Biosynthesis of polyhydroxybutyrate from methane and carbon dioxide using type II methanotrophs

Methane (CH₄) and carbon dioxide (CO₂) are the dominant greenhouse gases (GHGs) that are increasing at an alarming rate. Methanotrophs have emerged as potential CH₄ and CO₂ biorefineries. This study demonstrated the synchronous incorporation of CH₄ and CO₂ into polyhydroxybutyrate (PHB) for the first time using ¹³C-labeling experiments in methanotrophs. By supplying substantial amounts of CO₂, PHB content was enhanced in all investigated type II methanotrophic strains by 140 %, 146 %, and 162 %. The highest content of PHB from CH₄ and CO₂ in flask-scale cultivation reached 38 % dry cell weight in Methylocystis sp. MJC1, in which carbon percentage in PHB from CO₂ was 45 %. Flux balance analysis predicted the critical roles of crotonyl-CoA carboxylase/reductase and phosphoenolpyruvate carboxylase in CO₂ recycling. This study provided proof of the conversion of GHGs into a valuable and practical product using methanotrophic bacteria, contributing to addressing GHG emissions.