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Study of the microbial competition between homoacetogens and hydrogenotrophic methanogens in aerobic mixed cultures for acetate production from H2/CO2 : an experimental and modelling approach

  • Laguillaumie, Léa
Publication Date
Sep 09, 2022
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Capturing carbon dioxide (CO2) is a major challenge for developing a low-carbon economy. In addition, the reduction of CO2 allows the synthesis of platform molecules for the chemical and energy industry. Anaerobic mixed cultures contain homoacetogenic microorganisms (HAC) capable of reducing CO2 to acetate. However, one of the obstacles to their use is the understanding and control of their functional diversity. In particular, managing the competition between HACs and hydrogenotrophic methanogens (HMs) that convert CO2 into methane is crucial to produce acetate.The study contributes to bring new knowledge on the competition between HAC and HM. For this, notions of mass transfer between the gas phase where the substrates are located, and the liquid phase which contains the microbial catalysts (G/L), as well as kinetic and thermodynamic aspects of biological reactions have been integrated.Microbial competition was studied using data from an ex situ biological methanation reactor of H2/CO2, in mixed and thermophilic culture. In parallel, microbial growth modeling work specific to each microorganism was carried out in order to study their behaviour in batch and continuous reactors. Maintaining a G/L mass transfer rate higher than the gas consumption rate appeared decisive to promote the growth of HACs. These modeling results, coupling microbial kinetics and thermodynamics of reactions, made it possible to identify favourable environments for HAC selection. They were then confronted with an experimental plan in order to study the system in real conditions.As part of the experimental plan, batch and then continuous reactors at 25°C and 35°C were set up. In batches, the lifting of the limitation by the transfer of gas, as well as the lowering of the temperature to 25°C made it possible to select mixed anaerobic cultures for the HAC function by eliminating the methanogens, after four successive batches of 4 to 6 days each.In continuous reactors, the effect of pH between 5 and 7 and dilution rate between 0.1 and 1 d-1 has been studied. At neutral pH, a significant acetate productivity of 296 mmolC.L-1.d-1 could be observed over several residence times. Moreover, lowering the pH to 5 did not eliminate the HMs. On the other hand, a zone of critical dilution rate of the HMs could be identified between 0.25 and 0.38 d-1, while the HACs were maintained in the reactor up to the maximum tested value of 1 d-1.Interestingly, the model showed that these reactors were not limited by the G/L mass transfer, but by the reaction thermodynamics, thus setting a maximum acetate concentration, which imposes the gas consumption rate. These results demonstrate the potential of using continuous reactors to wash out methanogens and select HAC on H2/CO2 in anaerobic mixed cultures. Thanks to a work of optimization of the culture medium for the HAC, the mixed culture could be enriched up to 75% in C. autoethanogenum, after the addition of vitamins in the culture medium. At 25°C, the production of ethanol and the elongation in butyrate and hexanoate could be observed, although minor.During this study, an adaptable model allowing to describe the microbial competition was developed. It made it possible to show the different types of limitations of the system: the G/L mass transfer, the thermodynamics of the reactions, or the nutrients of the liquid phase. Understanding the system has allowed the development of techniques for enriching mixed cultures with HAC. These results constitute a database for the optimization of the production of platform molecules from H2/CO2, without recourse to chemical or thermal inhibitions of HMs.

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