New Biotechnology  Volume 00, Number 00  June 2015

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Research Paper

Enhancement of carbon dioxide reduction and methane production by an obligate anaerobe and gas dissolution device Seungjin Kim1, Kwangkeun Choi2, Jong-Oh Kim3 and Jinwook Chung1 1

R&D Center, Samsung Engineering Co. Ltd., Woncheon-Dong, Youngtong-Gu, Suwon, Gyeonggi-Do 443-823, Republic of Korea Central Research Center, Green and Global In Tech Co. Ltd., U-Tower 910 2039 Youngduk-Dong, Kiheung-Gu, Yongin, Gyeonggi-Do 446-908, Republic of Korea 3 Department of Civil and Environmental Engineering, Hanyang University, 222 Wangsimni-Ro, Seongdong-Gu, Seoul 133-791, Republic of Korea 2

The use of gas dissolution devices to improve the efficiency of H2 dissolution has enhanced CO2 reduction and CH4 production. In addition, the nutrients that initially existed in anaerobic sludge were exhausted over time, and the activities of anaerobic microorganisms declined. When nutrients were artificially injected, CO2 reduction and CH4 production rates climbed. Thus, assuming that the activity of the obligatory anaerobic microorganisms is maintained, a gas dissolution device will further enhance the efficiency of CO2 reduction and CH4 production.

Introduction Countries worldwide have recently been implementing programs to regulate carbon dioxide (CO2) emissions, which cause global warming. Korea is at high risk regarding greenhouse gas emissions, because its CO2 emission is the ninth highest globally, and its rate of CO2 emission is the world’s highest. Thus, high-efficiency technology that decreases CO2 is essential in addressing this problem. To this end, pan-governmental CO2 reduction plans are being prepared. CO2 is generally treated using collection/separation and conversion technologies. However, the former method has several disadvantages, such as its low feasibility and stability, high cost and poor recyclability [1,2]. In particular, the photocatalytic reduction of CO2 has recently attracted attention because CO2 can be reduced in water vapor or solvent by photocatalysts. However, photocatalytic reduction technologies remain in their infancy due to their low photocatalytic efficiency, the absence of a suitable form of photocatalyst, and their use of solar energy [3,4]. Also, the electrochemical reduction of CO2 to produce methanol is achievable but kinetically complex and requires effective electrocatalysis [5,6]. Thus, conversion technologies that are eco-friendly and generate energy are being developed. Photosynthetic microorganisms Corresponding author: Chung, J. ([email protected]) http://dx.doi.org/10.1016/j.nbt.2015.05.004 1871-6784/ß 2015 Elsevier B.V. All rights reserved.

and microalgae are used to fix and reduce CO2 biologically and convert it into useful substances. The photosynthetic method requires sufficient time, area and energy such as solar power and artificial light sources [7,8]. Anaerobic digestion is a biological process in which organic substrate is transformed into biogas, primarily comprising methane (CH4) (55–75%), CO2 (30–45%) and traces (1% of gas volume) of nitrogen (N2), hydrogen (H2), ammonia (NH3), and final stabilized organic products [9]. CH4 generates few atmospheric pollutants and less CO2 per unit energy compared with other fossil fuels. Although certain applications require high-purity CH4, various stages of purity can be used, and the efficiencies of transport and energy conversion might improve 15–20% and 25%, respectively, versus the generation of electricity generation [10–12]. Also, H2 is as good an energy source as CH4 and can be produced at rates of up to 8%. However, H2 is not well developed commercially for production and use and is more difficult to produce from biomass due to its high cost and the large area that is required for pretreatment. For example, the high cost is attributed in part to labor costs and investments in machines and fertilizers. Because most liquid biomass usually contains large amounts of water, the dewatering process should be introduced for its direct use as fuel [13–15]. For these reasons, we focused on minimizing that CO2 that is discharged from an anaerobic digestion reaction by enhancing www.elsevier.com/locate/nbt

