Production of poly-β-hydroxybutyrate (PHB) by Methylobacterium organophilum isolated from a methanotrophic consortium in a two-phase partition bioreactor

Production of poly(3-hydroxybutyrate) and poly(3-hydroxybutyrate-co-3-hydroxyvalerate) from methane and volatile fatty acids: properties, metabolic routes and current trend

Polyhydroxyalkanoates (PHAs) are biobased and biodegradable polymers that could effectively replace fossil-based and non-biodegradable plastics. However, their production is currently limited by the high production costs, mainly due to the costly carbon sources used, low productivity and quality of the materials produced. A potential solution lies in utilizing cheap and renewable carbon sources as the primary feedstock during the biological production of PHAs, paving the way for a completely sustainable and economically viable process. In this review, the opportunities and challenges related to the production of polyhydroxyalkanoates using methane and volatile fatty acids (VFAs) as substrates were explored, with a focus on poly(3-hydroxybutyrate) and poly(3-hydroxybutyrate-co-3-hydroxyvalerate). The discussion reports the current knowledge about promising Type II methanotrophs, the impact of process parameters such as limiting nutrients, CH4:O2 ratio and temperature, the type of co-substrate and its concentration. Additionally, the strategies developed until now to enhance PHA production yields were also discussed.

P3HB from CH4 using methanotrophs: aspects of bioreactor, fermentation process and modelling for cost-effective biopolymer production

P3HB (poly-β-hydroxybutyrate), an energy-storage compound of several microorganisms, can be used as bioplastics material. P3HB is completely biodegradable under aerobic and aerobic conditions, also in the marine environment. The intracellular agglomeration of P3HB was examined employing a methanotrophic consortium. Supplanting fossil, non-degradable polymers by P3HB can significantly reduce the environmental impact of plastics. Utilizing inexpensive carbon sources like CH₄ (natural gas, biogas) is a fundamental methodology to make P3HB production less costly, and to avoid the use of primary agricultural products such as sugar or starch. Biomass growth in polyhydroxyalkanoates (PHA) in general and in Poly (3-hydroxybutyrate) manufacture in specific could be a foremost point, so here the authors focus on natural gas as a proper carbon source and on the selection of bioreactors to produceP3HB, and in future further PHA, from that substrate. CH₄ can also be obtained from biomass, e.g., biogas, syngas methanation or power-to-gas (synthetic natural gas, SNG). Simulation software can be utilized for examination, optimizing and scale-up of the process as shown in this paper. The fermentation systems continuously stirred tank reactor (CSTR), forced-liquid vertical loop bioreactor (VTLB), forced-liquid horizontal tubular loop bioreactor (HTLB), airlift (AL) fermenter and bubble column (BC) fermenter were compared for their methane conversion, kLa value, productivity, advantages and disadvantages. Methane is compared to methanol and other feedstocks. It was discovered that under optimum processing circumstances and using Methylocystis hirsuta, the cells accumulated 51.6% cell dry mass of P3HB in the VTLB setup.

Microbial communities and their roles in the Cenozoic sulfurous oil reservoirs in the Southwestern Qaidam Basin, Western China

The latest discovery of sulfurous natural gas marked a breakthrough in the Cenozoic natural gas exploration in the southwestern margin of Qaidam Basin. The 16S rRNA analyses were performed on the crude oil samples from H₂S-rich reservoirs in the Yuejin, Shizigou and Huatugou profiles, to understand the sulfurous gas origin, which was also integrated with carbon and hydrogen isotopes of alkane and sulfur isotopes of H₂S collected from the Yingxiongling Area. Results show that the microorganisms in samples can survive in the hypersaline reservoirs, and can be classified into multiple phyla, including Proteobacteria, Planctomycetes, Firmicutes, Bacteroidetes, and Haloanaerobiaeota. Methanogens are abundant in all of the three profiles, while sulfate-reducing bacteria are abundant in Yuejin and Huatugou profiles, contributing to the methane and H₂S components in the natural gas. The carbon, hydrogen and sulfur isotopes of sulfurous natural gas in the Yingxiongling Area show that the natural gas is a mixture of coal-type gas and oil-type gas, which was primarily derived from thermal degradation, and natural gas from the Yuejin and Huatugou profiles also originated from biodegradation. The isotopic analysis agrees well with the 16S rRNA results, i.e., H₂S-rich natural gas from the Cenozoic reservoirs in the southwest margin of the Qaidam Basin was primarily of thermal genesis, with microbial genesis of secondary importance.

