The seabed is the largest anaerobic bioreactor on Earth. Ocean water contains 28 mM sulfate, which can penetrate metres down into the seabed. Dissimilatory sulfate reduction to sulfide is therefore globally the most important terminal pathway of organic matter mineralisation in the anoxic seabed. The sulfate reducing microorganisms feed on small organic molecules and H2 from which they transfer electrons to reduce sulfate to sulfide. The process of dissimilatory sulfate reduction thereby transforms the sulfur to its most reduced form, H2S, which drives the sulfur cycle in marine sediments.

Radiolabelled sulfate, 35SO42-, is...

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