Poly-3-hydroxybutyrate metabolism in the type II methanotroph Methylocystis parvus OBBP

Conventional methanotrophs are responsible for atmospheric methane oxidation in paddy soils

Soils serve as the biological sink of the potent greenhouse gas methane with exceptionally low concentrations of ∼1.84 p.p.m.v. in the atmosphere. The as-yet-uncultivated methane-consuming bacteria have long been proposed to be responsible for this ‘high-affinity' methane oxidation (HAMO). Here we show an emerging HAMO activity arising from conventional methanotrophs in paddy soil. HAMO activity was quickly induced during the low-affinity oxidation of high-concentration methane. Activity was lost gradually over 2 weeks, but could be repeatedly regained by flush-feeding the soil with elevated methane. The induction of HAMO activity occurred only after the rapid growth of methanotrophic populations, and a metatranscriptome-wide association study suggests that the concurrent high- and low-affinity methane oxidation was catalysed by known methanotrophs rather than by the proposed novel atmospheric methane oxidizers. These results provide evidence of atmospheric methane uptake in periodically drained ecosystems that are typically considered to be a source of atmospheric methane.

Atmospheric methane may be consumed by microorganisms in soil, but the mechanisms behind high-affinity methane oxidization remain poorly understood. Here, Jia et al. show that known methanotrophic bacteria are responsible for atmospheric methane uptake in periodically drained wetland ecosystems.

Low energy emulsion-based fermentation enabling accelerated methane mass transfer and growth of poly(3-hydroxybutyrate)-accumulating methanotrophs

Methane is a low-cost feedstock for the production of polyhydroxyalkanoate biopolymers, but methanotroph fermentations are limited by the low solubility of methane in water. To enhance mass transfer of methane to water, vigorous mixing or agitation is typically used, which inevitably increases power demand and operational costs. This work presents a method for accelerating methane mass transfer without agitation by growing methanotrophs in water-in-oil emulsions, where the oil has a higher solubility for methane than water does. In systems without agitation, the growth rate of methanotrophs in emulsions is five to six times that of methanotrophs in the medium-alone incubations. Within seven days, cells within the emulsions accumulate up to 67 times more P3HB than cells in the medium-alone incubations. This is achieved due to the increased interfacial area of the aqueous phase, and accelerated methane diffusion through the oil phase.

Influence of nutrients on oxidation of low level methane by mixed methanotrophic consortia

Low-level methane emissions from coal mine ventilation air (CMV-CH4; i.e., 1 % CH4) can significantly contribute to global climate change, and therefore, treatment is important to reduce impacts. To investigate CMV-CH4 abatement potential, five different mixed methanotrohic consortia (MMCs) were established from soil/sediment sources, i.e., landfill top cover soil, bio-solid compost, vegetated humus soil, estuarine and marine sediments. Enrichment conditions for MMCs were as follows: nitrate mineral salt (NMS) medium, pH ~ 6.8; 25 °C; 20-25 % CH4; agitation 200 rpm; and culture period 20 days, in mini-bench-top bioreactors. The enriched cultures were supplemented with extra carbon (methanol 0.5-1.5 %, formate 5-15 mM, and acetate 5-15 mM), nitrogen (nitrate 0.5-1.5 g L(-1), ammonium 0.1-0.5 g L(-1), or urea: 0.1-0.5 g L(-1)), and trace elements (copper 1-5 μM, iron 1-5 μM, and zinc 1-5 μM) in different batch experiments to improve low-level CH4 abatement. Average CH4 oxidation capacities (MOCs) of MMCs varied between 1.712 ± 0.032 and 1.963 ± 0.057 mg g(-1)DWbiomass h(-1). Addition of formate improved the MOCs of MMCs, but the dose-response varied for different MMCs. Acetate, nitrate and copper had no significant effect on MOCs, while addition of methanol, ammonium, urea, iron and zinc impacted negatively. Overall, MMCs enriched from marine sediments and landfill top cover soil showed high MOCs which were largely resilient to nutrient supplementation, suggesting a strong potential for biofilter development for industrial low-level CH4 abatement, such as those present in CMV.

