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Min Sub Sim1,2,*, Hideaki Ogata3,4, Wolfgang Lubitz3, Jess F. Adkins2, Alex L. Sessions2, Victoria J. Orphan2 & Shawn E. McGlynn2,5,*
1 School of Earth and Environmental Sciences, Seoul National University, Seoul 08826, South Korea. 2 Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA. 3 Max Planck Institute for Chemical Energy Conversion, Stiftstrasse 34-36, D-45470 Mulheim an der Ruhr, Germany. 4 Institute of Low Temperature Science, Hokkaido University, Sapporo 060-0819, Japan. 5 Earth-Life Science Institute, Tokyo Institute of Technology, Ookayama, Tokyo 152-8550, Japan.
*Correspondence and requests for materials should be addressed to M.S.S. or to S.E.M.
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.
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