Efficient production of a polyhydroxyalkanoate by Azotobacter vinelandii OP using apple residues as promising feedstock

Synthesis, Structure, and Properties of Polyhydroxybutyrate Derived from Azotobacter Vinelandii N-15

Biodegradable polymers are of great interest in addressing the current pollution problem caused by synthetic petroleum-based polymers. It is well known that various microorganisms synthesize and store high-molecular-weight polyhydroxyalkanoates in their cytoplasm as water-insoluble inclusions. In this study, the bacterium Azotobacter vinelandii N-15 strain is used for bioplastic production. The optimal polyhydroxybutyrate (PHB) yield (62% of biomass, 23.6 g L-1 dry cells) is achieved by cultivating the bacteria in Burke's medium with molasses as a carbon source (5 wt.%) at 30 °C, 220 rpm, for 50 h. The resulting polymer was characterized using thin-layer chromatography, UV-Vis, fourier transform infrared, nuclear magnetic resonance spectroscopy, gas chromatography, and X-ray diffraction. The results confirmed that the polymer is PHB with a purity of 98.9%, a molecular weight of 1.2 MDa, a crystallinity of 73%, a melting point of 179 °C, a decomposition temperature of 275 °C, a density of 1.22 g cm- 3, a melt flow index of 10 g 10 min-1, a Shore hardness of 82, a tensile strength of 30 MPa, and a relative elongation at break of 4.5%. Thus, a bioplastic with properties suitable for practical applications is successfully obtained using molasses-a byproduct of sugar production.

Sustainable polyhydroxybutyrate (PHB) production from biowastes by Halomonas sp. WZQ-1 under non-sterile conditions

Polyhydroxyalkanoates (PHA) are promising candidates for replacing petroleum-derived plastics; however, their high production costs limit their commercialisation. In this study, we successfully isolated an efficient PHA-producing strain from a salt lake, which was subsequently identified as Halomonas sp. WZQ-1. Notably, Halomonas sp. WZQ-1 could serve as a promising cell-factory platform for polyhydroxybutyrate (PHB) production, achieving a comparatively high PHB productivity (7.64 ± 0.4 g L-1) under moderate salt stress (60 g L-1 NaCl). We further realised semi-continuous PHB production in a bench-scale fermenter at a steady state by irregularly replenishing the organic substrate. The maximum PHB concentration reached 12.13 g L-1. Finally, we realised the non-sterile conversion of typical biowastes (e.g. pomelo and cantaloupe residues) to PHB using Halomonas sp. WZQ-1. Encouragingly, 4.36 g L-1 PHB was directly obtained from the hydrolysate of pomelo residues with a characteristic melting temperature of 174.0 °C. Life cycle assessment was employed to systematically evaluate the environmental sustainability and potential challenges of biowaste-driven PHB biorefineries. Overall, our findings could serve as a pivotal step toward the commercialisation of PHB and provide a valuable reference for PHB biorefineries.

The Absence of Phasins PhbP2 and PhbP3 in Azotobacter vinelandii Determines the Growth and Poly-3-hydroxybutyrate Synthesis

Phasins are proteins located on the surface of poly-3-hydroxybutyrate (P3HB) granules that affect the metabolism of the polymer, the size and number of the granules, and some also have stress-protecting and growth-promoting effects. This study evaluated the effect of inactivating two new phasins (PhbP2 or PhbP3) on the cellular growth, production, and molecular mass of P3HB in cultures under low or high oxygen transfer rates (OTR). The results revealed that under high OTRₘₐₓ conditions (between 8.1 and 8.9 mmol L-1 h-1), the absence of phasins PhbP2 and PhbP3 resulted in a strong negative effect on the growth rate; in contrast, the rates of specific oxygen consumption increased in both cases. This behavior was not observed under a low oxygen transfer rate (3.9 ± 0.71 mol L-1 h-1), where cellular growth and oxygen consumption were the same for the different strains evaluated. It was observed that at high OTR, the absence of PhbP3 affected the production of P3HB, decreasing it by 30% at the end of cultivation. In contrast, the molecular weight remained constant over time. In summary, the absence of phasin PhbP3 significantly impacted the growth rate and polymer synthesis, particularly at high maximum oxygen transfer rates (OTRₘₐₓ).

Production of poly-3-hydroxybutyrate (PHB) nanoparticles using grape residues as the sole carbon source

The production of poly-3-hydroxybutyrate (PHB) on an industrial scale remains a major challenge due to its higher production cost compared to petroleum-based plastics. As a result, it is necessary to develop efficient fermentative processes using low-cost substrates and identify high-value-added applications where biodegradability and biocompatibility properties are of fundamental importance. In this study, grape residues, mainly grape skins, were used as the sole carbon source in Azotobacter vinelandii OP cultures for PHB production and subsequent nanoparticle synthesis based on the extracted polymer. The grape residue pretreatment showed a high rate of conversion into reducing sugars (fructose and glucose), achieving up to 43.3 % w w-1 without the use of acid or external heat. The cultures were grown in shake flasks, obtaining a biomass concentration of 2.9 g L-1 and a PHB accumulation of up to 37.7 % w w-1. PHB was characterized using techniques such as Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC). The formation of emulsified PHB nanoparticles showed high stability, with a particle size between 210 and 240 nm and a zeta potential between -12 and - 15 mV over 72 h. Owing to these properties, the produced PHB nanoparticles hold significant potential for applications in drug delivery.