We continued our studies on the interaction of nitrate reduction with other microbial processes. For this purpose, enrichment cultures under sulfate reducing and nitrate reducing conditions were obtained using produced water from an oil-water separator as a source of microorganisms. We also continued to monitor laboratory incubations from the produced water supplemented with nitrate or sulfate and different electron donors. The incubations with the produced water showed that sulfate reduction was limited by electron donors. The addition of crude oil stimulated the process considerably (fig.1). Those incubations were used as a source for oil-degrading sulfate reducing enrichments (fig.2). In course of growth with oil, sulfate-reducing enrichments produced acetate. Acetate was accumulated as a transient metabolite and was gradually removed in 6 weeks (fig. 3). Interestingly, in nitrate reducing enrichments, nitrate reduction was observed only with acetate as an electron donor. Oil did not support this process. Nitrate reduction was sensitive to the level of sulfide. In the cultures containing trace level of sulfide all added nitrate was removed in one week with a rate of 2.2 ± 0.2 mmol/L/day. In the cultures containing 0.45 mM sulfide the rate of nitrate reduction decreased to 0.5± 0.03 mmol/L/day. In the incubations with produced water, where initial the sulfide level was around 3.5 mM the rate of nitrate reduction was 0.02 ± 0.01 mmol/lL/day, apparently due to sulfide toxicity. However, it may be that with the decrease in the pre-existing sulfide level the rate of nitrate reduction may increase.
Figure 1
Figure 2
Figure 3