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

Engineering bacterial microcompartments to enable sustainable microbial bioproduction from C1 greenhouse gases

One-carbon (C1) greenhouse gases are the primary driver of global climate change. Fermenting these gases into higher-value products is an attractive strategy for climate action and sustainable development. C1 gas-fermenting bacteria are promising chassis organisms, but various technical challenges hinder scale-up to industrial production levels. Bacterial microcompartments (MCPs), proteinaceous organelles that encapsulate enzymatic pathways, may confer several metabolic benefits to increase the industrial potential of these bacteria. Many species of gas-fermenting bacteria are already predicted to natively produce MCPs. Here, we describe how these organelles can be identified and engineered to encapsulate pathways that convert C1 gases into valuable chemical products.

Advancements in the production of value-added products via methane biotransformation by methanotrophs: Current status and future perspectives

Methane gas is recognized as a promising carbon substrate for the biosynthesis of value-added products due to its abundance and low price. Methanotrophs utilized methane as their sole source of carbon and energy, thus they can serve as efficient biocatalysts for methane bioconversion. Methanotrophs-catalyzed microbial bioconversion offer numerous advantages, compared to chemical processes. Current indirect chemical conversions of methane suffer from their energy-intensive processes and high capital expenditure. Methanotrophs can be cell factories capable of synthesizing various value-added products from methane such as methanol, organic acids, ectoine, polyhydroxyalkanoates, etc. However, the large-scale commercial implementation using methanotrophs remains a formidable challenge, primarily due to limitations in gas-liquid mass transfer and low metabolic capacity. This review explores recent advancements in methanotroph research, providing insights into their potential for enabling methane bioconversion.

Production of ectoine by Vreelandella boliviensis using non-aseptic repeated-batch and continuous cultivations in an air-lift bioreactor

Ectoine, an osmolyte produced by various microorganisms, has numerous commercial applications. Vreelandella boliviensis (formerly called Halomonas boliviensis) generates high ectoine concentrations, i.e., 78.6 g/L. This study investigated three cultivation strategies for ectoine production in a non-aseptic air-lift bioreactor. The first strategy was performed in a repeated-batch mode with 5% (w/v) NaCl to induce cell growth, followed by the addition of solid NaCl to a final concentration of 12.5% (w/v) to prompt ectoine production. A maximum dry cell weight of 13.8 g/L at 46.5 h, a maximum ectoine concentration of 1.37 g/L at 37.5 h, and a maximum volumetric productivity of 0.93 g/L/d at 34.5 h were reached. The second strategy employed a three-step repeated-batch cultivation method. In the first step, cells were grown at the optimum salt concentration, harvested by centrifugation, and cultivated in a replenished medium for the second step. In the third step, the cells were harvested again and grown in a fresh medium containing 12.5% (w/v) NaCl. This strategy improved dry cell weight to 32 g/L, ectoine concentration to 4.37 g/L, and productivity to 1.76 g/L/day at 60 h of cultivation. The third strategy consisted of continuous cultivations that were investigated using different NaCl concentrations. The highest ectoine concentration of 2.83 g/L and productivity of 3.49 g/L/d were obtained with 8.5% (w/v) NaCl at a dilution rate of 0.05 (1/h). This study is the first to report ectoine production by V. boliviensis in continuous air-lift bioreactors under non-aseptic conditions.

Microbial electrosynthesis technology for CO2 mitigation, biomethane production, and ex-situ biogas upgrading

Currently, global annual CO2 emissions from fossil fuel consumption are extremely high, surpassing tens of billions of tons, yet our capacity to capture and utilize CO2 remains below a small fraction of the amount generated. Microbial electrosynthesis (MES) systems, an integration of microbial metabolism with electrochemistry, have emerged as a highly efficient and promising bio-based carbon-capture-and-utilization technology over other conventional techniques. MES is a unique technology for lowering the atmospheric CO2 as well as CO2 in the biogas, and also simultaneously convert them to renewable bioenergy, such as biomethane. As such, MES techniques could be applied for biogas upgrading to generate high purity biomethane, which has the potential to meet natural gas standards. This article offers a detailed overview and assessment of the latest advancements in MES for biomethane production and biogas upgrading, in terms of selecting optimal methane production pathways and associated electron transfer processes, different electrode materials and types, inoculum sources and microbial communities, ion-exchange membrane, externally applied energy level, operating temperature and pH, mode of operation, CO2 delivery method, selection of inorganic carbon source and its concentration, start-up time, and system pressure. It also highlights the current MES challenges associated with upscaling, design and configuration, long-term stability, energy demand, techno-economics, achieving net negative carbon emission, and other operational issues. Moreover, we provide a summary of current and future opportunities to integrate MES with other unique biosystems, such as methanotrophic bioreactors, and incorporate quorum sensing, 3D printing, and machine learning to further develop MES as a better biomethane-producer and biogas upgrading technique.

Quantitative assessment of methane bioconversion based on kinetics and bioenergetics

The biological conversion of methane under ambient conditions can be performed by methanotrophs that utilize methane as both a sole source of energy and a carbon source. However, compared to the established microbial chassis used for general fermentation with sugar as a feedstock, the productivity of methanotrophs is low. The fundamental knowledge of their metabolic or cellular bottlenecks is limited. In this review, the industrial-scale potential of methane bioconversion was evaluated. In particular, the enzyme kinetics associated with the oxidation and assimilation of methane were investigated to evaluate the potential of methane fermentation. The kinetics of enzymes involved in methane metabolism were compared with those used in the metabolic processes of traditional fermentation (glycolysis). Through this analysis, the current limitations of methane metabolism were identified. Methods for increasing the efficiency of methane bioconversion and directions for the industrial application of methane-based fermentation were discussed.