Recent trends of biotechnological production of polyhydroxyalkanoates from C1 carbon sources

Growing concerns over the use of limited fossil fuels and their negative impacts on the ecological niches have facilitated the exploration of alternative routes. The use of conventional plastic material also negatively impacts the environment. One such green alternative is polyhydroxyalkanoates, which are biodegradable, biocompatible, and environmentally friendly. Recently, researchers have focused on the utilization of waste gases particularly those belonging to C1 sources derived directly from industries and anthropogenic activities, such as carbon dioxide, methane, and methanol as the substrate for polyhydroxyalkanoates production. Consequently, several microorganisms have been exploited to utilize waste gases for their growth and biopolymer accumulation. Methylotrophs such as Methylobacterium organophilum produced highest amount of PHA up to 88% using CH₄ as the sole carbon source and 52-56% with CH₃OH. On the other hand Cupriavidus necator, produced 71-81% of PHA by utilizing CO and CO₂ as a substrate. The present review shows the potential of waste gas valorization as a promising solution for the sustainable production of polyhydroxyalkanoates. Key bottlenecks towards the usage of gaseous substrates obstructing their realization on a large scale and the possible technological solutions were also highlighted. Several strategies for PHA production using C1 gases through fermentation and metabolic engineering approaches are discussed. Microbes such as autotrophs, acetogens, and methanotrophs can produce PHA from CO₂, CO, and CH₄. Therefore, this article presents a vision of C1 gas into bioplastics are prospective strategies with promising potential application, and aspects related to the sustainability of the system.

A shortcut to carbon-neutral bioplastic production: Recent advances in microbial production of polyhydroxyalkanoates from C1 resources

Since the 20th century, plastics that are widely being used in general life and industries are causing enormous plastic waste problems since improperly discarded plastics barely degrade and decompose. Thus, the demand for polyhydroxyalkanoates (PHAs), biodegradable polymers with material properties similar to conventional petroleum-based plastics, has been increased so far. The microbial production of PHAs is an environment-friendly solution for the current plastic crisis, however, the carbon sources for the microbial PHA production is a crucial factor to be considered in terms of carbon-neutrality. One‑carbon (C1) resources, such as methane, carbon monoxide, and carbon dioxide, are greenhouse gases and are abundantly found in nature and industry. C1 resources as the carbon sources for PHA production have a completely closed carbon loop with much advances; i) fast carbon circulation with direct bioconversion process and ii) simple fermentation procedure without sterilization as non-preferable nutrients. This review discusses the biosynthesis of PHAs based on C1 resource utilization by wild-type and metabolically engineered microbial host strains via biorefinery processes.

Imaging and modelling of poly(3-hydroxybutyrate) synthesis in Paracoccus denitrificans

Poly(3-hydroxybutyrate) (PHB) granule formation in Paracoccus denitrificans Pd1222 was investigated by laser scanning confocal microscopy (LSCM) and gas chromatography analysis. Cells that had been starved for 2 days were free of PHB granules but resynthesized them within 30 min of growth in fresh medium with succinate. In most cases, the granules were distributed randomly, although in some cases they appeared in a more organized pattern. The rates of growth and PHB accumulation were analyzed within the frame of a Genome-Scale Metabolic Model (GSMM) containing 781 metabolic genes, 1403 reactions and 1503 metabolites. The model was used to obtain quantitative predictions of biomass yields and PHB synthesis during aerobic growth on succinate as sole carbon and energy sources. The results revealed an initial fast stage of PHB accumulation, during which all of the acetyl-CoA originating from succinate was diverted to PHB production. The next stage was characterized by a tenfold lower PHB production rate and the simultaneous onset of exponential growth, during which acetyl-CoA was predominantly drained into the TCA cycle. Previous research has shown that PHB accumulation correlates with cytosolic acetyl-CoA concentration. It has also been shown that PHB accumulation is not transcriptionally regulated. Our results are consistent with the mentioned findings and suggest that, in absence of cell growth, most of the cellular acetyl-CoA is channeled to PHB synthesis, while during exponential growth, it is drained to the TCA cycle, causing a reduction of the cytosolic acetyl-CoA pool and a concomitant decrease of the synthesis of acetoacetyl-CoA (the precursor of PHB synthesis).

Biogas-based production of glycogen by Nostoc muscorum: Assessing the potential of transforming CO2 into value added products

The potential of the filamentous N₂-fixing cyanobacterium Nostoc muscorum for CO₂ capture from high-loaded streams (i.e. flue gas or biogas) combined with the accumulation of glycogen (GL) and polyhydroxybutyrate (PHB), was evaluated under nutrient-sufficient and nutrient-limited conditions. N. muscorum was able to grow under CO₂ contents from 0.03 up to 30% v/v, thus tolerating CO₂ concentrations similar to those found in raw biogas or flue-gas, with maximum CO₂-fixation rates of 191.9 ± 46 g m^-3 d^-1 at a biomass concentration of 733.3 ± 207.4 mg TSS L^-1. Despite N. muscorum was inhibited by the presence of H₂S, the co-inoculation with activated sludge resulted in both CO₂ and H₂S depletion. Moreover, N. muscorum accumulated GL up to ∼54% dcw under N and P-deprivation, almost 36 times higher than that recorded under nutrients sufficient condition. The addition of 10% extra carbon in the form of valeric acid not only did not hamper the growth of N. muscorum (336.0 ± 113.1 mg TSS L^-1) but also increased the GL content to ∼58% dcw. On the contrary, a negligible PHB accumulation was found under the tested conditions, likely due to the high CO₂ concentration of 30% v/v in the headspace and therefore the high availability of inorganic carbon for the cultures. N. muscorum cultures achieved VFAs degradations up to ∼78% under controlled pH. These results supported N. muscorum as a sustainable alternative for CO₂-capture and greenhouse gas mitigation or for photosynthetic biogas upgrading coupled with value added biomass production.

