Development of Methane-Utilizing Mixed Cultures for the Production of Polyhydroxyalkanoates (PHAs) from Anaerobic Digester Sludge

Aromatic acid metabolism in Methylobacterium extorquens reveals interplay between methylotrophic and heterotrophic pathways

Efforts towards microbial conversion of lignin to value-added products face many challenges because lignin's methoxylated aromatic monomers release toxic C1 byproducts such as formaldehyde. The ability to grow on methoxylated aromatic acids (e.g., vanillic acid) has recently been identified in certain clades of methylotrophs, bacteria characterized by their unique ability to tolerate and metabolize high concentrations of formaldehyde. Here, we use a phyllosphere methylotroph isolate, Methylobacterium extorquens SLI 505, as a model to identify the fate of formaldehyde during methylotrophic growth on vanillic acids. M. extorquens SLI 505 displays concentration-dependent growth phenotypes on vanillic acid without concomitant formaldehyde accumulation. We conclude that M. extorquens SLI 505 overcomes potential metabolic bottlenecks from simultaneous assimilation of multicarbon and C1 intermediates by allocating formaldehyde towards dissimilation and assimilating the ring carbons of vanillic acid heterotrophically. We correlate this strategy with maximization of bioenergetic yields and demonstrate that formaldehyde dissimilation for energy generation rather than formaldehyde detoxification is advantageous for growth on aromatic acids. M. extorquens SLI 505 also exhibits catabolite repression during growth on methanol and low concentrations of vanillic acid, but no diauxie during growth on methanol and high concentrations of vanillic acid. Results from this study outline metabolic strategies employed by M. extorquens SLI 505 for growth on a complex single substrate that generates both C1 and multicarbon intermediates and emphasizes the robustness of M. extorquens for biotechnological applications for lignin valorization.

Bioplastic polyhydroxyalkanoate conversion in waste activated sludge

Polyhydroxyalkanoates (PHA) have been proposed as a promising solution for plastic pollution due to their biodegradability and diverse applications. To promote PHA as a competitive commercial product, an attractive alternative is to produce and recover PHA in the use of mixed cultures such as waste activated sludge from wastewater treatment plants. PHA can accumulate in sludge with a potential range of 40%-65% g PHA/g VSS. However, wider challenges with PHA production efficiency, stability, and economic viability still persist for PHA application. This work provides an overview of the current understanding and status of PHA bioconversion in waste sludge with particular attention given to metabolic pathways, operation modes, factors affecting the process, and applications. Challenges and future prospectives for PHA bioconversion in sludge are discussed.

Polyhydroxyalkanoate Production by Methanotrophs: Recent Updates and Perspectives

Methanotrophs are bacteria that consume methane (CH4) as their sole carbon and energy source. These microorganisms play a crucial role in the carbon cycle by metabolizing CH4 (the greenhouse gas), into cellular biomass and carbon dioxide (CO2). Polyhydroxyalkanoates (PHAs) are biopolymers produced by various microorganisms, including methanotrophs. PHA production using methanotrophs is a promising strategy to address growing concerns regarding plastic pollution and the need for sustainable, biodegradable materials. Various factors, including nutrient availability, environmental conditions, and metabolic engineering strategies, influence methanotrophic production. Nutrient limitations, particularly those of nitrogen or phosphorus, enhance PHA production by methanotrophs. Metabolic engineering approaches, such as the overexpression of key enzymes involved in PHA biosynthesis or the disruption of competing pathways, can also enhance PHA yields by methanotrophs. Overall, PHA production by methanotrophs represents a sustainable and versatile approach for developing biomedical materials with numerous potential applications. Additionally, alternative feedstocks, such as industrial waste streams or byproducts can be explored to improve the economic feasibility of PHA production. This review briefly describes the potential of methanotrophs to produce PHAs, with recent updates and perspectives.

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.

