Effects of Methane to Oxygen Ratio on Cell Growth and Polyhydroxybutyrate Synthesis in High Cell Density Cultivation of Methylocystis Sp. MJC1

Polyhydroxybutyrate (PHB) production through CH conversion by methanotrophs offers a solution for greenhouse gas emissions and plastic waste concerns. In this study, we aimed to achieve high cell density cultivation of Methylocystis sp. MJC1 for efficient PHB production. Cultivating MJC1 using CH and air (3:7, v/v) yielded a final cell density of 52.9 g/L with a 53.7% (28.4 g/L) PHB content after 210 h, showcasing PHB mass production potential. However, long-term cultivation led to a low volumetric productivity of 0.200 g/L/h. To address this, we conducted cultivation at various O/CH ratios using O instead of air, which significantly improved the PHB productivity. Under high O conditions (O/CH ratio of 1.5), biomass productivity increased 1.51-fold compared to that under low O conditions in the same time frame; however, PHB accumulation was delayed. Using an equal ratio of CH and O induced active cell growth and selective PHB production, achieving the highest PHB productivity (0.365 g/L/h) with a final cell density of 55.9 g/L and PHB content of 61.7% (34.5 g/L) in 162 h. This study highlighted the significance of the O/CH ratio in CH conversion and PHB production by M. sp. MJC1.