Carbon-sulfur coupling in a seasonally hypoxic, high-sulfate reservoir in SW China: Evidence from stable CS isotopes and sulfate-reducing bacteria

Anthropogenic input of sulfate (SO42-) in reservoirs may enhance bacterial sulfate reduction (BSR) under seasonally hypoxic conditions in the water column. However, factors that control BSR and its coupling to organic carbon (OC) mineralization in seasonally hypoxic reservoirs remain unclear. The present study elucidates the coupling processes by analyzing the concentrations and isotopic composition of dissolved inorganic carbon (DIC) and sulfur (SO42-, sulfide) species, and the microbial community in water of the Aha reservoir, SW China, which has high SO(4)(2-)concentra-tion due to the inputs from acid mine drainage about twenty years ago. The water column at two sites in July and October revealed significant thermal stratification. In the hypoxic bottom water, the delta C-13-DIC decreased while the delta S-34-SO(4)(2-)increased, implying organic carbon mineralization due to BSR. The magnitude of S isotope fractionation (delta S-34, obtained from delta S-34(sulfate)-delta S-34(sulfide)) during the process of BSR fell in the range of 3.4 parts per thousand & nbsp;& nbsp;to 27.0 parts per thousand & nbsp; in July and 21.6 parts per thousand & nbsp; to 31.8 parts per thousand & nbsp; in October, suggesting a change in the community of sulfate-reducing bacteria (SRB). The relatively low water column stability in October compared to that in July weakened the difference of water chemistry and ultimately affected the SRB diversity. The production of DIC (delta DIC) scaled a strong positive relationship with the delta S-34 in July (p < 0.01), indicating that high OC availability favored the survival of incomplete oxidizers of SRB. However, in October, delta C-13-DIC was correlated with the delta S-34 in the bottom hypoxic water (p < 0.01), implying that newly degraded OC depleted in C-13 could favor the dominance of complete oxidizers of SRB which caused greater S isotope fractionation. Moreover, the sulfide supplied by BSR might stimulate the reductive dissolution of Fe and Mn oxides (Fe(O)OH and MnO2). The present study helps to understand the coupling of C and Sin seasonally hypoxic reservoirs characterized by high SO(4)(2-)concentration.