Poly-β-hydroxybutyrate Production by Methylosinus trichosporium OB3b at Different Gas-phase Conditions

The utilization of methane for production of Poly-β-hydroxybutyrate (PHB) not only cuts the emissions of greenhouse gases but also greatly reduces PHB production cost. The aim of this study was to determine the effects of gas-phase conditions on PHB production by Methylosinus trichosporium OB3b. Bacterial cultivation and PHB production were conducted in a series of sealed serum bottles. Nitrogen-free mineral salts medium was used to induce PHB production in the presence or absence of N2 in the headspace. In the absence of N2, the highest PHB content (i.e., 52.9% of the dry cell weight with a PHB concentration of 814.3 mg.L-1) was obtained at a ratio of CH4:O2=2:1. Further study at different O2 concentrations with a fixed CH4 partial pressure in absence of N2 showed that PHB accumulation by methanotroph could be tolerated high oxygen partial pressure and its respond to the variation of the oxygen concentration depends on the methane partial pressure. In presence of N2, with headspace gas replenished only when oxygen was almost depleted, the degradation of intracellular PHB has appeared. In the regimen of updating headspace gas at the point when the PHB content began to decrease, the highest PHB content (i.e., 55.5% of the dry cell weight with 901.8 mg.L-1 PHB concentration and 12.5 mg.L-1.h-1PHB productivity) was obtained at 0.2 atm O2 and PHB accumulation was depressed with an oxygen concentration greater than 0.3 atm. The methanotroph responses differentially to the increase in the oxygen partial pressure with regard to PHB accumulation either in the presence or in the absence of N2.