Optimization of methane gas-liquid mass transfer during biogas-based ectoine production in bubble column bioreactors

Nowadays, the utilization of biogas for energy generation is hindered by the declining production costs of solar and wind power. A shift towards the valorization of biogas into ectoine, a highly valuable bioproduct priced at 1000 €⸱kg-1, offers a novel approach to fostering a more competitive biogas market while contributing to carbon neutrality. This study evaluated the optimization of CH4 gas-liquid mass transfer in 10 L bubble column bioreactors for CH4 conversion into ectoine and hydroxyectoine using a mixed methanotrophic culture. The influence of the empty bed residence time (EBRTs of 27, 54, and 104 min) at different membrane diffuser pore sizes (0.3 and 0.6 mm) was investigated. Despite achieving CH4 elimination capacities (CH4-ECs) of 10-12 g⸱m-3⸱h-1, an EBRT of 104 min mediated CH4 limitation within the cultivation broth, resulting in a negligible biomass growth. Reducing the EBRT to 54 min entailed CH4-ECs of 21-24 g⸱m-3⸱h-1, concomitant to a significant increase in biomass growth (up to 0.17 g⸱L⸱d-1) and reaching maximum ectoine and hydroxyectoine accumulation of 79 and 13 mg⸱gVSS-1, respectively. Conversely, process operation at an EBRT of 27 min lead to microbial inhibition, resulting in a reduced biomass growth of 0.09 g⸱L⸱d-1 and an ectoine content of 47 mg⸱gVSS-1. While the influence of diffuser pore size was less pronounced compared to EBRT, the optimal process performance was observed with a diffuser pore size of 0.6 mm.

Metabolic Engineering of Corynebacterium glutamicum for Highly Efficient Production of Ectoine

Ectoine is a compatible solute that functions as a cell protector from various stresses, protecting cells and stabilizing biomolecules, and is widely used in medicine, cosmetics, and biotechnology. Microbial fermentation has been widely used for the large-scale production of ectoine, and a number of fermentation strategies have been developed to increase the ectoine yield, reduce production costs, and simplify the production process. Here, Corynebacterium glutamicum was engineered for ectoine production by heterologous expression of the ectoine biosynthesis operon ectBAC gene from Halomonas elongata, and a series of genetic modifications were implemented. This included introducing the de3 gene from Escherichia coli BL21 (DE3) to express the T7 promoter, eliminating the lysine transporter protein lysE to limit lysine production, and performing a targeted mutation lysCS301Y on aspartate kinase to alleviate feedback inhibition of lysine. The new engineered strain Ect10 obtained an ectoine titer of 115.87 g/L in an optimized fed-batch fermentation, representing the highest ectoine production level in C. glutamicum and achieving the efficient production of ectoine in a low-salt environment.

Survival strategies of aerobic methanotrophs under hypoxia in methanogenic lake sediments

BACKGROUND

Microbial methane oxidation, methanotrophy, plays a crucial role in mitigating the release of the potent greenhouse gas methane from aquatic systems. While aerobic methanotrophy is a well-established process in oxygen-rich environments, emerging evidence suggests their activity in hypoxic conditions. However, the adaptability of these methanotrophs to such environments has remained poorly understood. Here, we explored the genetic adaptability of aerobic methanotrophs to hypoxia in the methanogenic sediments of Lake Kinneret (LK). These LK methanogenic sediments, situated below the oxidic and sulfidic zones, were previously characterized by methane oxidation coupled with iron reduction via the involvement of aerobic methanotrophs.

RESULTS

In order to explore the adaptation of the methanotrophs to hypoxia, we conducted two experiments using LK sediments as inoculum: (i) an aerobic "classical" methanotrophic enrichment with ambient air employing DNA stable isotope probing (DNA-SIP) and (ii) hypoxic methanotrophic enrichment with repeated spiking of 1% oxygen. Analysis of 16S rRNA gene amplicons revealed the enrichment of Methylococcales methanotrophs, being up to a third of the enriched community. Methylobacter, Methylogaea, and Methylomonas were prominent in the aerobic experiment, while hypoxic conditions enriched primarily Methylomonas. Using metagenomics sequencing of DNA extracted from these experiments, we curated five Methylococcales metagenome-assembled genomes (MAGs) and evaluated the genetic basis for their survival in hypoxic environments. A comparative analysis with an additional 62 Methylococcales genomes from various environments highlighted several core genetic adaptations to hypoxia found in most examined Methylococcales genomes, including high-affinity cytochrome oxidases, oxygen-binding proteins, fermentation-based methane oxidation, motility, and glycogen use. We also found that some Methylococcales, including LK Methylococcales, may denitrify, while metals and humic substances may also serve as electron acceptors alternative to oxygen. Outer membrane multi-heme cytochromes and riboflavin were identified as potential mediators for the utilization of metals and humic material. These diverse mechanisms suggest the ability of methanotrophs to thrive in ecological niches previously thought inhospitable for their growth.

CONCLUSIONS

Our study sheds light on the ability of enriched Methylococcales methanotrophs from methanogenic LK sediments to survive under hypoxia. Genomic analysis revealed a spectrum of genetic capabilities, potentially enabling these methanotrophs to function. The identified mechanisms, such as those enabling the use of alternative electron acceptors, expand our understanding of methanotroph resilience in diverse ecological settings. These findings contribute to the broader knowledge of microbial methane oxidation and have implications for understanding and potential contribution methanotrophs may have in mitigating methane emissions in various environmental conditions.

High production of ectoine from methane in genetically engineered Methylomicrobium alcaliphilum 20Z by preventing ectoine degradation

BACKGROUND

Methane is a greenhouse gas with a significant potential to contribute to global warming. The biological conversion of methane to ectoine using methanotrophs represents an environmentally and economically beneficial technology, combining the reduction of methane that would otherwise be combusted and released into the atmosphere with the production of value-added products.

RESULTS

In this study, high ectoine production was achieved using genetically engineered Methylomicrobium alcaliphilum 20Z, a methanotrophic ectoine-producing bacterium, by knocking out doeA, which encodes a putative ectoine hydrolase, resulting in complete inhibition of ectoine degradation. Ectoine was confirmed to be degraded by doeA to N-α-acetyl-L-2,4-diaminobutyrate under nitrogen depletion conditions. Optimal copper and nitrogen concentrations enhanced biomass and ectoine production, respectively. Under optimal fed-batch fermentation conditions, ectoine production proportionate with biomass production was achieved, resulting in 1.0 g/L of ectoine with 16 g/L of biomass. Upon applying a hyperosmotic shock after high-cell-density culture, 1.5 g/L of ectoine was obtained without further cell growth from methane.

CONCLUSIONS

This study suggests the optimization of a method for the high production of ectoine from methane by preventing ectoine degradation. To our knowledge, the final titer of ectoine obtained by M. alcaliphilum 20ZDP3 was the highest in the ectoine production from methane to date. This is the first study to propose ectoine production from methane applying high cell density culture by preventing ectoine degradation.

