Evidence of the activity of dissimilatory sulfate-reducing prokaryotes in nonsulfidogenic tropical mobile muds

In spite of the nonsulfidic conditions and abundant reactive iron(III) commonly found in mobile tropical deltaic muds, genes encoding dissimilatory sulfite reductase (dsr) were successfully amplified from the upper ∼1 m of coastal deposits sampled along French Guiana and in the Gulf of Papua. The dsr sequences retrieved were highly diverse, were generally represented in both study regions and fell into six large phylogenetic groupings: Deltaproteobacteria, Thermodesulfovibrio groups, Firmicutes and three groups without known cultured representatives. The spatial and temporal distribution of dsr sequences strongly supports the contention that the sulfate-reducing prokaryote communities in mobile mud environments are cosmopolitan and stable over a period of years. The decrease in the ³⁵SO₄ ² tracer demonstrates that, despite abundant reactive sedimentary iron(III) (∼350-400 μmol g^-1), the sulfate-reducing prokaryotes present are active, with the highest levels of sulfide being generated in the upper zones of the cores (0-30 cm). Both the time course of the ³⁵S-sulfide tracer activity and the lack of reduced sulfur in sediments demonstrate virtually complete anaerobic loss of solid phase sulfides. We propose a pathway of organic matter oxidation involving at least 5-25% of the remineralized carbon, wherein sulfide produced by sulfate-reducing prokaryotes is cyclically oxidized biotically or abiotically by metal oxides.