Toxicological Effects Of Microcystin-LR On Earthworm In Soil

Microcystin-LR?MC-LR?is a cyclic heptapeptide toxin produced by cyanobacteria in eutrophic water.It can be transferred into soil–crop systems via irrigation,overflow and algae fertilization.MC-LR in soil can be absorbed and transported by soil organisms into the food chain,which is harmful to soil organisms and consequently pose severe risk to human health.So far,the studies available are focused on the pollution characteristics,environmental behavior and toxic effect of MC-LR in water.But the toxic effects of MC-LR on soil organisms are rarely reported,especially lack of the toxicity of MC-LR on soil animals.Earthworms are considered as an important bio-indicator of chemical toxicity in the soil ecosystem.In the present study,we detected the toxic effects of MC-LR on earthworms?Eisenia fetida?at levels from the individual to cells.The results would provide scientific theory basis for establishing the ecological risk evaluation system of MC-LR in soil.The main conclusions were as follows:?1?The acute toxicity and reproduction toxicity of MC-LR on earthworm were studied.The results showed that the LC₅₀ values for MC-LR at 48 h and 72 h were0.634?g cm^-2 and 0.149?g cm^-2,respectively.According to the toxicity classification of chemicals,MC-LR was classified as a super-toxic chemical.The LC₅₀ values for MC-LR in soil were 0.517 mg kg^-1 at 7 d and 0.460 mg kg^-1 at 14 d,indicating that MC-LR should be classified as a highly toxic chemical.In addition,the EC₅₀ of MC-LR was 0.268 mg kg^-1.Increasing concentrations of MC-LR led to a significant decrease in hatchability,mean cocoons per earthworm,mean hatchlings per cocoon,and cocoon weight compared to the control,especially when the concentrations of MC-LR above 0.2 mg kg^-1.The exposure of earthworms to MC-LR significantly inhibited their growth.Growth inhibition increased with increasing MC-LR concentration and exposure duration.?2?The effects of MC-LR on the changes in antioxidant defense system were investigated on earthworms.The results showed that the activities of superoxide dismutase?SOD?,peroxidase?POD?,catalase?CAT?,and glutathione peroxidase?GSH-Px?were first increased,and then returned to control levels.Furthermore,the CAT and GSH-Px activities finally decreased below control levels at day 28.In general,the glutathione?GSH?content in all treatment groups was markedly inhibited compared to that of control,especially at day 21 and 28.The malondialdehyde?MDA?content in all treatment groups was highly induced compared to that in the control.These results suggest that MC-LR caused major lipid peroxidation and oxidative injury in earthworms.?3?Ultrastructure of earthworm was examined at 14-day exposure of MC-LR.The results showed that MC-LR posed severe damage on earthworm tissues.The damage increased with increasing MC-LR concentration.The epidermis of earthworms was affected by MC-LR and the tissues appeared to be less organized,as well as deleterious effect on the circular and longitudinal muscular layers.Furthermore,the internal damage with a deformation of the dorsal blood vessel and a degradation of the muscular layer protecting the digestive system were observed when exposed to high concentration of MC-LR.These damage potentially resulted in a disorder of the nervous and digestive systems.?4?The effects of MC-LR on earthworm coelomocytes were detected by flow cytometry and neutral red retention time of lysosome.The results showed that coelomocyte apoptosis?including early and late apoptosis?increased markedly in the treatment groups compared with that in the control after 14 d incubation.The coelomocyte apoptosis increased with increasing MC-LR concentration.The NRRT in the treatment groups was significantly inhibited compared with that in the control,especially after exposure for 14 d.NRRT inhibition increased with increasing MC-LR concentration and exposure duration.This inhibition was indicative of a decrease in lysosomal membrane stability and severe damage to earthworm coelomocytes.