Please cite this article in press as: Kim, S. et al., Enhancement of carbon dioxide reduction and methane production by an obligate anaerobe and gas dissolution device, New Biotechnol. (2015), http://dx.doi.org/10.1016/j.nbt.2015.05.004

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CH4 production with residual H2 and CO2. The residual H2 and CO2 were obtained easily by gas and wastewater that were discharged from the electronics manufacturing industry. Specifically, hydrogen can be predominantly generated from the electrocoagulation process for removing fluoride ion in hydrofluoric acid (HF) wastewater among various types of wastewater that is discharged from electronics manufacturers [16,17]. When bioconversion technology (particularly anaerobic methanization) is used to treat CO2, methane can be produced, in addition to CO2 being reduced. The resulting methane can be used as an energy source, limiting fossil fuel consumption. Thus, such methods can also decrease the CO2 that is generated from the use of fossil fuels [18–22]. However, because H2, which can be used as a reducing agent in the bioconversion of CO2 to CH4, has low solubility in water, its use by anaerobic microorganisms is limited, creating a bottleneck in enhancing CO2 reduction and CO2–CH4 conversion rates [23–29]. This study was performed to develop a method of increasing the solubility of H2–CO2 mixed gas (particularly that of H2) and measure CO2 reduction and CH4 production rates using obligate anaerobes and a gas dissolution device over long-term operation. In addition, the molecular microbial ecology of this technology was analyzed to determine its microbial diversity.

Materials and methods CO2-reducing microorganisms The anaerobic microorganisms that were used to reduce CO2 were obtained from the anaerobic digestion tank of S municipal wastewater treatment plant in Yongin, Korea. As soon as sludge was transported to the laboratory, strict anaerobic conditions were established using H2–CO2 gas (4:1 ratio) to keep them active in the sludge. Many methanogens grow in simple medium, in which 4 mol H2 is oxidized and 1 mol CO2 is reduced to methane, resulting in negative pressure in the culture tube or vial. The H2:CO2 atmosphere is repressurized during growth of the methanogen. The following medium was used for the enrichment (g/L): NH4Cl, 1; NaCl, 0.6; NaHCO3, 5; KH2PO4, 0.3; K2HPO4, 0.3; MgCl26H2O, 0.16; CaCl22H2O, 0.009; cysteineHCl, 0.25 g; Na2S9H2O, 0.25 g; and resazurin 0.1% solution, 1 mL. Also, 10 mL of a solution of trace minerals (g/L) was added to 1 L of medium: trisodium nitrilotriacetic acid, 1.5; Fe(NH4)2(SO4)2, 0.8; NaSeO3, 0.2; CoCl26H2O, 0.1; MnSO4H2O, 0.1; Na2MoO42H2O, 0.1; NaWO42H2O, 0.1; ZnSO47H2O, 0.1; NiCl26H2O, 0.1; H3BO3, 0.01; and CuSO45H2O, 0.01 [30]. The medium was replaced every day to maintain microbial activity and minimize biological reactions due to cellular debris under anaerobic conditions.

H2 and CO2 gas supply In this study, hydrogen that was obtained from electrocoagulation treatment of wastewater from an eletronics manufacturer was supplied to the CO2 reduction tank with CO2 that was discharged from the manufactuer [17]. HF wastewater was used as an electrolyte without any additive to perform electrocoagulation from 20 to 24 V in the presence of aluminum electrodes in a reactor volume of 70 L. During the wastewater treatment (30-min hydraulic retention time), the electrodes that were connected to the anode solubilized aluminum by an electrochemical reaction, and the 2

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cathodes produced H2. In the electronics industry, CO2 is used in manufacturing applications, including semiconductor device manufacturing, surface cleaning, and circuit board assembly. During this process, the residual CO2 (purity >99%) was collected and supplied to the CO2 reduction tank.

