Selective bio-oxidation of propane to acetone using methane-oxidizing Methylomonas sp. DH-1

Methanotrophs mediated biogas valorization: Sustainable route to polyhydroxybutyrate production

This study explores the ability of methanotrophs to convert biogas into biopolymers, addressing H2S as a limitation in the utilization of biogas as a carbon source for bioconversion. Transcriptomic analysis was conducted to understand the growth and changes in the expression patterns of Type I and II methanotrophs under varying H2S concentrations. Results suggested that Type II methanotrophs can possess a native H2S utilization pathway. Both Type I and II methanotrophs were evaluated for their growth and polyhydroxybutyrate (PHB) production from biogas. Methylocystis sp. MJC1 and Methylocystis sp. OK1 exhibited a maximum biomass production of 4.0 and 4.5 gDCW/L, respectively, in fed-batch culture, aligning with the transcriptome data. Furthermore, Methylocystis sp. MJC1 produced 2.9 g PHB/L from biogas through gas fermentation. These findings underscore biogas-based biotechnology as an innovative solution for environmental and industrial challenges with further optimization and productivity enhancement research expected to broaden the potential in this field.

Regulation of acetate tolerance by small ORF-encoded polypeptides modulating efflux pump specificity in Methylomonas sp. DH-1

BACKGROUND

Methanotrophs have emerged as promising hosts for the biological conversion of methane into value-added chemicals, including various organic acids. Understanding the mechanisms of acid tolerance is essential for improving organic acid production. WatR, a LysR-type transcriptional regulator, was initially identified as involved in lactate tolerance in a methanotrophic bacterium Methylomonas sp. DH-1. In this study, we investigated the role of WatR as a regulator of cellular defense against weak organic acids and identified novel target genes of WatR.

RESULTS

By conducting an investigation into the genome-wide binding targets of WatR and its role in transcriptional regulation, we identified genes encoding an RND-type efflux pump (WatABO pump) and previously unannotated small open reading frames (smORFs), watS1 to watS5, as WatR target genes activated in response to acetate. The watS1 to watS5 genes encode polypeptides of approximately 50 amino acids, and WatS1 to WatS4 are highly homologous with one predicted transmembrane domain. Deletion of the WatABO pump genes resulted in decreased tolerance against formate, acetate, lactate, and propionate, suggesting its role as an efflux pump for a wide range of weak organic acids. WatR repressed the basal expression of watS genes but activated watS and WatABO pump genes in response to acetate stress. Overexpression of watS1 increased tolerance to acetate but not to other acids, only in the presence of the WatABO pump. Therefore, WatS1 may increase WatABO pump specificity toward acetate, switching the general weak acid efflux pump to an acetate-specific efflux pump for efficient cellular defense against acetate stress.

CONCLUSIONS

Our study has elucidated the role of WatR as a key transcription factor in the cellular defense against weak organic acids, particularly acetate, in Methylomonas sp. DH-1. We identified the genes encoding WatABO efflux pump and small polypeptides (WatS1 to WatS5), as the target genes regulated by WatR for this specific function. These findings offer valuable insights into the mechanisms underlying weak acid tolerance in methanotrophic bacteria, thereby contributing to the development of bioprocesses aimed at converting methane into value-added chemicals.

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.

Methylomonas rapida sp. nov., a novel species of fast-growing, carotenoid-producing obligate methanotrophs with high biotechnological potential

