Methanotrophic Poly(hydroxybutyrate) Through C1 Fermentation and Downstream Process Development: Molar Mass, Thermal...

Area of Science:

• Biotechnology and Bioplastics
• Polymer Science
• Microbial Fermentation

Background:

• Current polyhydroxybutyrate (PHB) production relies on sugar fermentation, which is not scalable to meet global plastic consumption demands.
• Utilizing methane (CH4) as a feedstock for PHB offers a sustainable pathway, with methane potentially sourced from waste biomass.
• High molar mass (Mw) of PHB is crucial for desirable mechanical properties, influenced by strain, culture conditions, and processing.

Purpose of the Study:

• To optimize conditions for polyhydroxybutyrate (PHB) accumulation and achieve high molar mass (Mw) using the methanotrophic strain *Methylocystis* sp. GB 25.
• To evaluate different downstream processing methods for PHB extraction and characterization.
• To assess the potential of methane-derived PHB as a sustainable bioplastics material.

Main Methods:

• Cultivation of *Methylocystis* sp. GB 25 on natural gas in a 350 L loop reactor under controlled temperature and pressure.
• Nutrient limitation (Phosphorus and Nitrogen) for varying durations (1, 2, 2.5 days) to induce PHB accumulation.
• Downstream processing involving centrifugation, spray-drying, chloroform solvent extraction, and enzymatic treatment.

Main Results:

• PHB yields of approximately 40% from biomass were achieved.
• Solvent extraction yielded PHB with high average weight molar masses (Mw) of 1.1-1.5 × 10^6 g mol^-1, significantly higher than commercial PHB.
• Optimal Mw was achieved after one day of nutrient limitation; enzymatic treatment resulted in degraded PHB, and cold chloroform extraction was inefficient for high-Mw fractions.
• Extracted PHB exhibited high crystallinity (~70%) and a melting temperature near 180 °C.
• Mechanical properties were characterized, and enzyme degradation was assessed.

Conclusions:

• Methanotrophic PHB production using *Methylocystis* sp. GB 25 is a viable route to sustainable bioplastics.
• The high Mw achieved enhances material robustness and versatility by potentially limiting degradation during processing.
• Optimized nutrient limitation and solvent extraction are effective for producing high-quality, high-molar-mass PHB.