Natural Polyhydroxyalkanoates-An Overview of Bacterial Production Methods

Polyhydroxyalkanoates (PHAs) are intracellular biopolymers that microorganisms use for energy and carbon storage. They are mechanically similar to petrochemical plastics when chemically extracted, but are completely biodegradable. While they have potential as a replacement for petrochemical plastics, their high production cost using traditional carbon sources remains a significant challenge. One potential solution is to modify heterotrophic PHA-producing strains to utilize alternative carbon sources. An alternative approach is to utilize methylotrophic or autotrophic strains. This article provides an overview of bacterial strains employed for PHA production, with a particular focus on those exhibiting the highest PHA content in dry cell mass. The strains are organized according to their carbon source utilization, encompassing autotrophy (utilizing CO2, CO) and methylotrophy (utilizing reduced single-carbon substrates) to heterotrophy (utilizing more traditional and alternative substrates).

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.

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.

Dynamics of Methane-Consuming Biomes from Wieliczka Formation: Environmental and Enrichment Studies

The rocks surrounding Wieliczka salt deposits are an extreme, deep subsurface ecosystem that as we studied previously harbors many microorganisms, including methanotrophs. In the presented research bacterial community structure of the Wieliczka Salt Mine was determined as well as the methanotrophic activity of the natural microbiome. Finally, an enrichment culture of methane-consuming methanotrophs was obtained. The research material used in this study consisted of rocks surrounding salt deposits in the Wieliczka Salt Mine. DNA was extracted directly from the pristine rock material, as well as from rocks incubated in an atmosphere containing methane and mineral medium, and from a methanotrophic enrichment culture from this ecosystem. As a result, the study describes the composition of the microbiome in the rocks surrounding the salt deposits, while also explaining how biodiversity changes during the enrichment culture of the methanotrophic bacterial community. The contribution of methanotrophic bacteria ranged from 2.614% in the environmental sample to 64.696% in the bacterial culture. The methanotrophic enrichment culture was predominantly composed of methanotrophs from the genera Methylomonas (48.848%) and Methylomicrobium (15.636%) with methane oxidation rates from 3.353 ± 0.105 to 4.200 ± 0.505 µmol CH4 mL-1 day-1.

The co-conversion of methane and mixtures of volatile fatty acids into poly(3-hydroxybutyrate-co-3-hydroxyvalerate) expands the potential of an integrated biorefinery

In this work, the potential of Methylocystis hirsuta to simultaneously use methane and volatile fatty acids mixtures for triggering PHBV accumulation was assessed for the first time batchwise. Biotic controls carried out with CH4 alone confirmed the inability of Methylocystis hirsuta to produce PHBV and achieved 71.2 ± 7 g m-3d-1 of PHB. Pure valeric acid and two synthetic mixtures simulating VFAs effluents from the anaerobic digestion of food waste at 35 °C (M1) and 55 °C (M2) were supplied to promote 3-HV inclusion. Results showed that pure valeric acid supported the highest polymer yields of 105.8 ± 9 g m-3d-1 (3-HB:3-HV=70:30). M1 mixtures led to a maximum of 103 ± 4 g m-3d-1 of PHBV (3-HB:3-HV=85:15), while M2 mixtures, which did not include valeric acid, showed no PHV synthesis. This suggested that the synthesis of PHBV from VFAs effluents depends on the composition of the mixtures, which can be tuned during the anaerobic digestion process.

Current and prognostic overview on the strategic exploitation of anaerobic digestion and digestate: A review