Application of microbial resources in biorefineries: Current trend and future prospects

The recent growing interest in sustainable and alternative sources of energy and bio-based products has driven the paradigm shift to an integrated model termed "biorefinery." Biorefinery framework implements the concepts of novel eco-technologies and eco-efficient processes for the sustainable production of energy and value-added biomolecules. The utilization of microbial resources for the production of various value-added products has been documented in the literatures. However, the appointment of these microbial resources in integrated resource management requires a better understanding of their status. The main of aim of this review is to provide an overview on the defined positioning and overall contribution of the microbial resources, i.e., algae, fungi and bacteria, for various bioprocesses and generation of multiple products from a single biorefinery. By utilizing waste material as a feedstock, biofuels can be generated by microalgae while sequestering environmental carbon and producing value added compounds as by-products. In parallel, fungal biorefineries are prolific producers of lignocellulose degrading enzymes along with pharmaceutically important novel products. Conversely, bacterial biorefineries emerge as a preferred platform for the transformation of standard cells into proficient bio-factories, developing chassis and turbo cells for enhanced target compound production. This comprehensive review is poised to offer an intricate exploration of the current trends, obstacles, and prospective pathways of microbial biorefineries, for the development of future biorefineries.

A comprehensive report on valorization of waste to single cell protein: strategies, challenges, and future prospects

The food insecurity due to a vertical increase in the global population urgently demands substantial advancements in the agricultural sector and to identify sustainable affordable sources of nutrition, particularly proteins. Single-cell protein (SCP) has been revealed as the dried biomass of microorganisms such as algae, yeast, and bacteria cultivated in a controlled environment. Production of SCP is a promising alternative to conventional protein sources like soy and meat, due to quicker production, minimal land requirement, and flexibility to various climatic conditions. In addition to protein production, it also contributes to waste management by converting it into food and feed for both human and animal consumption. This article provides an overview of SCP production, including its benefits, safety, acceptability, and cost, as well as limitations that constrains its maximum use. Furthermore, this review criticizes the downstream processing of SCP, encompassing cell wall disruption, removal of nucleic acid, harvesting of biomass, drying, packaging, storage, and transportation. The potential applications of SCP, such as in food and feed as well as in the production of bioplastics, emulsifiers, and as flavoring agents for baked food, soup, and salad, are also discussed.

Enrichment of mixed methanotrophic cultures producing polyhydroxyalkanoates (PHAs) from various environmental sources

Methanotrophic bacteria can use atmospheric methane (CH4) as a sole carbon source for the growth and production of polyhydroxyalkanoates (PHA). The development of CH4 bioconversion processes relies heavily on the selection of an efficient methanotrophic culture. This research assessed the effect of selected growth conditions, such as nitrogen sources on the enrichment of methanotrophic cultures from various environments for PHA accumulation. Nitrate-based medium favoured the culture growth and selection for PHA-producing methanotrophic cultures with Methylocystis sp. as a major genus and accumulation of up to 27 % polyhydroxybutyrate (PHB) in the biomass. Three PHB-producing cultures: enriched from waste activated sludge (AS), peat bog soil (PB) and landfill biocover soil (LB) were then tested for their ability to produce PHA copolymer at different CH4:O2 ratios. All enriched cultures were able to utilise valeric acid as a cosubstrate for the accumulation of PHA with a 3-hydroxyvaleric (3HV) fraction of 21-41 mol% depending on the inoculum source and CH4 concentration. The process performance of selected cultures was evaluated and compared to the culture of reference strain Methylocystis hirsuta DSM 18500. All mixed cultures irrespective of their inoculum source had similar levels of 3HV fraction in the PHA (38 ± 2 mol%). The highest poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) production was observed for AS culture at 10 % CH4 with an accumulation of 27 ± 3 % of dry cell weight (DCW), 3HV fraction of 39 ± 2 mol% and yield of 0.42 ± 0.02 g-PHA/g-substrate.

Direct Methane Oxidation by Copper- and Iron-Dependent Methane Monooxygenases

Methane is a potent greenhouse gas that contributes significantly to climate change and is primarily regulated in Nature by methanotrophic bacteria, which consume methane gas as their source of energy and carbon, first by oxidizing it to methanol. The direct oxidation of methane to methanol is a chemically difficult transformation, accomplished in methanotrophs by complex methane monooxygenase (MMO) enzyme systems. These enzymes use iron or copper metallocofactors and have been the subject of detailed investigation. While the structure, function, and active site architecture of the copper-dependent particulate methane monooxygenase (pMMO) have been investigated extensively, its putative quaternary interactions, regulation, requisite cofactors, and mechanism remain enigmatic. The iron-dependent soluble methane monooxygenase (sMMO) has been characterized biochemically, structurally, spectroscopically, and, for the most part, mechanistically. Here, we review the history of MMO research, focusing on recent developments and providing an outlook for future directions of the field. Engineered biological catalysis systems and bioinspired synthetic catalysts may continue to emerge along with a deeper understanding of the molecular mechanisms of biological methane oxidation. Harnessing the power of these enzymes will necessitate combined efforts in biochemistry, structural biology, inorganic chemistry, microbiology, computational biology, and engineering.

Carbon dioxide and methane as carbon source for the production of polyhydroxyalkanoates and concomitant carbon fixation

The currently used plastics are non-biodegradable, and cause greenhouse gases (GHGs) emission as they are petroleum-based. Polyhydroxyalkanoates (PHAs) are biopolymers with excellent biodegradability and biocompatibility, which can be used to replace petroleum-based plastics. A variety of microorganisms have been found to synthesize PHAs by using typical GHGs: carbon dioxide and methane as carbon sources. Converting carbon dioxide (CO2) and methane (CH4) to PHAs is an attractive option for carbon capture and biodegradable plastic production. In this review, the microorganisms capable of using CO2 and CH4 to produce PHAs were summarized. The metabolic mechanism, PHAs production process, and the factors influencing the production process are illustrated. The currently used optimization techniques to improve the yield of PHAs are discussed. The challenges and future prospects for developing economically viable PHAs production using GHGs as carbon source are identified. This work provides an insight for achieving carbon sequestration and bioplastics based circular economy.

