Plastic biodegradation: Frontline microbes and their enzymes

Plastic polymers with different properties have been developed in the last 150 years to replace materials such as wood, glass and metals across various applications. Nevertheless, the distinct properties which make plastic desirable for our daily use also threaten our planet's sustainability. Plastics are resilient, non-reactive and most importantly, non-biodegradable. Hence, there has been an exponential increase in plastic waste generation, which has since been recognised as a global environmental threat. Plastic wastes have adversely affected life on earth, primarily through their undesirable accumulation in landfills, leaching into the soil, increased greenhouse gas emission, etc. Even more damaging is their impact on the aquatic ecosystems as they cause entanglement, ingestion and intestinal blockage in aquatic animals. Furthermore, plastics, especially in the microplastic form, have also been found to interfere with chemical interaction between marine organisms, to cause intrinsic toxicity by leaching, and by absorbing persistent organic contaminants as well as pathogens. The current methods for eliminating these wastes (incineration, landfilling, and recycling) come at massive costs, are unsustainable, and put more burden on our environment. Thus, recent focus has been placed more on the potential of biological systems to degrade synthetic plastics. In this regard, some insects, bacteria and fungi have been shown to ingest these polymers and convert them into environmentally friendly carbon compounds. Hence, in the light of recent literature, this review emphasises the multifaceted roles played by microorganisms in this process. The current understanding of the roles played by actinomycetes, algae, bacteria, fungi and their enzymes in enhancing the degradation of synthetic plastics are reviewed, with special focus on their modes of action and probable enzymatic mechanisms. Besides, key areas for further exploration, such as the manipulation of microorganisms through molecular cloning, modification of enzymatic characteristics and metabolic pathway design, are also highlighted.