Toxicological Effects Of Polylactic Acid Microplastics(PLA-MPs) On Earthworms Of Two Living Habits

Microplastics(MPs)continue to be released into the environment as emerging pollutants and have been extensively studied for their environmental persistence and accumulation.The soil environment has been shown to be an important sink for the presence of microplastics,and ecotoxicity studies on soil organisms are currently focused on conventional microplastics,with little attention paid to the harmful effects of biodegradable microplastics in soil.In this study,polylactic acid microplastics(PLA-MPs)of different particle sizes(100,500 and 1000 mesh)and concentrations(0.5%,2%and 5%)were set up over 28 days to study the ecotoxicological effects on epigeic earthworms(Eisenia fetida)and endogeic earthworms(Pheretima guillelmi).The results of the study showed that:(1)The level of hunger and the type of earthworm affect their feeding choice of PLA-MPs.When satiated,PLA-MPs led to varying degrees of reduction in their habitat function,with Eisenia fetida showing marked avoidance behavior,more pronounced at high concentrations.In contrast,Pheretima guillelmi showed some feeding preference,especially at smaller particle sizes and higher concentrations.However,neither earthworm showed significant avoidance behavior under starvation.(2)Pheretima guillelmi was more resistant to the negative effects of PLA-MPs than Eisenia fetida.At the end of the 28-day experiment,there was a significant difference in mortality between the two earthworms.The mortality rate of Eisenia fetida was significantly higher than that of the control in all treatments,showing a trend of 100 mesh treatment>500 mesh treatment>1000 mesh treatment.It is noteworthy that the mortality of Pheretima guillelmi was not significantly different from that of the control.Under the stress of PLA-MPs,both earthworms showed significant inhibition of SOD,CAT and GST enzyme activities for the most part.The antioxidant enzyme activities of Pheretima guillelmi showed a tendency to gradually approach the control level with decreasing particle size compared to Eisenia fetida.Although significant physiological changes were also produced,they were not sufficient to cause lethality.(3)PLA-MPs undergo different degrees of degradation in the soil,which increases the soil organic matter content and promotes earthworm biomass and reproductive behavior.In the experiment of Eisenia fetida.the smaller the particle size and higher the concentration of the microplastic the more pronounced the promotion of soil carbon content was in the treatment.Also,earthworm biomass,number and weight of egg cocoons were positively correlated with soil C content.On the contrary,in the experiment of Pheretima guillelmi,as the concentration of PLA-MPs treatment increased.the soil C content showed a trend of 100 mesh treatment>500 mesh treatment>1000 mesh treatment.In contrast,the biomass of Pheretima guillelmi showed a trend of 1000 mesh treatment>500 mesh treatment>100 mesh treatment.(4)The addition of PLA-MPs played a significant role in promoting the C-related metabolic pathways of both earthworms,with different effects on the gut microbial community.The results showed that PLA-MPs of different particle sizes and concentrations did not significantly affect the diversity,richness and structure of the gut microbial community of Eisenia fetida,and played a role in promoting the richness and structure of the gut microbes of Pheretima guillelmi to some extent.Furthermore,some of the C-related metabolic pathways of Eisenia fetida were significantly and positively correlated with soil C content.biomass,and egg callus number and weight following degradation of PLA-MPs.In contrast,soil C content was negatively correlated with C-related metabolic pathways in Pheretima guillelmi.In summary,the particle size and concentration of PLA-MPs had a significant effect on the biological response of both earthworms.and unlike Eisenia fetida which were significantly affected by stress,PLA-MPs showed some environmental friendliness to Pheretima guillelmi.Furthermore,exposure of PLA-MPs to soil increased the organic carbon content of the soil.significantly affected C-related metabolic pathways in the gut and promoted increased earthworm biomass and reproductive behavior.