showed that anaerobic oxidation of methane predominantly coupled with sulfate reduction attenuated methane concentrations near the sediment‐water interface. Consistent with model results, 16S rRNA microbiome analysis revealed a high relative abundance of taxa capable of dissimilatory sulfate reduction. It further supported the conclusion that high simulated sulfate reduction rates could be maintained by a “cryptic” sulfur cycle coupled to iron and methane. Low relative abundance of known iron reducing bacteria raised the possibility of abiotic ferric iron (Fe) reduction driving sulfide reoxidation to intermediate‐valence sulfur forms; widespread potential for microbially mediated disproportionation, oxidation, and reduction of sulfur intermediates provided mechanisms for completing redox cycles; and archaea comprising up to 25% of the microbial community could include consortia capable of anaerobic oxidation of methane. These biogeochemical processes were found to be controlled by hyporheic fluxes. Lower‐magnitude fluxes in wetland compared to channel sediments created sharper geochemical gradients that generated greater heterogeneity in microbial distributions and reaction rates. Changes in upward flux caused fluctuations in sulfate concentrations that led to alternating simulations of methane production and transport. Our work supports the importance of hyporheic flux‐driven iron‐sulfur‐methane cycling in sulfate‐impacted wetlands and prompts further investigations under freshwater conditions.

Reactive Transport: A Review of Basic Concepts with Emphasis on Biochemical Processes

“…Reviews exists for the modelling of this particular phenomenon [22,190]. Modelling of carbon oxidation and redox sequence has also been done for domains closer to the surface, such as natural wetlands (e.g., [191,192]) and managed aquifer recharge (e.g., [193][194][195]).…”

Integrating periphyton and surfacewater–groundwatermethods to understand lake ecosystem processes

“…Steefel et al (2015) and Gamazo et al (2016) performed an extensive review of numerical reactive transport codes relevant to environmental modeling. More recently, Arora et al (2015), Li and S ¸engör (2020), Ng et al (2020), andRodríguez-Escales et al (2020) show case studies that have used reactive transport modeling in lakes and ponds.…”

“…However, exploration into periphyton representation in reactive transport models in lakes has been limited (Hua et al 2013;Ginder-Vogel et al 2019). By taking into account the residence times of hydrostatic and hydrodynamic factors of hyporheic flow, reactive transport models have been used to characterize relationships between vertical hydraulic gradients in groundwater and surface water that create chemical gradients in the hyporheic zone (Ng et al 2020;Wu et al 2020;Broecker et al 2021). Therefore if considering sediment microbiology and enhanced aqueous chemistry, a similar modeling path could be used to study nutrient fluxes into lakes from groundwater.…”

Modeling the Organic Carbon Oxidation and Redox Sequence Under the Partial‐Equilibrium Approach: A Discussion by Means of a Semi‐Analytical Solution

“…Contrarily to the fully kinetic strategy, which is disconnected from thermodynamics, the use of the PEA guarantees that TEAs will be reduced only when there exists a favorable Gibbs free energy in the solution. The PEA has been used in several models (Greskowiak et al., 2006; Jakobsen, 2007; Ng et al., 2020; Rodríguez‐Escales et al., 2020; van Breukelen et al., 2004). However, one should be careful about the equilibrium conditions.…”