Biodegradable plastics (BDPs) have been widely used as
substitutes
for traditional plastics, and their environmental fate is a subject
of intense research interest. Compared with the aerobic degradation
of BDPs, their biodegradability under anaerobic conditions in environmental
engineering systems remains poorly understood. This study aimed to
investigate the degradability of BDPs composed of poly(butylene adipate-co-terephthalate) (PBAT), poly(lactide acid) (PLA), and
their blends, and explore the mechanism underlying their microbial
degradation under conditions of anaerobic digestion (AD). The BDPs
readily depolymerized under thermophilic conditions but were hydrolyzed
at a slow rate under conditions of mesophilic AD. After 45 days of
thermophilic AD, a decrease in the molecular weight and significant
increase in the production of methane and carbon dioxide production
were observed. Network and metagenomics analyses identified AD as
reservoirs of plastic-degrading bacteria that produce multiple plastic-degrading
enzymes. PETase was identified as the most abundant plastic-degrading
enzyme. A potential pathway for the anaerobic biodegradation of BDPs
was proposed herein. The polymers of high molecular weight were subjected
to abiotic hydrolysis to form oligomers and monomers, enabling subsequent
microbial hydrolysis and acetogenesis. Ultimately, complete degradation
was achieved predominantly via the pathway involved in the conversion
of acetic acid to methane. These findings provide novel insight into
the mechanism underlying the anaerobic degradation of BDPs and the
microbial resources crucial for the efficient degradation of BDPs.