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

Abstract

The addition of 20 mM MoO 4 2− (molybdate) to a reduced marine sediment completely inhibited the SO 4 2− 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 MoO 4 2− addition and gave a measure of the preceding utilization rate of acetate by the SO 4 2− -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 H 2 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 H 2 by the absence of SO 4 2− reduction. If propionate and butyrate were completely oxidized by the SO 4 2− -reducing bacteria, the stoichiometry of the reactions would indicate that H 2 , 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 SO 4 2− -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 SO 4 2− reduction. The presence of MoO 4 2− seemed not to affect the fermentative and methanogenic activities; an MoO 4 2− inhibition technique seems promising in the search for the natural substrates of SO 4 2− reduction in sediments.