EFFECTS OF NITRATE ON BACTERIAL COMMUNITIES IN AN OIL FIELD ENVIRONMENT

ABSTRACT
The study aimed to determine whether nitrate utilising SRB thrive in the Saudi Aramco Hawtah oil field produced water re-injection system (PWRI). Bacterial populations were recovered through enrichments in selective growth media from samples collected at different locations in PWRI. Characterization of purified Hawtah field SRB isolates using 16S rRNA gene-based phylogeny, revealed the presence of an isolate, coded DD-H SRB, showing a high 16S rRNA sequence similarity (99%) with Desulfovibrio desulfuricans spp. desulfuricans.  The change from sulphate to nitrate reduction was tested using Aramco DD-H SRB isolate and a “wild” Hawtah SRB enrichment in the laboratory batch culture experiments. Control culture comprised a tri-species mixture of pure Desulfovibrio strains. Both, DD-H SRB isolate and control SRB tri-culture, when exposed to a gradually decreasing concentration of sulfate and a concurrently increasing concentration of nitrate over a 40-day period, changed slowly from sulfate to nitrate respiration. Upon inoculation into a sulfate-free medium, fast nitrate reduction was observed. In “wild” Hawtah field SRB population nitrate reduction was fast, while sulfate respiration ceased. The functional gene aprA, coding for APS-reductase enzyme, which is found in all known SRB genera, was present in all tested cultures grown with nitrate as a sole electron acceptor.