Electron flow dynamics in sulfur-based autotrophic bioreduction of Cr(VI) mediated by inorganic carbon species: Insights for environmental remediation

The deployment of sulfur-based autotrophic bioremediation for in situ groundwater remediation faces hurdles due to electron competition among electron acceptors, impacting contaminant removal efficiency and causing pH instability. Notably, the sulfur-based bioreduction of Cr(VI) [Cr(VI)-SAR] exemplifies gaps in our comprehension of electron competition dynamics with inorganic carbon (IC), and its subsequent influence on pH. Herein, we established a Cr(VI)-SAR system interfaced with diverse IC species, providing definitive insights into electron transfer mechanisms through rigorous multi-biocycle analysis and thermodynamically consistent half-reaction calculations. Through quantification of electron transfer pathways, we derived reaction equations for Cr(VI) reduction in conjunction with various IC species. Furthermore, metagenomics were used to quantify functional enzymes and identify diverse electron transport patterns alongside IC fixation pathways. Notably, the enrichment of genes associated with electron shuttles and conductive pili expands the paradigm of extracellular electron transfer, while the Wood-Ljungdahl pathway streamlines microbial metabolic proliferation with reduced energy expenditure. Quantitative analysis of these functional genes offers a plausible mechanism underlying the observed shifts in electron competition between IC and Cr(VI). This research marks an advancement in the Cr(VI)-SAR foundational theory, with a particular focus on the dynamics of electron competition, contributing to a deeper understanding of this environmentally significant process.