Mesophilic compostability of polylactic acid and the associated microbiome as revealed by metagenomics

Polylactic acid (PLA), the most popular bioplastic, has high sustainability potential as it is bio-sourced and also harbors biodegradability. A form of its biodegradability is via composting, and it was previously established that thermophilic temperatures are needed for PLA breakdown in composts. Here we report the development of composts that have overcome the temperature requirement needed for PLA composting. Our mesophilic composts exhibited clear PLA biodegradability, and this is due to specific biological activity enriched in our material. To investigate the nature of this mesophilic activity, we conducted metagenomics analysis to reveal the microbial composition and enzyme-coding potential associated with PLA biodegradation. These efforts revealed multiple enzyme subtypes with strong enrichment on PLA surfaces in our trained composts, and the top candidate was a type of hydro-lyase, an enzyme that can cleave carbon-carbon and carbon-oxygen bonds, both present in the chemical structure of PLA, in the absence of water. Hydro-lyases represent a novel class of enzymes that could facilitate PLA degradation, and our results point to the model that the combinatorial action of multiple types of enzymes is what drives PLA biodegradation and how the temperature barrier for PLA composting is overcome.