Polystyrene (PS), which accounts for a significant fraction of plastic wastes, is difficult to biodegrade due to its unique molecular structure. Therefore, biodegradation and chemical modification of PS is limited. In this study, we report PS biodegradation by the larvae of another darkling beetle, Plesiophthalmus davidis (Coleoptera: Tenebrionidae). In 14 days, P. davidis ingested 34.27 ± 4.04 mg of Styrofoam (PS foam) per larva, and survived by only feeding on Styrofoam. Fourier-transform infrared spectroscopy confirmed that the ingested Styrofoam was oxidized. Gel permeation chromatography analysis indicated the decrease in average molecular weight of the residual PS in the frass compared with the feed Styrofoam. When the extracted gut flora was cultured for 20 days with PS films, biofilm and cavities were observed by scanning electron microscopy and atomic force microscopy. X-ray photoelectron spectroscopy (XPS) studies revealed C-O bonding was introduced to the biodegraded PS film. Serratia sp. WSW (KCTC 82146), which was isolated from the gut flora, also formed a biofilm and cavities on the PS film in 20 days, but its degradation was less prominent than the gut flora. XPS confirmed that C-O and C=O bonds were introduced to the biodegraded PS film by Serratia sp. WSW. Microbial community analysis revealed that Serratia was in the gut flora in significant amounts and increased in 6-folds when the larvae were Styrofoam-fed for 2 weeks. This suggests that P. davidis larvae and its gut bacteria could be used to chemically modify and rapidly degrade PS.
Importance PS is widely produced in the modern world, but it is robust against biodegradation. A few studies reported the biodegradation of PS, but most of them merely observed its weight loss; fewer were able to find its chemical modifications, a rather direct evidence to the biodegradation, by using limited organisms. Therefore, it is required to find an effective way to decompose PS using various kinds of organisms. Herein, we discovered a new PS-degrading insect species and bacterial strain, and we found the genus that includes the PS-degrading bacterial strain occurs in significant amounts in the larval gut flora, which increases as the larvae were Styrofoam-fed. Our research offers a wider selection of PS-degrading insects and the possibility of using a certain mixture of bacteria that resemble the gut flora of a PS-degrading insect to biodegrade PS, and thus could contribute to solving the global plastic crisis.