Time-dependent Joint Toxicity Characteristics and Mechanisms of Five Carbamate Pesticides towards Chlorella pyrenoidosa

Abstract

Carbamate pesticides are widely used in agricultural production, so their residues in the environment and toxicity towards non-target organisms are concerned. Five kinds of carbamate pesticides, i.e. baygon, pirimicarb, methomyl, aldicarb and carbofuran were selected as research substances, the microplate toxicity analysis method was used to determine the growth inhibition toxicity of each pesticide and their mixtures towards Chlorella pyrenoidsa in different exposure times (0, 24, 48, 72 and 96 h). The effects of the five pesticides and their mixtures on the physiological characeristics such as chlorophyll content, protein content, superoxide dismutase (SOD) activity and lipid peroxidation product malondialdehyde (MDA) content in Chlorella pyrenoidosa were also investigated to explore possible toxicity mechanisms. The results showed that the five pesticides had time-dependent effects. The pesticide pirimicarb promoted the growth of green algae at low and medium concentrations, showing a non-monotonic J-type concentration-response curve (CRC). The CRCs of the other four pesticides showed classic S types. Selecting the negative logarithm of the half effect concentration (pEC50) as the toxicity index, toxicity order of the five pesticides in 96 h was as follows: carbofuran (pEC50=3.43) > baygon (pEC50=2.77) > pirimicarb (pEC50=2.12) > methomyl (pEC50=2.11) > aldicarb (pEC50=1.89). The chlorophyll and protein contents as well as SOD activity in Chlorella pyrenoidosa decreased gradually with the lengthening of exposure time. However, MDA contents increased with time lengthening, showing that the algae cells were destructed and the lipid peroxidation exceeded the self-repairing ability of the cells. This caused the inhibition of SOD activity, and thus harmed the antioxidant capacity of cells. Thus, the accumulation of H2O2 in algae cells resulted in the decreasing of SOD activity and increasing of MDA content. Toxicity of the five pesticide mixtures was also time-dependent and showed stimulatory effect, i.e. hormesis phenomenon. The maximum stimulating effects of the five-component mixtures appeared in 96 h, and the chlorophyll and protein content together with SOD activity also increased. As a result, the content of MDA was continuously decreased. Antagonism appeared in the five-component mixtures in 96 h, which was closely related to the concentrations or concentration ratios of mixture components.