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

Abstract

Background: Syngas 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).

Results: The 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.

Conclusions: These 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.

Keywords: Acetobacterium; Bioethanol; CO-enriched mixed culture; Carbon monoxide; Geosporobacter; Pleomorphomonas; Syngas.

Fig. 1

Fermentation of (a–d) CO, and (e–h) CO: CO₂: H₂ by the mixed and pure CO-consuming cultures. Panels d and h show that no CO, H₂, or CO₂ was abiotically consumed. The initial CO partial pressure was 30.4 kPa. The data are averages of triplicates; error bars indicate one standard deviation

Fig. 2

Fermentation of (a–c) CO: H₂ and (d–f) CO: CO₂ by the mixed and pure CO-consuming cultures. The initial partial pressure of CO was 30.4 kPa. The data are averages of triplicates; error bars indicate one standard deviation

Fig. 3

Electron distribution (%) from electron donors^a to acetate, ethanol, and/or H₂ produced in fermentation of CO and mixtures of CO with CO₂ and/or H₂ by (a) the CO-enriched mixed culture, (b) the Acetobacterium-like isolate, and (c) the Pleomorphomonas-like isolate. The data are averages with standard deviation of triplicates. The electron balance in (a) corresponds to day 21, and in (b, c) to the last day of fermentation. ^aCO, H₂, and/or yeast extract (YE), and initial concentration of organic matter. ^*Distribution of electrons to H₂ includes H₂ produced and not consumed. e⁻ meq.: electron milliequivalent

Fig. 4

Fermentation of CO in a batch membrane reactor with continuous CO supply in phosphate-buffered medium. a H₂, CO₂, and CO measured in the headspace, and acetate and ethanol concentrations in the liquid phase. b pH values over time. c Relative abundance of main phylotypes detected during fermentation. The data in a, b are averages of at least two measurements