Mineralization rates of C-labeled substrates were determined in the presence and absence of Na(2)MoO(4), an inhibitor of sulfate reduction, in the profundal sediments of a shallow eutrophic lake. Sulfate reduction was inhibited by Na(2)MoO(4) at all concentrations tested (0.2 to 200 mM), whereas methane production was inhibited at Na(2)MoO(4) concentrations greater than 20 mM. Initial mineralization rates of glucose were unaffected by Na(2)MoO(4); however, Na(2)MoO(4) decreased the mineralization rates of lactate (58%), propionate (52%), an amino acid mixture (85%), and acetate (14%). These decreases in the rates of mineralization were attributed to inhibition of sulfate reduction. Hydrogen stimulated the reduction of SO(4) 2.5- to 2.8-fold, demonstrating potential hydrogen oxidation by sulfate-reducing bacteria. These results indicate that sulfate reducers utilize an array of substrates as electron donors and are of potential significance to the in situ mineralization of lactate, propionate, and free amino acids in these sediments.