Reactor and gas dissolution device A reactor was developed to reduce CO2 and produce CH4 efficiently (Fig. 1). Two 5-L reactors were prepared as acryl cylinders – a control reactor (referred to as reactor C) and a working reactor (reactor R). The reactors were incorporated into a sturdy gas-tight system with stainless steel tubing and connecters (Swagelok-type, Whitey Co., USA). An agitator was installed in each reactor to ensure uniform mixing of the culture medium (100 mL), including trace minerals, and the H2 and CO2 that were dissolved in the medium. However, because H2 has low solubility (0.00155 g-gas/kg-water at 258C), the agitator alone could not increase its solubility. Thus, gas was injected into the center of the reactors. A gas circulation device was used at the joint between the liquid and gas flow, particularly in reactor R, to improve the solubility of the undissolved H2 and CO2. The effects of the gas dissolution device were confirmed by abiotic test before the microorganism was injected. On simultaneous injection of gas into the two reactors, the water level in reactor C without the gas dissolution device rose and that in reactor R with the gas dissolution device declined, because both reactors were connected. An aspirator-type gas dissolution device dissolved the gas as microbubbles using the differential pressure that was generated by controlling the flow of the fluid. The dissolution velocity of the injected gas was measured, based on differences in the water level. The test was conducted for 30 min with and without the gas dissolution device. H2 gas was injected, and the two reactors were agitated at the same speed. Figure 1 shows the injection of the H2–CO2 mixed gas (hereafter referred to as mixed gas). As shown in Fig. 1, the mixed gas was injected into the center of the reactor using sparger that was connected to a gas inlet line. In only for reactor R, an aspiratortype device was used to dissolve the gas. Circulation was achieved by pumping liquid from the bottom to the top of both reactors. The mixed gas dissolved in the water, and the resulting mixture was used by the methanogens to produce CH4. In addition, our aim was to examine CO2 reduction and CH4 production rates. Although the mixed gas was injected into the center of the reactor, its pressure would have exceeded that in the reactor. Also, most of the injected gas (especially H2) would not have dissolved in the water but would have been discharged, decreasing the amount of H2 and CO2 that was used by the methanogens and decreasing CO2 reduction and CH4 production. Thus, as much mixed gas as could be dissolved in the water was injected continuously into the center of the reactor.

Operational conditions The effects of the gas dissolution device and nutrient sources on CO2 reduction and CH4 production were examined for approximately 160 d. First, reactors R and C were operated at 37  18C and pH 7.1  0.2. The H2 and CO2 were mixed at a ratio of 5:1 but the ratio in the actual test differed from that in the enrichment culture

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FIGURE 1

Schematics of carbon dioxide removal reactor.

to compensate for the deficiency due to the low solubility of H2. In the enrichment culture, an H2:CO2 ratio of 4:1 was sufficient, because the mixed gas was not injected continuously. However, in the actual test, the solubility of H2 gas was low, and the quantity that was injected had to exceed the theoretical value [17]. The mixed gas was injected at 9.23 mL/min into reactor R and at 6.25 mL/min into reactor C. The injection rate of mixed gas to both reactors differed, because disparate amounts of mixed gas dissolved due to the gas dissolution device. Also, in a preliminary test, we verified that these lower injection rates seldom influenced the dissolution (data not shown). The mixed gas was dissolved in reactors R and C at 9.23 and 6.25 mL/min, respectively – lower than what would have affected counterflow outside of the reactor. Higher rates would have caused the gas to be discharged directly without being dissolved and would have impeded CO2 reduction and CH4 production.

to 2108C at 208C/min. The injection volume was 250 mL. The morphological differences between microorganisms in each reactor were analyzed on a phase-contrast microscope with a changecoupled device (CCD) camera and a fluorescence microscope (IN480T-FL-3MT, American Scope Co., USA).

