Simultaneous sulfide oxidation and sulfate reduction for intracellular redox homeostasis under highly acidic conditions

Sulfide oxidation and sulfate reduction are opposing processes in the microbial sulfur cycle, typically separated to avoid futile cycling. Here, we show that Mycobacterium sp. MAG-M116, identified from a biodesulfurization system under low pH, sulfate-rich, and H₂S-overload conditions, simultaneously performs sulfide oxidation and assimilatory sulfate reduction (ASR), challenging this paradigm. Under aerobic conditions, MAG-M116 oxidizes sulfide to elemental sulfur, channeling electrons into the quinone pool to drive forward electron transfer for ATP synthesis and reverse electron transfer (RET) for NADPH generation. ASR, acting as a redox homeostat, captures electrons from leak-prone RET to mitigate electron leakage and reduce reactive oxygen species production by 57.5%. This bidirectional sulfur metabolism enables rapid sulfide detoxification while maintaining intracellular redox homeostasis, allowing Mycobacterium to dominate the community (abundance increasing from 3.5% to 99%). These findings reveal an adaptive strategy wherein coupled opposing redox reactions contribute to maintaining intracellular redox homeostasis under substrate-excess conditions.