Bacteria-mediated sulfur cycle and cadmium stabilization in an anaerobic-aerobic coupled system

Although cadmium stabilization through dissimilatory sulfate reduction (DSR), mediated by sulfate-reducing bacteria (SRB) has been extensively studied, the mutual interactions between cadmium and sulfur-cycle systems co-regulated by SRB and sulfur-oxidizing bacteria (SOB) remain poorly characterized. We examined physiological responses (growth kinetics, sulfur valence transitions, Cd²⁺ tolerance) to determine how bacterially mediated bidirectional sulfur cycling influences cadmium speciation. Results demonstrate that SRB (Enterobacter quasihormaechei) immobilizes Cd²⁺ anaerobically, forming CdS, as confirmed by XRD, FTIR, and SEM/EDS analyses. At 40 mg/L Cd²⁺ over 7 days, Cd immobilization efficiency reached 75 %. Conversely, SOB (Pseudomonas protegens) aerobically oxidizes S₂O₃^2- and S^2- to replenish SO₄^2- for DSR while inducing limited dissolution of SRB-mineralized products. Treatment with 66.67 mg/L biogenic CdS for 7 days yielded only 18 % Cd²⁺ dissolution. Crucially, at 40 mg/L Cd²⁺, the actual stoichiometric ratios were SO₄^2-:S^2- = 2:1 (reduction) and S^2-:SO₄^2- = 1.29:1 (oxidation), enabling mutual substrate replenishment: SRB provides S^2- to SOB while SOB supplies SO₄^2- to SRB. This synergistic loop ultimately stabilizes cadmium as CdS. This study establishes a groundbreaking bioremediation strategy for aquatic cadmium contamination.