EMULSION-BASED FERMENTATION FOR ACCELERATED GAS SUBSTRATE MASS TRANSFER

Abstract

Described here is a method for increasing the transfer of a gas substrate in microbial fermentation, comprising incubating an emulsion comprising an oil phase and an aqueous phase droplet dispersed in the oil phase, and supplying the gas substrate to the oil phase, wherein the aqueous phase droplet comprises a microorganism, and wherein the emulsion is stabilized by a surfactant or an amphiphilic particle that is adsorbed to an interface of the oil phase and the aqueous phase. Also described is an emulsion for microbial fermentation, comprises an oil phase and an aqueous phase droplet dispersed in the oil phase, wherein the aqueous phase droplet comprises a microorganism, wherein the emulsion comprises a gas substrate externally-supplied to the oil phase, and wherein the emulsion is stabilized by a surfactant or an amphiphilic particle that is adsorbed to an interface of the oil phase and the aqueous phase.

Claims

1. A method for increasing the transfer of a gas substrate in microbial fermentation, comprising incubating an emulsion comprising an oil phase and an aqueous phase droplet dispersed in the oil phase, and supplying the gas substrate to the oil phase, wherein the aqueous phase droplet comprises a microorganism, and wherein the emulsion is stabilized by a surfactant or an amphiphilic particle that is adsorbed to an interface of the oil phase and the aqueous phase droplet.

2. The method of claim 1, wherein the aqueous phase droplet comprises a microorganism selected from the group consisting of carbon monoxide-utilizers, hydrogen utilizers, and alkane utilizers.

3. The method of claim 1, wherein the gas substrate is selected from the group consisting of carbon monoxide, hydrogen, and alkane.

4. The method of claim 1, wherein the aqueous phase droplet comprises a methanotroph, and wherein the gas substrate is methane.

5. The method of claim 4, wherein the methanotroph is adapted to produce a bioproduct of biodiesel generation, propylene oxide production, single cell protein production, extracellular polysaccharides production, or human health supplements production.

6. The method of claim 4, wherein the methanotroph is adapted to produce a polyhydroxyalkanoate, and the method further comprises recovering the polyhydroxyalkanoate produced.

7. The method of claim 1, wherein the oil phase comprises a fluid that has a higher solubility for the gas substrate than the aqueous phase and is capable of generating a stable emulsion with the aqueous phase.

8. The method of claim 1, wherein the oil phase comprises a hydrocarbon.

9. The method of claim 1, wherein the oil phase comprises a fluorocarbon.

10. The method of claim 1, wherein the oil phase comprises at least one of C.sub.9H.sub.5OF.sub.15 (HFE-7500), C.sub.21F.sub.48N.sub.2 (FC-40), or perfluoromethyldecalin (PFMD).

11. The method of claim 1, wherein the emulsion is stabilized by a fluoro-surfactant comprising a poly(ethylene glycol)-perfluorinated polyether (PEG-PFPE) amphiphilic block copolymer.

12. The method of claim 1, wherein the emulsion is stabilized by a partially fluorinated silica nanoparticle.

13. The method of claim 12, wherein the aqueous phase droplet further comprises a hydrophilic polymer adsorbed to the partially fluorinated silica nanoparticle at the interface.

14. The method of claim 12, wherein the partially fluorinated silica nanoparticle is covalently grafted with a hydrophilic polymer.

15. The method of claim 1, wherein the method is substantially free of mechanical stirring or agitation of the emulsion during incubation.

16. An emulsion for microbial fermentation, comprises an oil phase and an aqueous phase droplet dispersed in the oil phase, wherein the aqueous phase droplet comprises a microorganism, wherein the emulsion comprises a gas substrate externally-supplied to the oil phase, and wherein the emulsion is stabilized by a surfactant or an amphiphilic particle that is adsorbed to an interface of the oil phase and the aqueous phase droplet.

