The effects of CO2 and H2 on CO metabolism by pure and mixed microbial cultures

Abstract

BackgroundSyngas fermentation, the bioconversion of CO, CO2, and H2 to biofuels and chemicals, has undergone considerable optimization for industrial applications. Even more, full-scale plants for ethanol production from syngas fermentation by pure cultures are being built worldwide. The composition of syngas depends on the feedstock gasified and the gasification conditions. However, it remains unclear how different syngas mixtures affect the metabolism of carboxidotrophs, including the ethanol/acetate ratios. In addition, the potential application of mixed cultures in syngas fermentation and their advantages over pure cultures have not been deeply explored. In this work, the effects of CO2 and H2 on the CO metabolism by pure and mixed cultures were studied and compared. For this, a CO-enriched mixed culture and two isolated carboxidotrophs were grown with different combinations of syngas components (CO, CO:H2, CO:CO2, or CO:CO2:H2).ResultsThe CO metabolism of the mixed culture was somehow affected by the addition of CO2 and/or H2, but the pure cultures were more sensitive to changes in gas composition than the mixed culture. CO2 inhibited CO oxidation by the Pleomorphomonas-like isolate and decreased the ethanol/acetate ratio by the Acetobacterium-like isolate. H2 did not inhibit ethanol or H2 production by the Acetobacterium and Pleomorphomonas isolates, respectively, but decreased their CO consumption rates. As part of the mixed culture, these isolates, together with other microorganisms, consumed H2 and CO2 (along with CO) for all conditions tested and at similar CO consumption rates (2.6 ± 0.6 mmol CO L−1 day−1), while maintaining overall function (acetate production). Providing a continuous supply of CO by membrane diffusion caused the mixed culture to switch from acetate to ethanol production, presumably due to the increased supply of electron donor. In parallel with this change in metabolic function, the structure of the microbial community became dominated by Geosporobacter phylotypes, instead of Acetobacterium and Pleomorphomonas phylotypes.ConclusionsThese results provide evidence for the potential of mixed-culture syngas fermentation, since the CO-enriched mixed culture showed high functional redundancy, was resilient to changes in syngas composition, and was capable of producing acetate or ethanol as main products of CO metabolism.Electronic supplementary materialThe online version of this article (doi:10.1186/s13068-017-0910-1) contains supplementary material, which is available to authorized users.

Impact of carbon monoxide partial pressures on methanogenesis and medium chain fatty acids production during ethanol fermentation

“…Moreover, an Acetobacterium sp. isolated from sludge collected at the same wastewater treatment plant as this experiment produced acetate and ethanol from CO (Esquivel‐Elizondo, Delgado, Rittmann, et al, ). Other potential carboxidotrophs detected in these experiments were Pleomorphomonas , Blautia , and Oscillospira .…”

“…Moreover, an Acetobacterium sp. isolated from sludge collected at the same wastewater treatment plant as this experiment produced acetate and ethanol from CO (Esquivel-Elizondo, Delgado, Rittmann, et al, 2017). Other potential carboxidotrophs detected in these Blautia, a carboxidotrophic acetogen (Lorowitz & Bryant, 1984), were not detected in the controls, but only during the second phase of fermentations with CO (3.5-27.9%), during MCFA production.…”

“…The mcrA gene, marker of methanogens, and the fthfs gene, marker of acetogens and other microorganisms that utilize the Wood–Ljungdahl pathway, were quantified via quantitative real‐time PCR (qPCR) using SYBR green reactions as described in (Esquivel‐Elizondo, Delgado, Rittmann, & Krajmalnik‐Brown, ). The primers (mlas and mcrA‐rev for methanogens, and fhs1 and FTHFS‐r for acetogens) and conditions used are described in Steinberg and Regan () and Xu, Liu, Du, and Chen (), respectively.…”

Energy Conservation and Carbon Flux Distribution During Fermentation of CO or H2/CO2 by Clostridium ljungdahlii

“…On the other hand, Fd red and NADPH are generated by an electron bifurcation reaction in the H 2 -fermentation by the NADP-specific [FeFe]-hydrogenase complex (Wang et al, 2013;Schuchmann and Müller, 2014;Mock et al, 2015). In addition, the free standard enthalpy changes are different for the conversion of CO or H 2 /CO 2 to acetate and ethanol synthesis (Table 1; Wang et al, 2013;Mock et al, 2015;Esquivel-Elizondo et al, 2017). Therefore, the basic differences of energy conservation between CO and H 2 as energy sources reveal that fermentation profiles and products are distinct in gas fermentation of C. ljungdahlii (Bertsch and Müller, 2015;Valgepea et al, 2018).…”

One-step production of C6–C8 carboxylates by mixed culture solely grown on CO

“…Perez et al [ 11 ] converted carboxylic acids to their corresponding alcohols using syngas as electron donor by carboxydotrophic bacteria C. ljungdahlii and C. ragsdalei . However, pure cultures are inclined to be sensitive to environmental disturbance and susceptible to contamination, and hence, are not perfect for industrial application with regard to waste valorization [ 12 , 13 ]. Esquivel-Elizondo et al [ 14 ].…”