Strategies for the biological production of ectoine by using different chassis strains

As an amino acid derivative and a typical compatible solute, ectoine can assist microorganisms in resisting high osmotic pressure. Own to its long-term moisturizing effects, ectoine shows extensive applications in cosmetics, medicine and other fields. With the rapid development of synthetic biology and fermentation engineering, many biological strategies have been developed to improve the ectoine production and simplify the production process. Currently, the microbial fermentation has been widely used for large scaling ectoine production. Accordingly, this review will introduce the metabolic pathway for ectoine synthesis and also comprehensively evaluate both wild-type and genetically modified strains for ectoine production. Furthermore, process parameters affecting the ectoine production efficiency and adoption of low cost substrates will be evaluated. Lastly, future prospects on the improvement of ectoine production will be proposed.

Microbial gas fermentation technology for sustainable food protein production

The development of novel, sustainable, and robust food production technologies represents one of the major pillars to address the most significant challenges humanity is going to face on earth in the upcoming decades - climate change, population growth, and resource depletion. The implementation of microfoods, i.e., foods formulated with ingredients from microbial cultivation, into the food supply chain has a huge potential to contribute towards energy-efficient and nutritious food manufacturing and represents a means to sustainably feed a growing world population. This review recapitulates and assesses the current state in the establishment and usage of gas fermenting bacteria as an innovative feedstock for protein production. In particular, we focus on the most promising representatives of this taxon: the hydrogen-oxidizing bacteria (hydrogenotrophs) and the methane-oxidizing bacteria (methanotrophs). These unicellular microorganisms can aerobically metabolize gaseous hydrogen and methane, respectively, to provide the required energy for building up cell material. A protein yield over 70% in the dry matter cell mass can be reached with no need for arable land and organic substrates making it a promising alternative to plant- and animal-based protein sources. We illuminate the holistic approach to incorporate protein extracts obtained from the cultivation of gas fermenting bacteria into microfoods. Herein, the fundamental properties of the bacteria, cultivation methods, downstream processing, and potential food applications are discussed. Moreover, this review covers existing and future challenges as well as sustainability aspects associated with the production of microbial protein through gas fermentation.

Highly efficient production of ectoine via an optimized combination of precursor metabolic modules in Escherichia coli BL21

Ectoine is an osmotic pressure protectant observed in various microorganisms and is widely used in cosmetics and pharmaceuticals. The market value of ectoine has increased considerably with social progress, resulting in high demand for ectoine production technology. Herein, a microbial cell factory in Escherichia coli that produces ectoine at high titers is described as developing efficient and environmentally friendly bio-based ectoine production technology. The ectoine biosynthetic pathway of Halomonas hydrothermalis was introduced into E. coli BL21 (DE3). Subsequent overexpression of precursor metabolic modules, including aspartate branching, pyruvate-oxoacetate, and glutamate biosynthesis pathways, resulted in the final strain, E. coli BCT08, which produced ectoine at a titer of 36.58 g/L during 30 h of fermentation. Sugar feeding speed optimization improved the ectoine titer to 131.8 g/L after 96 h of cultivation. This represents a remarkable achievement in ectoine production from glucose under low-salt conditions and has vast potential for industrial applications.

Strong bacterial stochasticity and fast fungal turnover in Taihu Lake sediments, China

Understanding the assembly and turnover of microbial communities is crucial for gaining insights into the diversity and functioning of lake ecosystems, a fundamental and central issue in microbial ecology. The ecosystem of Taihu Lake has been significantly jeopardized due to urbanization and industrialization. In this study, we examined the diversity, assembly, and turnover of bacterial and fungal communities in Taihu Lake sediment. The results revealed strong bacterial stochasticity and fast fungal turnover in the sediment. Significant heterogeneity was observed among all sediment samples in terms of environmental factors, especially ORP, TOC, and TN, as well as microbial community composition and alpha diversity. For instance, the fungal richness index exhibited an approximate 3-fold variation. Among the environmental factors, TOC, TN, and pH had a more pronounced influence on the bacterial community composition compared to the fungal community composition. Interestingly, species replacement played a dominant role in microbial beta diversity, with fungi exhibiting a stronger pattern. In contrast, stochastic processes governed the community assembly of both bacteria and fungi, but were more pronounced for bacteria (R2 = 0.7 vs. 0.5). These findings deepen the understanding of microbial assembly and turnover in sediments under environmental stress and provide essential insights for maintaining the multifunctionality of lake ecosystems.

A bio-augmented system with Methylosarcina sp. LC-4 immobilized on bio-carriers: Towards an integrated approach to mitigate and valorize methane emissions from landfills to biodiesel

Bio-augmented systems based on methanotrophs are indispensable in curbing anthropogenic methane emissions from engineered landfills or dumpsites to curtail rising levels of greenhouse gases. Using a defined methanotroph culture immobilized on an inert material-based bio-carrier makes it possible to harness these methane emissions for creating value-added products, thus contributing to the circular bio-economy. The methane oxidation capacity of the model methanotroph Methylosarcina sp. LC-4, a prospective organism for biodiesel production using methane present in landfill gas, immobilized on several inert bio-carriers, was evaluated to identify a bio-carrier that provided optimum conditions for the process. Among the several bio-carriers evaluated, perlite and vermiculite were selected due to their high specific surface area and superior water-holding capacity, which result in the retention of nutrients and biomass and higher methane elimination capacity. While perlite showed high biomass holding capacity and methane transport, vermiculite supported a high growth of methanotrophs. LC-4 immobilized on perlite and vermiculite as the bio-carrier showed maximum methane elimination capacity (MEC) of 291.3 g m-2 day-1 and 155.5 g m-2 day-1, respectively. The low bed height of only 0.13 m and a short start-up period of 2-4 days are promising for use as alternate daily cover in a landfill. The recovered biomass had 12% (w/w) fatty acid methyl ester (FAME), with a high fraction of (∼85%) of C14-C18 saturated and monounsaturated fatty acids, suitable for biodiesel production. The combination of perlite and vermiculite increased MEC and FAME content levels. The current study demonstrated a new bio-augmented system designed with a pure methanotroph for methane elimination with a short start-up time and the valorization of the assimilated methane.