Cultivation processes to select microorganisms with high accumulation ability

The microbial ability to accumulate biomolecules is fundamental for different biotechnological applications aiming at the production of biofuels, food and bioplastics. However, high accumulation is a selective advantage only under certain stressful conditions, such as nutrient depletion, characterized by lower growth rate. Conventional bioprocesses maintain an optimal and stable environment for large part of the cultivation, that doesn't reward cells for their accumulation ability, raising the risk of selection of contaminant strains with higher growth rate, but lower accumulation of products. Here in this work the physiological responses of different microorganisms (microalgae, bacteria, yeasts) under N-starvation and energy starvation are reviewed, with the aim to furnish relevant insights exploitable to develop tailored bioprocesses to select specific strains for their higher accumulation ability. Microorganism responses to starvation are reviewed focusing on cell cycle, biomass production and variations in biochemical composition. Then, the work describes different innovative bioprocess configurations exploiting uncoupled nutrient feeding strategies (feast-famine), tailored to maintain a selective pressure to reward the strains with higher accumulation ability in mixed microbial populations. Finally, the main models developed in recent studies to describe and predict microbial growth and intracellular accumulation upon N-starvation and feast-famine conditions have been reviewed.

Biotransformation of Methane and Carbon Dioxide Into High-Value Products by Methanotrophs: Current State of Art and Future Prospects

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.

Valorization of methane from environmental engineering applications: A critical review

Wastewater and waste management sectors alone account for 18% of the anthropogenic methane (CH₄) emissions. This study presents a critical overview of methanotrophs ("methane oxidizing microorganisms") for valorizing typically discarded CH₄ from environmental engineering applications, focusing on wastewater treatment plants. Methanotrophs can convert CH₄ into valuable bioproducts including chemicals, biodiesel, DC electricity, polymers, and S-layers, all under ambient conditions. As discarded CH₄ and its oxidation products can also be used as a carbon source in nitrification and annamox processes. Here we discuss modes of CH₄ assimilation by methanotrophs in both natural and engineered systems. We also highlight the technical challenges and technological breakthroughs needed to enable targeted CH₄ oxidation in wastewater treatment plants.

Methanotroph-microalgae co-culture for greenhouse gas mitigation: Effect of initial biomass ratio and methane concentration

This work evaluated the effect of different initial biomass ratios in a co-culture of an alkaliphilic methanotrophic bacteria consortium (AMB) and the green microalga Scenedesmus obtusiusculus (GM) on the maximum CH₄ specific biodegradation rate and global carbon uptake. The highest maximum specific biodegradation rate was 589 ± 0.01 mg_CH4 g_biomass^-1 d^-1 obtained for a proportion of 3:1 AMB-GM (w w^-1) and 8% of initial CH₄ in the headspace. The methane degradation rate was 1.5 times lower than the value obtained solely by the AMB consortium, and it was associated with pH increases due to the evolved CO₂ consumption by the microalga. Increased activity of the AMB consortium along the experiments was due to progressive adaptation. Massive sequencing revealed the presence of methanotrophic/methylotrophic species such as Methylocystis sp., Methylomicrobium sp., Methylophaga sp., and Hyphomicrobium sp. Successful complete methane and carbon dioxide uptake was obtained with the 3:1, 4:1, and 5:1 AMB-GM biomass ratios, while for the rest of the ratios tested, more than 70% of the initial methane was transformed into biomass and inorganic carbon. This study showed that methanotrophic-microalgal co-cultures lead to a promising strategy for greenhouse gases mitigation in one step.

Modeling of nitrogen fixation and polymer production in the heterotrophic diazotroph Azotobacter vinelandii DJ

Nitrogen fixation is an important metabolic process carried out by microorganisms, which converts molecular nitrogen into inorganic nitrogenous compounds such as ammonia (NH₃). These nitrogenous compounds are crucial for biogeochemical cycles and for the synthesis of essential biomolecules, i.e. nucleic acids, amino acids and proteins. Azotobacter vinelandii is a bacterial non-photosynthetic model organism to study aerobic nitrogen fixation (diazotrophy) and hydrogen production. Moreover, the diazotroph can produce biopolymers like alginate and polyhydroxybutyrate (PHB) that have important industrial applications. However, many metabolic processes such as partitioning of carbon and nitrogen metabolism in A. vinelandii remain unknown to date.

Genome-scale metabolic models (M-models) represent reliable tools to unravel and optimize metabolic functions at genome-scale. M-models are mathematical representations that contain information about genes, reactions, metabolites and their associations. M-models can simulate optimal reaction fluxes under a wide variety of conditions using experimentally determined constraints. Here we report on the development of a M-model of the wild type bacterium A. vinelandii DJ (iDT1278) which consists of 2,003 metabolites, 2,469 reactions, and 1,278 genes. We validated the model using high-throughput phenotypic and physiological data, testing 180 carbon sources and 95 nitrogen sources. iDT1278 was able to achieve an accuracy of 89% and 91% for growth with carbon sources and nitrogen source, respectively. This comprehensive M-model will help to comprehend metabolic processes associated with nitrogen fixation, ammonium assimilation, and production of organic nitrogen in an environmentally important microorganism.