The depletion of fossil fuels and increasing demand for energy are encountered by generating renewable biogas. Anaerobic digestion (AD) produces not only biogas, also other value-added products from the digestate using various organic, municipal and industrial wastes which have several benefits like remediating waste, reduces greenhouse gas emissions, renewable energy generation and securing socio-economic status of bio-based industries. This review work critically analyzes the biorefinery approaches on AD process for the production of biogas and digestate, and their direct and indirect utilization. The left-out residue obtained from AD is called 'digestate' which enriched with organic matter, nitrogen, heavy metals and other valuable micronutrients. However, the direct disposal of digestate to the land as fertilizer/landfills creates various environmental issues. Keeping this view, the digestate should be upgraded or transformed into high valued products such as biofertilizer, pyrochar, biodiesel, syngas and soil conditioner that can aid to enrich the soil nutrients and ensures the safe environment as well. In this context, the present review focused to illustrate the current techniques and different strategic exploitations on AD proper management of digestate products for storage and further applications. Such a technology transfer provides a proven strategic mechanism towards the enhancement of the sustainability of bio-based industries, attaining the energy demand, safest waste management, protection of environment and reduces the socio-economic issues of the industrial sector.

Sustainable Process for the Production of Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) from Renewable Resources: A Simulation Study

Bacterially produced polyhydroxyalkanoates are valuable substitutes for petrochemical plastics, but their current production capacities are very scarce. Producing poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHB-co-HV) from methane and odd-chain carbon fatty acids could make the production of this biodegradable polymer cost-effective. This study analyzes the main factors affecting methanotrophic growth and poly(3-hydroxybutyrate-co-3-hydroxyvalerate) accumulation, simulating a pilot-scale process based on a double-stage approach. The effects of the nitrogen source and the oxygen partial pressure during a 20 day growth phase were studied; the cosubstrate concentration, the culture selected, and the methane partial pressure were investigated during the poly(3-hydroxybutyrate-co-3-hydroxyvalerate) production stage performed within 15 days under nutrient starvation. Methylocystis parvus OBBP and Methylosinus thricosporum OB3b reached the maximum growth productivities with ammonium as the nitrogen source and oxygen at high partial pressure. The simulation of the PHB-co-HV accumulation revealed that methanotrophs could better accumulate the polymer with low valeric acid concentrations. A methane-abundant gas stream (0.5 atm of methane) could increase process yields up to 0.32 kg m^-3 d^-1.

Biogas bioconversion into poly(3-hydroxybutyrate) by a mixed microbial culture in a novel Taylor flow bioreactor

Biogas-based biopolymer production represents an alternative biogas valorization route with potential to cut down plastic pollution and greenhouse gas emissions. This study investigated for the first time the continuous bioconversion of methane, contained in biogas, into poly(3-hydroxybutyrate) (PHB) by a mixed methanotrophic culture using an innovative high mass-transfer Taylor flow bioreactor. Following a hydrodynamic flow regime mapping, the influence of the gas residence time and the internal gas recirculation on CH₄ abatement was assessed under non nutrient limiting conditions. Under optimal operational conditions (gas residence time of 60 min and internal gas recycling ratio of 17), the bioreactor was able to support a CH₄ removal efficiency of 63.3%, a robust CH₄ elimination capacity (17.2 g-CH₄ m^-3h^-1) and a stable biomass concentration (1.0 g L^-1). The simultaneous CH₄ abatement and PHB synthesis was investigated under 24-h:24-h nitrogen feast/famine continuous operation. The cyclic nitrogen starvation and the Taylor flow imposed in the bioreactor resulted in a relatively constant biomass concentration of 0.6 g L^-1 with PHB contents ranging from 11 to 32% w w^-1 (on a dry weight basis), entailing an average PHB productivity of 5.9 g-PHB m^-3 d^-1 with an associated PHB yield of 19.8 mg-PHB g-CH₄^-1. Finally, the molecular analysis of the microbial population structure indicated that type II methanotrophs outcompeted non-PHB accumulating type I methanotrophs, with a heterotrophic-methanotrophic consortium enriched in Methylocystis, Hyphomicrobium, Rubinisphaeraceae SH PL14 and Pseudonocardia.

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.

