Temperature tactics: Targeting acetate or methane production in autotrophic H2/CO2 conversion with mixed cultures

Ex situ biomethanation is a promising technology that combines the concepts of Power-to-X and carbon capture and utilization (CCU) by employing microorganisms to convert hydrogen (H₂) and carbon dioxide (CO₂) into biomethane (CH₄). Mesophilic (37°C) and thermophilic (50°C) ex situ biomethanation processes in continuous stirred tank reactors (CSTRs), were evaluated for the first time under a high continuous H₂/CO₂ supply. Each process was evaluated with and without the addition of minerals and vitamins. In all thermophilic reactors, methanogenesis was favored, achieving stable methane yields of 110 ± 8 %. Additionally, the addition of minerals resulted in a 67 % increase in H₂ consumption rates. In contrast, mesophilic reactors showed low CH₄ yields (7.5 %) and acetogenesis as the dominant pathway, with acetate concentrations reaching up to 7.6 ± 2 g L⁻¹. The effect of free acetic acid (FAA), a rarely considered concept based on the correlation between pH and acetic acid concentration, was also tested on thermophilic ex situ CSTRs. A clear negative correlation was demonstrated between FAA concentration and the CH₄ productivity, with a 50 % inhibition of CH₄ productivity at 0.062 g L⁻¹ of FAA.