DNA extraction Anaerobic sludge from the reactors was analyzed by polymerase chain reaction-denaturing gradient gel electrophoresis (PCRDGGE). The samples were stored at low temperature, packed, and centrifuged for 20 min at 13,000 rpm to separate cells, and only solid matter was used in separating the supernatant. DNA was extracted from each sample using the Ultra CleanTM Soil DNA kit (MoBio Laboratories Inc., Solana Beach, CA, USA), which entailed beadbeating and spin column purification steps.

PCR Sampling and analysis The gas that was discharged outside of the reactor was measured immediately after its generation using a wet gas meter (Model WNK-0.5, SHINAGAWA, Japan), and its components were determined on a gas chromatograph (GC 6000, Younglin, Korea) that was equipped with a thermal conductivity detector and a carboxen-1000 column (4.5 m  3.2 mm, 60/80 mesh, Supelco). The temperature of the injector and detector was 2208C. The carrier gas was helium (30 mL/min). The analyses were performed using the following program: 5 min at 358C and a linear gradient from 358C

Two rounds of PCR were used to amplify partial 16S rDNA sequences that were representative of Archaea – a nested PCR approach using the primers PR A46f (50 -YTA AGC CAT GCR AGT-30 ) and PREA 1100r (50 -CCC TAC GGG GYG CAS CAG-30 ) in the first round and a second round with the internal primers PARCH 340f (50 -CCC TAC GGG GYG CAS CAG-30 ) with a GC clamp and PARCH 519r (50 -TTA CCG CGG CKG CTG-30 ). The amplicons were analyzed by denaturing gradient gel electrophoresis [31]. The reaction mixture (50 mL final volume) comprised 300 mM Tris–HCl (pH 8.8), 100 mM (NH4)2SO4, 100 mM KCl, 20 mM

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TABLE 1

Solubility of H2 gas based on use of the gas dissolution device Use of gas dissolution device

Yes

Research Paper

No

Time (min)

0 30

0 30

Water level (cm) First

Second

Third

Height (cm) Height (cm) Difference (cm) Mean (cm) Gas quantity (mL) Dissolution velocity (mL/min)

12.6 12.0 0.6 0.6 47.1

12.6 11.9 0.7

12.6 12.1 0.5

Height (cm) Height (cm) Difference (cm) Mean (cm) Gas quantity (mL) Dissolution velocity (mL/min)

12.6 12.4 0.2 0.17 13.3 0.44

1.57 12.6 12.4 0.2

12.6 12.5 0.1

FIGURE 2

Flow rate of input and output gases in each reactor. 4

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DGGE DGGE was performed using the Dcode Universal Mutation Detection System (Bio-Rad, USA). PCR samples were loaded onto 8% (w/v) polyacrylamide gels (bisacrylamide gel stock solution, 37.5:1) in 0.5 TAE (20 mM Tris, 10 mM acetate, and 0.5 mM Na-EDTA, at pH 7.4) that were prepared with a 20–80% gradient of denaturant

(7 M urea and 40% deionized formamide) and run at 608C for 3 hours at 200 V. The gels were stained for 45 min with SYBR Green I (Molecular Probe, Netherlands), inspected under UV light, and photographed on a Gel Doc 2000 (BIO-RAD).

Sequencing of PCR products The amplified 16S rDNA that was extracted from the DGGE was analyzed on an ABI Prizm 377 XL sequencer. The sequences were edited with SeqEd v1.0.3 (Applied Biosystems, Australia), and the percentage similarity to reference strains in GenBank was estimated within a 322-bp section of the gene. The retrieved sequences were compared with available sequences in the BLAST program (National Center for Biotechnology Information). The sequences were aligned with ClustalX [33].

