pH-dependent Zinc Toxicity Differentials in Species of Penicillium and Rhodotorula During Oil Biodegradation
The influence of pH (5.5, 7.0 and 8.5) on the response patterns of species of Penicillium and Rhodotorula to increasing concentrations of zinc during biodegradation of Nigerian light crude oil was assessed by the carbon dioxide (CO2) evolution technique, over a 16-day period. Mean rates of CO2 evolution decreased with increasing pH in control cultures of Penicillium sp., but increased with increasing pH in cultures containing 1000 mg L-1 of zinc. However, analysis of variance of data revealed that there was no statistically significant (p>0.05) difference among the metal concentrations, suggesting tolerance of this species of Penicillium to high levels of zinc. In contrast, mean rates of CO2 evolved in cultures containing Rhodotorula sp. decreased with increasing pH in both control and test cultures containing zinc. Efficiency of inhibition was minimal at pH 8.5 in cultures containing Penicillium sp., but maximal at the same pH in those containing Rhodotorula sp. Interestingly, low concentrations of zinc (≤10 mg L-1) stimulated CO2 evolution in Rhodotorula sp. at pH 5.5 and 7.0, but not 8.5, suggesting pH-dependent low level zinc requirement of the oil biodegradative enzymes of the organism.
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