Sulfate Regeneration by Sulfur-Oxidizing Bacteria Plays an Unrecognized Role in Reducing Methane Emissions from the Sediments of a Freshwater Lake

Sulfate dynamics strongly regulate methane emissions in eutrophic freshwater lakes, yet the mechanisms driving in situ sulfate regeneration remain poorly understood. This study examined the temporal and spatial variations of sulfate and sulfur-cycling microorganisms in Lake Taihu to elucidate the influence of dissolved oxygen (DO) on sulfate regeneration and methane fluxes. Pronounced seasonal sulfate fluctuations were observed with higher concentrations during oxic (spring and winter) and lower levels during anoxic (summer and autumn) periods. Under oxic conditions, sulfur-oxidizing bacteria (SOB) dominated surface sediments, whereas sulfate-reducing bacteria (SRB) prevailed in deeper anoxic layers, sustaining an active internal sulfur cycle. Microcosm experiments demonstrated that sulfate was regenerated from reduced sulfur species under oxic conditions, and inhibition of sulfate reduction by molybdate confirmed the oxidative recycling process. Increased sulfate availability substantially suppressed methane emissions by outcompeting methanogens for organic substrates. Quantitative analysis indicated that approximately 43% of the methane reduction was attributable to the regeneration of sulfate and other electron acceptors (e.g., nitrate and Fe(III)), while 57% was due to aerobic methane oxidation. These findings highlight a redox-driven “sulfur pump”, “iron pump”, and “nitrogen pump” mechanism that collectively mitigates methane emissions, offering new insights into biogeochemical feedbacks in shallow lake ecosystems.