Abstract
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 (, S0, S2, S4, and ), 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 S4 S2, which is consistent with previous measurements of , S4, and S2 consumption rates in marine sediments. In water column incubations, however, the relative reactivity was S2 S4. 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. 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.
Highlights
Cryptic sulfur cycling has recently been observed in a variety of different environments from pelagic oxygen minimum zones (Canfield et al, 2010) to marine sediments (Holmkvist et al, 2011; Glombitza et al, 2016) and salt marshes (Mills et al, 2016)
Thiosulfate S2O23− was consumed at an initial rate of 1.2 μM h−1
The initial consumption of S2O23− was accompanied by a small increase in SO23− concentrations, which decreased throughout the remainder of the experiment (Figure 2A)
Summary
Cryptic sulfur cycling (i.e., the simultaneous reduction of sulfate and reoxidation of sulfide) has recently been observed in a variety of different environments from pelagic oxygen minimum zones (Canfield et al, 2010) to marine sediments (Holmkvist et al, 2011; Glombitza et al, 2016) and salt marshes (Mills et al, 2016). Experimental work in low sulfate, iron rich sediments indicates that an active sulfur cycle exists even in environments in which microbial iron reduction is expected to be favorable to sulfate reduction (Hansel et al, 2015). In all cases, this cycle involves the concomitant reduction of sulfate, via microbial sulfate reduction (MSR), and Turnover Rates of Intermediate Sulfur Species oxidation of the sulfide thereby produced. Recent analytical advances have made accurate quantification of these compounds possible, the low concentrations typically observed likely do not correlate with their biogeochemical importance (Zopfi et al, 2004)
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