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Genome-scale metabolic model of Azotobacter vinelandii DJ achives over 90% accuracy.

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iDT1278 is the most comprehensive model to simulate diazotrophy.

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Determining the most suitable culture conditions to produce polymers A. vinelandii.

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Constraint-based modeling unravels links among nitrogen fixation and production of organic nitrogen.

Biological conversion of methane to polyhydroxyalkanoates: Current advances, challenges, and perspectives

Methane emissions and plastic pollution are critical global challenges. The biological conversion of methane to poly-β-hydroxybutyrate (PHB) not only mitigates methane emissions but also provides biodegradable polymer substitutes for petroleum-based materials used in plastics production. This work provides an early overview of the methane-based PHB advances and discusses challenges and related strategies. Recent advances of PHB, including PHB biosynthetic pathways, methanotrophs, bioreactors, and the performances of PHB materials are introduced. Major challenges of methane-based PHB production are discussed in detail; these include low efficiency of methanotrophs, low gas-liquid mass transfer efficiency, and poor material properties. To overcome these limitations, various approaches are also explored, such as feast-famine regimes, engineered microorganisms, gas-permeable membrane bioreactors, two-phase partitioning bioreactors, poly-β-hydroxybutyrate-co-hydroxyvalerate synthesis, and molecular weight manipulation.

Elucidating the influence of environmental factors on biogas-based polyhydroxybutyrate production by Methylocystis hirsuta CSC1

The valorization of biogas as a feedstock for the generation of added-value bioproducts will play a key role on the sustainability of anaerobic digestion. The present work assessed the influence of key environmental parameters (O₂:CH₄ ratio, temperature and nitrogen source) on the growth and polyhydroxybutyrate (PHB) synthesis under nitrogen limiting conditions of the type II methanotroph Methylocystis hirsuta CSC1 using biogas as a feedstock. The O₂:CH₄ ratios tested (1:1, 1.5:1 and 2:1) did not affect significantly M. hirsuta CSC1 growth yields (~5 g TSS mol^-1 CH₄), although lower CH₄ removal rates were reached under O₂-limiting conditions (ratio 1:1). The highest PHB content (45 wt%) was achieved at a ratio 2:1 and was threefold higher than those obtained at lower ratios (~15 wt%). The increase in temperature from 15 to 25 °C resulted in increases in the growth yield (from 5 to 6 g TSS mol^-1 CH₄) and PHB content (from 32 to 40 wt%). Conversely, the lowest PHB content (30 wt%) was reached at 37 °C, together with a negligible growth under nutrient sufficient conditions. The nitrogen source also played a key role on both M. hirsuta CSC1 growth and PHB synthesis. Thus, ammonium resulted in the highest growth yield (7 g TSS mol^-1 CH₄), although the maximum PHB content was achieved when biomass was previously grown in nitrate as the nitrogen source (41 wt%). Nitrite exerted an inhibitory effect on M. hirsuta CSC1 growth.

The effect of methane and odd-chain fatty acids on 3-hydroxybutyrate (3HB) and 3-hydroxyvalerate (3HV) synthesis by a Methylosinus-dominated mixed culture

A methanotrophic community was enriched in a semi-continuous reactor under non-aseptic conditions with methane and ammonia as carbon and nitrogen source. After a year of operation, Methylosinus sp., accounted for 80% relative abundance of the total sequences identified from potential polyhydroxyalkanoates (PHAs) producers, dominated the methane-fed enrichment. Prior to induction of PHA accumulation, cells harvested from the parent reactor contained low level of PHA at 4.0 ± 0.3 wt%. The cells were later incubated in the absence of ammonia with various combinations of methane, propionic acid, and valeric acid to induce biosynthesis of poly(3-hydroxybutyrate) (PHB) and poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV). Previous studies reported that methanotrophic utilization of odd-chain fatty acids for the production of PHAs requires reducing power from methane oxidation. However, our findings demonstrated that the PHB-containing methanotrophic enrichment does not require methane availability to generate 3-hydroxybutyrate (3HB) and 3-hydroxyvalerate (3HV)—when odd-chain fatty acids are presented. The enrichment yielded up to 14 wt% PHA with various mole fractions of 3HV monomer depending on the availability of methane and odd-fatty acids. Overall, the addition of valeric acid resulted in a higher PHA content and a higher 3HV fraction. The highest 3HV fraction (up to 65 mol%) was obtained from the methane–valeric acid experiment, which is higher than those previously reported for PHA-producing methanotrophic mixed microbial cultures.

Microbial consortia including methanotrophs: some benefits of living together

With the progress of biotechnological research and improvements made in bioprocessing with pure cultures, microbial consortia have gained recognition for accomplishing biological processes with improved effectiveness. Microbes are indispensable tool in developing bioprocesses for the production of bioenergy and biochemicals while utilizing renewable resources due to technical, economic and environmental advantages. They communicate with specific cohorts in close proximity to promote metabolic cooperation. Use of positive microbial associations has been recognized widely, especially in food industries and bioremediation of toxic compounds and waste materials. Role of microbial associations in developing sustainable energy sources and substitutes for conventional fuels is highly promising with many commercial prospects. Detoxification of chemical contaminants sourced from domestic, agricultural and industrial wastes has also been achieved through microbial catalysis in pure and co-culture systems. Methanotrophs, the sole biological sink of greenhouse gas methane, catalyze the methane monooxygenasemediated oxidation of methane to methanol, a high energy density liquid and key platform chemical to produce commodity chemical compounds and their derivatives. Constructed microbial consortia have positive effects, such as improved biomass, biocatalytic potential, stability etc. In a methanotroph-heterotroph consortium, non-methanotrophs provide key nutrient factors and alleviate the toxicity from the culture. Non-methanotrophic organisms biologically stimulate the growth and activity of methanotrophs via production of growth stimulators. However, methanotrophs in association with co-cultured microorganisms are in need of further exploration and thorough investigation to study their interaction mode and application with improved effectiveness.

