Development and Optimization of the Biological Conversion of Ethane to Ethanol Using Whole-Cell Methanotrophs Possessing Methane Monooxygenase

Nitrite-driven anaerobic ethane oxidation

Ethane, the second most abundant gaseous hydrocarbon in vast anoxic environments, is an overlooked greenhouse gas. Microbial anaerobic oxidation of ethane can be driven by available electron acceptors such as sulfate and nitrate. However, despite nitrite being a more thermodynamically feasible electron acceptor than sulfate or nitrate, little is known about nitrite-driven anaerobic ethane oxidation. In this study, a microbial culture capable of nitrite-driven anaerobic ethane oxidation was enriched through the long-term operation of a nitrite-and-ethane-fed bioreactor. During continuous operation, the nitrite removal rate and the theoretical ethane oxidation rate remained stable at approximately 25.0 mg NO2 -N L-1 d-1 and 11.48 mg C2H6 L-1 d-1, respectively. Batch tests demonstrated that ethane is essential for nitrite removal in this microbial culture. Metabolic function analysis revealed that a species affiliated with a novel genus within the family Rhodocyclaceae, designated as 'Candidatus Alkanivoras nitrosoreducens', may perform the nitrite-driven anaerobic ethane oxidation. In the proposed metabolic model, despite the absence of known genes for ethane conversion to ethyl-succinate and succinate-CoA ligase, 'Ca. A. nitrosoreducens' encodes a prospective fumarate addition pathway for anaerobic ethane oxidation and a complete denitrification pathway for nitrite reduction to nitrogen. These findings advance our understanding of nitrite-driven anaerobic ethane oxidation, highlighting the previously overlooked impact of anaerobic ethane oxidation in natural ecosystems.

Methanotrophs: Metabolic versatility from utilization of methane to multi-carbon sources and perspectives on current and future applications

The development of biorefineries for a sustainable bioeconomy has been driven by the concept of utilizing environmentally friendly and cost-effective renewable energy sources. Methanotrophic bacteria with a unique capacity to utilize methane as a carbon and energy source can serve as outstanding biocatalysts to develop C1 bioconversion technology. By establishing the utilization of diverse multi-carbon sources, integrated biorefinery platforms can be created for the concept of the circular bioeconomy. An understanding of physiology and metabolism could help to overcome challenges for biomanufacturing. This review summaries fundamental gaps for methane oxidation and the capability to utilize multi-carbon sources in methanotrophic bacteria. Subsequently, breakthroughs and challenges in harnessing methanotrophs as robust microbial chassis for industrial biotechnology were compiled and overviewed. Finally, capabilities to exploit the inherent advantages of methanotrophs to synthesize various target products in higher titers are proposed.

Sustainable biosynthesis of chemicals from methane and glycerol via reconstruction of multi-carbon utilizing pathway in obligate methanotrophic bacteria

Obligate methanotrophic bacteria can utilize methane, an inexpensive carbon feedstock, as a sole energy and carbon substrate, thus are considered as the only nature-provided biocatalyst for sustainable biomanufacturing of fuels and chemicals from methane. To address the limitation of native C1 metabolism of obligate type I methanotrophs, we proposed a novel platform strain that can utilize methane and multi-carbon substrates, such as glycerol, simultaneously to boost growth rates and chemical production in Methylotuvimicrobium alcaliphilum 20Z. To demonstrate the uses of this concept, we reconstructed a 2,3-butanediol biosynthetic pathway and achieved a fourfold higher titer of 2,3-butanediol production by co-utilizing methane and glycerol compared with that of methanotrophic growth. In addition, we reported the creation of a methanotrophic biocatalyst for one-step bioconversion of methane to methanol in which glycerol was used for cell growth, and methane was mainly used for methanol production. After the deletion of genes encoding methanol dehydrogenase (MDH), 11.6 mM methanol was obtained after 72 h using living cells in the absence of any chemical inhibitors of MDH and exogenous NADH source. A further improvement of this bioconversion was attained by using resting cells with a significantly increased titre of 76 mM methanol after 3.5 h with the supply of 40 mM formate. The work presented here provides a novel framework for a variety of approaches in methane-based biomanufacturing.

Type II methanotrophs: A promising microbial cell-factory platform for bioconversion of methane to chemicals

Methane, the predominant element in natural gas and biogas, represents a promising alternative to carbon feedstocks in the biotechnological industry due to its low cost and high abundance. The bioconversion of methane to value-added products can enhance the value of gas and mitigate greenhouse gas emissions. Methanotrophs, methane-utilizing bacteria, can make a significant contribution to the production of various valuable biofuels and chemicals from methane. Type II methanotrophs in comparison with Type I methanotrophs have distinct advantages, including high acetyl-CoA flux and the co-incorporation of two important greenhouse gases (methane and CO₂), making it a potential microbial cell-factory platform for methane-derived biomanufacturing. Herein, we review the most recent advances in Type II methanotrophs related to multi-omics studies and metabolic engineering. Representative examples and prospects of metabolic engineering strategies for the production of suitable products are also discussed.

Biomass: A Renewable Source of Fuels, Chemicals and Carbon Materials

Fossil fuels have been long used as a source of carbon for synthetizing the fuels, chemicals, and carbon-based materials we use on a daily basis [...].