Microbial Insights of Enhanced Anaerobic Conversion of Syngas
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Liu et al. Biotechnol Biofuels (2020) 13:53 https://doi.org/10.1186/s13068-020-01694-z Biotechnology for Biofuels RESEARCH Open Access Microbial insights of enhanced anaerobic conversion of syngas into volatile fatty acids by co-fermentation with carbohydrate-rich synthetic wastewater Chao Liu1,2, Wen Wang1* , Sompong O‑Thong2,3, Ziyi Yang1, Shicheng Zhang2,4, Guangqing Liu1 and Gang Luo2,4* Abstract Background: The co‑fermentation of syngas (mainly CO, H2 and CO2) and diferent concentrations of carbohydrate/ protein synthetic wastewater to produce volatile fatty acids (VFAs) was conducted in the present study. Results: It was found that co‑fermentation of syngas with carbohydrate‑rich synthetic wastewater could enhance the conversion efciency of syngas and the most efcient conversion of syngas was obtained by co‑fermentation of syngas with 5 g/L glucose, which resulted in 25% and 43% increased conversion efciencies of CO and H2, compared to syngas alone. The protein‑rich synthetic wastewater as co‑substrate, however, had inhibition on syngas conver‑ sion due to the presence of high concentration of NH4+‑N (> 900 mg/L) produced from protein degradation. qPCR analysis found higher concentration of acetogens, which could use CO and H2, was present in syngas and glucose co‑fermentation system, compared to glucose solo‑fermentation or syngas solo‑fermentation. In addition, the known acetogen Clostridium formicoaceticum, which could utilize both carbohydrate and CO/H2 was enriched in syngas solo‑ fermentation and syngas with glucose co‑fermentation. In addition, butyrate was detected in syngas and glucose co‑fermentation system, compared to glucose solo‑fermentation. The detected n‑butyrate could be converted from acetate and lactate/ethanol which produced from glucose in syngas and glucose co‑fermentation system supported by label‑free quantitative proteomic analysis. Conclusions: These results demonstrated that the co‑fermentation with syngas and carbohydrate‑rich wastewater could be a promising technology to increase the conversion of syngas to VFAs. In addition, the syngas and glucose co‑fermentation system could change the degradation pathway of glucose in co‑fermentation and produce fatty acids with longer carbon chain supported by microbial community and label‑free quantitative proteomic analysis. The above results are innovative and lead to achieve efective conversion of syngas into VFAs/longer chain fatty acids, which would for sure have a great interest for the scientifc and engineering community. Furthermore, the present study also used the combination of high‑throughput sequencing of 16S rRNA genes, qPCR analysis and label‑free *Correspondence: [email protected]; [email protected]; [email protected] 1 Biomass Energy and Environmental Engineering Research Center, Beijing University of Chemical Technology, Beijing 100029, China 2 Shanghai Key Laboratory of Atmospheric Particle Pollution and Prevention (LAP3), Department of Environment Science and Engineering, Fudan University, Shanghai 200433, China Full list of author information is available at the end of the article © The Author(s) 2020. This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. 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Keywords: Volatile fatty acid, Co‑fermentation, Acetogen, qPCR analysis, Label‑free quantitative proteomic analysis Background process for VFA production from integration process Gasifcation or pyrolysis of lignocellulosic wastes pro- with syngas and waste activated sludge (WAS) [18], it duces a gas mixture (syngas), which is attracting much was still unknown how the organic matters afect the attention for the efcient conversion of the refractory production of VFA from syngas in the mixed culture. On organic wastes [1]. Te produced syngas is mainly com- the one hand, the conversion of syngas might be inhib- posed of CO, H2, CO2 (70–80%) and other trace com- ited due to high concentrations of products generated pounds [2, 3]. Te low volumetric energy density of from the anaerobic fermentation of organic wastes. Pre- syngas (usually less than 13 MJ/m3), which is only about vious studies reported that high concentration of VFAs 40% of natural gas, is the limitation for syngas to be used could inhibit bacterial growth during the anaerobic