MICROBIAL-DEGRADATION OF POLY(3-HYDROXYBUTYRATE) AND POLY(3-HYDROXYBUTYRATE-CO-3-HYDROXYVALERATE) IN SOILS

The microbial degradation of tensile test pieces made of poly(3-hydroxybutyrate) [P(3HB)] or a copolymer of 90% 3.hydroxybutyric acid and 10% 3-hydroxyvaleric acid was studied in soils incubated at a constant temperature of 15, 28, or 40-degrees-C for up to 200 days. In addition, hydrolytic degradation in sterile buffer at temperatures ranging from 4 to 55-degrees-C was monitored for 98 days. Degradation was measured through loss of weight (surface erosion), molecular weight, and mechanical strength. While no weight loss was recorded in sterile buffer, samples incubated in soils were degraded at an erosion rate of 0.03 to 0.64% weight loss per day, depending on the polymer, the soil, and the incubation temperature. The erosion rate was enhanced by incubation at higher temperatures, and in most cases the copolymer lost weight at a higher rate than the homopolymer. The molecular weights of samples incubated at 40-degrees-C in soils and those incubated at 40-degrees-C in sterile buffer decreased at similar rates, while the molecular weights of samples incubated at lower temperatures remained almost unaffected, indicating that molecular weight decrease is due to simple hydrolysis and not to the action of biodegrading microorganisms. The degradation resulted in loss of mechanical properties. From the samples used in the biodegradation studies, 295 dominant microbial strains capable of degrading P(3HB) and the poly(3-hydroxybutyrate-co-3-hydroxyvalerate) copolymer in vitro were isolated and identified. They consisted of 105 gram-negative bacteria, mostly belonging to Acidovorax facilis and Variovorax paradoxus, 36 Bacillus strains, 68 Streptomyces strains, and 86 mold strains, mainly belonging to Aspergillus fumigatus and species of the genus Penicillium.