Sulfur disproportionation in engineered environmental systems: A critical review of mechanisms, applications, and emerging perspectives.

Sulfur disproportionation (SDP), a microbial metabolism that couples the oxidation and reduction of intermediate-valence sulfur compounds, is increasingly recognized as a key process in both natural sulfur cycling and engineered environmental systems. Despite growing interest, our understanding of SDP remains incomplete, and its practical significance has often been overlooked. This critical review integrates recent advances in ecophysiology, molecular mechanisms, and reactor-scale applications to establish a unified framework and provide practical guiding principles for SDP. We first summarize its ecological contexts and role in the cryptic sulfur cycle in diverse anoxic environments, followed by the details of its complex genetic and enzymatic basis. We then critically assess the use of SDP as an emerging strategy to overcome kinetic limitations in sulfur-based autotrophic denitrification, thereby enabling high-rate nitrogen removal, mitigating nitrous oxide (N2O) emissions, and facilitating nitrogen recovery via dissimilatory nitrate reduction to ammonium (DNRA). Furthermore, SDP provides an organic-carbon-free platform for the remediation of acid mine drainage and metal-contaminated groundwater by immobilizing toxic metals like chromium and antimony while reducing the risk of methylmercury formation associated with heterotrophic sulfate reduction. Its capacity to drive key subsurface biogeochemical transformations, such as the anaerobic oxidation of methane in deep aquifers, further underscores its broader opportunities for passive in situ bioremediation of other contaminants. Effective engineering control relies on creating a protected niche for these slow-growing sulfur-disproportionating microorganisms (SDM) through nitrate limitation, high-biomass-retention strategies, and staged management that decouples sulfide production from consumption. Despite this promise, SDP-enabled technologies still face challenges such as slow kinetics, process stability, and the significant uncultivated diversity of SDM. Future research should integrate systems biology to elucidate cryptic sulfur cycling, synthetic ecology to construct robust microbial consortia, and advanced reactor engineering to intensify SDP and ensure its reliable scale-up.