The beetle's gut microbiome and its power to ecologically decompose polystyrene waste

The intensive use of plastic, combined with ineffective waste management, has exacerbated the issue of plastic pollution. This phenomenon represents one of the most critical environmental emergencies of the past century, with well-documented negative effects on biodiversity and ecosystems. Recently, the scientific community has increasingly focused on biodegradation as a potential strategy for plastic disposal, particularly investigating the role of plastivorous insects and their gut microbiota. In this context, Alphitobius diaperinus (Coleoptera: Tenebrionidae) has emerged as a promising candidate for polystyrene (PS) degradation. The first chapter of this study explores three key aspects of PS biodegradation by this insect. First, rearing conditions were optimized to enhance larval degradation efficiency. Second, chemical analyses were conducted to characterize the structural modifications of PS after digestion and to confirm the presence of degradation byproducts, such as α-methylstyrene and cumyl alcohol, within the larvae's gut. These analyses were performed using two complementary techniques: micro-Fourier Transform Infrared Spectroscopy (micro-FTIR) and Gas Chromatography-Mass Spectrometry (GC-MS). Finally, gut microbiota composition was assessed through metabarcoding analysis using full-length PacBio HiFi sequencing of the 16S rDNA gene. Significant differences were observed between the microbiota of PS-fed larvae and that of the control group under different experimental conditions, highlighting the potential involvement of specific bacterial taxa in PS metabolism. The second chapter focuses on a bacterial strain isolated from the gut microbiota of PS-fed larvae through enrichment cultures in a medium where PS was the sole carbon source. This bacterium, identified as Stenotrophomonas indicatrix strain DAI2m/c, underwent whole-genome sequencing to identify enzymes-encoding genes involved in intracellular metabolic pathways responsible for styrene degradation. The study confirmed that the bacterial genome encodes all the enzymes required for one of the two recognized PS degradation pathways. The overall findings of this study provide new insights into the role of plastivorous insects, particularly their gut microbiota, in polystyrene degradation processes. Moreover, they open new perspectives for the development of innovative biotechnological solutions for sustainable plastic waste management.