Unusual PHA Biosynthesis

Characterization of P(3HB) from untreated raw palm oil mill effluent using Azotobacter vinelandii ΔAvin_16040 lacking S-layer protein

The production of poly(3-hydroxybutyrate) [P(3HB)] from untreated raw palm oil mill effluent (urPOME), the first wastewater discharge from crude palm oil extraction, is discussed. The mutant strain Azotobacter vinelandii ΔAvin_16040, which lacks the S-layer protein but has a better P(3HB) synthesis capability than the wild type strain ATCC 12,837, was chosen for this study. UrPOME substrate, with high biological oxygen demand (BOD), chemical oxygen demand (COD) and suspended solids, was used without pre-treatment. DSMZ-Azotobacter medium which was devoid of laboratory sugar(s) was used as the basal medium (BaM). Initially, Azotobacter vinelandii ΔAvin_16040 generated 325.5, 1496.3, and 1465.7 mg L^-1 of P(3HB) from BaM with 20% urPOME, 2BaM with 20% urPOME and 20 g L^-1 sucrose, and 2BaM with 20% urPOME and 2 mL L^-1 glycerol, respectively. P(3HB) generation was enhanced by nearly tenfold using statistical optimization, resulting in 13.9 g L^-1. Moreover, the optimization reduced the compositions of mineral salts and sugar in the medium by 48 and 97%, respectively. The urPOME-based P(3HB) product developed a yellow coloration most possibly attributed to the aromatic phenolics content in urPOME. Despite the fact that both were synthesised by ΔAvin_16040, thin films of urPOME-based P(3HB) had superior crystallinity and tensile strength than P(3HB) produced only on sucrose. When treated with 10 and 50 kGy of electron beam irradiation, these P(3HB) scissioned to half and one-tenth of their original molecular weights, respectively, and these cleavaged products could serve as useful base units for specific polymer structure construction.

Production of Polyhydroxybutyrate (PHB) and Factors Impacting Its Chemical and Mechanical Characteristics

Plastic pollution is fueling the grave environmental threats currently facing humans, the animal kingdom, and the planet. The pursuit of renewable resourced biodegradable materials commenced in the 1970s with the need for carbon neutral fully sustainable products driving important progress in recent years. The development of bioplastic materials is highlighted as imperative to the solutions to our global environment challenges and to the restoration of the wellbeing of our planet. Bio-based plastics are becoming increasingly sustainable and are expected to substitute fossil-based plastics. Bioplastics currently include both, nondegradable and biodegradable compositions, depending on factors including the origins of production and post-use management and conditions. Among the most promising materials being developed and evaluated is polyhydroxybutyrate (PHB), a microbial bioprocessed polyester belonging to the polyhydroxyalkanoate (PHA) family. This biocompatible and non-toxic polymer is biosynthesized and accumulated by a number of specialized bacterial strains. The favorable mechanical properties and amenability to biodegradation when exposed to certain active biological environments, earmark PHB as a high potential replacement for petrochemical based polymers such as ubiquitous high density polyethylene (HDPE). To date, high production costs, minimal yields, production technology complexities, and difficulties relating to downstream processing are limiting factors for its progression and expansion in the marketplace. This review examines the chemical, mechanical, thermal, and crystalline characteristics of PHB, as well as various fermentation processing factors which influence the properties of PHB materials.

Biosynthesis and Characteristics of Aromatic Polyhydroxyalkanoates

Polyhydroxyalkanoates (PHAs) are polyesters synthesized by bacteria as a carbon and energy storage material. PHAs are characterized by thermoplasticity, biodegradability, and biocompatibility, and thus have attracted considerable attention for use in medical, agricultural, and marine applications. The properties of PHAs depend on the monomer composition and many types of PHA monomers have been reported. This review focuses on biosynthesized PHAs bearing aromatic groups as side chains. Aromatic PHAs show characteristics different from those of aliphatic PHAs. This review summarizes the types of aromatic PHAs and their characteristics, including their thermal and mechanical properties and degradation behavior. Furthermore, the effect of the introduction of an aromatic monomer on the glass transition temperature (T_g) of PHAs is discussed. The introduction of aromatic monomers into PHA chains is a promising method for improving the properties of PHAs, as the characteristics of aromatic PHAs differ from those of aliphatic PHAs.

The loss of function of PhaC1 is a survival mechanism that counteracts the stress caused by the overproduction of poly-3-hydroxyalkanoates in Pseudomonas putidaΔfadBA

The poly-3-hydroxylkanoate (PHA)-overproducing mutant Pseudomonas putida U ΔfadBA (PpΔfadBA) lacks the genes encoding the main β-oxidation pathway (FadBA). This strain accumulates enormous amounts of bioplastics when cultured in chemically defined media containing PHA precursors (different n-alkanoic or n-aryl-alkanoic acids) and an additional carbon source. In medium containing glucose or 4-hydroxy-phenylacetate, the mutant does not accumulate PHAs and grows just as the wild type (P. putida U). However, when the carbon source is octanoate, growth is severely impaired, suggesting that in PpΔfadBA, the metabolic imbalance resulting from a lower rate of β-oxidation, together with the accumulation of bioplastics, causes severe physiological stress. Here, we show that PpΔfadBA efficiently counteracts this latter effect via a survival mechanism involving the introduction of spontaneous mutations that block PHA accumulation. Surprisingly, genetic analyses of the whole pha cluster revealed that these mutations occurred only in the gene encoding one of the polymerases (phaC1) and that the loss of PhaC1 function was enough to prevent PHA synthesis. The influence of these mutations on the structure of PhaC1 and the existence of a protein-protein (PhaC1-PhaC2) interaction that explains the functionality of the polymerization system are discussed herein.

Microbial synthesis of functional homo-, random, and block polyhydroxyalkanoates by β-oxidation deleted Pseudomonas entomophila

Functional polyhydroxyalkanoates (PHAs) allow chemical modifications to widen PHA diversity, promising to increase values of these biodegradable and biocompatible polyesters. Among functional PHAs, unsaturated PHA site chains can be easily grafted to add chemical groups, and to cross-link with other PHA polymer chains. However, it has been very difficult to obtain structurally controllable functional homo-, random, or block PHA. For the first time, a β-oxidation deleted Pseudomonas entomophila was used to successfully synthesize random copolymers of 3-hydroxydodecanoate (3HDD) and 3-hydroxy-9-decenoate (3H9D). Compositions of the random copolymers P(3HDD-co-3H9D) can be adjusted by ratios of dodecanoic acid (DDA) to 9-decenol (9DEO) fed to the culture of P. entomophila. Homopolymer P3H9D was formed when only 9DEO was added to the culture. Diblock copolymers of P3HDD-b-P3H9D were produced by feeding DDA as the first precursor to form a P3HDD block followed by adding 9DEO as the second precursor to form a second P3H9D block. It was demonstrated that random copolymers P(3HDD-co-3H9D) could be crossed-linked under UV-radiation due to the presence of the unsaturated bonds. Thermal and mechanical characterizations of the above homo-, random, and diblock PHA polymers were conducted. It was found that the diblock polymer P3HDD-b-P3H9D increased at least 2-fold on Young's modulus compared with its random copolymers consisting of similar 3HDD/3H9D ratios. This study demonstrates that PHA functionality could be controlled to meet various requirements.