Abstract
Methane and methanol are common industrial by-products that can be used as feedstocks for the production of value-added products by methylotrophic bacteria. Alphaproteobacterial methanotrophs are known to produce and accumulate the biopolymer polyhydroxybutyrate (PHB) under conditions of nutrient starvation. The present study determined optimal production of biomass and PHB by Methylosinus trichosporium OB3b as a function of carbon source (methane or methanol), nitrogen source (ammonium or nitrate), and nitrogen-to-carbon ratio during growth. Statistical regression analysis with interactions was performed to assess the importance of each factor, and their respective interactions, on biomass and PHB production. Higher biomass concentrations were obtained with methane as carbon source and with ammonium as nitrogen source. The nitrogen source that favored PHB production was ammonium for methane-grown cells and nitrate for methanol-grown cells. Response surface methodology (RSM) was used to determine conditions leading to optimal biomass and PHB production. As an example, the optimal PHB concentration was predicted to occur when a mixture of 30% methane and 70% methanol (molar basis) was used as carbon source with nitrate as nitrogen source and a nitrogen-to-carbon molar ratio of 0.017. This was confirmed experimentally, with a PHB concentration of 48.7 ± 8.3 mg/L culture, corresponding to a cell content of 52.5 ± 6.3% (cell dry weight basis). Using RSM to simultaneously interrogate multiple variables toward optimized growth and production of biopolymer serves as a guide for establishing more efficient industrial conditions to convert single-carbon feedstocks into value-added products.
Highlights
The impact of plastic pollution has motivated interest in economically viable biodegradable polymers
Methylosinus trichosporium OB3b cultivated in medium with nitrate produced more PHB upon N-limitation than cultures grown with ammonium, whereas the opposite result was observed with Methylocystis parvus OBBP (Rostkowski et al, 2013; Zhang et al, 2017)
The present study aims to optimize the production of biomass and PHB in M. trichosporium OB3b in relation to carbon source, nitrogen source, and the carbon to nitrogen ratio using statistical regression analysis combined with response surface methodology (RSM)
Summary
The impact of plastic pollution has motivated interest in economically viable biodegradable polymers. The present study aims to optimize the production of biomass and PHB in M. trichosporium OB3b in relation to carbon source (methane or methanol), nitrogen source (nitrate or ammonium), and the carbon to nitrogen ratio using statistical regression analysis combined with response surface methodology (RSM). Statistical regression analysis and RSM help determine the effects of multiple variables simultaneously (i.e., independent, or process, variables) toward the response of interest (i.e., response variable) and measure the significance of each independent variable on the response (Myers et al, 2016) This analysis enabled us to determine optimal carbon and nitrogen combinations toward the highest yield of either biomass or PHB per liter of culture of M. trichosporium OB3b. This study highlights the potential of RSM to optimize methanotrophic growth medium toward production of a valuable biopolymer despite the complex interacting effects of carbon and nitrogen sources on metabolism
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