Poly-β-hydroxybutyrate Production by Methylosinus trichosporium OB3b at Different Gas-phase Conditions

BACKGROUND

The utilization of methane for production of Poly-β-hydroxybutyrate (PHB) not only cuts the emissions of greenhouse gases but also greatly reduces PHB production cost.

OBJECTIVES

The aim of this study was to determine the effects of gas-phase conditions on PHB production by Methylosinus trichosporium OB3b.

MATERIALS AND METHODS

Bacterial cultivation and PHB production were conducted in a series of sealed serum bottles. Nitrogen-free mineral salts medium was used to induce PHB production in the presence or absence of N2 in the headspace.

RESULTS

In the absence of N2, the highest PHB content (i.e., 52.9% of the dry cell weight with a PHB concentration of 814.3 mg.L-1) was obtained at a ratio of CH4:O2=2:1. Further study at different O2 concentrations with a fixed CH4 partial pressure in absence of N2 showed that PHB accumulation by methanotroph could be tolerated high oxygen partial pressure and its respond to the variation of the oxygen concentration depends on the methane partial pressure. In presence of N2, with headspace gas replenished only when oxygen was almost depleted, the degradation of intracellular PHB has appeared. In the regimen of updating headspace gas at the point when the PHB content began to decrease, the highest PHB content (i.e., 55.5% of the dry cell weight with 901.8 mg.L-1 PHB concentration and 12.5 mg.L-1.h-1PHB productivity) was obtained at 0.2 atm O2 and PHB accumulation was depressed with an oxygen concentration greater than 0.3 atm.

CONCLUSIONS

The methanotroph responses differentially to the increase in the oxygen partial pressure with regard to PHB accumulation either in the presence or in the absence of N2.

Novel approaches and reasons to isolate methanotrophic bacteria with biotechnological potentials: recent achievements and perspectives

The recent drop in the price of natural gas has rekindled the interests in methanotrophs, the organisms capable of utilizing methane as the sole electron donor and carbon source, as biocatalysts for various industrial applications. As heterologous expression of the methane monooxygenases in more amenable hosts has been proven to be nearly impossible, future success in methanotroph biotechnology largely depends on securing phylogenetically and phenotypically diverse methanotrophs with relatively high growth rates. For long, isolation of methanotrophs have relied on repeated single colony picking after initial batch enrichment with methane, which is a very rigorous and time-consuming process. In this review, three unconventional isolation methods devised for facilitation of the isolation process, diversification of targeted methanotrophs, and/or screening of rapid growers are summarized. The soil substrate membrane method allowed for isolation of previously elusive methanotrophs and application of high-throughput extinction plating technique facilitated the isolation procedure. Use of a chemostat with gradually increased dilution rates proved effective in screening for the fastest-growing methanotrophs from environmental samples. Development of new isolation technologies incorporating microfluidics and single-cell techniques may lead to discovery of previously unculturable methanotrophs with unexpected metabolic potentials and thus, certainly warrant future investigation.

Microbial Methylotrophic Metabolism: Recent Metabolic Modeling Efforts and Their Applications In Industrial Biotechnology

Developing methylotrophic bacteria into cell factories that meet the chemical demand of the future could be both economical and environmentally friendly. Methane is not only an abundant, low-cost resource but also a potent greenhouse gas, the capture of which could help to reduce greenhouse gas emissions. Rational strain design workflows rely on the availability of carefully combined knowledge often in the form of genome-scale metabolic models to construct high-producer organisms. In this review, the authors present the most recent genome-scale metabolic models in aerobic methylotrophy and their applications. Further, the authors present models for the study of anaerobic methanotrophy through reverse methanogenesis and suggest organisms that may be of interest for expanding one-carbon industrial biotechnology. Metabolic models of methylotrophs are scarce, yet they are important first steps toward rational strain-design in these organisms.

Engineering the bioconversion of methane and methanol to fuels and chemicals in native and synthetic methylotrophs

Methylotrophy describes the ability of organisms to utilize reduced one-carbon compounds, notably methane and methanol, as growth and energy sources. Abundant natural gas supplies, composed primarily of methane, have prompted interest in using these compounds, which are more reduced than sugars, as substrates to improve product titers and yields of bioprocesses. Engineering native methylotophs or developing synthetic methylotrophs are emerging fields to convert methane and methanol into fuels and chemicals under aerobic and anaerobic conditions. This review discusses recent progress made toward engineering native methanotrophs for aerobic and anaerobic methane utilization and synthetic methylotrophs for methanol utilization. Finally, strategies to overcome the limitations involved with synthetic methanol utilization, notably methanol dehydrogenase kinetics and ribulose 5-phosphate regeneration, are discussed.

