What Controls the Sulfur Isotope Fractionation during Dissimilatory Sulfate Reduction?

Abstract

Sulfate often behaves conservatively in the oxygenated environments but serves as an electron acceptor for microbial respiration in a wide range of natural and engineered systems where oxygen is depleted. As a ubiquitous anaerobic dissimilatory pathway, therefore, microbial reduction of sulfate to sulfide has been of continuing interest in the field of microbiology, ecology, biochemistry, and geochemistry. Stable isotopes of sulfur are an effective tool for tracking this catabolic process as microorganisms discriminate strongly against heavy isotopes when cleaving the sulfur–oxygen bond. Along with its high preservation potential in environmental archives, a wide variation in the sulfur isotope effects can provide insights into the physiology of sulfate reducing microorganisms across temporal and spatial barriers. A vast array of parameters, including phylogeny, temperature, respiration rate, and availability of sulfate, electron donor, and other essential nutrients, has been explored as a possible determinant of the magnitude of isotope fractionation, and there is now a broad consensus that the relative availability of sulfate and electron donors primarily controls the magnitude of fractionation. As the ratio shifts toward sulfate, the sulfur isotope fractionation increases. The results of conceptual models, centered on the reversibility of each enzymatic step in the dissimilatory sulfate reduction pathway, are in qualitative agreement with the observations, although the underlying intracellular mechanisms that translate the external stimuli into the isotopic phenotype remain largely unexplored experimentally. This minireview offers a snapshot of our current understanding of the sulfur isotope effects during dissimilatory sulfate reduction as well as their potential quantitative applications. It emphasizes the importance of sulfate respiration as a model system for the isotopic investigation of other respiratory pathways that utilize oxyanions as terminal electron acceptors.

Bacterial Sulfate Reduction Facilitates Iodine Mobilization in the Deep Confined Aquifer of the North China Plain

“…Bacterial sulfate reduction preferentially utilizes the light isotope ( 32 S), leading to the enrichment of the heavy isotope ( 34 S) up to 70‰ in environments. 40 Thus, the observed distinct enrichment of groundwater δ 34 S SO4 indicates that bacterial sulfate reduction prevails in the geogenic highiodine groundwater of the NCP. 41 More importantly, groundwater δ 34 S SO4 values and detected sulfide content were significantly positively correlated with the concentrations of iodide and total iodine, suggesting that bacterial sulfate reduction may participate in iodine mobilization and iodide enrichment in the aquifer (Figures 2A,B and S2).…”

“…Stable isotopes of sulfur are an effective tool for tracking the bacterial sulfate reduction process in aquifers. , In this study, groundwater δ 34 S SO4 in the deep confined aquifer of the NCP ranged from 15.56 to 69.31‰ (Table S1). Bacterial sulfate reduction preferentially utilizes the light isotope ( 32 S), leading to the enrichment of the heavy isotope ( 34 S) up to 70‰ in environments . Thus, the observed distinct enrichment of groundwater δ 34 S SO4 indicates that bacterial sulfate reduction prevails in the geogenic high-iodine groundwater of the NCP .…”

In Situ Rb/Sr Geochronology and Stable Isotope Geochemistry Evidence for Neoproterozoic and Paleozoic Fracture‐Hosted Fluid Flow and Microbial Activity in Paleoproterozoic Basement, SW Sweden

“…When applied to the samples with a large variation in δ 34 S pyrite and assigning the minimum δ 34 S pyrite value of each sample, a δ 34 S sulfate,0 of 21‰ to assign an enrichment factor 34 ε , and a fractionation factor, it is estimated that a fraction of 40%, 44%, 61%, and 68% of the sulfate has been exhausted in the samples with highly varied δ 34 S composition, meaning that Rayleigh distillation had a substantial influence in the fracture system. It is also important to consider the relationship between temperature and the magnitude of microbial sulfur isotope fractionation, as studies suggest that 34 ε larger than 60‰ is restricted to temperatures <40°C, and 34 ε larger than 40‰ to temperatures <80°C (Sim et al., 2023). Such temperatures are not unreasonable for the studied samples, such as the sample with youngest Rb/Sr age, because south Sweden has experienced low‐temperature conditions throughout most of the Phanerozoic, with a hiatus during Caledonian foreland basin maximum (Guenthner et al., 2017), see below.…”

“…Such temperatures are not unreasonable for the studied samples, such as the sample with youngest Rb/Sr age, because south Sweden has experienced low‐temperature conditions throughout most of the Phanerozoic, with a hiatus during Caledonian foreland basin maximum (Guenthner et al., 2017), see below. The apparent high 34 ε of the studied samples further indicates that relatively high sulfate concentrations have occurred in the fracture system at the time of MSR (cf., Sim et al., 2023). High dissolved sulfate concentrations are not unreasonable as these type of Paleozoic fracture assemblages previously have been linked to the formation from sulfate‐rich basinal brines that resided beneath, now eroded, marine sedimentary rock successions (Drake & Tullborg, 2009).…”

Speciation and Possible Origins of Organosulfur Compounds in Rice Paddy Soils Affected by Acid Mine Drainage

“…Most studies of sulfur stable isotopes in AMD-contaminated areas concentrated on the source identification of sulfate , and process tracing associated with inorganic sulfur cycling. ,, The primary sources of sulfate in AMD are mineral dissolution and sulfide oxidation. , The formation of organosulfur compounds depends on microbial reduction of sulfate, which follows two pathways: (a) S­(-II) produced by assimilatory sulfate reduction leading to the incorporation into cell constituents and the synthesis of S-bearing amino acids (e.g., cysteine) and (b) S­(-II) produced by dissimilatory sulfate reduction, which is expelled from the cell and through the process of organic matter sulfurization, may subsequently form organosulfur compounds . Assimilatory sulfate reduction results in very little sulfur isotope fractionation (ε = −1 to −3‰), and organosulfur compounds produced through assimilatory processes are usually relatively enriched in 34 S. dissimilatory sulfate reduction results in a large extent of sulfur isotope fractionation (ε = −20 to −75‰), , and organosulfur compounds produced from dissimilatory sulfide are usually relatively depleted in 34 S.…”