Chemistry and Biochemistry: Research Progress Vol. 10

Background: Poly(3-hydroxybutyrate) (PHB) microbial degradation proceeds primarily through degradative enzymes secreted by bacteria and fungi, but there are differences in degradation mechanisms due to the specificity of the secreted enzymes. In bacterial PHB degradation, extracellular PHB depolymerases are secreted that specifically hydrolyse the ester bonds on the PHB surface, generating low-molecular-weight compounds such as 3-hydroxybutyric acid.

Aim: The study aims to investigate the effects of incineration ash-containing landfill soil on PHB biodegradation and to isolate and identify PHB-degrading bacteria under highly alkaline conditions.

Study Design: Comparative experimental study using incineration ash-containing landfill soil and university campus soil as control.

Place and Duration of Study: Nishi-Iburi Regional Union Final Disposal Site and Muroran Institute of Technology campus, Muroran, Hokkaido, Japan, 2024–2025.

Methodology: PHB films (0.02 g) were buried in landfill and university soils at 30°C for up to 63 days. Soil pH, bacterial and fungal CFU, and bacterial community structure were assessed. Degradation experiments were also conducted with Ralstonia sp. C1 inoculation. PHB-degrading bacteria were isolated on 0.1% PHB agar medium and identified via 16S rRNA gene sequencing.

Results: Landfill soil showed a strongly alkaline pH of 9.7 ± 0.05, with bacterial CFU approximately 40-fold lower [(2.1 ± 0.33) × 10⁶ vs. (4.4 ± 0.17) × 10⁸ CFU/g] and fungal CFU approximately 300-fold lower than university soil. PHB degradation reached only 65.2% in landfill soil vs. 100% in university soil by day 63. Even with Ralstonia sp. C1 inoculation, degradation remained suppressed (35.4% vs. 79.8% at day 28). Only 8 PHB-degrading strains were isolated from landfill soil vs. 16 from university soil, and all landfill isolates were Gram-positive (Actinomadura, Kitasatospora, Priestia), whereas university soil yielded both Gram-positive (Streptomyces) and Gram-negative (Ralstonia) strains.

Conclusion: Highly alkaline conditions in incineration ash-containing landfill soils markedly suppress PHB biodegradation by adversely affecting microbial community structure and viable cell counts. These findings underscore the importance of disposal-environment-specific material evaluation and the need for strategies to promote biodegradation under alkaline landfill conditions.