17. The emulsion for microbial fermentation of claim 16, wherein the aqueous phase droplet comprises a microorganism selected from the group consisting of carbon monoxide-utilizers, hydrogen utilizers, and alkane utilizers.

18. The emulsion for microbial fermentation of claim 16, wherein the gas substrate is selected from the group consisting of carbon monoxide, hydrogen, and alkane.

19. The emulsion for microbial fermentation of claim 16, wherein the aqueous phase droplet comprises a methanotroph, and wherein the gas substrate is methane.

20. The emulsion for microbial fermentation of claim 19, wherein the aqueous phase droplet further comprises a bioproduct of biodiesel generation, propylene oxide production, single cell protein production, extracellular polysaccharides production, or human health supplements production.

21. The emulsion for microbial fermentation of claim 19, wherein the aqueous phase droplet further comprises a polyhydroxyalkanoate produced by the methanotroph.

22. The emulsion for microbial fermentation of claim 16, wherein the oil phase comprises a fluid that has a higher solubility for the gas substrate than the aqueous phase and is capable of generating a stable emulsion with the aqueous phase.

23. The emulsion for microbial fermentation of claim 16, wherein the oil phase comprises a hydrocarbon.

24. The emulsion for microbial fermentation of claim 16, wherein the oil phase comprises a fluorocarbon.

25. The emulsion for microbial fermentation of claim 16, wherein the oil phase comprises at least one of HFE-7500, FC-40, or PFMID.

26. The emulsion for microbial fermentation of claim 16, wherein the emulsion is stabilized by a fluoro-surfactant comprising a PEG-PFPE amphiphilic block copolymer.

27. The emulsion for microbial fermentation of claim 16, wherein the emulsion is stabilized by a partially fluorinated silica nanoparticle.

28. The emulsion for microbial fermentation of claim 27, wherein the aqueous phase droplet further comprises a hydrophilic polymer adsorbed to the partially fluorinated silica nanoparticle at the interface.

29. The emulsion for microbial fermentation of claim 27, wherein the partially fluorinated silica nanoparticle is covalently grafted with a hydrophilic polymer.

30. A method for obtaining an emulsion for microbial fermentation, comprising mixing an oil phase and an aqueous phase, wherein the oil phase comprises (a) a fluorocarbon and (b) a surfactant or an amphiphilic particle adapted to adsorb to an interface of the oil phase and the aqueous phase, and wherein the aqueous phase comprises a microorganism and optionally a hydrophilic polymer.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0039] FIG. 1: Scheme of an example microfluidic device and process flow.

[0040] FIG. 2: Cell growth in drops. (a) Bright-field microscopy images of 40-pL drops containing cells at different time points. Arrows indicate two cells in the drop at t=0 h. (b) Comparison of OD600 of culture grown in emulsions composed of monodisperse drops (40 pL, 150 pL, 750 pL and 5 nL), polydisperse drops, polydisperse drops stabilized by NPs, bulk aqueous media, and bulk aqueous media mixed with HFE-7500 containing no emulsions. In each sample, the total volumes of the aqueous media and HFE-7500 were fixed at 1.8 mL respectively except for the samples containing aqueous media only. All experiments were performed with no shaking except the control. The control composed of 1.8-mL aqueous media under shaking conditions at 150 rpm.

[0041] FIG. 3: Comparison of P3HB accumulation in cells grown in bulk solution and cells grown in droplets after seven days of incubation. (a) TEM images. (b) P3HB wt % as measured by Nile red stain.

[0042] FIG. 4: (a) Partitioning of methane from gas phase to liquid phase in the absence of microbial growth. Lines in the figure were modelled using Eq. (1). For all experiments, the volume of water and HFE were kept constant at 1.8 mL (except in the sample that contained water only). The volume of the gas phase was 4.4 mL. (b) Partitioning of methane from methane-saturated HFE to water. The volume of water and HFE were both 4.0 mL.

