When sulfate-reducing bacteria use sulfur compounds instead of oxygen during respiration, the chemical reactions that generate energy are biased toward specific natural isotopes of sulfur. The sulfite produced is thus depleted in sulfur isotopes relative to its natural abundance-often by only a few additional atoms per thousand reactions. Sim et al. (p. 74) found that a marine bacterium isolated from coastal sediments can produce a remarkably high signal of isotopic fractionation-up to 68 times per thousand reactions. Similar high values that have been observed in sedimentary rocks were thought to require additional cycling of sulfur and oxygen; however, these results suggest that this may not always be the case.
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