Effect of nitrate on reduction of ferric iron by a bacterium isolated from crude oil.

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-.