DETAILED DESCRIPTION

[0043] In some embodiments, describe here is a method for accelerating methane mass transfer without the need for agitation. This effect can be achieved by growing methanotrophs in water-in-oil (W/O) emulsions suspended in an oil that has a higher solubility for methane than water does. The method provides enhanced mass transfer of methane or P3HB accumulation within an emulsion-based system. Here a fluorinated oil with biocompatibility was used, gas permeability and corresponding emulsion stabilizing agents were used to generate stable emulsions. Other oils can also be used so long as they possess similar qualities. For example, hydrocarbon-based oils having a higher solubility for methane than water and being capable of generating a stable emulsion can be used. It is shown that the growth rate of methanotrophs in such emulsions, in the absence of agitation, may increase by, for example, four to five times compared with growth rates in bulk aqueous solution where methane is delivered from the gas phase. It is found that cells incubated in the emulsions can accumulate, for example, up to 67 times more P3HB than in bulk within seven days. Emulsion-based fermentation thus can be used for high-density methanotroph fermentations.

[0044] Volumes of droplets generated. As shown in FIG. 1, cells suspended in the culture media were encapsulated into microdroplets. To generate uniform drops of different volumes, the flow rates of continuous and disperse phases were varied, and the geometry of flow-focusing nozzle. To generate polydisperse drops, the culture media were mixed with the continuous phase followed by manual shaking. The size of the drops obtained ranged from 5 pL to 1 nL. The drops generated were then collected in a gas-tight vial and incubated with methane and oxygen injected in the headspace. Small volumes of the drops were sampled and destabilized for the characterization of cell growth and P3HB accumulation at various time points.

[0045] The rate of cell growth increased in emulsion-based fermentation. FIG. 2a shows bright-field microscopy images of cells in 40 pL drops as a function of time. At the initial concentration of cells used (OD.sub.600 ˜0.1, corresponding to ˜9.28×10.sup.7 cells/mL), a 40-pL drop contained an average of 4 cells at t=0 h. The number of cells in a drop increased significantly after 48 h. No further increase was observed after 96 h. The qualitative trend of cell growth from these microcopy images was consistent with optical density measurements. FIG. 2b shows that cells replicated rapidly during the first 72 h to reach OD.sub.600 ˜0.7 (corresponding to 6.50×10.sup.8 cells/mL) and stabilized afterwards. In the control experiment where the cells were cultured with shaking at 150 rpm in the aqueous media without HFE-7500 or emulsions, the cells also grew to a maximum density of OD.sub.600 ˜0.7 in 72 h. The encapsulation of cells in emulsions thus did not have negative effect on the growth rate and final cell density reached. The observed time scale for cell growth was expected based on the composition of the culture media. The composition of the media was chosen such that nitrogen (ammonium) was the limiting nutrient. Nitrogen depletion is generally considered an effective trigger to start P3HB accumulation in methanotrophic bacteria. The amount of nitrogen used in the media was also designed to allow the growth of methanotrophs for three days, after which the cells switch to P3HB accumulation mode.

[0046] In the absence of agitation, the use of emulsions increased the rate of cell growth compared with growth in bulk solution. FIG. 2b shows that the cell density obtained from growth in emulsions was ˜2.2 times more than that in bulk media at t=168 hours. The maximum rate of growth (as measured by the biggest increase in OD.sub.600 between two time points [AOD.sub.600/Δt]max) increased by 4.6±0.4 times when the cells were incubated in emulsions, compared with their growth in bulk media and in a mixture of bulk media and neat HFE-7500 without the formation of emulsion. Previously, a small amount of paraffin oil (2.5-10% v/v) was added to the aqueous bulk media to increase the mass transfer of methane. As paraffin oil has a higher solubility of methane than water does, an increased cell density was obtained compared with the case without paraffin oil when the incubation was performed with shaking at 150 rpm. In the absence of shaking, however, the results show that the addition of an immiscible phase (HFE-7500) having a higher methane solubility alone was insufficient to increase the rate of cell growth. Due to the relatively high interfacial tension (˜50 mN/m) between water and HFE-7500, the mixture separated into top and bottom phases spontaneously with no agitation. Such mixture would at most double the interfacial area and the corresponding mass transfer between the aqueous phase and HFE-7500 or the gas phase in the headspace. On the other hand, the use of emulsions stabilized by surfactants or NPs increased the interfacial area between HFE-7500 and the aqueous phase significantly. For the size of drops used here, the interfacial area increased by approximately 160-800 times compared with bulk solution. So long as the emulsions were stable against coalescence, no shaking was required to maintain this large interfacial area for enhanced mass transfer of methane into the aqueous phase. Surprisingly, no noticeable difference in growth rates was observed in emulsions composed of monodisperse drops for the volumes tested and in polydisperse drops with volumes ranging from 5 pL to 5 nL, whether stabilized by surfactants or NPs, as shown in FIG. 4.

