One of the unique properties of bacterial synthesized PHAs is their biodegradability in various environments. Petrochemical-based plastics currently in wide use are being regarded as a major threat of pollution. Various investigations have been directed to the study of environmental factors that influence biodegradability of PHA (temperature, moisture level, pH and nutrient supply) and those related to the PHA materials themselves (composition, crystallinity, additives and surface area).
We have studied the degradation trends of selected PHA films in a tropical mangrove environment. The biodegradability of homopolymer poly(3-hydroxybutyrate) [P(3HB)] and its co-polymers; poly(3-hydroxybutyrate-co-5 mol% 3-hydroxyvalerate) [P(3HB-co-5 mol% 3HV)] and poly(3-hydroxybutyrate-co-5 mol% 3-hydroxyhexanoate) [P(3HB-co-5 mol% 3HHx)] were investigated along with P(3HB) films containing 38 wt% titanium dioxide (TiO2) [P(3HB)-38 wt% TiO2)]. The degradation of these formulations was monitored for 8 weeks at 3 different zones in an intermediate mangrove compartment.
The degradation rate was observed both on the surface and in the sediment and was expressed in percentage of weight loss. The microbial enumeration done using sediment from the different zones indicated similar colony-forming unit (CFU) counts even though differences were noticed in the degradation profile of the various films in the respective zones. The results obtained revealed that co-polymers disintegrated at similar or higher rate than the homopolymer, P(3HB). However, the incorporation of TiO2 into PHB films caused the degradation rate of P(3HB-38 wt% TiO2) composite film to be far slower than all the other PHA films. The overall rate of degradation of all PHA films placed on the sediment surface was slower than those buried in the sediment.
Biodegradability of P(3HB-co-3HHx) film in tropical mangrove ecosystem (after 0, 1, 2 or 3 weeks)
P(3HB-co-3HV) can be synthesized and stored in many microorganisms as a source of carbon and energy under nutrient limitation and in the presence of excess carbon. Besides the PHA biodegradation study in tropical mangrove environment, intracellular P(3HB-co-3HV) degradation, i. e. the mobilization of previously synthesized P(3HB-co-3HV) also being carried out. In our laboratory, Delftia acidovorans DS 17 (formerly known as Comamonas acidovorans) was used to study the accumulation and mobilization of P(3HB-co-3HV). Mobilization study is using bacterial cells containing medium (38 wt%) and high (75 wt%) amounts of the copolymer.
(Left photo) Before P(3HB-co-3HV) mobilization in Delftia acidovorans
(Right photo) After P(3HB-co-3HV) mobilization in Delftia acidovorans
The mobilization of 3HB and 3HV monomers occurred rapidly and simultaneously in cells that contained 30-40 wt% copolymer of the CDW. However, cells containing about 75 wt% P(3HB-co-3HV) of the CDW showed poor mobilizing ability. Analysis on the sizes and morphology of the P(3HB-co-3HV) granules using TEM revealed that very high accumulation of copolymer within cells affected mobilization efficiency in D. acidovorans.
