ABSTRACT

Toxicity is a measure of the degree to which a substance can be poisonous or harmful. It deals with the effects of chemical substances on organisms. The contamination of aquatic environment with various kinds of petroleum products has been a long-term issue and can as such affect the roles of microorganisms in food chain and as agents of degradation of substances in the aquatic environment.  The acute toxicity of petroleum products was assessed on some bacterial species. The petroleum products used include; (drilling fluid – Oil based drilling fluids (OBDF) and Water based drilling fluids (WBDF), degreasers – Aquabreak and Rigwash, Oil Spill dispersants – OSD seacare and Industrial detergents- Gamazyne Biological Toilet Cleanser (BTC) and Teepol). The test bacteria were isolated and identified using standard methods. The test isolates used were Nitrobacter sp, Pseudomonas sp, Vibrio sp, Escherichia coli and Bacillus sp. The percentage log survival was used as an index for acute toxicity. The results obtained from the percentage log survival revealed a decrease in the percentage log survival as the concentrations and exposure periods increased for all the test isolates. The decrease in the survival of the test isolates suggested that the functions of each of the test isolate in the aquatic habitat may as well be impaired like degradation capability of Pseudomonas sp. The toxic effect of these chemicals could also affect the ability of the affected aquatic habitat in supporting life of aquatic organisms. The sensitivity trend for the test organisms at 95% confidence level showed a decreasing order; respectively. Nitrobacter species was responsible for the oxidation of nitrites to nitrates in the nitrogen cycle. The study revealed that these petroleum products were toxic to the test organisms. Growth inhibition of these microorganisms by the petroleum product may have led to the disruption of the biogeochemical cycles where they acted as the key degraders. Such could have had an overall effect on the entire biodegradation activities. The significance of the toxicity of the petroleum products on the test isolates was analyzed by ANOVA using SPSS and the result revealed that there was significant difference between the isolates and the hydrocarbons concentrations. For Nitrobacter species in freshwater, BTC had the lowest toxicity (LC₅₀ 2547.24 ppm) while WBDF and Aquabreak had the lowest toxicities (LC₅₀ (454.02 ppm); (LC₅₀ 242.90 ppm) in marine and brackish water respectively. The higher the LC₅₀, the lower the toxicity of the chemical. For Pseudomonas sp, OBDF had the lowest lethal toxicity (2073.69 ppm), WBDF (5776.69 ppm) and Aquabreak (230.04 ppm) in freshwater, marine and brackish water respectively. For Escherichia coli, WBDF had the lowest (2192.73 ppm) lethal toxicity on the isolate from freshwater, Aquabreak in marine water (1358.45 ppm) and brackish (4778.71 ppm). In Vibrio sp BTC had the lowest lethal toxicity LC₅₀ in freshwater (2334.02 ppm), WBDF (1980.25 ppm) in marine water and Aquabreak (719.45 ppm) in brackish habitat. Same trend was also observed in Bacillus.