Metabolic engineering of the type I methanotroph Methylomonas sp. DH-1 for production of succinate from methane

Upcycling CO2 into succinic acid via electrochemical and engineered Escherichia coli

Converting CO2 into value-added chemicals still remains a grand challenge. Succinic acid has long been considered as one of the top building block chemicals. This study reported efficiently upcycling CO2 into succinic acid by combining between electrochemical and engineered Escherichia coli. In this process, the Cu-organic framework catalyst was synthesized for electrocatalytic CO2-to-ethanol conversion with high Faradaic efficiency (FE, 84.7 %) and relative purity (RP, 95 wt%). Subsequently, an engineered E. coli with efficiently assimilating CO2-derived ethanol to produce succinic acid was constructed by combining computational design and metabolic engineering, and the succinic acid titer reached 53.8 mM with the yield of 0.41 mol/mol, which is 82 % of the theoretical yield. This study effort to link the two processes of efficient ethanol synthesis by electrocatalytic CO2 and succinic acid production from CO2-derived ethanol, paving a way for the production of succinic acid and other value-added chemicals by converting CO2 into ethanol.

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.

Construction of a broad-host-range Anderson promoter series and particulate methane monooxygenase promoter variants expand the methanotroph genetic toolbox

Methanotrophic bacteria are currently used industrially for the bioconversion of methane-rich natural gas and anaerobic digestion-derived biogas to valuable products. These bacteria may also serve to mitigate the negative effects of climate change by capturing atmospheric greenhouse gases. Several genetic tools have previously been developed for genetic and metabolic engineering of methanotrophs. However, the available tools for use in methanotrophs are significantly underdeveloped compared to many other industrially relevant bacteria, which hinders genetic and metabolic engineering of these biocatalysts. As such, expansion of the methanotroph genetic toolbox is needed to further our understanding of methanotrophy and develop biotechnologies that leverage these unique microbes for mitigation and conversion of methane to valuable products. Here, we determined the copy number of three broad-host-range plasmids in Methylococcus capsulatus Bath and Methylosinus trichosporium OB3b, representing phylogenetically diverse Gammaproteobacterial and Alphaproteobacterial methanotrophs, respectively. Further, we show that the commonly used synthetic Anderson series promoters are functional and exhibit similar relative activity in M. capsulatus and M. trichosporium OB3b, but the synthetic series had limited range. Thus, we mutagenized the native M. capsulatus particulate methane monooxygenase promoter and identified variants with activity that expand the activity range of synthetic, constitutive promoters functional not only in M. capsulatus, but also in Escherichia coli. Collectively, the tools developed here advance the methanotroph genetic engineering toolbox and represent additional synthetic genetic parts that may have broad applicability in Pseudomonadota bacteria.

Production and characterization of a biodegradable polymer, poly(3-hydroxybutyrate-co-3-hydroxyvalerate), using the type II methanotroph, Methylocystis sp. MJC1

The production of polyhydroxyalkanoates (PHAs) through the biological conversion of methane is a promising solution to address both methane emissions and plastic waste. Type II methanotrophs naturally accumulate a representative PHA, poly(3-hydroxybutyrate) (PHB), using methane as the sole carbon source. In this study, we aimed to produce poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV copolymer) with improved properties compared with PHB, using the type II methanotroph, Methylocystis sp. MJC1. We optimized the pH, valerate concentration, and valerate supply time in a one-step cultivation process using a gas bioreactor to enhance PHBV copolymer production yield and the 3-hydroxyvalerate (3HV) molar fraction. Under the optimal conditions, the biomass reached 21.3 g DCW/L, and PHBV copolymer accumulation accounted for 41.9 % of the dried cell weight, with a 3HV molar fraction of 28.4 %. The physicochemical properties of the purified PHBV copolymer were characterized using NMR, FTIR, TGA, DSC, and GPC.

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.

