Biodegradation of aliphatic homopolyesters and aliphatic-aromatic copolyesters by anaerobic microorganisms

Abstract

The anaerobic degradability of natural and synthetic polyesters is investigated applying microbial consortia (3 sludges, 1 sediment) as well as individual strains isolated for this purpose. In contrast to aerobic conditions, the natural homopolyester polyhydroxybutyrate (PHB) degrades faster than the copolyester poly(hydroxybutyrate-co-hydroxyvalerate) (PHBV). For the synthetic polyester poly(epsilon-caroplacton) (PCL), microbial degradation in the absence of oxygen could be clearly demonstrated; however, the degradation rate is significantly lower than for PHB and PHBV. Other synthetic polyesters such as poly(trimethylene adipate) (SP3/6), poly(tetramethylene adipate) (SP4/6), and aliphatic-aromatic copolyesters from 1,4-butanediol, terephthalic acid, and adipic acid (BTA-copolymers) exhibit only very low anaerobic microbial susceptibility. A copolyester with high amount of terephthalic acid (BTA 40:60) resisted the anaerobic breakdown even under thermophilic conditions and/or when blended with starch. For the synthetic polymers, a number of individual anaerobic strain could be isolated which are able to depolymerize the polymers and selected strains where identified as new species of the genus Clostridium or Propionispora. Their distinguished degradation patterns point to the involvement of different degrading enzymes which are specialized to depolymerize either the natural polyhydroxyalkanoates (e.g., PHB), the synthetic polyester PCL, or other synthetic aliphatic polyesters such as SP3/6. It can be supposed that these enzymes exhibit comparable characteristics as those described to be responsible for aerobic polyester degradation (lipases, cutinases, and PHB-depolymerases).