[0047] P3HB accumulation increased in emulsion-based fermentation. Based on cell growth data, it is expect that cells grown in emulsions would accumulate more P3HB than cells in bulk media do in the absence of agitation. Indeed, FIG. 3 shows that cells grown inside droplets accumulated ˜32±4 wt % P3HB while cells grown in bulk media did not accumulate any P3HB, as indicated by both TEM images and Nile red staining. In the control experiment where the cells were cultured with shaking at 150 rpm in aqueous media without HFE-7500 or emulsions, the maximum amount of P3HB accumulated was ˜35±6 wt % of cell mass. As such, the emulsion was effective in increasing the rate of accumulation of P3HB with no significant effect on the final accumulated level of P3HB.

[0048] Emulsion-based fermentation is limited by methane consumption rather than mass transfer. FIGS. 2 and 3 show that the size of the drops used did not affect cell growth or P3HB accumulation. These results indicate that the culture may be rate-limited by cell metabolism, rather than the mass transport of methane. To verify this effect, the kinetics of mass transfer of methane from the gas phase into the liquid phase were measured. FIG. 4a shows the partial pressure of methane in the headspace of water and HFE-7500 respectively over 168 hours. The initial and equilibrium partial pressures of methane in the headspace were used to calculate the Henry's constant for quantifying the solubility of methane in different liquids. The Henry's constant calculated for HFE-7500 (1.1×10.sup.−2 mol atm.sup.−1) was about 10 times higher than that for water (1.2×10.sup.−3 mol atm.sup.−1). Methane is thus approximately 10 times more soluble in HFE-7500 than in water. The curves in FIG. 4a also allowed us to calculate the rate of methane transfer from the gas phase to the liquid phase using Eq. (1):

[00001]$\begin{array}{cc}\frac{\mathrm{dC}}{\mathrm{dt}}=K_L.Math.a\left(C^{*}-C\right)& \left(1\right)\end{array}$

where

[00002]$C=\frac{V_{\mathrm{gas}}}{V_{\mathrm{liquid}}.Math.\mathrm{RT}}.Math.\left(P_0-P\right)$

is the concentration of dissolved methane in the liquid phase (mg CH.sub.4 L.sup.−1), V.sub.gas is the volume of gas in the headspace, V.sub.liquid is the volume of the liquid, R is the universal gas constant, T is temperature, P is the partial pressure of methane at time t, and P.sub.0 is the partial pressure of methane at time t=0. C* is the saturated concentration of dissolved methane (mg CH.sub.4 L.sup.−1), K.sub.L is the mass transfer coefficient (cm h.sup.−1) and a is the gas/liquid interfacial area per volume of liquid (cm.sup.2 cm.sup.−3). Because of the difficulty in measuring K.sub.L and a separately, the product K.sub.La is treated as a single measureable variable representing the volumetric mass transfer coefficient. Integrating Eq. (1) yields Eq. (2):

[00003]$\begin{array}{cc}\ln \left(1-\frac{C}{C^{*}}\right)=-K_L.Math.\mathrm{at}& \left(2\right)\end{array}$