Achieving polyhydroxyalkanoate production from rubber wood waste using mixed microbial cultures and anaerobic-aerobic feeding regime

In the past few years, creating value-added products has become the best choice to pretreat biomass waste. For instance, the fermentable sugar obtained after pretreatment bioconversion into valuable bioproducts, biopolymer as a typical representative, has become a potential strategy. In particular, the production of biopolymer polyhydroxyalkanoate (PHA) by mixed microbial cultures in waste activated sludge can be regarded as a promising alternative to traditional petrochemical plastics. In this study, the enzymatic hydrolysate of rubber wood was utilized as substrate to explore the optimal process conditions for the accumulation of PHA under anaerobic-aerobic mode. The results showed that longer operation cycle (24 h), suitable anaerobic duration (3.5 h) and secondary feeding regimen (secondary addition without draining liquid) were more beneficial to PHA production. After accumulation, the highest PHA production, PHA storage yield (Y_PHA/S) and ratio to cell dry weight (CDW) reached 929.8 mg COD·L^-1, 0.24 g COD/g COD and 0.31 g PHA/g CDW, respectively. The Y_PHA/S values were similar to the previous reported 0.22 ∼ 0.24 g COD/g COD. The results demonstrated that the secondary feeding regimen was an effective approach to improve the production of PHA with rubber wood enzymatic hydrolysate as substrate.

Strategies for Biosynthesis of C1 Gas-derived Polyhydroxyalkanoates: A review

Biosynthesis of polyhydroxyalkanoates (PHAs) from C1 gases is highly desirable in solving problems such as climate change and microplastic pollution. PHAs are biopolymers synthesized in microbial cells and can be used as alternatives to petroleum-based plastics because of their biodegradability. Because 50% of the cost of PHA production is due to organic carbon sources and salts, the utilization of costless C1 gases as carbon sources is expected to be a promising approach for PHA production. In this review, strategies for PHA production using C1 gases through fermentation and metabolic engineering are discussed. In particular, autotrophs, acetogens, and methanotrophs are strains that can produce PHA from CO₂, CO, and CH₄. In addition, integrated bioprocesses for the efficient utilization of C1 gases are introduced. Biorefinery processes from C1 gas into bioplastics are prospective strategies with promising potential and feasibility to alleviate environmental issues.

Biomolecules Production from Greenhouse Gases by Methanotrophs

Harmful effects on living organisms and the environment are on the rise due to a significant increase in greenhouse gas (GHG) emissions through human activities. Therefore, various research initiatives have been carried out in several directions in relation to the utilization of GHGs via physicochemical or biological routes. An environmentally friendly approach to reduce the burden of significant emissions and their harmful effects is the bioconversion of GHGs, including methane (CH4) and carbon dioxide (CO2), into value-added products. Methanotrophs have enormous potential for the efficient biotransformation of CH4 to various bioactive molecules, including biofuels, polyhydroxyalkanoates, and fatty acids. This review highlights the recent developments in methanotroph-based systems for methanol production from GHGs and proposes future perspectives to improve process sustainability via biorefinery approaches.

Elucidating the key environmental parameters during the production of ectoines from biogas by mixed methanotrophic consortia

Anaerobic digestion (AD) is a robust biotechnology for the valorisation of organic waste into biogas. However, the rapid decrease in renewable electricity prices requires alternative uses of biogas. In this context, the engineering of innovative platforms for the bio-production of chemicals from CH₄ has recently emerged. The extremolyte and osmoprotectant ectoine, with a market price of ~1000€/Kg, is the industrial flagship of CH₄-based bio-chemicals. This work aimed at optimizing the accumulation of ectoines using mixed microbial consortia enriched from saline environments (a salt lagoon and a salt river) and activated sludge, and biogas as feedstock. The influence of NaCl (0, 3, 6, 9 and 12 %) and Na₂WO₄ (0, 35 and 70 μg L^-1) concentrations and incubation temperature (15, 25 and 35 °C) on the stoichiometry and kinetics of the methanotrophic consortia was investigated. Consortia enriched from activated sludge at 15 °C accumulated the highest yields of ectoine and hydroxyectoine at 6 % NaCl (105.0 ± 27.2 and 24.2 ± 5.4 mg_extremolyte g_biomass^-1, respectively). The consortia enriched from the salt lagoon accumulated the highest yield of ectoine and hydroxyectoine at 9 % NaCl (56.6 ± 2.5 and 51.0 ± 2.0 mg_extremolyte g_biomass^-1, respectively) at 25 °C. The supplementation of tungsten to the cultivation medium did not impact on the accumulation of ectoines in any of the consortia. A molecular characterization of the enrichments revealed a relative abundance of ectoine-accumulating methanotrophs of 7-16 %, with Methylomicrobium buryatense and Methylomicrobium japanense as the main players in the bioconversion of methane into ectoine.

