Anaerobic biodegradation of natural and synthetic polyesters

For an environmentally save application of degradable plastics, even the anaerobic biodegradability (e.g. in anaerobic biowaste treatment plants or landfills) must be guaranteed. This recent study revealed important aspects of the basic mechanism of anaerobic polyester break down as well as significant differences to aerobic mechanisms. The biodegradation of the natural aliphatic polyesters polyhydroxybutyrate (PHB) and polyhydroxybutyrate-co-hydroxyvalerate (PHBV) and synthetic aliphatic polyesters such as poly(e-caprolactone) (PCL), SP 3/6 and SP 4/6 (consisting of 1,3-propanediol/adibic acid and 1,4-butanediole/adipic acid, respectively) was ascertained. However, in contrast to aerobic systems, the homopolymer PHB showed higher degradation and mineralization rates than its copolymer with PHV (PHBV). A commercialized synthetic aliphatic?aromatic copolyester which is aerobically completely biodegradable resisted the anaerobic breakdown. The isolation of individual characterized polyester degrading anaerobes (new species of the genus Clostridium or Propionispora) rendered successively improved degradation experiments under controlled laboratory conditions possible. The degradation time was reduced compared to the degradation in unidentified complex environments. The involved depolymerizing enzymes seem to differ and are specialized to depolymerize either the natural polyalkanoates (e.g. PHB), the synthetic polyester PCL or synthetic aliphatic polyesters such as SP 3/6. The isolation and investigation of the anaerobic extracellular PHB-depolymerizing enzyme revealed interesting characteristics and significant differences to known aerobic PHB-depolymerases.