Biodegradation of polyesters by filamentous fungi.

Biopolymers have gained increasing interest as substitutes of “traditional”, non-biodegradable plastics. Polyesters are the favorites, thanks to the economic process technology and easy production of the starting monomers. Their behavior in the degradation process depends on their structure, their hydrophobic character, which limits the enzyme activity together with other factors like low surface area, high molecular weight and crystallinity. Several microorganisms are already known to be promising biopolymer degraders, fungi included, but rarely at low temperatures and under uncontrolled conditions, as in the case of littering. This study investigated the biodegradation by filamentous fungi, preserved at Mycotheca Universitatis Taurinensis of the University of Torino, of biodegradable aliphatic and aliphatic-aromatic polyesters, such as polybutylene succinate (PBS). The tested polymers were subjected to alkaline hydrolytic degradation concurrently to a primary solid screening, carried out to select fungal strains capable of growing in the presence of the polymers as sole carbon source, in form of films of 40 μm thickness at 15 °C and 24 °C. A good percentage of the tested fungal strains were able to grow on the different polyesters at 24°C, a lower ability was observed at 15°C. The materials have similar molecular weight, but exhibit a very different hydrolytic behavior which corresponds to the trend observed in the solid screening, due to their difference in hydrophobicity, crystalline grade and chemical structure. These aspects mostly influence the biodeterioration, in particular the biofilm formation, and the bioassimilation steps. For these reasons, the degradation and bioassimilation ability of the best performing fungus in the preliminary solid screening were investigated on polymers and monomers in two different aqueous media at 24 °C. During the degradation in both liquid media, a significant growth of the fungal biomass has been observed. From the HPLC-RI analyses of the media, the depolymerization ability of the microorganism was confirmed. The degradation products in the chromatograms completely matched the monomer components of the tested polyesters, no unknown peaks were detected. However, some monomers were detected only in trace and their concentration fell below the detection limit after 20 days of degradation. The trials with monomers at different concentrations highlighted a concentration effect and a different assimilation rate in enriched solution.