Assessing the influence of the carbon source on the abatement of industrial N2O emissions coupled with the synthesis of added-value bioproducts

The continuous abatement of a synthetic N₂O emission from a nitric acid plant coupled with the simultaneously production of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) copolymer and the coenzyme Q10 (CoQ₁₀) in a bubble column reactor (BCR) was tested using methanol, glycerol and a mixture of sodium acetate-acetic acid (Ac-HAc) as a carbon and electron donor source. The BCRs were inoculated with Paracoccus denitrificans and supplied with the carbon/electron donor at a loading rate of 139gCm^-3d^-1. High N₂O removal efficiencies (81-91%) were achieved, with glycerol supporting the highest abatement. The PHBV cell content ranged from 25 to 53%, with highest accumulation in the culture obtained with methanol and Ac-HAc. However, the greatest PHBV productivities were observed in the BCRs operated with glycerol and Ac-HAc (21.7 and 33.5gPHBVm^-3d^-1, respectively). Glycerol supply induced the highest molar ratio (23%) of the homopolymer 3-hydroxyvalerate in the composition of PHBV. In addition, the specific cell content of CoQ₁₀ ranged from 0.4 to 1mgg^-1. This work constitutes, to the best of our knowledge, the first study combining N₂O abatement with the simultaneous production of multiple bioproducts, which pave the way to the development of greenhouse gas biorefineries for climate change mitigation.

Draft Genome Sequence of Sphingobacterium sp. CZ-UAM, Isolated from a Methanotrophic Consortium

Sphingobacterium sp. CZ-UAM was isolated from a methanotrophic consortium in mineral medium using methane as the only carbon source. A draft genome of 5.84 Mb with a 40.77% G+C content is reported here. This genome sequence will allow the investigation of potential methanotrophy in this isolated strain.

Nitrous Oxide Abatement Coupled with Biopolymer Production As a Model GHG Biorefinery for Cost-Effective Climate Change Mitigation

N₂O represents ∼6% of the global greenhouse gas emission inventory and the most important O₃-depleting substance emitted in this 21st century. Despite its environmental relevance, little attention has been given to cost-effective and environmentally friendly N₂O abatement methods. Here we examined, the potential of a bubble column (BCR) and an internal loop airlift (ALR) bioreactors of 2.3 L for the abatement of N₂O from a nitric acid plant emission. The process was based on the biological reduction of N₂O by Paracoccus denitrificans using methanol as a carbon/electron source. Two nitrogen limiting strategies were also tested for the coproduction of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) coupled with N₂O reduction. High N₂O removal efficiencies (REs) (≈87%) together with a low PHBV cell accumulation were observed in both bioreactors in excess of nitrogen. However, PHBV contents of 38-64% were recorded under N limiting conditions along with N₂O-REs of ≈57% and ≈84% in the ALR and BCR, respectively. Fluorescence in situ hybridization analyses showed that P. denitrificans was dominant (>50%) after 6 months of experimentation. The successful abatement of N₂O concomitant with PHBV accumulation confirmed the potential of integrating biorefinery concepts into biological gas treatment for a cost-effective GHG mitigation.

Coupled effects of methane monooxygenase and nitrogen source on growth and poly-β-hydroxybutyrate (PHB) production of Methylosinus trichosporium OB3b

The coupled effects of nitrogen source and methane monooxygenase (MMO) on the growth and poly-β-hydroxybutyrate (PHB) accumulation capacity of methanotrophs were explored. The ammonia-supplied methanotrophs expressing soluble MMO (sMMO) grew at the highest rate, while N₂-fixing bacteria expressing particulate MMO (pMMO) grew at the lowest rate. Further study showed that more hydroxylamine and nitrite was formed by ammonia-supplied bacteria containing pMMO, which might cause their slightly lower growth rate. The highest PHB content (51.0%) was obtained under nitrogen-limiting conditions with the inoculation of nitrate-supplied bacteria containing pMMO. Ammonia-supplied bacteria also accumulated a higher content of PHB (45.2%) with the expression of pMMO, while N₂-fixing bacteria containing pMMO only showed low PHB production capacity (32.1%). The maximal PHB contents of bacteria expressing sMMO were low, with no significant change under different nitrogen source conditions. The low MMO activity, low cell growth rate and low PHB production capacity of methanotrophs continuously cultivated with N₂ with the expression of pMMO were greatly improved in the cyclic NO₃^-N₂ cultivation regime, indicating that long-term deficiency of nitrogen sources was detrimental to the activity of methanotrophs expressing pMMO.

