Disentangling Microbial Syntrophic Mechanisms for Hexavalent Chromium Reduction in Autotrophic Biosystems
Abstract Background: Hexavalent chromium [Cr(VI)] is one of ubiquitous heavy-metal contaminants in groundwater, and electron donors are considered to be a key parameter for Cr(VI) biotransformation. During autotrophic remediation process, however, much remains to be unveiled that how complex syntrophic microbial communities couple Cr(VI) reduction with other elemental cycles. Results: Two series of Cr(VI)-reducing groundwater bioreactors were independently amended by elemental sulfur (S0) and iron (Fe0), and inoculated with the same inoculum. After 160 days incubation, both bioreactors showed the similar archaea-dominating microbiota compositions, whereas a higher Cr(VI) reducing rate and more methane production were detected in the Fe0-driven one. Metabolic reconstruction of 23 retrieved genomes revealed complex symbiotic relationships driving distinct elemental cycles coupled with Cr(VI) reduction in bioreactors. In both bioreactors, these inferred Cr(VI) reducers were assumed to live in syntrophy with oxidizers of sulfur, iron, and volatile fatty acids (VFAs) and methane produced by carbon fixers and multi-trophic methanogens, while hydrogen slowly released via an Fe0 corrosion process might readily facilitate methanogenesis and more methane oxidation might be linked to Cr(VI) reduction in the Fe-bioreactor.Conclusion: These findings provide insights into mutualistic symbioses of carbon, sulfur, iron and chromium metabolisms in groundwater systems, providing implications for both in-situ and ex-situ bioremediation of contaminated groundwater.