Electron transfer mechanisms and microbiome stability of mixed-SRB-assembled Bio-FeS hybrids for enhanced chromium remediation

While biogenic iron sulfide nanoparticles (Bio-FeS NPs) exhibit exceptional efficacy in hexavalent chromium (Cr(VI)) remediation, their broader application is severely constrained by an overreliance on pure-culture synthesis that oversimplifies complex environmental realities, and a limited understanding of the underlying electron transfer and synergistic removal mechanisms within mixed microbial consortia. To bridge these knowledge gaps, we synthesized a biohybrid material (Bio-FeS@SRB) in situ using a mixed consortium of sulfate-reducing bacteria (SRB). Response surface analysis identified the optimal synthetic conditions (640.31 mg L⁻¹ SO₄^2-, 202.77 mg L⁻¹ Fe²⁺, pH 7.22), under which FeS NPs (24.46 ± 4.30 nm) formed evenly on the cell surface. Electrochemical analysis demonstrated a significantly greater electron transfer capacity for Bio-FeS@SRB compared to SRB alone, as evidenced by a 67.20% reduction in charge transfer resistance, an increased direct electron transfer ratio, and doubled electron transport system activity (ETSA). Consequently, Bio-FeS@SRB achieved > 99% removal of Cr(VI) at high concentrations (50 mg L⁻¹). The reaction followed first-order kinetics and also substantially promoted sulfate reduction. Mechanism investigations revealed that FeS NPs could substitute coenzyme Q, establishing an alternative high-efficiency intracellular electron transfer conduit. The in-situ formed FeS NPs protected key sulfate-reducing genera (e.g., Humidesulfovibrio), enriched Cr(VI) resistant groups (e.g., Enterococcus), and increased the abundance of functional genes involved in sulfur metabolism (e.g., sat, aprA, aprB) and Cr(VI) tolerance (chrA, yieF). Partial Least Squares Path Modeling (PLS-PM) identified ETSA as the key internal driver for the bio-mineral synergy, and the synergistic effect increased with initial Cr(VI) concentration (from 7.82 ± 0.51% at 50 mg L⁻¹ to 18.52 ± 0.75% at 100 mg L⁻¹). Our study provided systematic insights into the synergies between biogenic FeS and complex microbial communities, providing a robust foundation for efficient bioremediation technologies.