Effects of microplastic ingestion on hydrogen production and microbiomes in the gut of the terrestrial isopod Porcellio scaber

Microplastic (MP) pollution is an environmental burden. MP enters food webs via ingestion by macrofauna, including isopods ( Porcellio scaber ) in terrestrial ecosystems. However, MP-effects on the host and its gut microbiome are largely unknown. We tested the hypothesis that biodegradable (polylactic acid, PLA) and non-biodegradable (polyethylene terephthalate, PET; polystyrene, PS) MP have contrasting effects on P. scaber mediated by changes of the associated gut microbiome. Although the isopods avoided food containing PS, isopod fitness after eight-week MP-exposure was unaffected. Qualitative and quantitative 16S rRNA gene and 16S rRNA analyses of gut microbiomes indicated general MP effects, MP-type specific indicator taxa, and stimulation by PLA compared to MP-free controls. Isopods emitted hydrogen, and its production increased and decreased after PLA-food and PET- or PS-food ingestion, respectively, relative to controls as indicated by microsensor measurements. Gut pH was unaffected by MP. We identified the gut of P. scaber as significant mobile source of reductant for soil microbiomes likely due to Enterobacteriaceae related fermentation activities that were stimulated by lactate generated during PLA-degradation. The findings suggest negative effects of PET and PS on gut fermentation, modulation of isopod hydrogen emissions by MP pollution, and the potential of MP to affect terrestrial food webs. ### Competing Interest Statement The authors have declared no competing interest.