Transcriptional and metabolomic responses of Methylococcus capsulatus Bath to nitrogen source and temperature downshift

Methanotrophs play a significant role in methane oxidation, because they are the only biological methane sink present in nature. The methane monooxygenase enzyme oxidizes methane or ammonia into methanol or hydroxylamine, respectively. While much is known about central carbon metabolism in methanotrophs, far less is known about nitrogen metabolism. In this study, we investigated how Methylococcus capsulatus Bath, a methane-oxidizing bacterium, responds to nitrogen source and temperature. Batch culture experiments were conducted using nitrate or ammonium as nitrogen sources at both 37°C and 42°C. While growth rates with nitrate and ammonium were comparable at 42°C, a significant growth advantage was observed with ammonium at 37°C. Utilization of nitrate was higher at 42°C than at 37°C, especially in the first 24 h. Use of ammonium remained constant between 42°C and 37°C; however, nitrite buildup and conversion to ammonia were found to be temperature-dependent processes. We performed RNA-seq to understand the underlying molecular mechanisms, and the results revealed complex transcriptional changes in response to varying conditions. Different gene expression patterns connected to respiration, nitrate and ammonia metabolism, methane oxidation, and amino acid biosynthesis were identified using gene ontology analysis. Notably, key pathways with variable expression profiles included oxidative phosphorylation and methane and methanol oxidation. Additionally, there were transcription levels that varied for genes related to nitrogen metabolism, particularly for ammonia oxidation, nitrate reduction, and transporters. Quantitative PCR was used to validate these transcriptional changes. Analyses of intracellular metabolites revealed changes in fatty acids, amino acids, central carbon intermediates, and nitrogen bases in response to various nitrogen sources and temperatures. Overall, our results offer improved understanding of the intricate interactions between nitrogen availability, temperature, and gene expression in M. capsulatus Bath. This study enhances our understanding of microbial adaptation strategies, offering potential applications in biotechnological and environmental contexts.

Methanotrophic oxidation of organic micropollutants and nitrogen upcycling in a hybrid membrane biofilm reactor (hMBfR) for simultaneous O2 and CH4 supply

Pharmaceuticals and other organic micropollutants (OMPs) present in wastewater effluents are of growing concern, as they threaten environmental and human health. Conventional biological treatments lead to limited removal of OMPs. Methanotrophic bacteria can degrade a variety of OMPs. By employing a novel bubble-free hybrid membrane biofilm bioreactor (hMBfR), we grew methanotrophic bacteria at three CH4 loading rates. Biomass productivity and CH4 loading showed a linear correlation, with a maximum productivity of 372 mg-VSS·L-1·d-1, with corresponding biomass concentration of 1117.6 ± 56.4 mg-VSS·L-1. Furthermore, the biodegradation of sulfamethoxazole and 1H-benzotriazole positively correlated with CH4 oxidation rates, with highest biodegradation kinetic constants of 3.58 L·g-1·d-1 and 5.42 L·g-1·d-1, respectively. Additionally, the hMBfR recovered nutrients as microbial proteins, with an average content 39% DW. The biofilm community was dominated by Methylomonas, while the bulk was dominated by aerobic heterotrophic bacteria. The hMBfR removed OMPs, allowing for safer water reuse while valorising CH4 and nutrients.

Resource availability governs polyhydroxyalkanoate (PHA) accumulation and diversity of methanotrophic enrichments from wetlands

Aquatic environments account for half of global CH4 emissions, with freshwater wetlands being the most significant contributors. These CH4 fluxes can be partially offset by aerobic CH4 oxidation driven by methanotrophs. Additionally, some methanotrophs can convert CH4 into polyhydroxyalkanoate (PHA), an energy storage molecule as well as a promising bioplastic polymer. In this study, we investigate how PHA-accumulating methanotrophic communities enriched from wetlands were shaped by varying resource availability (i.e., C and N concentrations) at a fixed C/N ratio. Cell yields, PHA accumulation, and community composition were evaluated in high (20% CH4 and 10 mM NH4 +) and low resource (0.2% CH4 and 0.1 mM NH4 +) conditions simulating engineered and environmental settings, respectively. High resource availability decreased C-based cell yields, while N-based cell yields remained stable, suggesting nutrient exchange patterns differed between methanotrophic communities at different resource concentrations. PHA accumulation was only observed in high resource enrichments, producing approximately 12.6% ± 2.4% (m/m) PHA, while PHA in low resource enrichments remained below detection. High resource enrichments were dominated by Methylocystis methanotrophs, while low resource enrichments remained significantly more diverse and contained only a minor population of methanotrophs. This study demonstrates that resource concentration shapes PHA-accumulating methanotrophic communities. Together, this provides useful information to leverage such communities in engineering settings as well as to begin understanding their role in the environment.

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.

Polyhydroxybutyrate Production from Methane and Carbon Dioxide by a Syntrophic Consortium of Methanotrophs with Oxygenic Photogranules without an External Oxygen Supply

Here, a syntrophic process was developed to produce polyhydroxy-β-butyrate (PHB) from a gas stream containing CH₄ and CO₂ without an external oxygen supply using a combination of methanotrophs with the community of oxygenic photogranules (OPGs). The co-culture features of Methylomonas sp. DH-1 and Methylosinus trichosporium OB3b were evaluated under carbon-rich and carbon-lean conditions. The critical role of O₂ in the syntrophy was confirmed through the sequencing of 16S rRNA gene fragments. Based on their carbon consumption rates and the adaptation to a poor environment, M. trichosporium OB3b with OPGs was selected for methane conversion and PHB production. Nitrogen limitation stimulated PHB accumulation in the methanotroph but hindered the growth of the syntrophic consortium. At 2.9 mM of the nitrogen source, 1.13 g/L of biomass and 83.0 mg/L of PHB could be obtained from simulated biogas. These results demonstrate that syntrophy has the potential to convert greenhouse gases into valuable products efficiently.

High Cell-Density Cultivation of Methylococcus capsulatus Bath for Efficient Methane-Derived Mevalonate Production

The engineered Methylococcus capsulatus Bath presents a promising approach for converting methane, a potent greenhouse gas, into valuable chemicals. High cell-density culture (HCDC) is necessary for high-titer growth-associated bioproducts, but it often requires time-consuming and labor-intensive optimization processes. In this study, we aimed to achieve efficient HCDC of M. capsulatus Bath by measuring the residual nutrient levels during bioreactor operations and analyzing the specific uptake of each medium component. By controlling the concentrations of nutrients, particularly calcium and phosphorus via intermittent feeding, we achieved a high cell density of 28.2 g DCW/L and a significantly elevated production of mevalonate at a concentration of 1.8 g/L from methane. Our findings demonstrate that the methanotroph HCDC approach presented herein offers a promising strategy for promoting sustainable development, with an exceptional g-scale production titer for value-added synthetic biochemicals.