Microbial one‑carbon and nitrogen metabolisms are beneficial to the reservoir recovery after cyanobacterial bloom

Cyanobacterial blooms have profound effects on the structure and function of plankton communities in inland waters, but few studies have focused on the effects of microbial-based processes in one‑carbon and nitrogen cycling on water quality improvement following the bloom. Here, we compared the structure and function of the bacterial community, focusing on microbial one‑carbon and nitrogen metabolisms during and after a cyanobacterial Microcystis bloom in a deep subtropical reservoir. Our data showed that microbial one‑carbon and nitrogen cycles were closely related to different periods of the bloom, and the changes of functional genes in microbial carbon and nitrogen cycling showed the same consistent trend as that of Methylomonas sp. With the receding of the bloom, the abundance of Methylomonas as well as the functional genes of microbial one‑carbon and nitrogen cycling reached the peak and then recovered. Our results indicate that microbial one‑carbon and nitrogen metabolisms were beneficial to the recovery of water quality from the cyanobacterial bloom. This study lays a foundation for a deep understanding of the cyanobacterial decomposition mediated by microbes in one‑carbon and nitrogen cycles in inland freshwaters.

C1-based biomanufacturing: Advances, challenges and perspectives

One-carbon (C1) compounds have emerged as a key research focus due to the growth of metabolic engineering and synthetic biology as affordable and sustainable nonfood sugar feedstocks for energy-efficient and environmentally friendly biomanufacturing. This paper summarizes and discusses current developments in C1 compounds for biomanufacturing. First, two primary groups of microbes that use C1 compounds (native and synthetic) are introduced, and the traits, categorization, and functions of C1 microbes are summarized. Second, engineering strategies for C1 utilization are compiled and reviewed, including reconstruction of C1-utilization pathway, enzyme engineering, cofactor engineering, genome-scale modeling, and adaptive laboratory evolution. Third, a review of C1 compounds' uses in the synthesis of biofuels and high-value compounds is presented. Finally, potential obstacles to C1-based biomanufacturing are highlighted along with future research initiatives.

Comparative genomic analysis of Methylocystis sp. MJC1 as a platform strain for polyhydroxybutyrate biosynthesis

Biodegradable polyhydroxybutyrate (PHB) can be produced from methane by some type II methanotroph such as the genus Methylocystis. This study presents the comparative genomic analysis of a newly isolated methanotroph, Methylocystis sp. MJC1 as a biodegradable PHB-producing platform strain. Methylocystis sp. MJC1 accumulates up to 44.5% of PHB based on dry cell weight under nitrogen-limiting conditions. To facilitate its development as a PHB-producing platform strain, the complete genome sequence of Methylocystis sp. MJC1 was assembled, functionally annotated, and compared with genomes of other Methylocystis species. Phylogenetic analysis has shown that Methylocystis parvus to be the closest species to Methylocystis sp. MJC1. Genome functional annotation revealed that Methylocystis sp. MJC1 contains all major type II methanotroph biochemical pathways such as the serine cycle, EMC pathway, and Krebs cycle. Interestingly, Methylocystis sp. MJC1 has both particulate and soluble methane monooxygenases, which are not commonly found among Methylocystis species. In addition, this species also possesses most of the RuMP pathway reactions, a characteristic of type I methanotrophs, and all PHB biosynthetic genes. These comparative analysis would open the possibility of future practical applications such as the development of organism-specific genome-scale models and application of metabolic engineering strategies to Methylocystis sp. MJC1.

Engineering the glyoxylate cycle for chemical bioproduction

With growing concerns about environmental issues and sustainable economy, bioproduction of chemicals utilizing microbial cell factories provides an eco-friendly alternative to current petro-based processes. Creating high-performance strains (with high titer, yield, and productivity) through metabolic engineering strategies is critical for cost-competitive production. Commonly, it is inevitable to fine-tuning or rewire the endogenous or heterologous pathways in such processes. As an important pathway involved in the synthesis of many kinds of chemicals, the potential of the glyoxylate cycle in metabolic engineering has been studied extensively these years. Here, we review the metabolic regulation of the glyoxylate cycle and summarize recent achievements in microbial production of chemicals through tuning of the glyoxylate cycle, with a focus on studies implemented in model microorganisms. Also, future prospects for bioproduction of glyoxylate cycle-related chemicals are discussed.

The integration of bio-catalysis and electrocatalysis to produce fuels and chemicals from carbon dioxide

The dependence on fossil fuels has caused excessive emissions of greenhouse gases (GHGs), leading to climate changes and global warming. Even though the expansion of electricity generation will enable a wider use of electric vehicles, biotechnology represents an attractive route for producing high-density liquid transportation fuels that can reduce GHG emissions from jets, long-haul trucks and ships. Furthermore, to achieve immediate alleviation of the current environmental situation, besides reducing carbon footprint it is urgent to develop technologies that transform atmospheric CO₂ into fossil fuel replacements. The integration of bio-catalysis and electrocatalysis (bio-electrocatalysis) provides such a promising avenue to convert CO₂ into fuels and chemicals with high-chain lengths. Following an overview of different mechanisms that can be used for CO₂ fixation, we will discuss crucial factors for electrocatalysis with a special highlight on the improvement of electron-transfer kinetics, multi-dimensional electrocatalysts and their hybrids, electrolyser configurations, and the integration of electrocatalysis and bio-catalysis. Finally, we prospect key advantages and challenges of bio-electrocatalysis, and end with a discussion of future research directions.

