Toward Microbial Recycling and Upcycling of Plastics: Prospects and Challenges

Abstract

Annually, 400 Mt of plastics are produced of which roughly 40% is discarded within a year. Current plastic waste management approaches focus on applying physical, thermal, and chemical treatments of plastic polymers. However, these methods have severe limitations leading to the loss of valuable materials and resources. Another major drawback is the rapid accumulation of plastics into the environment causing one of the biggest environmental threats of the twenty-first century. Therefore, to complement current plastic management approaches novel routes toward plastic degradation and upcycling need to be developed. Enzymatic degradation and conversion of plastics present a promising approach toward sustainable recycling of plastics and plastics building blocks. However, the quest for novel enzymes that efficiently operate in cost-effective, large-scale plastics degradation poses many challenges. To date, a wide range of experimental set-ups has been reported, in many cases lacking a detailed investigation of microbial species exhibiting plastics degrading properties as well as of their corresponding plastics degrading enzymes. The apparent lack of consistent approaches compromises the necessary discovery of a wide range of novel enzymes. In this review, we discuss prospects and possibilities for efficient enzymatic degradation, recycling, and upcycling of plastics, in correlation with their wide diversity and broad utilization. Current methods for the identification and optimization of plastics degrading enzymes are compared and discussed. We present a framework for a standardized workflow, allowing transparent discovery and optimization of novel enzymes for efficient and sustainable plastics degradation in the future.

Keywords: biodegradation; biorecycling; comprehensive workflow; enzymes; plastics.

FIGURE 1

Plastics and their specific chemical bonds.

FIGURE 2

Comprehensive workflow for the identification of plastic degrading enzymes. New plastic degrading enzymes can be found by (A) environmental screening of interesting strains (green arrows) and (B) the computational methods (magenta arrows). In the case of computational methods, the enzymes need to be expressed in a suitable expression system (2) before high throughput enzyme assays can be performed. Via high throughput enzymatic assays, active strains/enzymes can be pinpointed. For the strains isolated via environmental screening, the specific enzymes need to be identified (2). The enzymes can then be further characterized to identify potential targets for enzyme optimization approaches. The protein depicted in this figure is based on PDB ID 6ANE (Fecker et al., 2018).