Molecular-Weight-Dependent Degradation of Plastics: Deciphering Host–Microbiome Synergy Biodegradation of High-Purity Polypropylene Microplastics by Mealworms

Abstract

The biodegradation of polypropylene (PP), a highly persistent nonhydrolyzable polymer, by Tenebrio molitor has been confirmed using commercial PP microplastics (MPs) (M n 26.59 and M w 187.12 kDa). This confirmation was based on the reduction of the PP mass, change in molecular weight (MW), and a positive Δδ 13 C in the residual PP. A MW-dependent biodegradation mechanism was investigated using five high-purity PP MPs, classified into low (0.83 and 6.20 kDa), medium (50.40 and 108.0 kDa), and high (575.0 kDa) MW categories to access the impact of MW on the depolymerization pattern and associated gene expression of gut bacteria and the larval host. The larvae can depolymerize/biodegrade PP polymers with high MW although the consumption rate and weight losses increased, and survival rates declined with increasing PP MW. This pattern is similar to observations with polystyrene (PS) and polyethylene (PE), i.e., both M n and M w decreased after being fed low MW PP, while M n and/or M w increased after high MW PP was fed. The gut microbiota exhibited specific bacteria associations, such as Kluyvera sp. and Pediococcus sp. for high MW PP degradation, Acinetobacter sp. for medium MW PP, and Bacillus sp. alongside three other bacteria for low MW PP metabolism. In the host transcriptome, digestive enzymes and plastic degradation-related bacterial enzymes were up-regulated after feeding on PP depending on different MWs. The T. molitor host exhibited both defensive function and degradation capability during the biodegradation of plastics, with high MW PP showing a relatively negative impact on the larvae.

Microplastics Biofragmentation and Degradation Kinetics in the Plastivore Insect Tenebrio molitor

“…molitor larvae modified the gene expression of both their gut bacteria and the larval host to establish a system favorable for plastic degradation when fed PP diets . The larval gut could function as an efficient bioreactor, initiating innate digestive enzymes, defensive mechanisms, and carbohydrate transport to synergistically degrade plastic. , The respective role of gut microorganisms and larval host in modulating the internal environment in response to in vivo MPs biodegradation warrants further in-depth research. Notably, the highest acidification level was observed in the PVC-fed group (6533 ± 189.4 and 7613 ± 266.2), which was significantly ( p < 0.005) higher than those observed in PE-fed and PLA-fed larvae (Figure ).…”

“…Our results indicate that concentrations and distribution of digestive functional enzymes underwent significant alterations in response to the interactions of MPs within this bioreactor in the plastivore insect. Additionally, previous research suggested that lipases, Chitinase, monooxygenase, dioxygenase, and other enzymes may be involved in the degradation and mineralization of different polymers. ,,, Future research should focus on developing highly sensitive biocompatible probes to directly visualize the concentrations and distribution of these enzymes during MP biodegradation processes.…”