Results and discussion Effect of gas dissolution Table 1 shows the dissolution velocity of gas that was measured, based on the difference in water level in the abiotic test, which was

FIGURE 3

Contents of each gas in input and output of the reactor. www.elsevier.com/locate/nbt 5 Please cite this article in press as: Kim, S. et al., Enhancement of carbon dioxide reduction and methane production by an obligate anaerobe and gas dissolution device, New Biotechnol. (2015), http://dx.doi.org/10.1016/j.nbt.2015.05.004

Research Paper

MgSO4, 20 pM of each primer, 10 mM of each dNTP, 2 U Taq polymerase (Super-Therm, England), and 20 ng template. The thermocycling program for the touchdown PCR program was as follows: initial denaturation at 908C for 5 min and 958C for 1 min; touchdown primer annealing from 558C to 458C (the annealing temperature was decreased 18C every second cycle for the first 22 cycles); extension at 728C for 1 min (for each of the 22 cycles); 18 additional cycles at 958C for 1 min, 458C for 1 min, and 728C for 1 min; and a final extension step at 728C for 10 min. Amplicons were analyzed by horizontal electrophoresis on an agarose gel (1–1.5%) and visualized with ethidium bromide per Sambrook et al. [32]. Fragment sizes were calculated on a Gel Doc 2000 (BIO-RAD).

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performed to determine the effects of the gas dissolution device. As shown in Table 1, the test was repeated three times, and the results were averaged. The gas dissolution device increased the H2 dissolution velocity by approximately 3.6-fold. The lower agitation speeds and lack of agitation led to decreased gas dissolution velocities [34]. These results indicated that the gas dissolution device could be used to dissolve H2–CO2 mixed gas and significantly enhance the solubility of H2. The CO2 did not need to be passed through the device, because its solubility is high, but H2 and CO2 were forced through it to prepare the gas saturation and prevent insufficient agitation. To examine the effects of the gas dissolution device on CO2 reduction and CH4 production, the reactors were tested with and

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without the device (reactors R and C, respectively). Figure 2 shows the results of the abiotic test to verify the efficiency of the gas dissolution device and the quantities of gas that were injected and discharged. The H2–CO2 mixed gas was injected continuously into reactors R and C at 9.23 mL/min and 6.25 mL/min, respectively. However, the quantity of H2 and CO2 that was reduced was smaller in reactor C versus reactor R – the H2–CO2 mixed gas was injected into reactor R at 9.23 mL/min, and the mixed gas (H2 and CO2 gas) was discharged at 3.5 mL/min. Conversely, mixed gas was discharged at 4.0 mL/min when H2–CO2 was injected into reactor C at 6.25 mL/min. These data indicate that the gas dissolution device in reactor R dissolved more H2 and CO2 than in reactor C. Thus, the quantity of CH4 in the gas that was discharged from reactor R was expected to exceed that from reactor C.

FIGURE 4

Rates of CO2 reduction and CH4 production. 6

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In addition, the gas discharge rate was initially high in both reactors at the early stages of operation (until approximately 40 d), decreasing afterward and plateauing. In the later stages (approximately 100 d later), the gas discharge rate increased. This pattern appears to be attributed to the nutrient concentration and the coexistence of acetoclastic methanogens and hydrogenotrophic methanogens – that is, the activities of two anaerobic CH4-producing bacteria were initially stronger due to the sufficient levels of nutrients in the anaerobic sludge. Over time, the acetic acid concentration fell, reducing the activity of CH4-producing acetoclastic methanogens, which can use acetic acid as the carbon source, and subsequently weakening the hydrogenotrophic methanogens due to the shortage of nutrients. This evidence has been reported by us. Thus, we did not consider or measure acetic acid concentrations, because only CO2 and H2 were fed as the electron acceptor and donor for CH4 production, respectively. As a result, Methanosaeta sp. and Methanospirillum were detected as

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the acetoclastic and hydrogenotrophic methanogens, respectively, in the CH4 production reactor to which only CO2 and H2 were added [17]. After the addition of nutrients, except for carbon sources (approximately 100 d later), much gas was produced, because the activity of hydrogenotrophic methanogens had recovered.

