Turnover rates of intermediate sulfur species (Sx2-, S⁰, S₂O32-, S₄O62-, SO32-) in anoxic freshwater and sediments

The microbial reduction of sulfate to sulfide coupled to organic matter oxidation followed by the transformation of sulfide back to sulfate drives a dynamic sulfur cycle in a variety of environments. The oxidative part of the sulfur cycle in particular is difficult to constrain because the eight electron oxidation of sulfide to sulfate occurs stepwise via a suite of biological and chemical pathways and produces a wide variety of intermediates (Sx2-, S⁰, S₂O32-, S₄O62-, and SO32-), which may in turn be oxidized, reduced or disproportionated. Although the potential processes affecting these intermediates are well-known from microbial culture and geochemical studies, their significance and rates in the environment are not well constrained. In the study presented here, time-course concentration measurements of intermediate sulfur species were made in amended freshwater water column and sediment incubation experiments in order to constrain consumption rates and processes. In sediment incubations, consumption rates were Scolloidal0 > Sx2- > SO32-≈ S₄O62- > S₂O32-, which is consistent with previous measurements of SO32-, S₄O62-, and S₂O32- consumption rates in marine sediments. In water column incubations, however, the relative reactivity was Scolloidal0 > SO32- > Sx2- > S₂O32- > S₄O62- Consumption of thiosulfate, tetrathionate and sulfite was primarily biological, whereas it was not possible to distinguish between abiotic and biological polysulfide consumption in either aqueous or sediment incubations. Scolloidal0 consumption in water column experiments was biologically mediated, however, rapid sedimentary consumption was likely due to reactions with iron minerals. These experiments provide important constraints on the biogeochemical reactivity of intermediate sulfur species and give further insight into the diversity of biological and geochemical processes that comprise (cryptic) environmental sulfur cycling.