Repeated Digestion by Earthworms on Tetracycline Degrading and Biological Responses in Tetracycline-Contaminated Sewage Sludge

The simulated culture method was conducted in the current study to investigate the effect of repeated ingestion by earthworms on tetracycline (TC) degradation and the characteristics of the biological responses at different TC concentrations in sewage sludge. The repeated ingestion by earthworms reduced TC residues. The TC degradation rate in sludge with low TC concentration (2.75 mg/kg) was superior to that with high TC concentration (125 mg/kg), and the degradation rate reached 64.91% after digestion by earthworms three times. The activity of antioxidant defense enzymes in earthworms increased after repeated ingestion under TC stress. The soluble protein (SP) content, and the activities of CAT, SOD, and POD in earthworms first increased and then decreased at low concentrations, while the SP content increased and the activities of the three defense enzymes first increased and then decreased at high concentrations after three cycles of digestion. Repeated digestion reduced the species richness of the bacterial communities in the intestines of the earthworms and vermicompost. Furthermore, the abundance of Intrasporangiaceae in the earthworm intestines remarkably increased, while that in the vermicompost decreased. Repeated digestion could also stimulate the growth of Xanthomonadaceae , Chitinophagaceae , Flavobacteriaceae , and Sphingobacteriaceae , which was positively correlated with the degradation of TC in vermicompost. In conclusion, earthworms effectively improved the TC degradation rate and reduced the adverse effects of TC stress by changing the activity of their antioxidant defense enzyme systems. Finally, the abundance of TC degradation-related bacteria in the earthworm intestines and vermicompost was enhanced by repeated digestion.