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.

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.

Screening for Methane Utilizing Mixed Communities with High Polyhydroxybutyrate (PHB) Production Capacity Using Different Design Approaches

With the adverse environmental ramifications of the use of petroleum-based plastic outweighing the challenges facing the industrialization of bioplastics, polyhydroxyalkanoate (PHA) biopolymer has gained broad interest in recent years. Thus, an efficient approach for maximizing polyhydroxybutyrate (PHB) polymer production in methanotrophic bacteria has been developed using the methane gas produced in the anaerobic digestion process in wastewater treatment plants (WWTPS) as a carbon substrate and an electron donor. A comparison study was conducted between two experimental setups using two different recycling strategies, namely new and conventional setups. The former setup aims to recycle PHB producers into the system after the PHB accumulation phase, while the latter recycles the biomass back into the system after the exponential phase of growth or the growth phase. The goal of this study was to compare both setups in terms of PHB production and other operational parameters such as growth rate, methane uptake rate, and biomass yield using two different nitrogen sources, namely nitrate and ammonia. The newly proposed setup is aimed at stimulating PHB accumulating type II methanotroph growth whilst enabling other PHB accumulators to grow simultaneously. The success of the proposed method was confirmed as it achieved highest recorded PHB accumulation percentages for a mixed culture community in both ammonia- and nitrate-enriched media of 59.4% and 54.3%, respectively, compared to 37.8% and 9.1% for the conventional setup. Finally, the sequencing of microbial samples showed a significant increase in the abundance of type II methanotrophs along with other PHB producers, confirming the success of the newly proposed technique in screening for PHB producers and achieving higher PHB accumulation.

Biopolymer production using volatile fatty acids as resource: Effect of feast-famine strategy and lignin reinforcement

The present study aimed to investigate the biopolymer production using VFA's as carbon source through feast and famine strategy in a sequencing batch reactor. Famine condition with nutrients and oxygen limitation resulted in high polyhydroxybutyrate yield (PHB: 2.65 ± 0.012 g/L; 0.36 ± 0.015 gPHB/gVFA) than feast mode (0.26 ± 0.02 g/L; 0.034 ± 0.013 gPHB/gVFA). Repeated batch operations induced substrate consumption, wherein acetate utilization was high in both the conditions (feast: 83%, famine 74%) followed by butyrate (feast: 74%, famine 72%). Besides, high biomass concentration was also observed in feast condition (3.45 ± 0.14 g/L VSS), while oxygen and nutrients limitation in famine mode regulated the carbon use for biomass growth (2.46 ± 0.15 g/L VSS). Further, PHB grafting with lignin (3% and 5%) exhibited increased thermal stability than pristine PHB. Biopolymer production using VFA's as carbon source and utilization of lignin as functional filler for strengthening PHB offer lignin valorization also wider its applications specifically in the biomedical field.