The genus Methylomonas accommodates strictly aerobic, obligate methanotrophs, with their sole carbon and energy sources restricted to methane and methanol. These bacteria inhabit oxic-anoxic interfaces of various freshwater habitats and have attracted considerable attention as potential producers of a single-cell protein. Here, we characterize two fast-growing representatives of this genus, strains 12 and MP1^T, which are phylogenetically distinct from the currently described Methylomonas species (94.0-97.3 % 16S rRNA gene sequence similarity). Strains 12 and MP1^T were isolated from freshwater sediments collected in Moscow and Krasnodar regions, respectively. Cells of these strains are Gram-negative, red-pigmented, highly motile thick rods that contain a type I intracytoplasmic membrane system and possess a particulate methane monooxygenase (pMMO) enzyme. These bacteria grow between 8 and 45 °C (optimum 35 °C) in a relatively narrow pH range of 5.5-7.3 (optimum pH 6.6-7.2). Major carotenoids synthesized by these methanotrophs are 4,4'-diaplycopene-4,4'-dioic acid, 1,1'-dihydroxy-3,4-didehydrolycopene and 4,4'-diaplycopenoic acid. High biomass yield, of up to 3.26 g CDW/l, is obtained during continuous cultivation of MP1^T on natural gas in a bioreactor at a dilution rate of 0.22 h^-1. The complete genome sequence of strain MP1^T is 4.59 Mb in size; the DNA G + C content is 52.8 mol%. The genome encodes four rRNA operons, one pMMO operon and 4,216 proteins. The genome sequence displays 82-85 % average nucleotide identity to those of earlier described Methylomonas species. We propose to classify these bacteria as representing a novel species of the genus Methylomonas, M. rapida sp. nov., with the type strain MP1^T (=KCTC 92586^T = VKM B-3663^T).

Biological production of 2-propanol from propane using a metabolically engineered type I methanotrophic bacterium

2-Propanol is a widely used industrial solvents. Herein, we employed a unique feature of type I methanotrophic bacterium Methylotuvimicrobium alcaliphilum 20Z possessing only particulate methane monooxygenase (pMMO) for one-step direct production of pure 2-propanol from propane. By maintaining cell growth on glycerol, and after deletion of both Ca²⁺-dependent and La³⁺-dependent methanol dehydrogenases, propane was converted to 2-propanol by pMMO. Although most of the 2-propanol produced was further oxidized to acetone, deletion of active alcohol dehydrogenase, concomitant with synchronous overexpression of secondary alcohol dehydrogenase, significantly inhibited such undesirable oxidation. As a result, a remarkable enhancement (263 mg/L) of 2-propanol was achieved for 120 h by increasing cell growth with a supply of 50% (v/v) propane in headspace. This is the first demonstration to develop an engineered methanotrophic strain for the one-step direct production of pure 2-propanol from propane using one-phase cultivation without the supply of chemical inhibitors or additional reducing-power sources.

Boosting the acetol production in methanotrophic biocatalyst Methylomonas sp. DH-1 by the coupling activity of heteroexpressed novel protein PmoD with endogenous particulate methane monooxygenase

BACKGROUND

Methylacidiphilum sp. IT6 has been validated its C3 substrate assimilation pathway via acetol as a key intermediate using the PmoCAB3, a homolog of the particulate methane monooxygenase (pMMO). From the transcriptomic data, the contribution of PmoD of strain IT6 in acetone oxidation was questioned. Methylomonas sp. DH-1, a type I methanotroph containing pmo operon without the existence of its pmoD, has been deployed as a biocatalyst for the gas-to-liquid bioconversion of methane and propane to methanol and acetone. Thus, Methylomonas sp. DH-1 is a suitable host for investigation. The PmoD-expressed Methylomonas sp. DH-1 can also be deployed for acetol production, a well-known intermediate for various industrial applications. Microbial production of acetol is a sustainable approach attracted attention so far.

RESULTS

In this study, bioinformatics analyses elucidated that novel protein PmoD is a C-terminal transmembrane-helix membrane with the proposed function as a transport protein. Furthermore, the whole-cell biocatalyst was constructed in Methylomonas sp. DH-1 by co-expression the PmoD of Methylacidiphilum sp. IT6 with the endogenous pMMO to enable acetone oxidation. Under optimal conditions, the maximum accumulation, and specific productivity of acetol were 18.291 mM (1.35 g/L) and 0.317 mmol/g cell/h, respectively. The results showed the first coupling activity of pMMO with a heterologous protein PmoD, validated the involvement of PmoD in acetone oxidation, and demonstrated an unprecedented production of acetol from acetone in type I methanotrophic biocatalyst. From the data achieved in batch cultivation conditions, an assimilation pathway of acetone via acetol as the key intermediate was also proposed.

