Extracellular electron supply from titanium( <scp>III</scp> )nitrilotriacetic acid promotes production of reduced fermentation end products in <i>Clostridium ljungdahlii</i>

Abstract Interest in the microbial production of bioethanol from renewable sources has prompted efforts to improve its production in homoacetogenic bacteria. It has been suggested that a strongly reducing environment might support the formation of more reduced products. This study investigated the effects of titanium(III)nitrilotriacetic acid (Ti(III)NTA), a highly electronegative redox mediator, on Clostridium ljungdahlii cultures grown in fructose‐based media with either an N 2 or a 20% CO 2 /80% H 2 atmosphere, and in autotrophic cultures with 20% CO 2 /80% H 2 . An atmosphere containing a 4:1 H 2 :CO 2 ratio, rather than the usual 3:1 ratio required for autotrophic acetate production in C. ljungdahlii , was used to determine the effect of excess H 2 electrons on autotrophic versus mixotrophic fermentation. Excess H 2 did not affect the metabolic end‐product profiles observed but the use of Ti(III)NTA resulted in increased production of reduced metabolites such as ethanol, 2,3‐butanediol and lactic acid beyond those reported when CO was used as the sole carbon and energy source. These results reveal a possible correlation between the redox potential of supplied electrons and their metabolic effect. The electronegativity of supplied electrons is crucial, as electrons with sufficient redox potential to reduce ferredoxin gave the observed result.