Improving Carbon Fixation and Acetate Production from Syngas Fermentation: On-Demand Versus Continuous Feeding

Syngas fermentation is a promising carbon capture and utilization (CCU) technology for producing carboxylic acids while transforming low-cost waste gas into high-value products. This study evaluates the two bioreactor feeding strategies for synthesis gas (syngas) fermentation by Eubacterium callanderi (formerly Butyribacterium methylotrophicum) strain Marburg—on-demand feeding (ODF) and continuous feeding (CF)—with a synthetic syngas mixture of 23 vol% CO2, 29 vol% CO, 32 vol% H2, and 16 vol% CH4, mimicking the syngas from lignocellulosic gasification. The ODF assay achieved a maximum syngas consumption rate of 112 mL/h, yielding 24.1 g/L acids, namely 22.9 g/L acetate and 1.3 g/L butyrate. CF of syngas at 223 mL/h required more gas (62.9 L) to produce 22.7 g/L total acids, from which 19.0 g/L acetate and 3.7 g/L butyrate were achieved. The CF-specific production rate (gproduct/gdry_cell_weight/hour) reached 0.5 g/gDCW/h (acetate) and 0.17 g/gDCW/h (butyrate), outperforming ODF with 0.3 and 0.02 g/gDCW/h, respectively. ODF minimized gas wastage and enabled CH4 accumulation inside the bioreactor up to approximately 78 vol%, while CF led to CO2 accumulation, indicating a need for more efficient CO2 utilization strategies, such as sequential fermentations. This work highlights the critical impact of the two feeding options studied with regard to scaling up the carbon-efficient production of carboxylic acids, and indicates that both strategies can have potential applications. ODF is ideal for increasing carbon fixation and achieving, simultaneously, gas cleaning, while CF fermentations are better suited to maximizing the acid production rate.