Unravelling the potential of thermophiles as chassis organisms for bioplastic upcycling

Bio-based plastics are necessary to reduce the carbon footprint of everyday materials, but prominent polymers like polylactide (PLA) currently pose a challenge in traditional recycling strategies. In order to enable efficient and economically viable biorecycling of bioplastics, we envisage a consolidated bioprocess in which the depolymerizing enzymes are produced and secreted by the microbial biocatalyst that simultaneously converts the released monomers into value-added products. Thermostable enzymes and thermophilic microbial hosts are required in order to operate closer to the glass transition temperature of the polymer and thus make it more accessible to the depolymerizing enzymes. In order to identify novel thermostable polylactide-depolymerizing enzymes, we pursued a database-assisted screening of roughly 20 000 proteins within our thermophilic strain library and additionally enriched natural producers that grow on PLA at elevated temperatures. The most promising enzymes are being characterized in vitro and compared to benchmark enzymes with regard to PLA degradation activity. The best performing enzymes will be overexpressed in Parageobacillus thermoglucosidasius while characterizing the lactic acid monomer consumption kinetics in vivo. Since the availability of genome editing methods for Geobacillus spp. is limited, such methods are being developed especially with regard to CRISPR-Cas systems. The combination of the proof of principle consolidated bioprocess and CRISPR-Cas genome editing methods will pave the way for engineering genetically stable chassis organisms of the genus Geobacillus, capable of PLA degradation and funneling the resulting carbon flux towards value-added compounds. This concept will be finally expanded towards other bioplastics such as PBAT and PBSA.