Deconstructing the Dissimilatory Sulfate Reduction Pathway: Isotope Fractionation of a Mutant Unable of Growth on Sulfate

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  • Emma Bertran, Harvard Univ, Harvard University, Dept Earth & Planetary Sci
  • ,
  • William D. Leavitt, Dartmouth Coll, Dartmouth College, Dept Biol Sci
  • ,
  • Andre Pellerin
  • Grant M. Zane, Univ Missouri, University of Missouri System, University of Missouri Columbia, Dept Biochem
  • ,
  • Judy D. Wall, Univ Missouri, University of Missouri System, University of Missouri Columbia, Dept Biochem
  • ,
  • Itay Halevy, Weizmann Inst Sci, Weizmann Institute of Science, Dept Environm Sci & Energy Res
  • ,
  • Boswell A. Wing, Univ Colorado, University of Colorado System, University of Colorado Boulder, Dept Geol Sci
  • ,
  • David T. Johnston, Harvard Univ, Harvard University, Dept Earth & Planetary Sci

The sulfur isotope record provides key insight into the history of Earth's redox conditions. A detailed understanding of the metabolisms driving this cycle, and specifically microbial sulfate reduction (MSR), is crucial for accurate paleoenvironmental reconstructions. This includes a precise knowledge of the step-specific sulfur isotope effects during MSR. In this study, we aim at resolving the cellular-level fractionation factor during dissimilatory sulfite reduction to sulfide within MSR, and use this measured isotope effect as a calibration to enhance our understanding of the biochemistry of sulfite reduction. For this, we merge measured isotope effects associated with dissimilatory sulfite reduction with a quantitative model that explicitly links net fractionation, reaction reversibility, and intracellular metabolite levels. The highly targeted experimental aspect of this study was possible by virtue of the availability of a deletion mutant strain of the model sulfate reducer Desulfovibrio vulgaris (strain Hildenborough), in which the sulfite reduction step is isolated from the rest of the metabolic pathway owing to the absence of its QmoABC complex (Delta Qmo). This deletion disrupts electron flux and prevents the reduction of adenosine phosphosulfate (APS) to sulfite. When grown in open-system steady-state conditions at 10% maximum growth rate in the presence of sulfite and lactate as electron donor, sulfur isotope fractionation factors averaged -15.9 parts per thousand (1 sigma = 0.4), which appeared to be statistically indistinguishable from a pure enzyme study with dissimilatory sulfite reductase. We coupled these measurements with an understanding of step-specific equilibrium and kinetic isotope effects, and furthered our mechanistic understanding of the biochemistry of sulfite uptake and ensuing reduction. Our metabolically informed isotope model identifies flavodoxin as the most likely electron carrier performing the transfer of electrons to dissimilatory sulfite reductase. This is in line with previous work on metabolic strategies adopted by sulfate reducers under different energy regimes, and has implications for our understanding of the plasticity of this metabolic pathway at the center of our interpretation of modern and palaeo-environmental records.

Original languageEnglish
Article number3110
JournalFrontiers in Microbiology
Volume9
Number of pages11
ISSN1664-302X
DOIs
Publication statusPublished - 14 Dec 2018

    Research areas

  • chemostat, deletion mutant, metabolic pathway, sulfite reduction, sulfur isotope fractionation, BACTERIAL REDUCTION, SULFUR ISOTOPES, BISULFITE ION, MODEL, BIOTURBATION, EVOLUTION, PROTEIN, MARINE

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