Integrative Genome-Scale Metabolic Modeling Reveals Versatile Metabolic Strategies for Methane Utilization in Methylomicrobium album BG8

Methylomicrobium album BG8 is an aerobic methanotrophic bacterium with promising features as a microbial cell factory for the conversion of methane to value-added chemicals. However, the lack of a genome-scale metabolic model (GEM) of M. album BG8 has hindered the development of systems biology and metabolic engineering of this methanotroph. To fill this gap, a high-quality GEM was constructed to facilitate a system-level understanding of the biochemistry of M. album BG8. Flux balance analysis, constrained with time-series data derived from experiments with various levels of methane, oxygen, and biomass, was used to investigate the metabolic states that promote the production of biomass and the excretion of carbon dioxide, formate, and acetate. The experimental and modeling results indicated that M. album BG8 requires a ratio of ∼1.5:1 between the oxygen- and methane-specific uptake rates for optimal growth. Integrative modeling revealed that at ratios of  >2:1 oxygen-to-methane uptake flux, carbon dioxide and formate were the preferred excreted compounds, while at ratios of <1.5:1 acetate accounted for a larger fraction of the total excreted flux. Our results showed a coupling between biomass production and the excretion of carbon dioxide that was linked to the ratio between the oxygen- and methane-specific uptake rates. In contrast, acetate excretion was experimentally detected during exponential growth only when the initial biomass concentration was increased. A relatively lower growth rate was also observed when acetate was produced in the exponential phase, suggesting a trade-off between biomass and acetate production.

IMPORTANCE A genome-scale metabolic model (GEM) is an integrative platform that enables the incorporation of a wide range of experimental data. It is used to reveal system-level metabolism and, thus, clarify the link between the genotype and phenotype. The lack of a GEM for Methylomicrobium album BG8, an aerobic methane-oxidizing bacterium, has hindered its use in environmental and industrial biotechnology applications. The diverse metabolic states indicated by the GEM developed in this study demonstrate the versatility in the methane metabolic processes used by this strain. The integrative GEM presented here will aid the implementation of the design-build-test-learn paradigm in the metabolic engineering of M. album BG8. This advance will facilitate the development of a robust methane bioconversion platform and help to mitigate methane emissions from environmental systems.

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.

Recent trends in methane to bioproduct conversion by methanotrophs

Methane is an abundant and low-cost gas with high global warming potential and its use as a feedstock can help mitigate climate change. Variety of valuable products can be produced from methane by methanotrophs in gas fermentation processes. By using methane as a sole carbon source, methanotrophic bacteria can produce bioplastics, biofuels, feed additives, ectoine and variety of other high-value chemical compounds. A lot of studies have been conducted through the years for natural methanotrophs and engineered strains as well as methanotrophic consortia. These have focused on increasing yields of native products as well as proof of concept for the synthesis of new range of chemicals by metabolic engineering. This review shows trends in the research on key methanotrophic bioproducts since 2015. Despite certain limitations of the known production strategies that makes commercialization of methane-based products challenging, there is currently much attention placed on the promising further development.

Syntrophic co-culture of a methanotroph and heterotroph for the efficient conversion of methane to mevalonate

As the bioconversion of methane becomes increasingly important for bio-industrial and environmental applications, methanotrophs have received much attention for their ability to convert methane under ambient conditions. This includes the extensive reporting of methanotroph engineering for the conversion of methane to biochemicals. To further increase methane usability, we demonstrated a highly flexible and efficient modular approach based on a synthetic consortium of methanotrophs and heterotrophs mimicking the natural methane ecosystem to produce mevalonate (MVA) from methane. In the methane-conversion module, we used Methylococcus capsulatus Bath as a highly efficient methane biocatalyst and optimized the culture conditions for the production of high amounts of organic acids. In the MVA-synthesis module, we used Escherichia coli SBA01, an evolved strain with high organic acid tolerance and utilization ability, to convert organic acids to MVA. Using recombinant E. coli SBA01 possessing genes for the MVA pathway, 61 mg/L (0.4 mM) of MVA was successfully produced in 48 h without any addition of nutrients except methane. Our platform exhibited high stability and reproducibility with regard to cell growth and MVA production. We believe that this versatile system can be easily extended to many other value-added processes and has a variety of potential applications.

