Global rates of marine sulfate reduction and implications for sub–sea-floor metabolic activities

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Science  23 May 2014:
Vol. 344, Issue 6186, pp. 889-891
DOI: 10.1126/science.1249213

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Sulfate reduction is a globally important redox process in marine sediments, yet global rates are poorly quantified. We developed an artificial neural network trained with 199 sulfate profiles, constrained with geomorphological and geochemical maps to estimate global sulfate-reduction rate distributions. Globally, 11.3 teramoles of sulfate are reduced yearly (~15% of previous estimates), accounting for the oxidation of 12 to 29% of the organic carbon flux to the sea floor. Combined with global cell distributions in marine sediments, these results indicate a strong contrast in sub–sea-floor prokaryote habitats: In continental margins, global cell numbers in sulfate-depleted sediment exceed those in the overlying sulfate-bearing sediment by one order of magnitude, whereas in the abyss, most life occurs in oxic and/or sulfate-reducing sediments.

Mapping sub–sea-floor communities

The sea floor is teeming with microbes, whose sheer numbers produce a major effect on the global biogeochemical cycles of carbon, sulfur, and other important nutrients. Bowles et al. constructed a map showing how deeply sulfates penetrate marine sediments worldwide and how quickly that sulfate is chemically reduced by microbes in the sub–sea-floor. Globally, almost a third of the organic carbon that reaches the sea floor is consumed during sulfate reduction, and the vast majority of microbial cells in the sub–sea-floor at continental margins get their energy through the biochemical processes of fermentation and methanogenesis.

Science, this issue p. 889.

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