Metabolic engineering of Methylorubrum extorquens AM1 for poly (3-hydroxybutyrate-co-3-hydroxyvalerate) production using formate

Formate is a promising environmentally friendly and sustainable feedstock synthesized from syngas or carbon dioxide. Methylorubrum extorquens is a type II methylotroph that can use formate as a carbon source. It accumulates polyhydroxyalkanoates (PHAs) inside the cell, mainly producing poly-3-hydroxybutyrate (PHB), a degradable biopolymer. Owing to its high melting point and stiff nature, however, mechanical property improvement is warranted in the form of copolymerization. To produce the PHA copolymer, poly (3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV), the endogenous gene phaC was deleted and the pathway genes bktB, phaJ1, and phaC2, with broader substrate specificities, were heterologously expressed. To improve the incorporation of 3-hydroxyvalerate (3HV), the expression level of bktB was improved by untranslated region (UTR) engineering, and the endogenous gene phaA was deleted. The engineered M. extorquens produced PHBV with 8.9% 3HV using formate as the sole carbon source. In addition, when propionate and butyrate were supplemented, PHBVs with 3HV portions of up to 70.6% were produced. This study shows that a PHBV copolymer with a high proportion of 3HV can be synthesized using formate, a C1 carbon source, through metabolic engineering and supplementation with short-chain fatty acids.

The indispensable role of assimilation in methane driven nitrate removal

Methane is a greenhouse gas that can be released from sludge anaerobic fermentation in wastewater treatment plants. Methane is also an alternative additional carbon source for deep nitrate removal of secondary effluent. A sequencing experiment was conducted to study the efficacy of nitrate removal with methane as the sole carbon source. The maximum nitrate removal rate was 17.2 mg-N·L^-1·d^-1. Nitrate removal was confirmed to arise via two pathways: aerobic methane oxidation coupled to denitrification (AME-D) contributed to 55% of the nitrate removal with the rest stemming from assimilation by methanotrophs. Additional study revealed that nitrate assimilated by methanotrophs was used for the synthesis of proteins, resulting in a protein content of 52.2% dry weight. Metagenomic sequencing revealed a high abundance of nitrate assimilation and glutamine synthetase genes, which were primarily provided by methanotrophs (mainly Methylomonas). Assimilatory nitrate removal by methanotrophs has a high potential for advanced nitrogen removal and for alleviating methane emissions. The nitrogen-rich biomass produced by nitrate absorption could also be used as a biofertilizer for nitrogen recycling.

Recent advancement on biological technologies and strategies for resource recovery from swine wastewater

Swine wastewater is categorized as one of the agricultural wastewater with high contents of organics and nutrients including nitrogen and phosphorus, which may lead to eutrophication in the environment. Insufficient technologies to remove those nutrients could lead to environmental problems after discharge. Several physical and chemical methods have been applied to treat the swine wastewater, but biological treatments are considered as the promising methods due to the cost effectiveness and performance efficiency along with the production of valuable products and bioenergies. This review summarizes the characteristics of swine wastewaters in the beginning, and briefly describes the current issues on the treatments of swine wastewaters. Several biological techniques, such as anaerobic digestion, A/O process, microbial fuel cells, and microalgae cultivations, and their future aspects will be addressed. Finally, the potentials to reutilize biomass produced during the treatment processes are also presented under the consideration of circular economy.

Towards biodegradable polyhydroxyalkanoate production from wood waste: Using volatile fatty acids as conversion medium

Production of polyhydroxyalkanoate (PHA) via mixed microbial consortia is a potential economic alternative responding to the current demand for functional greener materials to replace traditional petroleum-basedpolymers. The goal of this study was to synthesize PHA using volatile fatty acids (VFAs) obtained from the co-fermentation of pretreated wood waste and sewage as carbon source. High PHA yield of 0.71 g COD PHA/g COD VFAs and PHA content of 50.3 g PHA/100 g VSS were obtained at VFAs ratio (even:odd) of 88:12 after seven cycles cultivation. Even acids were more suitable for accumulating PHA as the preferred carbon source than odd acids, resulting in 3-hydroxybutyrate being the main monomer. PHA production achieved to the highest value of about 2639 mg COD/L at 1400 mg COD/L VFAs concentration. The bacterial genera displayed a highly diverse of the microbial community for the synthesis of PHA.

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.