Gut Microbiome Associating with Carbon and Nitrogen Metabolism during Biodegradation of Polyethene in Tenebrio larvae with Crop Residues as Co-Diets

Tenebrio molitor and Tenebrio obscurus (Coleoptera: Tenebrionidae) larvae are two commercial insects that eat plant and crop residues as diets and also biodegrade synthetic plastics polyethylene (PE). We examined biodegradation of low-density PE (LDPE) foam (Mn = 28.9 kDa and Mw = 342.0 kDa) with and without respective co-diets, i.e., wheat brain (WB) or corn flour (CF), corn straw (CS), and rice straw (RS) at 4:1 (w/w), and their gut microbiome and genetic metabolic functional groups at 27.0 ± 0.5 °C after 28 days of incubation. The presence of co-diets enhanced LDPE consumption in both larvae and broad-depolymerized the ingested LDPE. The diet type shaped gut microbial diversity, potential pathways, and metabolic functions. The sequence of effectiveness of co-diets was WB or CF > CS > RS for larval development and LDPE degradation. Co-occurrence networks indicated that the larvae co-fed with LDPE displayed more complex correlations of gut microbiome than the larvae fed with single diets. The primary diet of WB or CF and crop residues CS and RS provided energy and nitrogen source to significantly enhance LDPE biodegradation with synergistic activities of the gut microbiota. For the larvae fed LDPE and LDPE plus co-diets, nitrogen fixation function was stimulated compared to normal diets and associated with LDPE biodegradation.