Effect of azo dyes on nitrite assimilation by Nitrobacter. – C. J. Ogugbue and N. A. Oranusi
Five azo dyes were tested for their toxic effects on nitrite-N assimilation by Nitrobacter sp. Nitrite-N assimilation was inhibited by the azo dyes tested: Mordant Black 17; Direct Red 2; Direct Blue 14; Reactive Red 4 and Reactive Yellow 2 at various concentrations (0.01mgL-1 – 100.00mgL-1). Statistical analysis (ANOVA) showed that there was significant difference between values obtained as nitrite-N assimilation for various concentrations of each dye at specific exposure times. However, stimulation effects on assimilation (when compared with control) were obtained at low concentrations and short exposure time with Reactive Red 4 and Reactive Yellow 2. This was attributed to hyperactivity caused by increase in metabolic activity of cells under stress to cope with toxic effects of the toxicants at sub-lethal concentrations. The ranges of EC50 values (mgL-1) obtained at various exposure times were: Mordant Black 17 (0.375-0.965); Direct Red 2 (0.379-2.078); Direct Blue 14 (0.141-11.00); Reactive Red 4 (323.97-6455.50) and Reactive Yellow 2 (23.41-116.01). Differences in degree of toxicity exerted were attributed to any/or all of the following factors: molecular size and impurities in the toxicants. Generally, EC50 values increased with increase in exposure time for each toxicant and this trend was attributed to acclimatization and/or detoxification.
Keywords: toxicity, azo dyes, assimilation, Nitrobacter