Toxicity of Selected Pesticides on the Microbial Pollution Bio-indicator, Pseudomonas aeruginosa

Abstract

Pesticides are well known to impact on non-target organisms such as vulnerable bio-indicator species in aquatic and non-aquatic environments. Pseudomonas aeruginosa is a known bioindicator for heavy metal, wastewater and polycyclic aromatic hydrocarbon pollution. This study explored the effects of varying concentrations (0.01 mg/L – 100 mg/L) of the organophosphorus insecticide, dichlorvos and the organochlorine herbicide, paraquat on P. aeruginosa isolated from river water to ascertain mortality rates, median lethal concentrations (LC₅₀), lowest observed effect concentrations (LOEC) and the no observed effect concentrations (NOEC) using a 96 h acute toxicity bioassay. P. aeruginosa was isolated and enumerated using cetrimide agar while the toxicity parameters were determined via probit regression analysis. There was a general decline in abundance of P. aeruginosa, and correspondingly, a rise mortality levels, as exposure period and pesticide concentrations increased. Survival rates of 0% were obtained at 72 h and 48 h and above for paraquat and dichlorvos respectively at the highest pesticide concentration of 100 mg/L. Dichlorvos was found to be more toxic to P. aeruginosa than paraquat as the LC₅₀, LOEC and NOEC were determined as 0.032 mg/L, 0.0042 mg/L and 0.0015 mg/L respectively for dichlorvos. The values obtained for paraquat were 0.44 mg/L, 0.0053 mg/L and 0.0017 mg/L for LC₅₀, LOEC and NOEC respectively. The findings of this study raise concerns for the indiscriminate use of pesticides as they may impact negatively on vital ecosystem players.