Nitrogen fixation (C(2)H(2) reduction) in a sediment-water system was studied under anaerobic incubation conditions. Sodium sulfide at low concentrations stimulated activity, with a twofold increase in C(2)H(4) production occurring in the presence of 8 mumol of S per ml of stream water. Sodium sulfide at concentrations of 16 mumol of S per ml or greater inhibited nitrogen fixation, with 64 mumol of S per ml being completely inhibitory. Sulfide at levels of 16 mumol/ml or above inhibited CO(2) production, and the degree of inhibition increased with increasing concentration of sulfide. Titanium (III) citrate (used to modify Eh levels) stimulated both nitrogen fixation and CO(2) production, but could not duplicate, at any concentration tested, the twofold increase in nitrogen fixation caused by 8 mumol of S per ml. Sulfide additions caused pH changes in the sediment, and when the sediment was adjusted and maintained at pH 7.0 all concentrations of sulfide inhibited nitrogen fixation activity. From considerations of the redox equilibria of H(2), H(2)S, and other sulfur species at various pH values, it appeared that H(2)S was the toxic entity and that HS was less toxic. The observed stimulation of activity was apparently due to a pH change coupled with the concurrent production of HS from H(2)S.