K.sub.La for different liquids in FIG. 4a were found readily from the slope of Eq. (2) (Table 1). The K.sub.La of methane from HFE-7500 to water was found using a similar procedure (FIG. 4b, Table 1). The effective K.sub.La from the gas phase into the emulsions was then estimated. Briefly, the emulsion system can be treated as a three-phase system where methane has to be transported, in series, from the gas phase to the oil phase (HFE-7500) and then into the aqueous phase. A resistance-in-series model is applied to estimate the total resistance to mass transfer, which equals the sum of the resistances at the gas-HFE and HFE-water interfaces. The overall mass transfer coefficient is the reciprocal of the overall resistance.

TABLE-US-00001 TABLE 1 K.sub.La values for different liquids tested. Liquids K.sub.La (h.sup.−1) Water + HFE-7500 0.051 Water 0.051 HFE-7500 0.115 Emulsion composed of 40 pL drops stabilized by surfactants 1.149 Emulsion composed of 5 nL drops stabilized by surfactants 1.146 Emulsion composed of polydisperse drops stabilized by NP 1.147

[0049] To identify whether mass transfer or cell metabolism was the rate limiting step in cell growth, the Damkohler number (Da) were calculated. Da is a dimensionless number that characterizes the reaction rate relative to the transport rate, and is defined as maximum possible methane utilization rate (MUR.sub.max) divided by the maximum mass transfer rate (MTR.sub.max):

[00004]$\begin{array}{cc}\mathrm{Da}=\frac{{\mathrm{MUR}}_{\max }}{{\mathrm{MTR}}_{\max }}=\frac{q_{\max ,\mathrm{CH}.Math..Math.4}.Math.X}{K_L.Math.{a\left[{\mathrm{CH}}_4\right]}^{*}}& \left(3\right)\end{array}$

where q.sub.max,CH4 is the specific methane utilization rate, X is the average cell concentration measured throughout the seven days of incubation, and [CH4]* is the saturation concentration of CH4 in the aqueous media. The value of q.sub.max,CH4were measured to be 0.044 mg CH.sup.4 mg TSS-1 h.sup.−1. The average Da numbers calculated for water (Da=8.1) and water and HFE-7500 without surfactant (Da=8.7) were both higher than 1, which indicate that cell growth was mass-transfer limited. On the other hand, the average Da numbers calculated for emulsions composed of drops of volumes 5 nL (Da=0.62), 40 pL (Da=0.62) and NP-stabilized polydisperse droplets (Da=0.63) were all less than 1, which indicate that cell growth was limited by reaction-rate or cell metabolism. These values also explain why no significant differences were observed when drops of different sizes were used in FIGS. 2 and 3. The lack of dependence on droplet size and uniformity allows the use of emulsification techniques that are more easily scalable than microfluidic approach. Indeed, the use of polydisperse emulsions formed by manual shaking gave similar results as monodisperse emulsions did. Furthermore, replacing surfactants with NPs rendered similar results. The use of NPs as stabilizers is advantageous because NPs are cheaper than surfactants, and can be potentially recovered and reused more easily than surfactants.

[0050] Scaling-up of the emulsion-based fermentation. In moving towards the scaling-up of the emulsion method shown here, further optimization can be implemented to destabilize the emulsion by applying an electric field. As the addition of chemical destabilizers is not necessary in this case, the oil phase and the NPs used for stabilizing the emulsion can be recycled to reduce cost and waste reagents. Although a single type of oil was used in this embodiment, the principle can be applied to other oils that are potentially more cost-effective and environmentally friendly than HFE-7500, so long as they satisfy the following: (1) compatibility with the growth of methanotrophs; (2) high solubility of methane, and low mass transfer resistance with the gas phase and with the water phase respectively; and (3) availability of the corresponding emulsion stabilizing agents. The emulsion-based method shown here can be used for the high-density production of P3HB from methanotrophs.