Cellulose nanocrystals coated with a tannic acid-Fe3+ complex as a significant medium for efficient CH4 microbial biotransformation

Microbial biotransformation of CH₄ gas has been attractive for the production of energy and high-value chemicals. However, insufficient supply of CH₄ in a culture medium needs to be overcome for the efficient utilization of CH₄. Here, we utilized cellulose nanocrystals coated with a tannic acid-Fe³⁺ complex (TA-Fe³⁺CNCs) as a medium component to enhance the gas-liquid mass-transfer performance. TA-Fe³⁺CNCs were well suspended in water without agglomeration, stabilized gas bubbles without coalescence, and increased the gas solubility by 20 % and the k_La value at a rapid inlet gas flow rate. Remarkably, the cell growth rate of Methylomonas sp. DH-1 as model CH₄-utilizing bacteria improved with TA-Fe³⁺CNC concentration without any cytotoxic or antibacterial properties, resulting in higher metabolite production ability such as methanol, pyruvate, formate, and succinate. These results showed that TA-Fe³⁺CNCs could be utilized as a significant component in the culture medium applicable as a promising nanofluid for efficient CH₄ microbial biotransformation.

Identification of efficient prokaryotic cell-penetrating peptides with applications in bacterial biotechnology

In bacterial biotechnology, instead of producing functional proteins from plasmids, it is often necessary to deliver functional proteins directly into live cells for genetic manipulation or physiological modification. We constructed a library of cell-penetrating peptides (CPPs) capable of delivering protein cargo into bacteria and developed an efficient delivery method for CPP-conjugated proteins. We screened the library for highly efficient CPPs with no significant cytotoxicity in Escherichia coli and developed a model for predicting the penetration efficiency of a query peptide, enabling the design of new and efficient CPPs. As a proof-of-concept, we used the CPPs for plasmid curing in E. coli and marker gene excision in Methylomonas sp. DH-1. In summary, we demonstrated the utility of CPPs in bacterial engineering. The use of CPPs would facilitate bacterial biotechnology such as genetic engineering, synthetic biology, metabolic engineering, and physiology studies.

Lee et al. construct a cell-penetrating peptides (CPP) library and identify CPPs that can penetrate bacterial cells with minimum or no impact on cell viability. For the identified top CPP candidates, their abilities to deliver macromolecules such as I-SceI and Cre recombinase proteins to bacteria are evaluated as proof-of-concept studies for potential applications.

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.

Laboratory scale bioreactor designs in the processes of methane bioconversion: Mini-review

Global methane emissions have been steadily increasing over the past few decades, exerting a negative effect on the environment. Biogas from landfills and sewage treatment plants is the main anthropogenic source of methane. This makes methane bioconversion one of the priority areas of biotechnology. This process involves the production of biochemical compounds from non-food sources through microbiological synthesis. Methanotrophic bacteria are a promising tool for methane bioconversion due to their ability to use this greenhouse gas and to produce protein-rich biomass, as well as a broad range of useful organic compounds. Currently, methane is used not only to produce biomass and chemical compounds, but also to increase the efficiency of water and solid waste treatment. However, the use of gaseous substrates in biotechnological processes is associated with some difficulties. The low solubility of methane in water is one of the major problems. Different approaches have been involved to encounter these challenges, including different bioreactor and gas distribution designs, solid carriers and bulk sorbents, as well as varying air/oxygen supply, the ratio of volumetric flow rate of gas mixture to its consumption rate, etc. The aim of this review was to summarize the current data on different bioreactor designs and the aspects of their applications for methane bioconversion and wastewater treatment. The bioreactors used in these processes must meet a number of requirements such as low methane emission, improved gas exchange surface, and controlled substrate supply to the reaction zone.

