Long-chain polyester degradaing microbial communities from industrial composting samples
Biodegradable plastics are an important tool in the attempt to achieve a circular polymer economy. Over the years, various biodegradable synthetic polymers were developed, which at the regulatory level must pass standardized biodegradation tests (such as  EN13432) under simulated industrial composting conditions (ISO 14855-1). Such thermophilic composting can result in more rapid degradation of synthetic polymers than under mesophilic conditions, but in most cases, not much is known about the microorganisms responsible for the degradation. Recently, long-chain aliphatic polyesters (LCPE), which can be sourced from plant oils and exhibit polyethylene-like material properties while being amenable to chemical recycling, were shown to biodegrade under industrial composting conditions in tests adhering to ISO 14855-1. In this work, we analyzed the microbial community composition in the different compost treatments based on amplicon-sequencing of taxonomic marker genes (bacterial 16S and fungal ITS2 rDNA). This revealed a significantly increased abundance of thermophilic bacterial genus Thermomonospora in all compost reactors degrading LCPE, with highest similarity to T. curvata DSM43183, which is known to encode for a depolymerase enzyme (Tcur1278) that is capable of hydrolyzing polycaprolactone (PCL) and polyethylene terephthalate (PET). While reference strain T. curvata DSM43183 did not grow with LCPE as sole carbon and energy source in the laboratory under the conditions we tested, the heterologously expressed PCL/PET-depolymerase Tcur1278 of T. curvata was confirmed to hydrolyze also LCPE. Hence, we revealed a first bacterial taxon potentially degrading LCPE bioplastics under industrial composting conditions and the first known bacterial depolymerase that can hydrolyze LCPE into monomers.