Reduction of CO2 and H2 and production of CH4 The total amounts of gas that was produced varied, based on the use of the gas dissolution device. Accordingly, assuming that the CH4 production depended on the reduction of H2 and CO2, the composition of the gas that was discharged from each reactor was determined (see Fig. 3). H2 and CO2 were injected into the reactors at a ratio of 5:1. As shown in Fig. 3, more H2 and CO2 were dissolved in reactor R than in reactor C, and the CH4 content was higher in reactor R. This finding indicates that the quantity of H2 and CO2 that were

FIGURE 5

Theoretical and real methane production rates in reactors C and R. www.elsevier.com/locate/nbt 7 Please cite this article in press as: Kim, S. et al., Enhancement of carbon dioxide reduction and methane production by an obligate anaerobe and gas dissolution device, New Biotechnol. (2015), http://dx.doi.org/10.1016/j.nbt.2015.05.004

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reduced was greater in reactor R (equipped with the gas dissolution device) versus reactor C. In addition, the nutrients that were injected after approximately 50 d and 100 d had a significant impact on the increase in H2/CO2 reduction and CH4 production. The nutrients were injected once after approximately 50 d and weekly after 100 d. In particular, the difference in H2, CO2, and CH4 content between reactors C and R after 100 d rose versus before 100 d. This result implies that the gas dissolution device

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increases the solubility of H2 and CO2 and improves CO2 reduction and CH4 production rates when methanogenic activity is maintained. As the CO2 and CH4 contents in Fig. 3 were converted into the gas that was discharged from each reactor, it became clearer that the gas dissolution device improved CO2 reduction and CH4 production (see Fig. 4). As shown in Fig. 4, CO2 reduction and CH4 production rates were higher in reactor R compared with

Research Paper FIGURE 6

Photographs of microorganisms in reactors C and R by fluorescent and phase-contrast microscope (1000). 8

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reactor C. In reactor R, the CO2 reduction was excellent (over 0.3 m3/m3 d) throughout the entire operation and was linked to the production of CH4, indicating that the activity of methanogens predominated among the anaerobic bacteria. The CH4 production rate demonstrated in this study was higher than those in previous studies [27,29]. Because these results were obtained from a closed system without any gas dissolution facility, CH4 production rates might depend on the solubility of hydrogen – low solubility could have resulted in a low CH4 production rate. Conversely, CH4 was produced from reactor C, regardless of the reduction in CO2 at the early stages of operation, probably because the nutrient levels were sufficient and because the microorganisms that produced CH4 from acetic acid dominated. After 30 d, however, the CO2 reduction rates became proportional to CH4 production rates, as in reactor R, probably because the nutrients for acetoclastic bacteria, such as acetate, were exhausted in the reactor, allowing microorganisms that only required CO2 as the nutrient to flourish. Acetic acid was not detected from either reactor, and only CH4 was produced, even after the injection of H2 and CO2 was temporarily halted. This finding implies that the activity of acetoclastic bacteria was significantly lower in the reactor. Figure 5 shows the theoretical and experimental quantities of the CH4 that was produced. The theoretical values always exceeded the actual production, but the differences decreased as the experiment progressed – CH4-producing bacteria did not exist initially at a high concentration, and not much of the mixed gas that was injected dissolved in the reactor, but as the test proceeded, the CH4-producing bacteria became dominant and more mixed gas was dissolved, which narrowed the difference between the actual and theoretical values. The actual production was approximately twice as high in reactor R than in reactor C, probably because the solubility of H2 and CO2 was higher in reactor R, as shown in Fig. 2, and because more dissolved gas was provided to the methanogens. We conclude that an undisturbed process between hydrogenproducing bacteria and hydrogen-consuming-methanogenics is narrow from the viewpoint of phylogenetic tree. A balanced hydrogen concentration is required, because methanogenics need enough H2 for CH4 production, and the partial pressure of H2 should be sufficiently low to prevent acetoclastic bacteria from surrounding too much H2 and consequently stopping H2 production. Although our system can be improved CH4 production via CO2 reduction in a typical anaerobic digestion system, we did not consider the reduction in acetoclastic methanogenesis as a major route of CH4 formation because we focused on the enhancement of CH4 production.

