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Abstract: The nitrate-reducing microbial community in oil reservoirs was examined by PCR using primers to amplify a segment of napA gene encoding for a subunit of the nitrate reductase, and the effects of different organic carbon additions on the nitrate-reducing community were also evaluated. The orders Rhodocyclales and Burkholderiales within Betaproteobacteria and Pseudomonadales within Gammaproteobacteria were recovered in production water of all three oil reservoirs. Amendment of organic acids promoted Enterobacteriales within Gammaproteobacteria and orders of Rhodocyclales, Pseudomonadales, and Burkholderiales, while alkanes favored the Rhizobiaceae family within Alphaproteobacteria and orders of Rhodocyclales and Pseudomonadales. Results indicated that the functional gene napA can be used as a valuable biomarker in analyzing the diversity of nitrate reducers in oil reservoirs. Nitrate-reducing microbial community shifts following the available carbon sources. Information about napA gene in oil reservoirs environment is scarce. This is the first study that combines molecular and culture-dependent approaches to reveal the diversity of the nitrate-reducing microbial community in production water of oil reservoirs by using the napA gene. [Copyright &y& Elsevier]