The effect of organophosphorous pesticides on acetylcholinesterase activity in Daphnia carinata and Paratya australiensis
Creators: name: Arunachalam Gandhi, Saikrithika
Publisher: name: RMIT University
Year: 2024

description: Australian freshwater environments and the organisms that inhabit them are unique. A potential threat to the structure and functioning of organisms within aquatic habitats is the presence of anthropogenic pollutants such as organophosphorous (OP) insecticides. The application of insecticides to crops within close proximity to freshwater habitats increases the likelihood that insecticides will be transported from their sites of application into the surrounding aquatic environment. This study evaluated the effects of two insecticides and their mixtures on two Australian native freshwater species. The study focused on the sublethal effects of these pesticides by investigating the acetylcholinesterase (AChE) activity in these species on exposure to the pesticides individually and in mixtures of different proportions. Chlorpyrifos and fenitrothion were the two pesticides used in this study for evaluation. <br><br>These pesticides are currently two of the most commonly used OP pesticides in Australian agriculture. Chlorpyrifos and fenitrothion are used both in urban areas and in agriculture to control insect pests. The discharge of these OP pesticides contaminates the freshwater environment, thus affecting non- target species in Australian freshwaters. Two Australian native freshwater species (Daphnia carinata and Paratya australiensis) were chosen to conduct the study, since they are excellent bioindicators of environmental stress and are widely distributed in Australian freshwater environments. These test species are also major components of the food chain in aquatic systems. Reduction in the activity of AChE in these species could result in reduced fitness in individuals and affect population numbers, thereby affecting freshwater food webs. The study aimed to determine if the AChE activity of the selected Australian native aquatic invertebrate species could be used as a biomarker to estimate adverse effects of OP pesticides in the Australian aquatic environment.<br><br> Initially the lethal concentration that affected 50% of the test population (LC50) of chlorpyrifos and fenitrothion was estimated for the two crustacean species. The LC50 was determined according to OECD guidelines (2004) with modifications as stated for 11 D. carinata and according to the ASTM guidelines (1998) for P. australiensis. Both test species were more sensitive to chlorpyrifos than to fenitrothion. The sublethal effects of the pesticides were estimated using the AChE assay, modified from Ellman et al. (1961). The assay was first optimised for each species. For P. australiensis, the contribution of butyrylcholinesterase (BChE) activity to total ChE (cholinesterase) activity was also estimated. The contribution of BChE was very low for P. australiensis and was assumed to be the same for daphnids; hence cholinesterase measured throughout the study was assumed to be predominantly AChE and is reported as AChE only. Each species was exposed to three sublethal concentrations individually and seven mixtures of the pesticides at different proportions. <br><br>The effect of the pesticides, individually and as mixtures, on AChE activity of the two species was evaluated. Exposure to chlorpyrifos resulted in a slow and steady decrease in AChE activity, which was dependent on time and concentration, whereas exposure to fenitrothion resulted in an immediate dose-dependent depression in AChE activity. When both test species were exposed to mixtures of the two pesticides, results indicated an antagonistic effect of chlorpyrifos and fenitrothion in relation to each other. AChE activity of the two test species was very sensitive to exposure to these two pesticides and could be estimated accurately. These two species and the depression in their AChE activity could therefore be used for biomonitoring sublethal exposure to OP insecticides in the environment. D. carinata was more sensitive than P. australiensis, since the AChE activity was reduced at lower OP concentrations. AChE is also a useful tool to estimate contamination by mixtures of these OP pesticides, although effects of chlorpyrifos and fenitrothion are antagonistic and thus maybe less detectable by this biomarker.