Plastics have become integral to our daily lives, offering convenience and accessibility across various sectors, including packaging and clothing. The stable properties of the large plastic polymers result in a lack of digestion by nature- it is unlikely you would catch a group of friends feasting upon some plastic cups- because our bodies are not accustomed to plastic assimilation. Subsequently, the stable quality of plastics has led to it topping the charts regarding world pollution. One word which describes plastics is probably familiar to most people- ‘Non-Biodegradable’. Thankfully, this statement would change with recent expansive studies of different organisms and biological molecules- plastics have become biodegradable.
G.mellonella
One intriguing discovery involves the Galleria mellonella larva, which has shown the ability to oxidize and depolymerize polyethylene (C2H4)n using its saliva. The remarkable nature of this discovery is due to how independent this process is- no prior abiotic treatment is involved, and the saliva can break down this polymer without the requirement of additional energy (such as temperature) in neutral, aqueous conditions. The process only takes a few hours. Without the aid of this Lepidopteran saliva, the natural decomposition of polyethylene could take up to nearly five centuries.
On a deeper dive into this unique process, researchers have found degrading enzymes (PEases) that allow for polyethylene breakdown capacity. Four macromolecule proteins from the hexameric superfamily were identified, which all aid in the oxidation of polyethylene, in varying proportions: Demetra, Cibeles, Ceres, and Cora.
The experiment compared a control group and the saliva of a different lepidopteran larva- to which only the G.mellonella buccal secretions were adequate in depolymerization. A possible explanation for this phenomenon lies in the diet of G.mellonella, beeswax- which shares a structure similar to polyethylene. This research is still in progress, with attempts to identify the differing catalytic qualities of the proteins involved and other aspects. Their success in identification is hugely anticipated to serve as a possible solution for polyethylene pollution.
A.diaperinus
Another species, Alphitobius diaperinus, a Coleopteran beetle, possesses the necessary mouthparts and gut microbiota to degrade polystyrene (C8H8)n. Commonly known as the lesser mealworm, the beetle possesses similar abilities in terms of polystyrene breakdown as the mealworm and superworm. Results from Yang et al. 2015 and Yang et al. 2020 showed that up to 0.12mg/d and 0.58mg/d of polystyrene could be depolymerized by the mealworm and superworm, respectively. Additionally, it is discovered that the mealworm and superworm have capacities to live off of a sole diet of polystyrene- depolymerizing it to low-molecular-weight products.
The underlying mechanism of polystyrene degradation relies upon two major bacteria within the insect’s gut colony- the Pseudomonadaceae and the Enterobacteriaceae. However, environmental conditions promoting the process are yet unclear.
PET46
Polyethylene terephthalate (PET), (C10H8O4)n, makes some of the most abundant plastic items (such as plastic bottles) and, consequently, constitutes a large proportion of all plastic pollution. In addition to the 80 known PETases (PET degrading enzymes), the deep sea enzyme PET46 is a discovery.
PET46 exhibits the ability to depolymerize both long-chain and short-chain PETs, leading to capacities for continuous degradation. However, unlike the action of the G.mellonella, abiotic treatment is required. The enzyme works more efficiently at 70 degrees Celsius.
PETs have a similar structure to lignin found in nature. Ferulic acid esterase decomposes lignin and possesses a similar construct to PET46. Therefore, lignin decomposers may be capable of acting on both polymers.
A.terreus
The fungus A.terreus, known for its lignin decomposition abilities, has shown promise in the depolymerization of polypropylene (C3H6)n. Polypropylene has higher heat resistance than PET and is a significant constituent of plastic pollution.
However, before breakdown by the fungi, the polypropylene sample must first be treated with UV light or heat to ensure the effectiveness of the degradation by A. terreus.
Plastic pollution remains one of the most significant issues affecting our environment. While these groundbreaking discoveries offer potential solutions, they are still in the developmental stage. Whether involving abiotic treatments or specific conditions, these studies indicate significant progress toward finding a long-lasting solution. With continued research and the refinement of these techniques, we can hope for increased biodegradability of plastics and the eventual mass degradation of plastic waste.