Abstract
Pesticides have been a cornerstone in increasing crop yields and assisting in the fight against vector-borne diseases. There are many different pesticides on the market, with South Africa having 500 registered pesticides and 3 000 registered pesticide products. The aim of the study was to determine the effects of two pesticides (individually and in a combination) on an indigenous freshwater shrimp, Caridina africana. The selected pesticides were a commonly used herbicide – atrazine (ATZ), and the infamous insecticide – dichlorodiphenyltrichloroethane (4,4’-DDT). Selection of the pesticides was based on the probability of them co-occurring in some of South Africa’s freshwater systems. The project comprises of integrated sections with different endpoints, the first of which determined the median lethal concentration (LC50) of 4,4’-DDT, ATZ and a combination of both to C. africana. Preliminary LC50 results showed ATZ to be toxic at levels that were not environmentally relevant (14 019.22 μg/l), so a standard 350 μg/l was set for all remaining combined group experimentation. The surviving organisms from this exposure were tested for biomarker responses to determine if there is a concentration-dependent response. A bimodal distribution was seen in the organism’s responses, with the greatest deviation, from the control group, calculated in organisms exposed to 106.8 μg/l and 5 557 μg/l ATZ. To establish the LC50 values for 4,4’-DDT and the combination exposure, a range of 4,4’-DDT concentrations was used. A logistic regression resulted in an LC50 value for 4,4’-DDT of 4.55 μg/l and an LC50 for the combination of 4.62 μg/l, indicating there is little effect of ATZ on the toxicity of 4,4’-DDT to C. africana. Following on from the LC50 determination, two, 7-day, sublethal exposures were performed to measure bioaccumulation and biomarker responses in the test organisms, from three exposure groups: ATZ, 4,4’-DDT and a combination of the two. The first exposure used 0.33 μg/l 4,4’-DDT in the 4,4’-DDT and combined group, while in the second exposure, the concentration of 4,4’-DDT was increased to 0.66 μg/l. The concentration of ATZ (used in the ATZ and combined group) over both exposures was 350 μg/l. The bioaccumulation of both pesticides in the organism’s tissue was below detection for the instrumentation, though, ATZ was
detected in all of the exposure water samples that it was added to. Biomarker responses were used to determine biochemical alterations, even at such low exposure concentrations. Using univariate and multivariate statistical analyses of the biomarker responses, alterations were seen when compared to a control group. Glutathione-S-transferase and energetic biomarkers were strong drivers in group separation in the principal component analysis, while the combined exposure of 0.66 μg/l 4,4’-DDT and 350 μg/l ATZ, resulted in the highest deviation score when using the integrated biomarker response index. Caridina africana is akin to the approximately one-third of freshwater shrimp species that are threatened or near threatened. Providing insight into the effects that pesticides, individually and in combination, are having on freshwater invertebrates is of great value. With so many synthetic chemicals entering our freshwater systems, mixture toxicity is an important facet in ecotoxicology.