Sulfide oxidation in marine sediments: Geochemistry meets microbiology

The main pathways of sulfide oxidation in marine sediments involve complex interactions of chemical reaction and microbial metabolism. Sulfide becomes partly oxidized and bound by Fe(III), and the resulting iron-sulfur minerals are transported toward the oxic sediment-water interface by bioturbating and irrigating fauna. Although oxygen is the main oxidant for pyrite or amorphous iron sulfide, oxidation reactions may also take place with nitrate or manganese oxide. Intermediate oxidation products such as elemental sulfur or thiosulfate undergo disproportionation reactions and thereby provide shunts in the sedimentary sulfur cycle. Although of widespread occurrence, chemolithoautotrophic sulfide oxidizing bacteria, such as Thiobacillus spp. or Thiomicrospira spp., appear to be of minor significance relative to heterotrophic or mixotrophic sulfide oxidizers of diverse phylogenetic lineages. As a unique group, the large sulfur bacteria of the genera Beggiatoa, Thioploca, and Thiomargarita have developed specialized modes of sulfide oxidation using nitrate stored in intracellular vacuoles. By commuting between electron acceptor and donor, or by temporally bridging their occurrences in the environment through a great storage potential for both nitrate and elemental sulfur, these bacteria compete efficiently with other microbial pathways of sulfide oxidation. Dissimilatory nitrate reduction in these bacteria leads preferentially to ammonium rather than to dinitrogen, as in the denitrifying bacteria. Beggiatoa appears to be widely distributed in coastal sediments with a high organic load. In such sediments where Beggiatoa often occurs unnoticed in the anoxic, oxidized zone rather than growing as a visible mat on the sediment surface, dissimilatory nitrate reduction to ammonium may dominate over denitrification