The addition of 20 mM MoO2-(molybdate) to a reduced marine sediment completely inhibited the SOreduction activity by about 50 nmol g-1 h-' (wet sediment). Acetate accumulated at a constant rate of about 25 nmol g-1 h-1 immediately afterMoO4addition and gave a measure of the preceding utilization rate of acetate by the SO4-reducing bacteria. Similarly, propionate and butyrate (including isobutyrate) accumulated at constant rates of 3 to 7 and 2 to 4 nmol g' h-1, respectively. The rate of H2 accumulation was variable, and a range of 0 to 16 nmol g-1 h-' 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 SO4reduction. If propionate and butyrate were completely oxidized by the SO4-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
A method is presented for quantifying the in situ rates of substrate oxidation by sulfate‐reducing bacteria in sediment. Molybdate (100 µl of 2 M Na2MoO4) injected into intact sediment cores inhibited sulfate reduction efficiently. The subsequent accumulation of acetate was linear. The accumulation rate in molybdate‐inhibited cores was determined from linear regressions of the acetate concentrations as a function of incubation time. The acetate contributed about 65% of the electron donor for sulfate reduction. In the surface layers the acetate accumulation rate leveled off with time, indicating that acetate was transported by diffusion to the surface where it could be oxidized by O2 and NO3‒ respiration. On the basis of linear regressions of the accumulation of propionate, butyrate, and isobutyrate, it was calculated that these substrates contributed 14, 5, and 8% of the electron donors for sulfate reduction.
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