CONCLUSION

Using bioinformatics tools, the protein PmoD has been elucidated as the membrane protein with the proposed function as a transport protein. Furthermore, results from the assays of PmoD-heteroexpressed Methylomonas sp. DH-1 as a whole-cell biocatalyst validated the coupling activity of PmoD with pMMO to convert acetone to acetol, which also unlocks the potential of this recombinant biocatalyst for acetol production. The proposed acetone-assimilated pathway in the recombinant Methylomonas sp. DH-1, once validated, can extend the metabolic flexibility of Methylomonas sp. DH-1.

Construction of a tunable promoter library to optimize gene expression in Methylomonas sp. DH-1, a methanotroph, and its application to cadaverine production

BACKGROUND

As methane is 84 times more potent than carbon dioxide in exacerbating the greenhouse effect, there is an increasing interest in the utilization of methanotrophic bacteria that can convert harmful methane into various value-added compounds. A recently isolated methanotroph, Methylomonas sp. DH-1, is a promising biofactory platform because of its relatively fast growth. However, the lack of genetic engineering tools hampers its wide use in the bioindustry.

RESULTS

Through three different approaches, we constructed a tunable promoter library comprising 33 promoters that can be used for the metabolic engineering of Methylomonas sp. DH-1. The library had an expression level of 0.24-410% when compared with the strength of the lac promoter. For practical application of the promoter library, we fine-tuned the expressions of cadA and cadB genes, required for cadaverine synthesis and export, respectively. The strain with P_rpmB-cadA and P_DnaA-cadB produced the highest cadaverine titre (18.12 ± 1.06 mg/L) in Methylomonas sp. DH-1, which was up to 2.8-fold higher than that obtained from a non-optimized strain. In addition, cell growth and lysine (a precursor of cadaverine) production assays suggested that gene expression optimization through transcription tuning can afford a balance between the growth and precursor supply.

CONCLUSIONS

The tunable promoter library provides standard and tunable components for gene expression, thereby facilitating the use of methanotrophs, specifically Methylomonas sp. DH-1, as a sustainable cell factory.

Verrucomicrobial methanotrophs grow on diverse C3 compounds and use a homolog of particulate methane monooxygenase to oxidize acetone

Short-chain alkanes (SCA; C2-C4) emitted from geological sources contribute to photochemical pollution and ozone production in the atmosphere. Microorganisms that oxidize SCA and thereby mitigate their release from geothermal environments have rarely been studied. In this study, propane-oxidizing cultures could not be grown from acidic geothermal samples by enrichment on propane alone, but instead required methane addition, indicating that propane was co-oxidized by methanotrophs. “Methylacidiphilum” isolates from these enrichments did not grow on propane as a sole energy source but unexpectedly did grow on C3 compounds such as 2-propanol, acetone, and acetol. A gene cluster encoding the pathway of 2-propanol oxidation to pyruvate via acetol was upregulated during growth on 2-propanol. Surprisingly, this cluster included one of three genomic operons (pmoCAB3) encoding particulate methane monooxygenase (PMO), and several physiological tests indicated that the encoded PMO3 enzyme mediates the oxidation of acetone to acetol. Acetone-grown resting cells oxidized acetone and butanone but not methane or propane, implicating a strict substrate specificity of PMO3 to ketones instead of alkanes. Another PMO-encoding operon, pmoCAB2, was induced only in methane-grown cells, and the encoded PMO2 could be responsible for co-metabolic oxidation of propane to 2-propanol. In nature, propane probably serves primarily as a supplemental growth substrate for these bacteria when growing on methane.

Stable-isotopic and metagenomic analyses reveal metabolic and microbial link of aerobic methane oxidation coupled to denitrification at different O2 levels