Resource Recovery from Wastewater by Biological Technologies: Opportunities, Challenges, and Prospects

Limits in resource availability are driving a change in current societal production systems, changing the focus from residues treatment, such as wastewater treatment, toward resource recovery. Biotechnological processes offer an economic and versatile way to concentrate and transform resources from waste/wastewater into valuable products, which is a prerequisite for the technological development of a cradle-to-cradle bio-based economy. This review identifies emerging technologies that enable resource recovery across the wastewater treatment cycle. As such, bioenergy in the form of biohydrogen (by photo and dark fermentation processes) and biogas (during anaerobic digestion processes) have been classic targets, whereby, direct transformation of lipidic biomass into biodiesel also gained attention. This concept is similar to previous biofuel concepts, but more sustainable, as third generation biofuels and other resources can be produced from waste biomass. The production of high value biopolymers (e.g., for bioplastics manufacturing) from organic acids, hydrogen, and methane is another option for carbon recovery. The recovery of carbon and nutrients can be achieved by organic fertilizer production, or single cell protein generation (depending on the source) which may be utilized as feed, feed additives, next generation fertilizers, or even as probiotics. Additionlly, chemical oxidation-reduction and bioelectrochemical systems can recover inorganics or synthesize organic products beyond the natural microbial metabolism. Anticipating the next generation of wastewater treatment plants driven by biological recovery technologies, this review is focused on the generation and re-synthesis of energetic resources and key resources to be recycled as raw materials in a cradle-to-cradle economy concept.

A methanotroph-based biorefinery: Potential scenarios for generating multiple products from a single fermentation

Methane, a carbon source for methanotrophic bacteria, is the principal component of natural gas and is produced during anaerobic digestion of organic matter (biogas). Methanotrophs are a viable source of single cell protein (feed supplement) and can produce various products, since they accumulate osmolytes (e.g. ectoine, sucrose), phospholipids (potential biofuels) and biopolymers (polyhydroxybutyrate, glycogen), among others. Other cell components, such as surface layers, metal chelating proteins (methanobactin), enzymes (methane monooxygenase) or heterologous proteins hold promise as future products. Here, scenarios are presented where ectoine, polyhydroxybutyrate or protein G are synthesised as the primary product, in conjunction with a variety of ancillary products that could enhance process viability. Single or dual-stage processes and volumetric requirements for bioreactors are discussed, in terms of an annual biomass output of 1000 tonnesyear(-1). Product yields are discussed in relation to methane and oxygen consumption and organic waste generation.

The Opportunity for High-Performance Biomaterials from Methane

Polyhydroxyalkanoate (PHA) biopolymers are widely recognised as outstanding candidates to replace conventional petroleum-derived polymers. Their mechanical properties are good and can be tailored through copolymer composition, they are biodegradable, and unlike many alternatives, they do not rely on oil-based feedstocks. Further, they are the only commodity polymer that can be synthesised intracellularly, ensuring stereoregularity and high molecular weight. However, despite offering enormous potential for many years, they are still not making a significant impact. This is broadly because commercial uptake has been limited by variable performance (inconsistent polymer properties) and high production costs of the raw polymer. Additionally, the main type of PHA produced naturally is poly-3-hydroxybutyrate (PHB), which has limited scope due to its brittle nature and low thermal stability, as well as its tendency to embrittle over time. Production cost is strongly impacted by the type of the feedstock used. In this article we consider: the production of PHAs from methanotrophs using methane as a cost-effective substrate; the use of mixed cultures, as opposed to pure strains; and strategies to generate a poly(3-hydroxybutyrate-co-3-hydroxyvalerate) copolymer (PHBV), which has more desirable qualities such as toughness and elasticity.

Effect of CH4/O2 ratio on fatty acid profile and polyhydroxybutyrate content in a heterotrophic-methanotrophic consortium

Understanding the role of heterotrophic-methanotrophic (H-Meth) communities is important for improvement of methane (CH4) oxidation capacities (MOC) particularly in conjunction with bio-product development in industrial bio-filters. Initially, a H-Meth consortium was established and enriched from marine sediments and characterized by next generation sequencing of the 16s rDNA gene. The enriched consortium was subjected to 10-50% CH4 (i.e., 0.20-1.6 CH4/O2 ratios) to study the effects on MOCs, biomass growth, fatty acid profiles and biopolymer (e.g. polyhydroxybutyrate; PHB) content. Methylocystis, Methylophaga and Pseudoxanthomonas dominated the H-Meth consortium. Culture enrichment of the H-Meth consortium resulted in 15-20-folds higher MOC compared to seed sediments. Increasing CH4 concentration (and decreased O2 levels) yielded higher MOCs, but did not improve total fatty acid contents. PHB contents varied between 2.5% and 8.5% independently of CH4/O2 ratios. The results suggest that H-Meth consortia could potentially be used in industrial bio-filters for production of biopolymer/biofuel precursors from CH4.

Methane as a resource: can the methanotrophs add value?

Methane is an abundant gas used in energy recovery systems, heating, and transport. Methanotrophs are bacteria capable of using methane as their sole carbon source. Although intensively researched, the myriad of potential biotechnological applications of methanotrophic bacteria has not been comprehensively discussed in a single review. Methanotrophs can generate single-cell protein, biopolymers, components for nanotechnology applications (surface layers), soluble metabolites (methanol, formaldehyde, organic acids, and ectoine), lipids (biodiesel and health supplements), growth media, and vitamin B12 using methane as their carbon source. They may be genetically engineered to produce new compounds such as carotenoids or farnesene. Some enzymes (dehydrogenases, oxidase, and catalase) are valuable products with high conversion efficiencies and can generate methanol or sequester CO2 as formic acid ex vivo. Live cultures can be used for bioremediation, chemical transformation (propene to propylene oxide), wastewater denitrification, as components of biosensors, or possibly for directly generating electricity. This review demonstrates the potential for methanotrophs and their consortia to generate value while using methane as a carbon source. While there are notable challenges using a low solubility gas as a carbon source, the massive methane resource, and the potential cost savings while sequestering a greenhouse gas, keeps interest piqued in these unique bacteria.

