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Effect Of Nitrate On Reduction Of Ferric Iron By A Bacterium Isolated From Crude Oil

Christian O. Obuekwe, Donald W. S. Westlake, Fred D. Cook

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A Pseudomonas sp. isolated from crude oil reduced ferric ions (Fe(III)) to ferrous ions (Fe(II)). In the presence of nitrate (NO3) after prolonged incubation, the amount of Fe(II) was lower than in its absence. However, during short incubation periods, the presence of NO3 significantly increased (99.5% confidence limit) the amount of Fe(II) produced. The decrease in Fe(II) on prolonged incubation was associated with increased production and accumulation of nitrite (NO2). Under low NO3 levels, where the production of NO2 was limited, a decrease in NO2 concentration was accompanied by an increase in Fe(II) production to levels comparable with those obtained in the absence of NO3. Preinduction of cells for nitrate reductase, which favoured rapid NO2 production, resulted in a more rapid decrease in Fe(II) production than in cells that were not preinduced. It is proposed that the inhibitory effect of NO3 on microbial reduction of Fe(III) is due to a secondary reaction, which involves the chemical oxidation of Fe(II) by NO2.