Aerobic methane (CH₄) oxidation coupled to denitrification (AME-D) can not only mitigate CH₄ emission into the atmosphere, but also potentially alleviate nitrogen pollution in surface waters and engineered ecosystems, and it has attracted substantial research interest. O₂ concentration plays a key role in AME-D, yet little is understood about how it impacts microbial interactions. Here, we applied isotopically labeled K¹⁵NO₃ and ¹³CH₄ and metagenomic analyses to investigate the metabolic and microbial link of AME-D at different O₂ levels. Among the four experimental O₂ levels of 21%,10%, 5% and 2.5% and a CH₄ concentration of 8% (i.e., the O₂/CH₄ ratios of 2.62, 1.26, 0.63 and 0.31), the highest NO₃^--N removal occurred in the AME-D system incubated at the O₂ concentration of 10%. Methanol and acetate may serve as the trophic linkage between aerobic methanotrophs and denitrifers in the AME-D systems. Methylotrophs including Methylophilus, Methylovorus, Methyloversatilis and Methylotenera were abundant under the O₂-sufficient condition with the O₂ concentration of 21%, while denitrifiers such as Azoarcus, Thauera and Thiobacillus dominated in the O₂-limited environment with the O₂ concentration of 10%. The competition of denitrifiers and methylotrophs in the AME-D system for CH₄-derived carbon, such as methanol and acetate, might be influenced by chemotactic responses. More methane-derived carbon flowed into methylotrophs under the O₂-sufficient condition, while more methane-derived carbon was used for denitrification in the O₂-limited environment. These findings can aid in evaluating the distribution and contribution of AME-D and in developing strategies for mitigating CH₄ emission and nitrogen pollution in natural and engineered ecosystems.

Identification of a cytosine methyltransferase that improves transformation efficiency in Methylomonas sp. DH-1

BACKGROUND

Industrial biofuels and other value-added products can be produced from metabolically engineered microorganisms. Methylomonas sp. DH-1 is a candidate platform for bioconversion that uses methane as a carbon source. Although several genetic engineering techniques have been developed to work with Methylomonas sp. DH-1, the genetic manipulation of plasmids remains difficult because of the restriction-modification (RM) system present in the bacteria. Therefore, the RM system in Methylomonas sp. DH-1 must be identified to improve the genetic engineering prospects of this microorganism.

RESULTS

We identified a DNA methylation site, TGGCCA, and its corresponding cytosine methyltransferase for the first time in Methylomonas sp. DH-1 through whole-genome bisulfite sequencing. The methyltransferase was confirmed to methylate the fourth nucleotide of TGGCCA. In general, methylated plasmids exhibited better transformation efficiency under the protection of the RM system than non-methylated plasmids did. As expected, when we transformed Methylomonas sp. DH-1 with plasmid DNA harboring the psy gene, the metabolic flux towards carotenoid increased. The methyltransferase-treated plasmid exhibited an increase in transformation efficiency of 2.5 × 103 CFU/μg (124%). The introduced gene increased the production of carotenoid by 26%. In addition, the methyltransferase-treated plasmid harboring anti-psy sRNA gene exhibited an increase in transformation efficiency by 70% as well. The production of carotenoid was decreased by 40% when the psy gene was translationally repressed by anti-psy sRNA.

CONCLUSIONS

Plasmid DNA methylated by the discovered cytosine methyltransferase from Methylomonas sp. DH-1 had a higher transformation efficiency than non-treated plasmid DNA. The RM system identified in this study may facilitate the plasmid-based genetic manipulation of methanotrophs.

Kerr P, Ali S, Sultana N, Jia Z. Multifunctional Periphytic Biofilms: Polyethylene Degradation and Cd2+ and Pb2+ Bioremediation under High Methane Scenario

Priority pollutants such as polyethylene (PE) microplastic, lead (Pb²⁺), and cadmium (Cd²⁺) have attracted the interest of environmentalists due to their ubiquitous nature and toxicity to all forms of life. In this study, periphytic biofilms (epiphyton and epixylon) were used to bioremediate heavy metals (HMs) and to biodegrade PE under high (120,000 ppm) methane (CH₄) doses. Both periphytic biofilms were actively involved in methane oxidation, HMs accumulation and PE degradation. Epiphyton and epixylon both completely removed Pb²⁺ and Cd²⁺ at concentrations of 2 mg L⁻¹ and 50 mg L⁻¹, respectively, but only partially removed these HMs at a relatively higher concentration (100 mg L⁻¹). Treatment containing 12% ¹³CH₄ proved to be most effective for biodegradation of PE. A synergistic effect of HMs and PE drastically changed microbial biota and methanotrophic communities. High-throughput 16S rRNA gene sequencing revealed that Cyanobacteria was the most abundant class, followed by Gammaproteobacteria and Alphaproteobacteria in all high-methane-dose treatments. DNA stable-isotope probing was used to label ¹³C in a methanotrophic community. A biomarker for methane-oxidizing bacteria, pmoA gene sequence of a ¹³C-labeled fraction, revealed that Methylobacter was most abundant in all high-methane-dose treatments compared to near atmospheric methane (NAM) treatment, followed by Methylococcus. Methylomonas, Methylocystis, Methylosinus, and Methylocella were also found to be increased by high doses of methane compared to NAM treatment. Overall, Cd⁺² had a more determinantal effect on methanotrophic activity than Pb²⁺. Epiphyton proved to be more effective than epixylon in HMs removal and PE biodegradation. The findings proved that both epiphyton and epixylon can be used to bioremediate HMs and biodegrade PE as an efficient ecofriendly technique under high methane concentrations.

