Utilization of Polyhydroxybutyrate (PHB) as intracellular reducing power for methanol production to alleviate the reliance on external energy sources by Methylocystis hirsuta

Methanol production is a promising biotechnology for the valorization of methane into a higher value product using methanotrophic bacteria. A major challenge facing the methane to methanol conversion is the need for external electron donor. As intracellular polyhydroxybutyrate (PHB) storage seems a prominent solution to circumvent such a challenge. In this study, a PHB accumulating methanotrophic strain i.e. Methylocystis hirsuta was employed to investigate the potential of coupling PHB consumption with methanol production. In addition, diversified batch experiments were performed to evaluate the optimum cultivation conditions for methanotrophs to increase methanol production. It was attained that the highest methanol concentrations were observed for cells harvested after shorter cultivation periods regardless their PHB content. Moreover, increasing formate concentrations had a negative effect on PHB consumption which was completely hindered at formate concentrations above 100 mM. Meanwhile, cells depending on PHB solely were able to produce up to 124 +/- 6 mg/L which was improved upon adjusting the growth, PHB accumulation and methanol production periods at which it reached 241 +/- 15 mg/L. PHB-rich cells supplied with only 25 mM formate was able to produce methanol concentration of 399 +/- 15 mg/L. Interestingly, this is similar to the methanol produced using PHB-lacking cells incubated in a medium having 100 mM of formate. Moreover, PHB-rich cells with only 25 mM formate had higher methane conversion efficiency (69 +/- 6%) compared to PHB-lacking cells with 100 mM formate (54 +/- 4%). Further studies are required to improve the methanol productivity of this process.

Automated summary

The study explores the potential of coupling PHB consumption with methanol production using a PHB accumulating methanotrophic strain. Optimum cultivation conditions play a key role in increasing methanol production for methanotrophs. Cells depending on PHB solely showed improved methanol production when growth, PHB accumulation, and methanol production periods were adjusted. Additionally, PHB-rich cells supplied with 25 mM formate were able to produce a higher methanol concentration than PHB-lacking cells with 100 mM formate.