Genome-scale revealing the central metabolic network of the fast growing methanotroph Methylomonas sp. ZR1

Methylomonas sp. ZR1 was an isolated new methanotrophs that could utilize methane and methanol growing fast and synthesizing value added compounds such as lycopene. In this study, the genomic study integrated with the comparative transcriptome analysis were taken to understanding the metabolic characteristic of ZR1 grown on methane and methanol at normal and high temperature regime. Complete Embden-Meyerhof-Parnas pathway (EMP), Entner-Doudoroff pathway (ED), Pentose Phosphate Pathway (PP) and Tricarboxy Acid Cycle (TCA) were found to be operated in ZR1. In addition, the energy saving ppi-dependent EMP enzyme, coupled with the complete and efficient central carbon metabolic network might be responsible for its fast growing nature. Transcript level analysis of the central carbon metabolism indicated that formaldehyde metabolism was a key nod that may be in charge of the carbon conversion efficiency (CCE) divergent of ZR1 grown on methanol and methane. Flexible nitrogen and carotene metabolism pattern were also investigated in ZR1. Nitrogenase genes in ZR1 were found to be highly expressed with methane even in the presence of sufficient nitrate. It appears that, higher lycopene production in ZR1 grown on methane might be attributed to the higher proportion of transcript level of C₄₀ to C₃₀ metabolic gene. Higher transcript level of exopolysaccharides metabolic gene and stress responding proteins indicated that ZR1 was confronted with severer growth stress with methanol than with methane. Additionally, lower transcript level of the TCA cycle, the dramatic high expression level of the nitric oxide reductase and stress responding protein, revealed the imbalance of the central carbon and nitrogen metabolic status, which would result in the worse growth of ZR1 with methanol at 30 °C.

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.

Adaptive laboratory evolution of Escherichia coli lacking cellular byproduct formation for enhanced acetate utilization through compensatory ATP consumption

Acetate has attracted great attention as a carbon source to develop economically feasible bioprocesses for sustainable bioproducts. Acetate is a less-preferred carbon source and a well-known growth inhibitor of Escherichia coli. In this study, we carried out adaptive laboratory evolution of an E. coli strain lacking four genes (adhE, pta, ldhA, and frdA) involved in acetyl-CoA consumption, allowing the efficient utilization of acetate as its sole carbon and energy source. Four genomic mutations were found in the evolved strain through whole-genome sequencing, and two major mutations (in cspC and patZ) mainly contributed to efficient utilization of acetate and tolerance to acetate. Transcriptomic reprogramming was examined by analyzing the genome-wide transcriptome with different carbon sources. The evolved strain showed high levels of intracellular ATP by upregulation of genes involved in NADH and ATP biosynthesis, which facilitated the production of enhanced green fluorescent protein, mevalonate, and n-butanol using acetate alone. This new strain, given its high acetate tolerance and high ATP levels, has potential as a starting host for cell factories targeting the production of acetyl-CoA-derived products from acetate or of products requiring high ATP levels.

Engineered Methanotrophy: A Sustainable Solution for Methane-Based Industrial Biomanufacturing

Methane is a promising feedstock with high abundance and low cost for the sustainable production of biochemicals and biofuels. Methanotrophic bacteria are particularly interesting platforms for methane bioconversion as they can utilize methane as a carbon substrate. Recently, breakthroughs in the understanding of methane metabolism in methanotrophs as well as critical advances in systems metabolic engineering of methanotrophic bacteria have been reported. Here, we discuss the important gaps in the understanding of methanotrophic metabolism that have been uncovered recently and the current trends in systems metabolic engineering in both methanotrophic bacteria and non-native hosts to advance the potential of methane-based biomanufacturing.

Metabolic engineering of type II methanotroph, Methylosinus trichosporium OB3b, for production of 3-hydroxypropionic acid from methane via a malonyl-CoA reductase-dependent pathway