[0051] It has been demonstrated an emulsion-based fermentation method for increasing the rates of cell growth and P3HB accumulation without the need for agitation. The use of emulsion effectively increased the interfacial area of the aqueous phase to accelerate the mass transfer of methane. The fermentation process became rate-limited by cell metabolism rather than the mass transport of methane. The use of microfluidics did not negatively affect cell growth and P3HB accumulation.

WORKING EXAMPLES

Example 1.1

[0052] Culture conditions. Methylocystis parvus OBBP, a P3HB-producing Type II methanotrophs, was used for all experiments in this study. M. parvus OBBP was grown in medium JM2. JM2 contained the following chemicals per liter of solution: 2.4 mM MgSO.sub.4. 7H.sub.2O, 0.26 mM CaCl.sub.2, 36 mM NaHCO.sub.3, 4.8 mM KH.sub.2PO.sub.4, 6.8 mM K.sub.2HPO.sub.4, 10.5 μM Na.sub.2MoO.sub.4.2H.sub.2O, 7 μM CuSO.sub.4.5H2O, 200 μM Fe-EDTA, 530 μM Ca-EDTA, 5 mL trace metal solution and 20 mL vitamin solution. The trace metal stock solution contained the following chemicals per liter of solution: 500 mg FeSO.sub.4.7H2O, 400 mg ZnSO.sub.4.7H.sub.2O, 20 mg MnCl.sub.2.7H.sub.2O, 50 mg CoCl.sub.2.6H.sub.2O, 10 mg NiCl.sub.2.6H.sub.2O, 15 mg H.sub.3BO.sub.3 and 250 mg EDTA. The vitamin stock solution contained the following chemicals per liter of solution: 2.0 mg biotin, 2.0 mg folic acid, 5.0 mg thiamine.HC1, 5.0 mg calcium pantothenate, 0.1 mg vitamin B12, 5.0 mg riboflavin and 5.0 mg nicotinamide. Ammonium chloride (4 mM) was added as a nitrogen source. M. parvus OBBP was grown in this liquid culture to reach an exponential phase with an optical density (OD.sub.600) of approximately 0.1. This concentration of cells was used in all subsequent experiments.

Example 1.2

[0053] Generation of emulsions. Microfluidic flow-focusing devices were used for the generation of emulsions composed of uniform microdroplets with controlled volumes. Soft lithography were used to fabricate microfluidic channels in poly(dimethylsiloxane) (PDMS). The microchannels were rendered hydrophobic by treatment with Aquapel (Pittsburgh Glass Works LLC, Pittsburgh, Pa., USA) to avoid droplet wetting of the wall. The droplets generated had a size dispersity<5%. The continuous phase was a hydrofluoroether HFE-7500 (3M, St. Paul, Minn., USA) containing a biocompatible “EA-surfactant” (RAN Biotechnologies Inc., Beverly, Mass., USA) (2% w/w), a PEG-PFPE amphiphilic block copolymer, to stabilize the drops against coalescence. These drops were stable for weeks. HFE-7500 was inert and permeable to gases, and had been shown to be compatible with cell cultures in drops. To study the effect of droplet size on the growth rate of the cells and the accumulation of P3HB, four different sizes of droplets were generated ranging from 40 pL to 5 nL in volume.

[0054] For the generation of emulsions composed of polydisperse drops, vigorous shaking of a mixture was manually performed of bacterial media (M. parvus OBBP culture, OD600 ˜0.1) and HFE-7500 containing EA-surfactant (2% w/w) at 2:3 volume ratio for 1 min. The size distribution of the drops formed was characterized. 100 nm amphiphilic silica nanoparticles (NPs) were also used to replace EA-surfactant for stabilizing the drops. The synthesis of these NPs was described in Examples 1.1. To prevent the adhesion of cells to NP surface, polyethylene glycol (MW=8000, 10 mg/mL) were also introduced into the aqueous phase prior to droplet formation.

