Freeze-thaw aging increases the toxicity of microplastics to earthworms and enriches pollutant-degrading microbial genera.

Freeze-thaw (FT) aging can change the physicochemical characteristics of microplastics (MPs). The toxic impacts of FT-aged-MPs to soil invertebrates are poorly understood. Here the toxic mechanisms of FT-aged-MPs were investigated in earthworms after 28 d exposure. Results showed that FT 50 µm PE-MPs significantly increased reactive oxygen species (ROS) by 5.78-9.04 % compared to pristine 50 µm PE-MPs (41.80-45.05 ng/mgprot), whereas FT 500 µm PE-MPs reduced ROS by 7.52-7.87 % compared to pristine 500 µm PE-MPs (51.44-54.46 ng/mgprot). FT-PP-MPs significantly increased ROS and malondialdehyde (MDA) content in earthworms by 14.82-44.06 % and 46.75-110.21 %, respectively, compared to pristine PP-MPs (40.56-44.66 ng/mgprot, 0.41-2.53 nmol/mgprot). FT-aged PE- and PP-MPs caused more severe tissue damage to earthworms. FT-aged PE-MPs increased the alpha diversity of the gut flora of earthworms compared to pristine MPs. Earthworm guts exposed to FT-aged-MPs were enriched with differential microbial genera of contaminant degradation capacity. FT-PE-MPs affected membrane translocation by up-regulating lipids and lipid-like molecules, whereas FT-PP-MPs changed xenobiotic biodegradation and metabolism by down-regulating organoheterocyclic compounds compared to the pristine PE- and PP-MPs. This study concludes that FT-aged MPs cause greater toxicity to earthworms compared to pristine MPs.