Abstract

This study was undertaken to investigate the effects of Bonny light crude oil (specific gravity = 0.81; API gravity 43.2⁰) on the numerical composition of soil bacteria and fungi and relative distribution of Gram positive and Gram negative soil bacteria. Eight different levels of the crude oil (0.5, 1.0, 2.0, 2.5, 5.0, 10.0, 15.0 or 20.0% v/w of soil) were used for the controlled pollution of soil samples obtained from the Botanical Garden, University of Nigeria Nsukka. Studies on the effects of crude oil on bacterial and fungal populations were carried out by plate count procedures using nutrient agar and Sabouraud dextrose agar respectively. Crude oil significantly (p<0.05) inhibited bacterial and fungal growth in a dose-dependent manner. Bacterial counts in the control soil sample ranged between 2.32 × 10⁹ and 2.80 × 10⁹ cfu/g while those of the contaminated samples ranged between 2.00 × 10⁸ and 2.77 × 10⁹ cfu/g. Fungal counts ranged from 1.02 × 10⁷ to 1.39 × 10⁷ cfu/g in the control and 1.60 × 10⁵ to 1.18 × 10⁷ cfu/g in the contaminated samples. At 15.0 and 20.0% levels of crude oil, the growth inhibitory effects of crude oil were maximum for bacteria and fungi respectively. Microbial respiration decreased concomitantly with increase in crude oil pollution during the first four weeks of the study. There was a prevalence of Gram positive bacteria over Gram negatives in the unpolluted soil but a preponderance of Gram negative rods over Gram positives and other morphological forms of Gram negatives in the polluted samples.

Key words: Bonny light crude oil, bacteria, fungi, contamination, hydrocarbon, biodegradation.