Bioactivity profiling of the extremolyte ectoine as a promising protectant and its heterologous production

Ectoine is a compatible solutes that is diffusely dispersed in bacteria and archaea. It plays a significant role as protectant against various external pressures, such as high temperature, high osmolarity, dryness and radiation, in cells. Ectoine can be utilized in cosmetics due to its properties of moisturizing and antiultraviolet. It can also be used in the pharmaceutical industry for treating various diseases. Therefore, strong protection of ectoine creates a high commercial value. Its current market value is approximately US$1000 kg-1. However, traditional ectoine production in high-salinity media causes high costs of equipment loss and wastewater treatment. There is a growing attention to reduce the salinity of the fermentation broth without sacrificing the production of ectoine. Thus, heterologous production of ectoine in nonhalophilic microorganisms may represent the new generation of the industrial production of ectoine. In this review, we summarized and discussed the biological activities of ectoine on cell and human health protection and its heterologous production.

Synthetic design of methanotroph co-cultures and their immobilization within polymers containing magnetic nanoparticles to enhance methanol production from wheat straw-based biogas

In this study, various methanotroph co-cultures were designed to enhance methanol production from biogas produced through the anaerobic digestion of wheat straw (WS). Furthermore, whole-cell immobilization was performed using magnetic nanoparticle (MNP)-loaded polymers to develop an efficient bioprocess. The anaerobic digestion of WS by cattle dung yielded 219 L/kg of total solids reduced. Methanol produced was 5.08 and 6.39 mmol/L by pure- and co-cultures from biogas, respectively. The optimization of process parameters enhanced methanol production to 6.82 mmol/L by co-culturing Mithylosinus sporium and Methylocella tundrae. The immobilized co-culture within the MNP-doped polymers exhibited much higher cumulative methanol of up to 70.74 mmol/L than the production of 22.34 mmol/L by free cells after ten cycles of reuse. This study suggests that MNP-doped polymer-based immobilization of methanotrophs is a unique approach for producing renewable fuels from biomass-derived biogas, a greenhouse gas.

Nitrate/nitrite-dependent anaerobic oxidation of methane (N-AOM) as a technology platform for greenhouse gas abatement in wastewater treatment plants: State-of-the-art and challenges

Nitrate/nitrite-dependent anaerobic oxidation of methane (N-AOM) is a metabolic process recently discovered and partially characterized in terms of the microorganisms and pathways involved. The N-AOM process can be a powerful tool for mitigating the impacts of greenhouse gas emissions from wastewater treatment plants by coupling the reduction of nitrate or nitrite with the oxidation of residual dissolved methane. Besides specific anaerobic methanotrophs such as bacteria members of the phylum NC10 and archaea belonging to the lineage ANME-2d, recent reports suggested that other methane-oxidizing bacteria in syntrophy with denitrifiers can also perform the N-AOM process, which facilitates the application of this metabolic process for the oxidation of residual methane under realistic scenarios. This work constitutes a state-of-art review that includes the fundamentals of the N-AOM process, new information on process microbiology, bioreactor configurations, and operating conditions for process implementation in WWTP. Potential advantages of the N-AOM process over aerobic methanotrophic biotechnologies are presented, including the potential interrelation of the N-AOM with other nitrogen removal processes within the WWTP, such as the anaerobic ammonium oxidation. This work also addressed the challenges of this biotechnology towards its application at full scale, identifying and discussing critical research niches.

Batch Experiments Demonstrating a Two-Stage Bacterial Process Coupling Methanotrophic and Heterotrophic Bacteria for 1-Alkene Production From Methane

Methane (CH₄) is a sustainable carbon feedstock for value-added chemical production in aerobic CH₄-oxidizing bacteria (methanotrophs). Under substrate-limited (e.g., oxygen and nitrogen) conditions, CH₄ oxidation results in the production of various short-chain organic acids and platform chemicals. These CH₄-derived products could be broadened by utilizing them as feedstocks for heterotrophic bacteria. As a proof of concept, a two-stage system for CH₄ abatement and 1-alkene production was developed in this study. Type I and Type II methanotrophs, Methylobacter tundripaludum SV96 and Methylocystis rosea SV97, respectively, were investigated in batch tests under different CH₄ and air supplementation schemes. CH₄ oxidation under either microaerobic or aerobic conditions induced the production of formate, acetate, succinate, and malate in M. tundripaludum SV96, accounting for 4.8–7.0% of consumed carbon from CH₄ (C-CH₄), while M. rosea SV97 produced the same compounds except for malate, and with lower efficiency than M. tundripaludum SV96, accounting for 0.7–1.8% of consumed C-CH₄. For the first time, this study demonstrated the use of organic acid-rich spent media of methanotrophs cultivating engineered Acinetobacter baylyi ADP1 ‘tesA-undA cells for 1-alkene production. The highest yield of 1-undecene was obtained from the spent medium of M. tundripaludum SV96 at 68.9 ± 11.6 μmol mol C_substrate^–1. However, further large-scale studies on fermenters and their optimization are required to increase the production yields of organic acids in methanotrophs.

Employing methylotrophs for a green economy: one-carbon to fuel them all and through metabolism redesign them

Microbial platforms are currently being optimized to revolutionize industrial energy production while mitigating shortages of global resources and food supplies. Here, we address recent advances to develop bacterial methylotrophic platforms as promising platforms enabling the reuse of products and materials (at their highest value) while reducing waste and pollution.

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.

Industrial production of microbial protein products

Microbial proteins provide a sustainable and nutritious alternative to traditional animal and plant-based proteins. Various strains have been demonstrated to generate biomass from a wide variety of substrates, from organic waste (e.g. banana peel) to gases (e.g. methane). Industrial production of microbial protein has proven difficult from both design (e.g. production rate) and regulatory (e.g. allergenicity of product) perspectives for both feed and food-grade products. Through use of low-cost microfluidics devices, early bioprocess design can be intensified, achieving high strain screening throughput with low titres. Integration of industrial waste streams (e.g. flue-gases, lignocellulosic residues) can reduce cost and carbon footprint of feedstock, while bespoke reactor design (e.g. Quorn's airlift U-loop fermenter) can remediate issues of low mass-transfer and product quality.