Methanotrophic production of polyhydroxybutyrate-co-hydroxyvalerate with high hydroxyvalerate content

Type II methanotrophic bacteria are a promising production platform for PHA biopolymers. These bacteria are known to produce pure poly-3-hydroxybutyrate homopolymer (PHB). We isolated a strain, Methylocystis sp. WRRC1, that was capable of producing a wide range of polyhydroxybutyrate-co-hydroxyvalerate copolymers (PHB-co-HV) when co-fed methane and valerate or n-pentanol. The ratio of HB to HV monomer was directly related to the concentration of valeric acid in the PHA accumulation media. We observed increased incorporation of HV and total polymer under copper-free growth conditions. The PHB-co-HV copolymers produced had decreased melting temperatures and crystallinity compared with methanotroph-produced PHB.

The Opportunity for High-Performance Biomaterials from Methane

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

Long-term cultivation of a stable Methylocystis-dominated methanotrophic enrichment enabling tailored production of poly(3-hydroxybutyrate-co-3-hydroxyvalerate)

Methane (CH4) is a readily available feedstock for production of polyhydroxyalkanoates (PHAs). The structure and PHA production capacity of a Methylocystis-dominated methanotrophic enrichment was stable in long-term operation (>175 days) when grown exponentially under non-aseptic conditions in fill-and-draw batch cultures with ammonium as nitrogen source. Cells harvested in the draw step were incubated in the absence of nitrogen with various combinations of CH4 and valerate to assess capacity for synthesis of poly(3-hydroxybutyrate) (P3HB) and poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV). When fed CH4 alone, only P3HB was produced. When fed CH4 plus valerate, PHBV was synthesized. The mol% of 3-hydroxyvalerate (3HV) increased with added valerate. Oxidation of CH4 was required for valerate assimilation, and the fraction of CH4 oxidized increased with increased mol% 3 HV. By separating PHA accumulation from cell replication, tailored PHA-rich biomass can be generated by addition of co-substrate, while retaining a large inoculum for the next cycle of cell division.

Biogenic concrete protection driven by the formate oxidation by Methylocystis parvus OBBP

The effectiveness of Microbiologically Induced Carbonate Precipitation (MICP) from the formate oxidation by Methylocystis parvus OBBP as an alternative process for concrete protection was investigated. MICP was induced on Autoclaved Aerated Concrete (AAC), the model material, by immersing the material in 10⁹M. parvus cells mL⁻¹ containing 5 g L⁻¹ of calcium formate. A 2 days immersion of the material gave the maximum weight increase of the specimens (38 ± 19 mg) and this was likely due to the deposition of calcium carbonate, biomass, and unconverted calcium formate. The solid deposition mainly occurred in the micropores of the specimen, close to the outer surface. A significantly lower water absorption was observed in the bacterially treated specimens compared to the non-treated ones (up to 2.92 ± 0.91 kg m⁻²) and this could be attributed to the solid deposition. However, the sonication test demonstrated that the bacterial treatment did not give a consolidating effect to the material. Overall, compared to the currently employed urea hydrolysis process, the formate-based MICP by M. parvus offers a more environmentally friendly approach for the biotechnological application to protect concrete.

Formate oxidation-driven calcium carbonate precipitation by Methylocystis parvus OBBP

