Diversity decoupled from sulfur isotope fractionation in a sulfate‐reducing microbial community

Abstract

The extent of fractionation of sulfur isotopes by sulfate‐reducing microbes is dictated by genomic and environmental factors. A greater understanding of species‐specific fractionations may better inform interpretation of sulfur isotopes preserved in the rock record. To examine whether gene diversity influences net isotopic fractionation in situ, we assessed environmental chemistry, sulfate reduction rates, diversity of putative sulfur‐metabolizing organisms by 16S rRNA and dissimilatory sulfite reductase (dsrB) gene amplicon sequencing, and net fractionation of sulfur isotopes along a sediment transect of a hypersaline Arctic spring. In situ sulfate reduction rates yielded minimum cell‐specific sulfate reduction rates < 0.3 × 10−15 moles cell−1 day−1. Neither 16S rRNA nor dsrB diversity indices correlated with relatively constant (38‰–45‰) net isotope fractionation (ε34Ssulfide‐sulfate). Measured ε34S values could be reproduced in a mechanistic fractionation model if 1%–2% of the microbial community (10%–60% of Deltaproteobacteria) were engaged in sulfate respiration, indicating heterogeneous respiratory activity within sulfate‐reducing populations. This model indicated enzymatic kinetic diversity of Apr was more likely to correlate with sulfur fractionation than DsrB. We propose that, above a threshold Shannon diversity value of 0.8 for dsrB, the influence of the specific composition of the microbial community responsible for generating an isotope signal is overprinted by the control exerted by environmental variables on microbial physiology.

Cryptic sulfur cycling during the formation of giant gypsum deposits

“…5). The laboratory-observed csSRR at which these relationships have been established are often 1-3 orders of magnitude higher than the csSRR usually reported from natural marine environments (Colangelo-Lillis et al, 2019;Jørgensen, 2021 and references therein). Some authors therefore argue that csSRR in marine environments are so low that cells always express fractionation factors close to equilibrium values (ƐMSR»-70‰ and λMSR»0.515) (e.g.…”

“…Ɛnet>-70‰ and λnet<0.515 are often observed in systems where depositional controls are absent or negligible, implying that natural populations of microbial sulfate reducers can produce a large range of isotopic fractionations (Colangelo-Lillis et al, 2019;Gilhooly et al, 2016;Gomes and Hurtgen, 2015 and references therein;litterature review in Sim et al, 2011a). In addition, modeling studies also suggest a first order control of csSRR on fractionation during MSR (Bradley et al, 2016;Wing and Halevy, 2014).…”

“…Howbeit, secondary factors independent from csSRR might also influence the extent of isotopic fractionation expressed during MSR (e.g. Bradley et al, 2016;Brüchert et al, 2001;Colangelo-Lillis et al, 2019;Stam et al, 2006;Wing and Halevy, 2014).…”

“…Although it is not within the scope of this work to reconcile laboratory experiments with field observations and modelling studies, we point out that the correct estimation of csSRR in natural environments (in mol.cell-1.day-1) relies on: (i) a precise estimate of the number of physiologically active sulfate reducing microbial cells, and (ii) the absence of significative reoxidative sulfur cycling. Any overestimation of active MSR cell abundance (Colangelo-Lillis et al, 2019) and/or underestimation of the rate of sulfide oxidation (Jørgensen, 2021;Treude et al, 2021) might lead to a large underestimation of csSRR. It is possible that the current observed range of csSRR in natural environments is biased towards low rates (Jørgensen, 2021), whereas csSRR is directly measured in pure culture experiments that allow the entire range of possible MSR-induced fractionations to be explored ) (Fig.…”