Abstract
Conventional chemical methods to transform methane and carbon dioxide into useful chemicals are plagued by the requirement for extreme operating conditions and expensive catalysts. Exploitation of microorganisms as biocatalysts is an attractive alternative to sequester these C1 compounds and convert them into value-added chemicals through their inherent metabolic pathways. Microbial biocatalysts are advantageous over chemical processes as they require mild-operating conditions and do not release any toxic by-products. Methanotrophs are potential cell-factories for synthesizing a wide range of high-value products via utilizing methane as the sole source of carbon and energy, and hence, serve as excellent candidate for methane sequestration. Besides, methanotrophs are capable of capturing carbon dioxide and enzymatically hydrogenating it into methanol, and hence qualify to be suitable candidates for carbon dioxide sequestration. However, large-scale production of value-added products from methanotrophs still presents an overwhelming challenge, due to gas-liquid mass transfer limitations, low solubility of gases in liquid medium and low titer of products. This requires design and engineering of efficient reactors for scale-up of the process. The present review offers an overview of the metabolic architecture of methanotrophs and the range of product portfolio they can offer. Special emphasis is given on methanol biosynthesis as a potential biofuel molecule, through utilization of methane and alternate pathway of carbon dioxide sequestration. In view of the gas-liquid mass transfer and low solubility of gases, the key rate-limiting step in gas fermentation, emphasis is given toward reactor design consideration essential to achieve better process performance.
Highlights
Emission of greenhouse gases has been increasing globally at an alarming rate
The limitations associated with biological production of methanol from methane can be overcome by utilizing CO2 as the substrate for some methanotrophic species (Xin et al, 2007; Patel et al, 2016a)
Methanotrophs can synthesize methanol directly from methane, the associated limitations have driven the researchers to look for alternate pathways
Summary
Emission of greenhouse gases has been increasing globally at an alarming rate. Global anthropogenic emissions of CO2 and CH4 have almost hit 43.1 billion tons and 9390 mmtCO2e, respectively (Global Carbon Project Budget-20191; Global Methane Initiative Report-2018)2. Methanotrophs utilize methane via a metabolic cascade comprising of four enzymes, namely, methane monooxygenase (MMO), methanol dehydrogenase (MDH), formaldehyde dehydrogenase (FADH), and formate Few studies have reported the effect of Cu2+ and Fe2+ ions on the production of PHB and methanol by methanotrophs (Zhang et al, 2008; Patel et al, 2016b).
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