Microbially induced carbonate precipitation (MICP) applied in the construction industry poses several disadvantages such asammonia release to the air and nitric acid production. An alternative MICP from calcium formate by Methylocystis parvus OBBP is presented here to overcome these disadvantages. To induce calcium carbonate precipitation, M. parvus was incubated at different calcium formate concentrations and starting culture densities. Up to 91.4% ± 1.6% of the initial calcium was precipitated in the methane-amended cultures compared to 35.1% ± 11.9% when methane was not added. Because the bacteria could only utilize methane for growth, higher culture densities and subsequently calcium removals were exhibited in the cultures when methane was added. A higher calcium carbonate precipitate yield was obtained when higher culture densities were used but not necessarily when more calcium formate was added. This was mainly due to salt inhibition of the bacterial activity at a high calcium formate concentration. A maximum 0.67 ± 0.03 g of CaCO3 g of Ca(CHOOH)2(-1) calcium carbonate precipitate yield was obtained when a culture of 10(9) cells ml(-1) and 5 g of calcium formate liter(-)1 were used. Compared to the current strategy employing biogenic urea degradation as the basis for MICP, our approach presents significant improvements in the environmental sustainability of the application in the construction industry.

Optimization of Methanotrophic Growth and Production of Poly(3-Hydroxybutyrate) in a High-Throughput Microbioreactor System

Production of poly(3-hydroxybutyrate) (P3HB) from methane has economic and environmental advantages over production by agricultural feedstock. Identification of high-productivity strains and optimal growth conditions is critical to efficient conversion of methane to polymer. Current culture conditions, including serum bottles, shake flasks, and agar plates, are labor-intensive and therefore insufficient for systematic screening and isolation. Gas chromatography, the standard method for analysis of P3HB content in bacterial biomass, is also incompatible with high-throughput screening. Growth in aerated microtiter plates coupled with a 96-well Nile red flow-cytometric assay creates an integrated microbioreactor system for high-throughput growth and analysis of P3HB-producing methanotrophic cultures, eliminating the need for individual manipulation of experimental replicates. This system was tested in practice to conduct medium optimization for P3HB production in pure cultures of Methylocystis parvus OBBP. Optimization gave insight into unexpected interactions: for example, low calcium concentrations significantly enhanced P3HB production under nitrogen-limited conditions. Optimization of calcium and copper concentrations in the growth medium increased final P3HB content from 18.1% to 49.4% and P3HB concentration from 0.69 g/liter to 3.43 g/liter while reducing doubling time from 10.6 h to 8.6 h. The ability to culture and analyze thousands of replicates with high mass transfer in completely mixed culture promises to streamline medium optimization and allow the detection and isolation of highly productive strains. Applications for this system are numerous, encompassing analysis of biofuels and other lipid inclusions, as well as analysis of heterotrophic and photosynthetic systems.

Methane as a resource: can the methanotrophs add value?

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

Disassembly and reassembly of polyhydroxyalkanoates: recycling through abiotic depolymerization and biotic repolymerization

An abiotic-biotic strategy for recycling of polyhydroxyalkanoates (PHAs) is evaluated. Base-catalyzed PHA depolymerization yields hydroxyacids, such as 3-hydroxybutyrate (3HB), and alkenoates, such as crotonate; catalytic thermal depolymerization yields alkenoates. Cyclic pulse addition of 3HB to triplicate bioreactors selected for an enrichment of Comamonas, Brachymonas and Acinetobacter. After each pulse, poly(3-hydroxybutyrate) (P3HB) transiently appeared: accumulation of P3HB correlated with hydrolysis of polyphosphate; consumption of P3HB correlated with polyphosphate synthesis. Cells removed from the cyclic regime and incubated with 3HB under nitrogen-limited conditions produced P3HB (molecular weight>1,000,000Da) at 50% of the cell dry weight (<8h). P3HB also resulted from incubation with acetate, crotonate, or a mixture of hydrolytic depolymerization products. Poly(3-hydroxybutyric acid-co-3-hydroxyvaleric acid) (PHBV) resulted from incubation with valerate or 2-pentenoate. A recycling strategy where abiotic depolymerization of waste PHAs yields feedstock for customized PHA re-synthesis appears feasible, without the need for energy-intensive feedstock purification.

