Two hydrocarbon utilizers Pseudomonas fluorescens and Vibrio fluvialis isolated from crude oil impacted mesotidal water and sediment samples and identified using Microgen ID Kit (Microgen Bioproduct Limited, UK), and 16s rRNA were exposed to different concentrations of nickel (Ni) (0.00, 1.17, 2.34, 4.68, 9.375 and 18.75mM). Following exposure to the various Ni concentrations, the isolates were monitored for effect on growth every six hours for 48 hours, percentage survability, and LC50 determined. The generation times for P. fluorescens and V. fluvialis increased from 1.55 to 3.73 and 1.56 to 3.75 (h/gen) as Ni concentration increased from 0.00 to 1.17Mm, respectively. Furthermore, growth rates decreased from 0.447 to 0.186 and 0.443 to 0.185 (h-1 ) for P. fluorescens and V. fluvialis, respectively. Without the toxicant, the peak numbers of cells were 21.77 and 21.68 log number of cells for P. fluorescens and V. fluvialis, respectively. However, in the presence of Ni, the peak log numbers of viable cells were 15.52 and 15.477 after 24 hours for P. fluorescens and V. fluvialis, respectively. Comparism of the number of surviving cells at 1.17mM showed that V. fluvialis had 76.69% of surviving cells against 9.32 % for P. fluorescens. Linear regression line fit plots revealed R2 values of 0.5372 for P. fluorescens and 0.9341 for V. fluvialis. The LC50 for V. fluvialis was 5.77 mM while that of P. fluorescens could not be determined from the model. Generally, pH values above 6.8 favoured both isolates even in Ni concentrations above 1.17mM. The findings in our study indicate that Ni in concentrations above 1.17mM has the potential to reduce the growth of hydrocarbonoclastic isolates.

Keywords : Niger Delta, Hydrocarbon Utilizers, Heavy Metals, Bacteria, Toxicity.