Limited organics shaped microbial mutualism for enhanced nitrate and Cr(VI) co-reduction in a sulfur-driven biosystem

Sulfur-driven autotrophic biosystems effectively remove nitrate or Cr(VI) individually, both being prevalent and hazardous groundwater contaminants, yet their co-remediation remains challenging. This study uncovered the concealed cause limiting co-removal and proposed a viable regulation strategy. Long-term operations of a sulfur packed biofilter (S⁰PB) exhibited suboptimal and heterogeneous performance, with nitrate removal rates varying by 25±9 % under a constant load of approximately 50.0 g NO₃^--N/(m³·d) and 0.5 g Cr(VI)/(m³·d). This impairment was attributed to Cr(VI)-induced suppression of NAR and NIR activities, coupled with microbial encapsulation by Cr(III)-precipitates (mainly CrOOH), resulting from both biotic Cr(VI) reduction and abiotic reduction mediated by sulfur-derived thiosulfate. Limited organic matter (OM) amendment alleviated this inhibition, increasing nitrate and Cr(VI) removal rates by 40±8 % and 28±9 %, respectively. Performance recovery was correlated with mitigated enzyme suppression and induced cellular encapsulation, driven by mutualistic interactions among autotrophic nitrate and Cr(VI) reducers (Thiobacillus and Ferritrophicum), heterotrophic nitrate and Cr(VI) reducers (Rhodoblastus), as well as sulfate-reducing bacteria (Desulfatirhabdium). These findings clarify the constraints impairing their efficiencies and provide a reliable approach to optimize nitrate and Cr(VI) co-removal in sulfur-involved biosystems.