Microbial distribution A microbial sample that was used to reduce CO2 and generate CH4 was collected from the anaerobic sludge containing several microbial species. However, over time, we expected a species of hydrogenotrophic methanogens to become dominant. Figure 6 shows the microorganisms in each reactor, which were viewed under a phase-contrast microscope and fluorescent microscope at 1000 magnification. The distribution of microbes between reactors was expected to be similar, because they were inoculated with the same anaerobic sludge.

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FIGURE 7

DGGE profiles of microorganisms in reactors C and R.

Yet, as shown in Fig. 6, the gas dissolution device affected disparate microorganism patterns between reactors. The microorganisms in reactor R were morphologically similar to Methanobacterium bryantii, whereas those in reactor C were similar to Methanosarcina mazei. In addition, the microorganisms in reactor R had a mean length of 4.7 mm and were Gram-negative; in reactor C, the microorganisms had a mean length of 1.3 mm and were Gram-positive. Because more H2 and CO2 dissolved in reactor R versus reactor C, microbial species that utilize H2 predominated easily in reactor R. M. bryantii appears to use H2 and CO2 more favorably for CH4 production than M. mazei. Further, as shown in Fig. 7, the dominant microorganism in reactor R was Methanosarcina bryantii, based on PCR-DGGE and molecular biological distribution assay. The similarity was 97% as shown in Table 2. In reactor C, M. mazei or Methanobacterium formicicum was dominant, because the species in reactor C was TABLE 2

Corresponding strains of DGGE bands using the 16s rDNA sequence Accession no.

bp

Strains

Similarity (%)

JF155845

622

Methanosarcina mazei

99

JF155844

619

Methanobacterium formicicum

98

U53419

556

Methanobacterium bryantii

97

AB353221

770

Methanococcoides alaskense

98

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cultured using a mixture of H2 and CO2 gas as an energy source. However, various strains were detected in the anaerobic sludge by DGGE assay. When H2 and CO2 were used as the sole energy source, hydrogenotrophic methanogens were dominant. The gas dissolution device facilitated the culture of hydrogenotrophic methanogens due to the large amounts of H2 that were dissolved.

Conclusion Research Paper

In this study, obligate anaerobic microorganisms were used to reduce CO2 in the air and produce CH4 as a useful energy source efficiently. To improve the CO2-to-CH4 conversion, large amounts of CO2 and H2 must be dissolved, which can be used by an obligate anaerobic strain that converts CO2 to CH4. However, the biological conversion of CO2 to CH4 is poor, due to the low solubility of H2.

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Thus, we developed a technology to increase the solubility of H2 and CO2. With this gas dissolution device, the solubility increased approximately threefold, which doubled the CO2 that was reduced and resulted in threefold higher CH4 production. With and without the gas dissolution device, the rates of CO2 reduction were 0.15 and 0.3 m3/m3 d, and those of CH4 production were 0.2 and 0.6 m3/m3 d, respectively. By DGGE assay, hydrogenotrophic methanogens were dominant in the reactor with the gas dissolution device. Thus, our device enhances the reduction of CO2 and aids in the production of CH4 as a useful energy source.

Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.nbt.2015.05. 004.

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www.elsevier.com/locate/nbt Please cite this article in press as: Kim, S. et al., Enhancement of carbon dioxide reduction and methane production by an obligate anaerobe and gas dissolution device, New Biotechnol. (2015), http://dx.doi.org/10.1016/j.nbt.2015.05.004

Enhancement of carbon dioxide reduction and methane production by an obligate anaerobe and gas dissolution device.

The use of gas dissolution devices to improve the efficiency of H2 dissolution has enhanced CO2 reduction and CH4 production. In addition, the nutrien...
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