The role of methanotrophy in the microbial carbon metabolism of temperate lakes

Previous stable isotope and biomarker evidence has indicated that methanotrophy is an important pathway in the microbial loop of freshwater ecosystems, despite the low cell abundance of methane-oxidizing bacteria (MOB) and the low methane concentrations relative to the more abundant dissolved organic carbon (DOC). However, quantitative estimations of the relative contribution of methanotrophy to the microbial carbon metabolism of lakes are scarce, and the mechanism allowing methanotrophy to be of comparable importance to DOC-consuming heterotrophy remained elusive. Using incubation experiments, microscopy, and multiple water column profiles in six temperate lakes, we show that MOB play a much larger role than their abundances alone suggest because of their larger cell size and higher specific activity. MOB activity is tightly constrained by the local methane:oxygen ratio, with DOC-rich lakes with large hypolimnetic volume fraction showing a higher carbon consumption through methanotrophy than heterotrophy at the whole water column level. Our findings suggest that methanotrophy could be a critical microbial carbon consumption pathway in many temperate lakes, challenging the prevailing view of a DOC-centric microbial metabolism in these ecosystems.

Ectoine Production from Biogas in Waste Treatment Facilities: A Techno-Economic and Sensitivity Analysis

The capacity of haloalkaliphilic methanotrophic bacteria to synthesize ectoine from CH₄-biogas represents an opportunity for waste treatment plants to improve their economic revenues and align their processes to the incoming circular economy directives. A techno-economic and sensitivity analysis for the bioconversion of biogas into 10 t ectoine·y^-1 was conducted in two stages: (I) bioconversion of CH₄ into ectoine in a bubble column bioreactor and (II) ectoine purification via ion exchange chromatography. The techno-economic analysis showed high investment (4.2 M€) and operational costs (1.4 M€·y^-1). However, the high margin between the ectoine market value (600-1000 €·kg^-1) and the estimated ectoine production costs (214 €·kg^-1) resulted in a high profitability for the process, with a net present value evaluated at 20 years (NPV₂₀) of 33.6 M€. The cost sensitivity analysis conducted revealed a great influence of equipment and consumable costs on the ectoine production costs. In contrast to alternative biogas valorization into heat and electricity or into low added-value bioproducts, biogas bioconversion into ectoine exhibited high robustness toward changes in energy, water, transportation, and labor costs. The worst- and best-case scenarios evaluated showed ectoine break-even prices ranging from 158 to 275 €·kg^-1, ∼3-6 times lower than the current industrial ectoine market value.

In vivo quantification of polyhydroxybutyrate (PHB) in the alphaproteobacterial methanotroph, Methylocystis sp. Rockwell

Methane is a common industrial by-product that can be used as feedstock for production of the biopolymer polyhydroxybutyrate (PHB) by alphaproteobacterial methanotrophs. In vivo assessment of PHB production would shed light on the biosynthesis process and guide design of improved production strategies, but it is currently difficult to perform efficiently. In this study, the alphaproteobacterial methanotroph Methylocystis sp. Rockwell was grown on methane with three different nitrogen sources (ammonium, nitrate, and atmospheric nitrogen), and biomass samples were harvested at defined time points during lag, exponential, and stationary growth phases. PHB cell content was analyzed at these sampling points via a standard gas chromatography-flame ionization detector method, which requires hydrolysis of PHB and esterification of the resulting monomer under acidic conditions, and a novel, rapid, cost-effective approach based on fixation and staining of bacterial cells via Nile Blue A fluorescent dye enabling differential staining of cell membranes and intracellular PHB granules for single-cell analysis through fluorescence microscopy. Overall, the two PHB quantification approaches were in agreement at all stages of growth and in all three growing conditions tested. The PHB cell content was greatest with atmospheric nitrogen as a nitrogen source, followed by ammonium and nitrate. Under atmospheric nitrogen and ammonium conditions, PHB cell content decreased with growth progression, while under nitrate conditions PHB cell content remained unchanged in all growth phases. In addition to presenting a rapid, efficient method enabling in vivo quantification of PHB production, the present study highlights the impact of nitrogen source on PHB production by Methylocystis sp. Rockwell. KEY POINTS: • A novel fluorescence microscopy method to quantify PHB in single cells was developed • The microscopy method was validated by the derivation/gas chromatography method • Methylocystis sp. Rockwell synthesizes PHB granules without nutrient stress.

Methylocystis silviterrae sp.nov., a high-affinity methanotrophic bacterium isolated from the boreal forest soil

A novel species is proposed for a high-affinity methanotrophic representative of the genus Methylocystis. Strain FS^T was isolated from a weakly acidic (pH 5.3) mixed forest soil of the southern Moscow area. Cells of FS^T are aerobic, Gram-negative, non-motile, curved coccoids or short rods that contain an intracytoplasmic membrane system typical of type-II methanotrophs. Only methane and methanol are used as carbon sources. FS^T grew at a temperature range of 4-37 °C (optimum 25-30 °C) and a pH range of 4.5 to 7.5 (optimum pH 6.0-6.5). The major fatty acids were C_{18  :  1}ω8c, C_{18  :  1}ω7c and C_{18  :  0}; the major quinone as Q-8. FS^T displays 16S rRNA gene sequences similarity to other taxonomically recognized members of the genus Methylocystis, with Methylocystis hirsuta CSC1^T (99.6 % similarity) and Methylocystis rosea SV97^T (99.3 % similarity) as its closest relatives. The genome comprises 3.85 Mbp and has a DNA G+C content of 62.6 mol%. Genomic analyses and DNA-DNA relatedness with genome-sequenced members of the genus Methylocystis demonstrated that FS^T could be separated from its closest relatives. FS^T possesses two particulate methane monooxygenases (pMMO): low-affinity pMMO1 and high-affinity pMMO2. In laboratory experiments, it was demonstrated that FS^T might oxidize methane at atmospheric concentration. The genome contained various genes for nitrogen fixation, polyhydroxybutyrate synthesis, antibiotic resistance and detoxification of arsenic, cyanide and mercury. On the basis of genotypic, phenotypic and chemotaxonomic characteristics, it is proposed that the isolate represents a novel species, Methylocystis silviterrae sp. nov. The type strain is FS^T (=KCTC 82935^T=VKM B-3535^T).

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

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

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.

