Volatile Fatty Acids and Hydrogen as Substrates for Sulfate-Reducing Bacteria in Anaerobic Marine Sediment

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The addition of 20 mM MoO42− (molybdate) to a reduced marine sediment completely inhibited the SO42− reduction activity by about 50 nmol g−1 h−1 (wet sediment). Acetate accumulated at a constant rate of about 25 nmol g−1 h−1 immediately after MoO42− addition and gave a measure of the preceding utilization rate of acetate by the SO42−-reducing bacteria. Similarly, propionate and butyrate (including isobutyrate) accumulated at constant rates of 3 to 7 and 2 to 4 nmol g−1 h−1, respectively. The rate of H2 accumulation was variable, and a range of 0 to 16 nmol g−1 h−1 was recorded. An immediate increase of the methanogenic activity by 2 to 3 nmol g−1 h−1 was apparently due to a release of the competition for H2 by the absence of SO42− reduction. If propionate and butyrate were completely oxidized by the SO42−-reducing bacteria, the stoichiometry of the reactions would indicate that H2, acetate, propionate, and butyrate account for 5 to 10, 40 to 50, 10 to 20, and 10 to 20%, respectively, of the electron donors for the SO42−-reducing bacteria. If the oxidations were incomplete, however, the contributions by propionate and butyrate would only be 5 to 10% each, and the acetate could account for as much as two-thirds of the SO42− reduction. The presence of MoO42− seemed not to affect the fermentative and methanogenic activities; an MoO42− inhibition technique seems promising in the search for the natural substrates of SO42− reduction in sediments.

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