Low O2 level enhances CH4-derived carbon flow into microbial communities in landfill cover soils

CH₄ oxidation in landfill cover soils plays a significant role in mitigating CH₄ release to the atmosphere. Oxygen availability and the presence of co-contaminants are potentially important factors affecting CH₄ oxidation rate and the fate of CH₄-derived carbon. In this study, microbial populations that oxidize CH₄ and the subsequent conversion of CH₄-derived carbon into CO₂, soil organic C and biomass C were investigated in landfill cover soils at two O₂ tensions, i.e., O₂ concentrations of 21% ("sufficient") and 2.5% ("limited") with and without toluene. CH₄-derived carbon was primarily converted into CO₂ and soil organic C in the landfill cover soils, accounting for more than 80% of CH₄ oxidized. Under the O₂-sufficient condition, 52.9%-59.6% of CH₄-derived carbon was converted into CO₂ (CE_CO2-C), and 29.1%-39.3% was converted into soil organic C (CE_organic-C). A higher CE_organic-C and lower CE_CO2-C occurred in the O₂-limited environment, relative to the O₂-sufficient condition. With the addition of toluene, the carbon conversion efficiency of CH₄ into biomass C and organic C increased slightly, especially in the O₂-limited environment. A more complex microbial network was involved in CH₄ assimilation in the O₂-limited environment than under the O₂-sufficient condition. DNA-based stable isotope probing of the community with ¹³CH₄ revealed that Methylocaldum and Methylosarcina had a higher relative growth rate than other type I methanotrophs in the landfill cover soils, especially at the low O₂ concentration, while Methylosinus was more abundant in the treatment with both the high O₂ concentration and toluene. These results indicated that O₂-limited environments could prompt more CH₄-derived carbon to be deposited into soils in the form of biomass C and organic C, thereby enhancing the contribution of CH₄-derived carbon to soil community biomass and functionality of landfill cover soils (i.e. reduction of CO₂ emission).

Biological conversion of propane to 2-propanol using group I and II methanotrophs as biocatalysts

Propane is the main component of liquefied petroleum gas and is derived from crude oil processing. Methanotrophic bacteria can convert various alkanes using methane monooxygenase enzyme to primary alcohols. These are further oxidized to various aldehydes by alcohol dehydrogenases or methanol dehydrogenases. In this study, 2-propanol was produced from propane using the whole cells of Methylosinus trichosporium OB3b, Methylomicrobium alcaliphilum 20Z, and Methylomonas sp. DH-1 as the biocatalysts. The biocatalytic process of converting propane to 2-propanol was optimized by the use of several inhibitors and additives, such as EDTA, sodium phosphate, and sodium formate to prevent oxidation of 2-propanol to acetone and to enhance conversion of propane to propanol. The maximum titer of 2-propanol was 0.424 g/L, 0.311 g/L, and 0.610 g/L for Methylomonas sp. DH-1, M. alcaliphilum 20Z, and M. trichosporium OB3b whole cells, respectively. These results showed that type I and type II methanotrophs could be used as the potent biocatalyst for conversion of propane to propanol.

