Biodegradation of Graphene Oxide by Insects (Tenebrio molitor Larvae): Role of the Gut Microbiome and Enzymes

Abstract

Biodegradation of graphene materials is critical for understanding their environmental process and fate. Thus, biodegradation and mineralization of graphene oxide (GO) by an insect (yellow mealworms, Tenebrio molitor larvae) were investigated. Twenty mealworms could eat up a piece of GO film (1.5 × 1.5 cm) in 15 days. The ingested GO film underwent degradation, and the residual GO sheets were observed in the frass. Raman imaging confirmed that the residual GO (I D /I G , 1.16) was more defective than the pristine GO film (I D /I G , 0.95). 14 C analysis showed that GO sheets were partially mineralized into CO 2 (0.26%) and assimilated into biomass compositions (e.g., lipid and protein) (0.36%). Gut microbes and extracellular enzymes in yellow mealworms played crucial roles in GO degradation, and the predominant gut microbes for GO biodegradation were identified as Enterobacteriaceae bacteria (e.g., Escherichia−Shigella sp.). Two biodegradation products belonging to hydroxylated or carboxylated aromatic compounds were formed with the assistance of electrons and hydroxyl radicals in mealworm guts. These findings are useful for better understanding the environmental and biological fate of graphene materials.

Reactive Oxygen Species Triggered Oxidative Degradation of Polystyrene in the Gut of Superworms (Zophobas atratus Larvae)

“…However, we were unable to exclude an important role of other functional microbes, extracellular enzymes, and enhancing factors as reported in previous works. 7,11,21,30 In fact, other functional gut microbes reported previously 21,30 could also be found abundantly in the PS-feeding groups of this work, such as Enterobacteriaceae, Lactococcus, and Spiroplasma. We therefore hypothesize that the rapid and effective degradation of PS was achieved by the synergistic effect of ROS and complex functional microbes and enzymes as well as other enhancing factors in the gut of larvae.…”

“…Accordingly, the transformation sequence of the residual PS chemical structure within the gut of superworms was as follows: C–H > C–O > CC > C–OH > CO. It was notable that the sequential order of the PS chemical structure transformation in the superworm gut was similar to the key process of PS aging or oxidative depolymerization in various environments mediated by ROS, ,, which is commonly present and participates in the immunity and metabolism processes of biological systems. − …”

“…In addition, it was speculated that the ROS could also be generated as a response to the toxicity of ingested small PS fragments (such as nano/micro-PS particles) as well as their degradation intermediates and additives 39 or induced by some extracellular enzymes of the gut as suggested by a previous study. 21 More research will be needed to get a comprehensive understanding of the generation of ROS in the gut of plastic-eating insect larvae.…”

“…Previous works suggested that photo- or thermo-oxidation pretreatment was able to largely accelerate biodegradation of non-hydrolyzable or vinyl plastics by microorganisms by decreasing the hydrophobicity and introducing CO or −OH functionalities, which make it more compatible to microorganisms and prone to enzyme-mediated degradation. , Recently, several studies also have demonstrated that the continuous production of reactive oxygen species (ROS; e.g., ·OH, H 2 O 2 , and ·O 2 – ) driven by microbes through aerobic and anaerobic respiration in various natural systems was able to cause the aging or oxidative degradation of plastics with depolymerization or cleavage of the long-chain structure and increase of oxygen-containing functional groups. − Strikingly, current studies also demonstrated that the plastic underwent strong oxidative decomposition in the gut environment with a decrease in hydrophobicity and molecular weight as well as the formation of carbonyl groups, − which was similar to the oxidation reaction in photo-based, thermal-based, and ROS pretreatments. Liu et al suggested that not easily decomposing carbon materials (e.g., graphene oxide) were able to be biodegraded into hydroxylated or carboxylated aromatic compounds with the assistance of hydroxyl radicals (·OH) in mealworm guts . Considering that ROS are commonly generated in biological systems and are participating in immunity and metabolic processes, − it is reasonable to assume that high levels of ROS were able to be generated in the gut of insect larvae after stimulation by plastic and microbial symbionts, which would significantly accelerate the oxidative decomposition of plastic, and facilitate further biodegradation by microbial symbionts in the gut environment.…”

“…Liu et al suggested that not easily decomposing carbon materials (e.g., graphene oxide) were able to be biodegraded into hydroxylated or carboxylated aromatic compounds with the assistance of hydroxyl radicals (·OH) in mealworm guts . Considering that ROS are commonly generated in biological systems and are participating in immunity and metabolic processes, − it is reasonable to assume that high levels of ROS were able to be generated in the gut of insect larvae after stimulation by plastic and microbial symbionts, which would significantly accelerate the oxidative decomposition of plastic, and facilitate further biodegradation by microbial symbionts in the gut environment. However, no study has been conducted to demonstrate the presence of ROS in the gut environment, as well as its role in accelerating plastic biodegradation.…”