Bioconversion of methane to biodegradable plastic: Metabolic route from greenhouse gas to green product

Methane is a highly potent greenhouse gas that accounts for about two-thirds of the global warming effect. However, from a biological perspective, methane represents carbon and energy source for a group of bacteria known as methanotrophs which convert methane into its cellular components, drive the biochemical processes during active bacterial metabolism and incorporate carbon into polyhydroxybutyrate (PHB), a type of biopolymer under nutrient starvation. This biopolymer is a precursor for production of biodegradable plastic (bioplastic). However, strain-to-strain variation in substrate range, optimum growth parameters and metabolic regulation are the major obstacles for scale up experiments. It is necessary to find out the appropriate proportion of carbon and nitrogen sources which could have synergistic effect on bacterial growth and accumulation of PHB. Thus, the study aimed to analyze the metabolic potential of Methylocystis sp. Rockwell for production of value-added compounds and to optimize the carbon and nitrogen sources for higher biomass and PHB production. Intracellular metabolites, evaluated by liquid chromatography/tandem accurate mass spectrometry (HPLC-MS) and flame ionization detection gas chromatography (FID-GC), were compared to determine how different carbon and nitrogen source combinations affected the production of metabolites. The biomass and PHB production were optimized via response surface method (RSM) using two factors central composite design. The results from this study revealed how Methylocystis sp. Rockwell alters its metabolism with different carbon and nitrogen sources, with implications to produce industrially useful metabolites. Moreover, the metabolic map showed that methane-growing conditions direct the bacterial metabolism toward primary energy-conserving pathways, whereas methanol-growing conditions direct it toward stress-related pathway- favouring the PHB production. Furthermore, Methylocystis sp. Rockwell preferred methane in ammonium mineral salts (AMS) than nitrate mineral salts (NMS) media for its growth and PHB production. However, a significant biomass reduction was observed under nitrogen starvation followed by enhanced PHB production. The PHB production was 1.43 folds higher in AMS (195.78 ± 18.9 mg/L) than NMS (136.34 ± 21.36 mg/L) media with the PHB cell content of 38 ± 9.5% in AMS and 28.8 ± 4.37% in NMS media. The analysis of variance of 3D surface responses were significant with p < 0.05 and R2 = 0.8 – 0.9 showed a best fit of our models. Our next step is to scale up the net PHB production by combining the projections from metabolomic map and RSM model to convert a greenhouse gas (methane) into the green products (bioplastic).