Isotopic Fractionation Associated With Sulfate Import and Activation by Desulfovibrio vulgaris str. Hildenborough.

Derek A Smith,Alexander S Bradley,David T Johnston,David A Fike

doi:10.3389/fmicb.2020.529317

Abstract

The use of stable isotopes to trace biogeochemical sulfur cycling relies on an understanding of how isotopic fractionation is imposed by metabolic networks. We investigated the effects of the first two enzymatic steps in the dissimilatory sulfate reduction (DSR) network – sulfate permease and sulfate adenylyl transferase (Sat) – on the sulfur and oxygen isotopic composition of residual sulfate. Mutant strains of Desulfovibrio vulgaris str. Hildenborough (DvH) with perturbed expression of these enzymes were grown in batch culture, with a subset grown in continuous culture, to examine the impact of these enzymatic steps on growth rate, cell specific sulfate reduction rate and isotopic fractionations in comparison to the wild type strain. Deletion of several permease genes resulted in only small (∼1‰) changes in sulfur isotope fractionation, a difference that approaches the uncertainties of the measurement. Mutants that perturb Sat expression show higher fractionations than the wild type strain. This increase probably relates to an increased material flux between sulfate and APS, allowing an increase in the expressed fractionation of rate-limiting APS reductase. This work illustrates that flux through the initial steps of the DSR pathway can affect the fractionation imposed by the overall pathway, even though these steps are themselves likely to impose only small fractionations.

Highlights

Dissimilatory sulfate reduction (DSR) is associated with the oxidation of organic matter in modern and ancient oceans, and is essential to regulating the redox state of the Earth’s surface
The actual fractionations imposed during dissimilatory sulfate reduction (DSR) are usually smaller, and depend on a number of environmental and biological factors such as sulfate reduction rate, sulfate concentration, temperature and the fluxes and fractionations imposed by the particular set of enzymes in a given organism (Thode et al, 1951; Rudnicki et al, 2001; Farquhar et al, 2003; Bradley et al, 2011, 2016; Sim et al, 2011b; Leavitt et al, 2013)
Hildenborough (DvH) is still able to carry out sulfate reduction following the deletion of DVU0053, DVU0279, and DVU1999, which is consistent with the presence of additional mechanisms for sulfate transport (Marietou et al, 2018)

Summary

Introduction

Dissimilatory sulfate reduction (DSR) is associated with the oxidation of organic matter in modern and ancient oceans, and is essential to regulating the redox state of the Earth’s surface. Sulfur and oxygen stable isotope ratios in sulfate and other sulfur-bearing species are largely a product of the biogeochemical sulfur cycle, they can be affected by abiotic processes like hydrothermal activity and photochemical reactions in the atmosphere. DSR is influential on the sulfur isotope ratios The actual fractionations imposed during DSR are usually smaller, and depend on a number of environmental and biological factors such as sulfate reduction rate, sulfate concentration, temperature and the fluxes and fractionations imposed by the particular set of enzymes in a given organism (Thode et al, 1951; Rudnicki et al, 2001; Farquhar et al, 2003; Bradley et al, 2011, 2016; Sim et al, 2011b; Leavitt et al, 2013). An essential approach to understanding the fractionations involves defining the fluxes of sulfur molecules through the DSR pathway, the fractionations of isotopes at each enzymatic step, and the controls on these in various organisms

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Conclusion