Production of nitrous oxide from anaerobic digester centrate and its use as a co-oxidant of biogas to enhance energy recovery

Coupled Aerobic-anoxic Nitrous Decomposition Operation (CANDO) is a new process for wastewater treatment that removes nitrogen from wastewater and recovers energy from the nitrogen in three steps: (1) NH4(+) oxidation to NO2(-); (2) NO2(-) reduction to N2O gas; and (3) N2O conversion to N2 with energy production. In this work, we optimize Steps 1 and 2 for anaerobic digester centrate, and we evaluate Step 3 for a full-scale biogas-fed internal combustion engine. Using a continuous stirred reactor coupled to a bench-scale sequencing batch reactor, we observed sustained partial oxidation of NH4(+) to NO2(-) and sustained (3 months) partial reduction of NO2(-) to N2O (75-80% conversion, mass basis), with >95% nitrogen removal (Step 2). Alternating pulses of acetate and NO2(-) selected for Comamonas (38%), Ciceribacter (16%), and Clostridium (11%). Some species stored polyhydroxybutyrate (PHB) and coupled oxidation of PHB to reduction of NO2(-) to N2O. Some species also stored phosphorus as polyphosphate granules. Injections of N2O into a biogas-fed engine at flow rates simulating a full-scale system increased power output by 5.7-7.3%. The results underscore the need for more detailed assessment of bioreactor community ecology and justify pilot- and full-scale testing.

Genome analysis coupled with physiological studies reveals a diverse nitrogen metabolism in Methylocystis sp. strain SC2

BACKGROUND

Methylocystis sp. strain SC2 can adapt to a wide range of methane concentrations. This is due to the presence of two isozymes of particulate methane monooxygenase exhibiting different methane oxidation kinetics. To gain insight into the underlying genetic information, its genome was sequenced and found to comprise a 3.77 Mb chromosome and two large plasmids.

PRINCIPAL FINDINGS

We report important features of the strain SC2 genome. Its sequence is compared with those of seven other methanotroph genomes, comprising members of the Alphaproteobacteria, Gammaproteobacteria, and Verrucomicrobia. While the pan-genome of all eight methanotroph genomes totals 19,358 CDS, only 154 CDS are shared. The number of core genes increased with phylogenetic relatedness: 328 CDS for proteobacterial methanotrophs and 1,853 CDS for the three alphaproteobacterial Methylocystaceae members, Methylocystis sp. strain SC2 and strain Rockwell, and Methylosinus trichosporium OB3b. The comparative study was coupled with physiological experiments to verify that strain SC2 has diverse nitrogen metabolism capabilities. In correspondence to a full complement of 34 genes involved in N2 fixation, strain SC2 was found to grow with atmospheric N2 as the sole nitrogen source, preferably at low oxygen concentrations. Denitrification-mediated accumulation of 0.7 nmol (30)N2/hr/mg dry weight of cells under anoxic conditions was detected by tracer analysis. N2 production is related to the activities of plasmid-borne nitric oxide and nitrous oxide reductases.

CONCLUSIONS/PERSPECTIVES

Presence of a complete denitrification pathway in strain SC2, including the plasmid-encoded nosRZDFYX operon, is unique among known methanotrophs. However, the exact ecophysiological role of this pathway still needs to be elucidated. Detoxification of toxic nitrogen compounds and energy conservation under oxygen-limiting conditions are among the possible roles. Relevant features that may stimulate further research are, for example, absence of CRISPR/Cas systems in strain SC2, high number of iron acquisition systems in strain OB3b, and large number of transposases in strain Rockwell.

Conceptualizing functional traits and ecological characteristics of methane-oxidizing bacteria as life strategies

