In this study, two primer sets were at first designed specific to P-17 and NOX, the two bacterial species used for measurements of assimilable organic carbon (AOC) in water, based on their respective 16S rDNA sequences. These two primer sets were then used to measure the AOC content in 41 water samples by quantitative real time polymerase chain reaction (qRT-PCR). Results showed that P-17 and NOX cell numbers quantified by qRT-PCR using the designed primer sets were consistently higher (about 40.8% for P-17 and 54.8% for NOX measurements) than those by the traditional but time-consuming plate culture method. This proposed method not only saves the 5 days needed for plate culture, but also may detect AOC at concentrations as low as 0.3 μg-acetate-C/l and 0.6 μg-oxalate-C/l, which are lower than those measured by the plate culture and other methods. Using this new method, the water from the tap and a local reservoir were found containing 122 μg/l and 194 μg/l AOC, respectively, accounting for 5.7% and 7.8% of TOC.

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