[0055] As water has a lower density than HFE-7500 does (p=1.63 g/mL), the generated drops creamed to the top of the collection vial to form a concentrated emulsion with volume fraction φ ˜80% within minutes upon collection. The volume fraction φ is defined as φ=V.sub.aq/V.sub.oil, where V.sub.aq is the total volume of the aqueous drops, and V.sub.oil is the volume of the HFE-7500 containing surfactants or NPs. For all subsequent experiments, 1.8 mL of this concentrated emulsion (φ ˜80%) was collected into a gas-tight glass vial (Wheaton, Mealville, N.J., USA) capped with butyl-rubber stoppers which were then crimp-sealed. The headspace in each vial was over-pressured at approximately 1.5 atm with 1:1.5 molar ratio of methane and oxygen. The emulsion was incubated at 30° C. for 0-7 days without shaking.

Example 1.3

[0056] Optical density measurements and imaging of cells in drops. To measure the optical density (OD.sub.600) of the cells, the emulsions were destabilized by adding 200 μL of 1H,1H,2H,2H-perfluorooctanol (Sigma-Aldrich, St Louis, Mo., USA) to each 200 μL of emulsion to merge the droplets into a single aqueous phase. For NP-stabilized drops, 300 μL of fluorinert FC-40 (Sigma-Aldrich, St Louis, Mo., USA) was added to destabilize 50 μL of the emulsion. For imaging the cells within the drops, the drops were injected into a wide microfluidic channel and imaged using an inverted optical microscope and an Electron Multiplying Charge Coupled Device (Andor iXon Ultra 897, Andor Technology Ltd., Belfast, UK).

Example 1.4

[0057] Visualization and quantification of P3HB. Transmission electron microscopy (TEM) was used to evaluate the morphology of the P3HB granules. After seven days of incubation, bacterial pellets were fixed with 2% glutaraldehyde and 4% paraformaldehyde in 0.1 M sodium cacodylate buffer (Na(CH.sub.3).sub.2 AsO.sub.2.3H.sub.2O), pH 7.4 for 48 h at 4° C. To coat cells in gelatin, cells were washed in the buffer and resuspended in 10% warm (˜50° C.) gelatin for 5 min, placed on ice for 5 min, then cut into blocks and post-fixed using cold osmium tetroxide (OsO.sub.4). Post-fixed samples were dehydrated using ethanol and acetonitrile, embedded in an epoxy resin mixture, then cut into ultra-thin sections, which were then mounted on copper grids. The grids were observed with a JEOL TEM 1400 microscope equipped with a Gatan 967 slow-scan, cooled CCD camera. Images were processed using Digital Micrograph, Digital Montage and TEM Auto tune.

[0058] To quantify P3HB content accumulated in the cells, Nile red was used to stain P3HB in the cells following a protocol published previously. Nile red staining requires small amounts of cells (<0.1 mg), and has been demonstrated to be a reliable method for measuring P3HB level in methanotrophs. Fluorescence intensity was measured from the cells after Nile red staining using a Scanford flow cytometer and 561 nm laser. The corresponding P3HB concentration (mg P3HB mg TSS.sup.1) was then extracted by comparing the measured fluorescence intensity with a calibration curve obtained previously. The calibration showed linear relationship between Nile red fluorescence intensity and P3HB content in cells (mg P3HB mg TSS.sup.−1) as measured by gas chromatography equipped with a flame ionization detector (GC-FID).