Methane-oxidizing bacteria (MOB) possess the ability to use methane for energy generation and growth, thereby, providing a key ecosystem service that is highly relevant to the regulation of the global climate. MOB subgroups have different responses to key environmental controls, reflecting on their functional traits. Their unique features (C1-metabolism, unique lipids and congruence between the 16S rRNA and pmoA gene phylogeny) have facilitated numerous environmental studies, which in combination with the availability of cultured representatives, yield the most comprehensive ecological picture of any known microbial functional guild. Here, we focus on the broad MOB subgroups (type I and type II MOB), and aim to conceptualize MOB functional traits and observational characteristics derived primarily from these environmental studies to be interpreted as microbial life strategies. We focus on the functional traits, and the conditions under which these traits will render different MOB subgroups a selective advantage. We hypothesize that type I and type II MOB generally have distinct life strategies, enabling them to predominate under different conditions and maintain functionality. The ecological characteristics implicated in their adopted life strategies are discussed, and incorporated into the Competitor-Stress tolerator-Ruderal functional classification framework as put forward for plant communities. In this context, type I MOB can broadly be classified as competitor-ruderal while type II MOB fit more within the stress tolerator categories. Finally, we provide an outlook on MOB applications by exemplifying two approaches where their inferred life strategies could be exploited thereby, putting MOB into the context of microbial resource management.

Global Molecular Analyses of Methane Metabolism in Methanotrophic Alphaproteobacterium, Methylosinus trichosporium OB3b. Part I: Transcriptomic Study

Methane utilizing bacteria (methanotrophs) are important in both environmental and biotechnological applications, due to their ability to convert methane to multicarbon compounds. However, systems-level studies of methane metabolism have not been carried out in methanotrophs. In this work we have integrated genomic and transcriptomic information to provide an overview of central metabolic pathways for methane utilization in Methylosinus trichosporium OB3b, a model alphaproteobacterial methanotroph. Particulate methane monooxygenase, PQQ-dependent methanol dehydrogenase, the H₄MPT-pathway, and NAD-dependent formate dehydrogenase are involved in methane oxidation to CO₂. All genes essential for operation of the serine cycle, the ethylmalonyl-CoA (EMC) pathway, and the citric acid (TCA) cycle were expressed. PEP-pyruvate-oxaloacetate interconversions may have a function in regulation and balancing carbon between the serine cycle and the EMC pathway. A set of transaminases may contribute to carbon partitioning between the pathways. Metabolic pathways for acquisition and/or assimilation of nitrogen and iron are discussed.

Stoichiometry and kinetics of the PHB-producing Type II methanotrophs Methylosinus trichosporium OB3b and Methylocystis parvus OBBP

In this study, modeling is used to describe how oxygen and nitrogen source affect the stoichiometry and kinetics of growth and PHB production in the Type II methanotrophs Methylosinus trichosporium OB3b and Methylocystis parvus OBBP. Significant differences were observed, with major implications for the use of these species in biotechnology applications. Such analyses can better inform bioreactor design, scale-up models, and life cycle assessments (LCAs).

Microbial production of poly(hydroxybutyrate) from C₁ carbon sources

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

Genome sequence of the methanotrophic poly-β-hydroxybutyrate producer Methylocystis parvus OBBP

Methylocystis parvus OBBP is an obligate methylotroph considered the type species of the genus Methylocystis. Two pmoCAB particulate methane monooxygenase operons and one additional singleton pmoC paralog were identified in the sequence. No evidence of genes encoding soluble methane monooxygenase was found. Comparison of M. parvus OBBP and Methylocystis sp. strain Rockwell (ATCC 49242) suggests that both species should be taxonomically classified in different genera.

Microaerophilic microenvironment at biocathode enhances electrogenesis with simultaneous synthesis of polyhydroxyalkanoates (PHA) in bioelectrochemical system (BES)

Microaerophilic microenvironment at biocathode was evaluated for electrogenesis along with the polyhydroxyalkanoates (PHA) accumulation in bio-electrochemical system (BES). The electrogenic activity (512 mV; 15.2 mW/m(2)) was extended for longer periods (144 h) which might be attributed to the lowering of losses due to the controlled microbial metabolism. Growth limiting stress at cathode due to lower oxygen levels and its effective utilization by the protons and electrons coming from anode, might have diverted the microbial metabolism towards PHA synthesis instead of oxidation. PHA accumulation (19% of dry cell weight (DCW)) was observed with higher hydroxy butyrate (HB) (89%) concentration at 48 th h in the cathodic biocatalyst and was re-utilized by the end of experiment. Bio-electro kinetics studied through voltammetry and Tafel analysis further supported the observed electrogenesis in microaerophilic reduction microenvironment, in terms of redox catalytic currents, Tafel slopes, exchange current densities and polarization resistance.

