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Biodegradation of PHA

One of the unique properties of microbial PHAs is their biodegradability in various environments. Various studies have been directed to the determine the 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 commercially important PHA films in a tropical mangrove environment. The biodegradability of P(3HB) and its co-polymers; P(3HB-co-5 mol% 3HV) and 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 eight weeks at three different zones in an intermediate mangrove compartment.

The degradation of PHA was observed both on the surface and in the mangrove sediment. PHA co-polymers disintegrated at similar or higher rate than the homopolymer, P(3HB). However, the incorporation of TiO2 into P(3HB) 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.

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P(3HB) film


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P(3HB-co-3HV) film


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P(3HB-co-3HHx) film
Biodegradation of P(3HB), P(3HB-co-3HV) and P(3HB-co-3HHx) film in tropical mangrove environment for six, four and three weeks respectively

 

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) was also studied. Delftia acidovorans DS 17 (formerly known as Comamonas acidovorans) was used to study the accumulation and mobilization of P(3HB-co-3HV).

 

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(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 DCW. However, cells containing about 75 wt% P(3HB-co-3HV) of the DCW 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.