Influence of biogas flow rate on biomass composition during the optimization of biogas upgrading in microalgal-bacterial processes

The influence of biogas flow rate (0, 0.3, 0.6, and 1.2 m(3) m(-2) h(-1)) on the elemental and macromolecular composition of the algal-bacterial biomass produced from biogas upgrading in a 180 L photobioreactor interconnected to a 2.5 L external bubbled absorption column was investigated using diluted anaerobically digested vinasse as cultivation medium. The influence of the external liquid recirculation/biogas ratio (0.5 < L/G < 67) on the removal of CO2 and H2S, and on the concentrations of O2 and N2 in the upgraded biogas, was also evaluated. A L/G ratio of 10 was considered optimum to support CO2 and H2S removals of 80% and 100%, respectively, at all biogas flow rates tested. Biomass productivity increased at increasing biogas flow rate, with a maximum of 12 ± 1 g m(-2) d(-1) at 1.2 m(3) m(-2) h(-1), while the C, N, and P biomass content remained constant at 49 ± 2%, 9 ± 0%, and 1 ± 0%, respectively, over the 175 days of experimentation. The high carbohydrate contents (60-76%), inversely correlated to biogas flow rates, would allow the production of ≈100 L of ethanol per 1000 m(3) of biogas upgraded under a biorefinery process approach.

Assessing the influence of CH4 concentration during culture enrichment on the biodegradation kinetics and population structure

Methanotrophic communities were enriched in three stirred tank reactors continuously supplied with CH4-laden air at 20, 2 and 0.2 gCH4 m(-3) in order to evaluate the influence of CH4 concentration on the biodegradation kinetics, population structure and potential polyhydroxyalkanoate production under sequential nitrogen limitations. The population structure of the enriched cultures, dominated by type I methanotrophs, was influenced by CH4 concentration. No significant correlation between CH4 concentration and the maximum specific degradation rate (qmax) or the half-saturation constant (KS) was recorded, microorganisms enriched at 2 gCH4 m(-3) presenting the highest qmax and those enriched at 20 and 0.2 gCH4 m(-3) exhibiting the lowest KS. Maximum polyhydroxybutyrate (PHB) contents of 1.0% and 12.6% (w/w) were achieved at 20 and 2 g CH4 m(-3), respectively. Polyhydroxyvalerate (PHV) was also detected at PHV:PHB ratios of up to 12:1 and 4:1 in the communities enriched at 20 and 0.2 gCH4 m(-3), respectively.

Polyhydroxyalkanoates accumulation by Methylobacterium organophilum CZ-2 during methane degradation using citrate or propionate as cosubstrates

Methylobacterium organophilum CZ-2 synthesized polyhydroxyalkanoates (PHAs) under nitrogen limitation with CH4 as carbon source and when either citrate or propionate was added as cosubstrates. The highest PHAs content (yPHA) in closed flasks was obtained in the CH4-citrate and CH4-propionate experiments attaining values of 0.82 and 0.68, respectively. M. organophilum CZ-2 cultivated in bioreactors with citrate and continuous CH4 addition yielded a final PHAs concentration of 143 gm(-3) containing hydroxybutyrate (HB), hydroxyvalerate (HV) and hydroxyoctanoate (HO), in a 55:35:10 ratio, with, yPHA of 0.88 and a CH4 elimination capacity (EC) of 20 gm(-3) h(-1). With propionate, the yPHA was 0.3 and the EC around 8 gm(-3) h(-1). From 1H and 13C NMR experiments it was found that the polymer produced with CH4-citrate contained six different monomers: 3HB, 3HV, 4HV, 4-hydroxyheptanoate (4HH), 3HO and 4HO, showing the great versatility of this PHAs producing bacterium.

Microbial production of poly(hydroxybutyrate) from C₁ carbon sources

Polyhydroxybutyrate (PHB) is an attractive substitute for petrochemical plastic due to its similar properties, biocompatibility, and biodegradability. The cost of scaled-up PHB production inhibits its widespread usage. Intensive researches are growing to reduce costs and improve thermomechanical, physical, and processing properties of this green biopolymer. Among cheap substrates which are used for reducing total cost of PHB production, some C₁ carbon sources, e.g., methane, methanol, and CO₂ have received a great deal of attention due to their serious role in greenhouse problem. This article reviews the fundamentals of strategies for reducing PHA production and moves on to the applications of several cheap substrates with a special emphasis on methane, methanol, and CO₂. Also, some explanation for involved microorganisms including the hydrogen-oxidizing bacteria and methanotrophs, their history, culture condition, and nutritional requirements are given. After description of some important strains among the hydrogen-oxidizing and methanotrophic producers of PHB, the article is focused on limitations, threats, and opportunities for application and their future trends.