fer- as fuel directly [3]. Alternatively, further conversion of mentation of syngas due to the presence of high concen- syngas to more valuable biochemicals and biofuels is trations of undissociated acids [4, 19]. On the other hand, preferable, which can be metabolized by various micro- the growth of some anaerobic bacteria such as acetogens organisms in anaerobic digestion [4, 5]. In recent years, can be accelerated by the presence of organic wastes many studies have been conducted to convert syngas to which could also utilize CO and H2/CO2 by autotrophic methane by anaerobic digestion (AD) [6, 7]. Compared pathway [4, 20]. Acevedo et al. compared the growth of with methane, the storage and transportation of vola- Clostridium ljungdahlii between the syngas–fructose tile fatty acid (VFA) are easier and safer. In addition, it is reactors and syngas reactors, and showed that the cell estimated that the value of VFA from 1t biomass (150$) densities of C. ljungdahlii in syngas–fructose reactors is much higher than that of methane (31$) [8]. Hence, were approximately 2.5 times higher than that in syngas anaerobic fermentation of syngas to produce VFA has reactors [16]. attracted growing interests which can be used as an It is known protein and carbohydrate are the two main excellent carbon source for biological nutrients removal organic compounds in organic waste/wastewater, e.g., of wastewater directly, or as precursor to synthesize com- WAS and cassava stillage are the model protein-rich and plex polymers such as PHA and longer chain fatty acids carbohydrate-rich organic wastes which consisted of 65% which are hydrophobic [9, 10]. Te conversion of syn- protein and 60% carbohydrate, respectively [21, 22]. On gas to VFA by pure cultures was studied previously and the basis of the above considerations, the main objec- Clostridium spp. (e.g., Clostridium ljungdahlii, Clostrid- tive of this work was to investigate the process perfor- ium carboxidivorans, etc.) are the model microorganisms mances of co-fermentation of syngas with carbohydrate for VFA/ethanol production converted from syngas via or protein synthetic wastewater for VFA production, and Wood–Ljungdahl biochemical pathway [4, 11]. However, to reveal the efects of diferent organic compounds on mixed culture is more promising compared with pure syngas fermentation. Label-free quantitative proteomic culture, considering no sterilization requirements, more analysis was used to provide deep insights on the co-fer- adaptive capacity in various conditions due to microbial mentation process from a microbiological point of view. diversity and the possibility of using wastewater as nutri- ents in mixed culture fermentation. Previous studies also Results Performances of the continuous bottles showed that H2 and CO can be converted to VFA metab- olized by homoacegenesis and acetate was the main Syngas fermentation with carbohydrate‑rich synthetic product using anaerobic mixed sludge [12, 13]. wastewater for VFA production In recent years, researches on the VFA production by As shown in Fig. 1a and Additional fle 1: Fig. S2, the anaerobic fermentation of various organic wastes (such CO and H2 conversion efciencies in S were 57% and as food waste, wastewater) attract considerable atten- 53%, respectively, while the conversion efciencies tions [14, 15], since it is the simple process for the simul- of CO and H2 in GS5 were 25% and 43% (P < 0.05) taneous conversion of organic wastes and syngas to VFA. higher compared to S. In the present study, no meth- Te VFAs/ethanol production from syngas and organic ane was detected, because 2-BES was added to the wastes has been performed using anaerobic fermenta- reactors at the beginning of the operation process. tion, while which was mainly achieved by pure cultures Te time curves of CO and H 2 consumption during [16, 17]. Although our previous study proposed a novel 24 h in GS5 (P-value = 0.001, F = 28.3 > Fcrit = 5.6 for Liu et al. Biotechnol Biofuels (2020) 13:53 Page 3 of 14 a b 90 180 CO Acetate H2 Propionate 75 150 iso-butyrate n-Butyrate iso-valerate 60 120 n-Valerate 45 90 30 60 VFAs concentration (mM) Syngas Consumpution (%) 15 30 0 0 SGS5 GS15 SG5GS5 G15GS15 c d 12 90 e G5 GS5-CO GS5-H2 GS5 10 GS15-CO GS15-H2 G15 75 S-CO S-H2 8 GS15 60 (mL) 6 2 H 45 4 CO and 2 30 Degradation efficiency of glucos 0 15 036912 15 18 21 24 036912 15 18 21 24 Time/h Time/h Fig.