Chain elongation in continuous microbial electrosynthesis cells: the effect of pH and precursors supply

Microbial electrosynthesis (MES) enables the production of carbon-neutral chemicals using CO₂ as a carbon source. Acetic acid is the main MES product, but recent studies show the direct production of elongated carboxylic acids, e.g., butyric and caproic acid. However, the production of elongated acids in MES systems is still inefficient due to the low growth rates of acetogenic bacteria and to limited solventogenic rates. Subsequently, researchers have produced elongated carboxylic acids directly from acetic acid or have operated MES systems at low pH to favor solventogenesis. However, the effect the addition of different chain elongation precursors and the operation pH exerts in the bioelectrochemical production of elongated acids remains unclear. To investigate this, three pH-controlled MES systems were operated in this study with continuous liquid and gas supply. MES systems elongating acetic acid at pH 6 achieved higher butyric (0.71 vs. 0.42 g L⁻¹) and caproic acid (0.71 vs. 0.42 g L⁻¹) titers in the absence of CO₂ sparging. Additionally, lowering the pH to 5 in the MES systems fed with CO₂ and acetic acid improved the elongated acids titers, reaching 0.72 g L⁻¹ butyric and 0.33 g L⁻¹ caproic acid. The 16 S rRNA analysis showed the community was dominated by Oscillibacter at pH 6, and by Clostridium at pH 5. Furthermore, the first scanning electron microscopy pictures revealing biofilm stratification in MES cathodes were taken in this study, where homogeneous rod-shaped bacteria biofilm layers, in contact with the graphite cathode, were covered by heterogeneous biofilm layers.