Role of APS reductase in biogeochemical sulfur isotope fractionation

Min Sub Sim,Shawn E Mcglynn,Hideaki Ogata,Alex L Sessions,Wolfgang Lubitz,Jess F Adkins,Victoria J Orphan

doi:10.1038/s41467-018-07878-4

Abstract

Sulfur isotope fractionation resulting from microbial sulfate reduction (MSR) provides some of the earliest evidence of life, and secular variations in fractionation values reflect changes in biogeochemical cycles. Here we determine the sulfur isotope effect of the enzyme adenosine phosphosulfate reductase (Apr), which is present in all known organisms conducting MSR and catalyzes the first reductive step in the pathway and reinterpret the sedimentary sulfur isotope record over geological time. Small fractionations may be attributed to low sulfate concentrations and/or high respiration rates, whereas fractionations greater than that of Apr require a low chemical potential at that metabolic step. Since Archean sediments lack fractionation exceeding the Apr value of 20‰, they are indicative of sulfate reducers having had access to ample electron donors to drive their metabolisms. Large fractionations in post-Archean sediments are congruent with a decline of favorable electron donors as aerobic and other high potential metabolic competitors evolved.

Highlights

Sulfur isotope fractionation resulting from microbial sulfate reduction (MSR) provides some of the earliest evidence of life, and secular variations in fractionation values reflect changes in biogeochemical cycles
Kinetic isotope effect of the adenosine phosphosulfate reductase (Apr) enzyme. 34S/32S isotope fractionation during the in vitro reduction of adenosine phosphosulfate (APS) to sulfite was determined at two different temperatures, 20 and 32 °C, using purified APS reductase isolated from the mesophilic deltaproteobacteria Desulfovibrio vulgaris Miyazaki[32]
Regression analysis of δ34S values of APS and sulfite based on the Rayleigh distillation model yielded a straight line at both temperatures (Fig. 2c, d), indicating that the reactions were unidirectional without significant back reaction, and that the sulfur isotope effect remained unchanged over the course of the experiment (Supplementary Note 1, Supplementary Fig. 1)

Summary

Introduction

Sulfur isotope fractionation resulting from microbial sulfate reduction (MSR) provides some of the earliest evidence of life, and secular variations in fractionation values reflect changes in biogeochemical cycles. Kinetic isotope fractionation between sulfate and sulfide is, in theory, dependent on the isotope effects of the enzymes involved, the relative rates of each enzymatic reaction[25,26], and fluxes through potential branch-points[27].

Results

Conclusion