A comparative transcriptome analysis of the novel obligate methanotroph Methylomonas sp. DH-1 reveals key differences in transcriptional responses in C1 and secondary metabolite pathways during growth on methane and methanol

Background

Methanotrophs play an important role in biotechnological applications, with their ability to utilize single carbon (C1) feedstock such as methane and methanol to produce a range of high-value compounds. A newly isolated obligate methanotroph strain, Methylomonas sp. DH-1, became a platform strain for biotechnological applications because it has proven capable of producing chemicals, fuels, and secondary metabolites from methane and methanol. In this study, transcriptome analysis with RNA-seq was used to investigate the transcriptional change of Methylomonas sp. DH-1 on methane and methanol. This was done to improve knowledge about C1 assimilation and secondary metabolite pathways in this promising, but under-characterized, methane-bioconversion strain.

Results

We integrated genomic and transcriptomic analysis of the newly isolated Methylomonas sp. DH-1 grown on methane and methanol. Detailed transcriptomic analysis indicated that (i) Methylomonas sp. DH-1 possesses the ribulose monophosphate (RuMP) cycle and the Embden–Meyerhof–Parnas (EMP) pathway, which can serve as main pathways for C1 assimilation, (ii) the existence and the expression of a complete serine cycle and a complete tricarboxylic acid (TCA) cycle might contribute to methane conversion and energy production, and (iii) the highly active endogenous plasmid pDH1 may code for essential metabolic processes. Comparative transcriptomic analysis on methane and methanol as a sole carbon source revealed different transcriptional responses of Methylomonas sp. DH-1, especially in C1 assimilation, secondary metabolite pathways, and oxidative stress. Especially, these results suggest a shift of central metabolism when substrate changed from methane to methanol in which formaldehyde oxidation pathway and serine cycle carried more flux to produce acetyl-coA and NADH. Meanwhile, downregulation of TCA cycle when grown on methanol may suggest a shift of its main function is to provide de novo biosynthesis, but not produce NADH.

Conclusions

This study provides insights into the transcriptomic profile of Methylomonas sp. DH-1 grown on major carbon sources for C1 assimilation, providing in-depth knowledge on the metabolic pathways of this strain. These observations and analyses can contribute to future metabolic engineering with the newly isolated, yet under-characterized, Methylomonas sp. DH-1 to enhance its biochemical application in relevant industries.

Electronic supplementary material

The online version of this article (10.1186/s12864-019-5487-6) contains supplementary material, which is available to authorized users.

Catalysis for the synthesis of methacrylic acid and methyl methacrylate

Methyl methacrylate (MMA) is a specialty monomer for poly methyl methacrylate (PMMA) and the increasing demand for this monomer has motivated industry to develop clean technologies based on renewable resources. The dominant commercial process reacts acetone and hydrogen cyanide to MMA (ACH route) but the intermediates (hydrogen cyanide, and acetone cyanohydrin) are toxic and represent an environmental hazard. Esterification of methacrylic acid (MAA) to MMA is a compelling alternative together with ethylene, propylene, and isobutene/t-butanol as feedstocks. Partially oxidizing isobutane or 2-methyl-1,3-propanediol (2MPDO) over heteropolycompounds to MAA in a single-step is nascent technology to replace current processes. The focus of this review is on catalysts and their role in the development of processes herein described. Indeed, in some cases remarkable catalysts were studied that enabled considerable steps forward in both the advancement of catalysis science and establishing the basis for new technologies. An emblematic example is represented by Keggin-type heteropolycompounds with cesium and vanadium, which are promising catalysts to convert isobutane and 2MPDO to MAA. Renewable sources for the MMA or MAA route include acetone, isobutanol, ethanol, lactic, itaconic, and citric acids. End-of-life PMMA is expected to grow as a future source of MMA.

Biological conversion of methane to chemicals and fuels: technical challenges and issues

Methane is a promising next-generation carbon feedstock for industrial biotechnology due to its low price and huge availability. Biological conversion of methane to valuable products can mitigate methane-induced global warming as greenhouse gas. There have been challenges for the conversion of methane into various chemicals and fuels using engineered non-native hosts with synthetic methanotrophy or methanotrophs with the reconstruction of synthetic pathways for target products. Herein, we analyze the technical challenges and issues of potent methane bioconversion technology. Pros and cons of metabolic engineering of methanotrophs for methane bioconversion, and perspectives on the bioconversion of methane to chemicals and liquid fuels are discussed.