# Microbial and Enzymatic Biodegradation of Plastic Waste for a Circular Economy
## 5 Conclusions and Future Directions  
Traditional waste management methods are costly and hazardous. However, microbial and enzymatic degradation of plastic presents a viable and sustainable solution to global plastic waste problems. Biodegradation by microorganisms, which produce enzymes like cutinase, lipases, esterases, and PETases, offers a more sustainable approach. PHA and PLA are promising bioplastics which not only degrade under specific conditions but also produce valuable products following degradation (e.g., 3-hydroxyalkanoates and lactic acid). These products can be upcycled (to enhance their value) and recycled into various industrial applications contributing to a circular economy. A circular economy is a closed-loop system where products can be easily extracted, recycled, and reused. However, in the context of soil, it becomes challenging, as once plastics enter the soil, they become difficult to retrieve and reuse.  
Additionally, investigating the interactions between the polymer structure, composition, and enzymatic degradation mechanisms will further aid in developing enzymes which effectively degrade specific plastics. This will provide critical information for further rational enzyme design and improvements in the biocatalytic performance of enzymes in terms of selectivity, activity, stability, and scalability. In addition, the development of specific enzymes capable of digesting one plastic in the presence of another will contribute towards addressing the expensive sorting step. Selective hydrolysis of plastic mixtures would greatly ameliorate the costs associated with plastic sorting. The cost and impact of the production of degradative enzymes also needs to be considered as part of a circular  
Figure 7: Lifecycle of biodegradable PLA and PHB contributing to a circular economy.  
economy. One approach is to enhance enzyme affordability by economies of scale. Another approach is via increasing the activity of enzymes by mutagenesis or solvent engineering.  
Integrating biological processes into the entire lifecycle of plastic products would enhance sustainability and favour environmental remediation. Creating a closed loop from production to end of use for plastics in non-polluting and regenerative ways is a must to achieve this goal of a circular economy. This requires continued technological advancements, the development of appropriate infrastructures, and policy support to integrating biological solutions into global waste management.  
Conceptualisation, G.T.H., M.A.A., R.S., K.N., B.J.R. and G.K.K.; methodology, M.A.A. and R.S.; investigation, M.A.A., R.S. and G.T.H. Writing--original draft preparation, M.A.A.; writing--review and editing, G.T.H., M.A.A., R.S., K.N., B.J.R. and G.K.K.; supervision, G.T.H., B.J.R. and G.K.K.; project administration, G.T.H.; funding acquisition, G.T.H., K.N., B.J.R. and G.K.K. All authors have read and agreed to the published version of the manuscript.  
This research was funded by Science Foundation Ireland, Programme Frontiers for the Future: grant number 21/FFP-A/9898.  
No new data were created or analysed in this study. Data sharing is not applicable to this article.  
The authors note that there are no conflicts of interest.