Example 1.5

[0059] Measurement of methane solubility and partitioning kinetics from gas phase to liquid phase. To measure the solubility of methane in various liquids, the partial pressure of methane was monitored in the headspace of water, HFE-7500 and the emulsions (φ ˜80%) respectively. Methane was injected in the headspace (4.4 mL) of a glass vial containing these liquids at a fixed volume of 3.6 mL. All vials were subject to same conditions at 30° C. To measure the concentration of methane, 0.5 mL of gas was extracted from the headspace and injected into a GOW-MAC gas chromatograph (GOW-Mac Instrument Co., Bethlehem, Pa., USA) with an Alltech CTR 1 column (Alltech Associates Inc., Deerfield, Ill., USA) and a thermal conductivity detector. The following parameters were used: injector, 120° C.; column, 60° C.; detector, 120° C.; and current, 150 mV. Peak areas of methane were compared to standards and quantified using the software ChromPerfect (Justice Laboratory Software, Denville, N.J., USA).

Example 1.6

[0060] Measurement of methane partitioning kinetics from HFE-7500 to water. To determine the methane partitioning kinetics between HFE-7500 and water, 4.0 mL of HFE-7500 was first saturated with methane by injection into a gas-tight glass vial (Wheaton, Mealville, N.J., USA) filled with 100% methane in the headspace. After saturation, HFE-7500 was added to separate bottles containing 4.0 mL of water and a vacuum headspace, allowing HFE-7500 to fill the vacant volume. All vials were subject to same conditions at 30° C. To measure the concentration of methane in water, 2.0 mL of methane-dissolved water was sampled using a syringe needle, and the chemical oxygen demand (COD) of the water sample was analyzed using a test kit (COD Reagent, TNT Plus, ULR; Hach Company, Loveland, Colo., USA).

Example 1.7

[0061] Measurement of maximum specific methane utilization rate. The maximum specific methane utilization rate (q.sub.max,CH4) for M. parvus OBBP in the culture media was evaluated. Briefly, cells were incubated in 160-mL serum bottle (Wheaton, Mealville, N.J., USA) capped with butyl-rubber stoppers and crimp-sealed under excess CH.sub.4:O.sub.2 headspace (molar ratio of 1:1.5). All bottles were incubated horizontally on orbital shaker tables at 150 rpm. The incubation temperature was 30° C. Bottle headspace was analyzed periodically to evaluate methane consumption.

[0062] As used herein, the singular terms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to a molecule can include multiple molecules unless the context clearly dictates otherwise.

[0063] As used herein, the terms “substantially,” “substantial,” and “about” are used to describe and account for small variations. When used in conjunction with an event or circumstance, the terms can refer to instances in which the event or circumstance occurs precisely as well as instances in which the event or circumstance occurs to a close approximation. For example, when used in conjunction with a numerical value, the terms can refer to a range of variation of less than or equal to ±10% of that numerical value, such as less than or equal to ±5%, less than or equal to ±4%, less than or equal to ±3%, less than or equal to ±2%, less than or equal to ±1%, less than or equal to ±0.5%, less than or equal to ±0.1%, or less than or equal to ±0.05%.

[0064] Additionally, amounts, ratios, and other numerical values are sometimes presented herein in a range format. It is to be understood that such range format is used for convenience and brevity and should be understood flexibly to include numerical values explicitly specified as limits of a range, but also to include all individual numerical values or sub-ranges encompassed within that range as if each numerical value and sub-range is explicitly specified. For example, a ratio in the range of about 1 to about 200 should be understood to include the explicitly recited limits of about 1 and about 200, but also to include individual ratios such as about 2, about 3, and about 4, and sub-ranges such as about 10 to about 50, about 20 to about 100, and so forth.

[0065] In the foregoing description, it will be readily apparent to one skilled in the art that varying substitutions and modifications may be made to the invention disclosed herein without departing from the scope and spirit of the invention. The invention illustratively described herein suitably may be practiced in the absence of any element or elements, limitation or limitations, which is not specifically disclosed herein. The terms and expressions which have been employed are used as terms of description and not of limitation, and there is no intention that in the use of such terms and expressions of excluding any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope of the invention. Thus, it should be understood that although the present invention has been illustrated by specific embodiments and optional features, modification and/or variation of the concepts herein disclosed may be resorted to by those skilled in the art, and that such modifications and variations are considered to be within the scopes of this invention.