Burkholderia sacchari (synonym Paraburkholderia sacchari): An industrial and versatile bacterial chassis for sustainable biosynthesis of polyhydroxyalkanoates and other bioproducts

This is the first review presenting and discussing Burkholderia sacchari as a bacterial chassis. B. sacchari is a distinguished polyhydroxyalkanoates producer strain, with low biological risk, reaching high biopolymer yields from sucrose (0.29 g/g), and xylose (0.38 g/g). It has great potential for integration into a biorefinery using residues from biomass, achieving 146 g/L cell dry weight containing 72% polyhydroxyalkanoates. Xylitol (about 70 g/L) and xylonic acid [about 390 g/L, productivity 7.7 g/(L.h)] are produced by the wild-type B. sacchari. Recombinants were constructed to allow the production and monomer composition control of diverse tailor-made polyhydroxyalkanoates, and some applications have been tested. 3-hydroxyvalerate and 3-hydroxyhexanoate yields from substrate reached 80% and 50%, respectively. The genome-scale reconstruction of its metabolic network, associated with the improvement of tools for genetic modification, and metabolic fluxes understanding by future research, will consolidate its potential as a bioproduction chassis.

Biomedical Applications of Biomolecules Isolated from Methanotrophic Bacteria in Wastewater Treatment Systems

Wastewater treatment plants and other remediation facilities serve important roles, both in public health, but also as dynamic research platforms for acquiring useful resources and biomolecules for various applications. An example of this is methanotrophic bacteria within anaerobic digestion processes in wastewater treatment plants. These bacteria are an important microbial source of many products including ectoine, polyhydroxyalkanoates, and methanobactins, which are invaluable to the fields of biotechnology and biomedicine. Here we provide an overview of the methanotrophs' unique metabolism and the biochemical pathways involved in biomolecule formation. We also discuss the potential biomedical applications of these biomolecules through creation of beneficial biocompatible products including vaccines, prosthetics, electronic devices, drug carriers, and heart stents. We highlight the links between molecular biology, public health, and environmental science in the advancement of biomedical research and industrial applications using methanotrophic bacteria in wastewater treatment systems.

Metabolome profiles of the alphaproteobacterial methanotroph Methylocystis sp. Rockwell in response to carbon and nitrogen source

Methanotrophs use methane as a sole carbon source and thus play a critical role in its global consumption. Intensified interest in methanotrophs for their low-cost production of value-added products and large-scale industrialization has led to investigations of strain-to-strain variation in parameters for growth optimization and metabolic regulation. In this study, Methylocystis sp. Rockwell was grown with methane or methanol as a carbon source and ammonium or nitrate as a nitrogen source. The intracellular metabolomes and production of polyhydroxybutyrate, a bioplastic precursor, were compared among treatments to determine how the different combinations of carbon and nitrogen sources affected metabolite production. The methane-ammonium condition resulted in the highest growth, followed by the methane-nitrate, methanol-nitrate and methanol-ammonium conditions. Overall, the methane-ammonium and methane-nitrate conditions directed metabolism toward energy-conserving pathways, while methanol-ammonium and methanol-nitrate directed the metabolic response toward starvation pathways. Polyhydroxybutyrate was produced at greater abundances in methanol-grown cells, independent of the nitrogen source. Together, the results revealed how Methylocystis sp. Rockwell altered its metabolism with different combinations of carbon and nitrogen source, with implications for production of industrially relevant metabolites.

Effect of COD on methanotrophic growth and the anaerobic digestibility of its sludge as a further step for integration in WWTPS

Within wastewater treatment plants (WWTPs), the anaerobically produced biogas is often underutilized. Fortunately, methanotrophic based biotechnologies can be the remedy for on-site exploitation and recovery of unused biogas. In this regard, efforts have been placed on evaluating the suitably of methanotrophs to be deployed in WWTPs. Even so, the effect of chemical oxygen demand (COD) on methanotrophic activity and methanotrophic sludge digestibility have not been studied, which is the focus of the present study. A methanotrophic culture enriched from activated sludge was exposed to four different COD concentrations (0-540 mg/L) to evaluate the effect of COD on the culture activity in batch mode. It was attained that the presence of COD concentrations up to 540 mg/L has limited influence on methanotrophic activity. This finding was supported by the similar average methane uptake rate (between 2.48 and 2.53 mgCH₄/hr) and consumption (61.4 ± 1.5%) observed under the different COD concentrations. On the other hand, methanotrophic sludge was digested in comparison to waste activated sludge (WAS) collected from a WWTP for more than 40 days to evaluate its digestibility. It was obtained that the methanotrophic sludge had a methane specific yield of approximately 1.72 times greater than WAS and had a higher solids destruction rate. This research is another step demonstrating the feasibility of methanotrophs integration in WWTP.

Microbial lipid biosynthesis from lignocellulosic biomass pyrolysis products

Lipids are a biorefinery platform to prepare fuel, food and health products. They are traditionally obtained from plants, but those of microbial origin allow for a better use of land and C resources, among other benefits. Several (thermo)chemical and biochemical strategies are used for the conversion of C contained in lignocellulosic biomass into lipids. In particular, pyrolysis can process virtually any biomass and is easy to scale up. Products offer cost-effective, renewable C in the form of readily fermentable molecules and other upgradable intermediates. Although the production of microbial lipids has been studied for 30 years, their incorporation into biorefineries was only described a few years ago. As pyrolysis becomes a profitable technology to depolymerize lignocellulosic biomass into assimilable C, the number of investigations on it raises significantly. This article describes the challenges and opportunities resulting from the combination of lignocellulosic biomass pyrolysis and lipid biosynthesis with oleaginous microorganisms. First, this work presents the basics of the individual processes, and then it shows state-of-the-art processes for the preparation of microbial lipids from biomass pyrolysis products. Advanced knowledge on separation techniques, structure analysis, and fermentability is detailed for each biomass pyrolysis fraction. Finally, the microbial fatty acid platform comprising biofuel, human food and animal feed products, and others, is presented. Literature shows that the microbial lipid production from anhydrosugars, like levoglucosan, and short-chain organic acids, like acetic acid, is straightforward. Indeed, processes achieving nearly theoretical yields form the latter have been described. Some authors have shown that lipid biosynthesis from different lignin sources is biochemically feasible. However, it still imposes major challenges regarding strain performance. No report on the fermentation of pyrolytic lignin is yet available. Research on the microbial uptake of pyrolytic humins remains vacant. Microorganisms that make use of methane show promising results at the proof-of-concept level. Overall, despite some issues need to be tackled, it is now possible to conceive new versatile biorefinery models by combining lignocellulosic biomass pyrolysis products and robust oleaginous microbial cell factories.