Genomic and Physiological Analysis of Carbon Storage in the Verrucomicrobial Methanotroph "Ca. Methylacidiphilum Fumariolicum" SolV

“Candidatus Methylacidiphilum fumariolicum” SolV is a verrucomicrobial methanotroph that can grow in extremely acidic environments at high temperature. Strain SolV fixes carbon dioxide (CO₂) via the Calvin–Benson–Bassham cycle with methane as energy source, a trait so far very unusual in methanotrophs. In this study, the ability of “Ca. M. fumariolicum” to store carbon was explored by genome analysis, physiological studies, and electron microscopy. When cell cultures were depleted for nitrogen, glycogen storage was clearly observed in cytoplasmic storage vesicles by electron microscopy. After cessation of growth, the dry weight kept increasing and the bacteria were filled up almost entirely by glycogen. This was confirmed by biochemical analysis, which showed that glycogen accumulated to 36% of the total dry weight of the cells. When methane was removed from the culture, this glycogen was consumed within 47 days. During the period of glycogen consumption, the bacteria kept their viability high when compared to bacteria without glycogen (from cultures growing exponentially). The latter bacteria lost viability already after a few days when starved for methane. Analysis of the draft genome of “Ca. M. fumariolicum” SolV demonstrated that all known genes for glycogen storage and degradation were present and also transcribed. Phylogenetic analysis of these genes showed that they form a separate cluster with “Ca. M. infernorum” V4, and the most closely related other sequences only have an identity of 40%. This study presents the first physiological evidence of glycogen storage in the phylum Verrucomicrobia and indicates that carbon storage is important for survival at times of methane starvation.

Cradle-to-gate life cycle assessment for a cradle-to-cradle cycle: biogas-to-bioplastic (and back)

At present, most synthetic organic materials are produced from fossil carbon feedstock that is regenerated over time scales of millions of years. Biobased alternatives can be rapidly renewed in cradle-to-cradle cycles (1-10 years). Such materials extend landfill life and decrease undesirable impacts due to material persistence. This work develops a LCA for synthesis of polyhydroxybutyrate (PHB) from methane with subsequent biodegradation of PHB back to biogas (40-70% methane, 30-60% carbon dioxide). The parameters for this cradle-to-cradle cycle for PHB production are developed and used as the basis for a cradle-to-gate LCA. PHB production from biogas methane is shown to be preferable to its production from cultivated feedstock due to the energy and land required for the feedstock cultivation and fermentation. For the PHB-methane cycle, the major challenges are PHB recovery and demands for energy. Some or all of the energy requirements can be satisfied using renewable energy, such as a portion of the collected biogas methane. Oxidation of 18-26% of the methane in a biogas stream can meet the energy demands for aeration and agitation, and recovery of PHB synthesized from the remaining 74-82%. Effective coupling of waste-to-energy technologies could thus conceivably enable PHB production without imported carbon and energy.

Cyclic, alternating methane and nitrogen limitation increases PHB production in a methanotrophic community

To identify feast-famine strategies that favor PHB accumulation in Type II methanotrophic proteobacteria, three sequencing batch reactors seeded with a defined inoculum of Type II methanotrophs were subjected to 24-h cycles consisting of (1) repeated nitrogen limitation, (2) repeated nitrogen and oxygen limitation, and (3) repeated nitrogen and methane limitation. PHB levels within each reactor and capacity to produce PHB in offline batch incubations were monitored over 11 cycles. PHB content increased only in the reactor limited by both nitrogen and methane. This reactor became dominated by Methylocystis parvus OBBP with no detectable minority populations. It was concluded that repeated nitrogen and methane limitations favored PHB accumulation in strain OBBP and provided it with a competitive advantage under the conditions imposed.