Two marine hydrocarbonoclastic bacterial isolates, Pseudomonas fluorescens and Vibrio fluvialis, obtained from water and sediment samples from Qua Iboe Estuary, Niger Delta, Nigeria were exposed to different concentrations of trivalent chromium (Cr) (0.00, 6.25, 12.50, 25.00, 50.00, 100.00 and 200.00 mM) and a control (0.00mM). After exposure, the isolates were monitored for effect of Cr on growth and LC50 also determined using a linear regression line fit plot. The generation times for P. fluorescens and V. fluvialis increased from 1.55 to 6.24 and 1.56 to 6.93 (h/gen), respectively as Cr concentration increased from 0.00 to 12.50mM. In contrast, the growth rates decreased from 0.447 to 0.111 and 0.443 to 0.100 (h-1 ) for P. fluorescens and V. fluvialis, respectively. At concentrations ≥ 25mM, the generation times and growth rates for both isolates dropped to zero. Control assay showed a short lag phase and extended exponential phase, however, Cr concentrations of 6.25 and 12.50mM showed a longer acclimatization and short exponential phases of growth for both isolates. Percentage survivability after one 1h of exposure showed 0% of survived cells for both isolates at concentrations of 100 and 200 mM. Quadratic regression line fit plot of survivability for both isolates gave R2 values of 0.763 and 0.905, respectively for P. fluorescens and V. fluvialis. Linear regression line fit plot gave LC50 values of 80.70 and 45.85 for P. fluorescens and V. fluvialis, respectively. Cr concentrations 100mM and above were very toxic to the test isolates in this study.

Keywords : Niger Delta, Hydrocarbon Utilizers, Chromium, Bacteria, Toxicity.