Biodegradation of polypropylene by yellow mealworms (Tenebrio molitor) and superworms (Zophobas atratus) via gut-microbe-dependent depolymerization

Polypropylene (PP), a fossil-based polyolefin plastics widely used worldwide, is non-hydrolyzable and resistant to biodegradation as a major source of plastic pollutants in environment. This study focused on feasibility of PP biodegradation in the larvae of two species of darkling beetles (Coleoptera: Tenebrionidae) i.e., yellow mealworms (Tenebrio molitor) and superworms (Zophobas atratus) using PP foam with number-, weight-, and size-average molecular weights (Mn, Mw, and Mz) of 109.8, 356.2, and 765.0 kDa, respectively. The tests were conducted in duplicates with respective larvae (300 T. molitor and 200 Z. atratus each incubator) at 25 °C and 65% humidity for over a 35-day period. The larvae of T. molitor and Z. atratus fed with PP foam as sole diet consumed PP at 1.0 ± 0.4 and 3.1 ± 0.4 mg 100 larvae−1 days−1, respectively; when fed the PP foam plus wheat bran, the consumption rates were enhanced by 68.11% and 39.70%, respectively. Gel permeation chromatography analyses of the frass of T. molitor and Z. atratus larvae fed PP only indicated that Mw was decreased by 20.4 ± 0.8% and 9.0 ± 0.4%; Mn was increased by 12.1 ± 0.4% and 61.5 ± 2.5%; Mz was decreased by 33.8 ± 1.5% and 32.0 ± 1.1%, indicating limited extent depolymerization. Oxidation and biodegradation of PP was confirmed through analysis of the residual PP in frass. Depression of gut microbes with the antibiotic gentamicin inhibited PP depolymerization in both T. molitor and Z. atratus larvae. High throughput 16S rRNA sequencing revealed that Citrobacter sp. and Enterobacter sp. were associated with PP diets in the gut microbiome of Z. atratus larvae while Kluyvera was predominant in the T. molitor larvae. The results indicated that PP can be biodegraded in both T. molitor and Z. atratus larvae via gut microbe-dependent depolymerization with diversified microbiomes.