We engineered a type II methanotroph, Methylosinus trichosporium OB3b, for 3-hydroxypropionic acid (3HP) production by reconstructing malonyl-CoA pathway through heterologous expression of Chloroflexus aurantiacus malonyl-CoA reductase (MCR), a bifunctional enzyme. Two strategies were designed and implemented to increase the malonyl-CoA pool and thus, increase in 3HP production. First, we engineered the supply of malonyl-CoA precursors by overexpressing endogenous acetyl-CoA carboxylase (ACC), substantially enhancing the production of 3HP. Overexpression of biotin protein ligase (BPL) and malic enzyme (NADP⁺-ME) led to a ∼22.7% and ∼34.5% increase, respectively, in 3HP titer in ACC-overexpressing cells. Also, the acetyl-CoA carboxylation bypass route was reconstructed to improve 3HP productivity. Co-expression of methylmalonyl-CoA carboxyltransferase (MMC) of Propionibacterium freudenreichii and phosphoenolpyruvate carboxylase (PEPC), which provides the MMC precursor, further improved the 3HP titer. The highest 3HP production of 49 mg/L in the OB3b-MCRMP strain overexpressing MCR, MMC and PEPC resulted in a 2.4-fold improvement of titer compared with that in the only MCR-overexpressing strain. Finally, we could obtain 60.59 mg/L of 3HP in 42 h using the OB3b-MCRMP strain through bioreactor operation, with a 6.36-fold increase of volumetric productivity compared than that in the flask cultures. This work demonstrates metabolic engineering of type II methanotrophs, opening the door for using type II methanotrophs as cell factories for biochemical production along with mitigation of greenhouse gases.

Genome-scale evaluation of core one-carbon metabolism in gammaproteobacterial methanotrophs grown on methane and methanol

Methylotuvimicrobium alcaliphilum 20Z is a promising platform strain for bioconversion of one-carbon (C1) substrates into value-added products. To carry out robust metabolic engineering with methylotrophic bacteria and to implement C1 conversion machinery in non-native hosts, systems-level evaluation and understanding of central C1 metabolism in methanotrophs under various conditions is pivotal but yet elusive. In this study, a genome-scale integrated approach was used to provide in-depth knowledge on the metabolic pathways of M. alcaliphilum 20Z grown on methane and methanol. Systems assessment of core carbon metabolism indicated the methanol assimilation pathway is mostly coupled with the efficient Embden-Meyerhof-Parnas (EMP) pathway along with the serine cycle. In addition, an incomplete TCA cycle operated in M. alcaliphilum 20Z on methanol, which might only supply precursors for de novo synthesis but not reducing powers. Instead, it appears that the direct formaldehyde oxidation pathway supply energy for the whole metabolic system. Additionally, a comparative transcriptomic analysis in multiple gammaproteobacterial methanotrophs also revealed the transcriptional responses of central metabolism on carbon substrate change. These findings provided a systems-level understanding of carbon metabolism and new opportunities for strain design to produce relevant products from different C1-feedstocks.

Efficient production of d-lactate from methane in a lactate-tolerant strain of Methylomonas sp. DH-1 generated by adaptive laboratory evolution

BACKGROUND

Methane, a main component of natural gas and biogas, has gained much attention as an abundant and low-cost carbon source. Methanotrophs, which can use methane as a sole carbon and energy source, are promising hosts to produce value-added chemicals from methane, but their metabolic engineering is still challenging. In previous attempts to produce lactic acid (LA) from methane, LA production levels were limited in part due to LA toxicity. We solved this problem by generating an LA-tolerant strain, which also contributes to understanding novel LA tolerance mechanisms.

RESULTS

In this study, we engineered a methanotroph strain Methylomonas sp. DH-1 to produce d-lactic acid (d-LA) from methane. LA toxicity is one of the limiting factors for high-level production of LA. Therefore, we first performed adaptive laboratory evolution of Methylomonas sp. DH-1, generating an LA-tolerant strain JHM80. Genome sequencing of JHM80 revealed the causal gene watR, encoding a LysR-type transcription factor, whose overexpression due to a 2-bp (TT) deletion in the promoter region is partly responsible for the LA tolerance of JHM80. Overexpression of the watR gene in wild-type strain also led to an increase in LA tolerance. When d form-specific lactate dehydrogenase gene from Leuconostoc mesenteroides subsp. mesenteroides ATCC 8293 was introduced into the genome while deleting the glgA gene encoding glycogen synthase, JHM80 produced about 7.5-fold higher level of d-LA from methane than wild type, suggesting that LA tolerance is a critical limiting factor for LA production in this host. d-LA production was further enhanced by optimization of the medium, resulting in a titer of 1.19 g/L and a yield of 0.245 g/g CH4.

CONCLUSIONS

JHM80, an LA-tolerant strain of Methylomonas sp. DH-1, generated by adaptive laboratory evolution was effective in LA production from methane. Characterization of the mutated genes in JHM80 revealed that overexpression of the watR gene, encoding a LysR-type transcription factor, is responsible for LA tolerance. By introducing a heterologous lactate dehydrogenase gene into the genome of JHM80 strain while deleting the glgA gene, high d-LA production titer and yield were achieved from methane.