Actinobacillus succinogenes in bioelectrochemical systems – Comparative study of redox mediators

A promising strategy to enhance biotechnological succinate production with Actinobacillus succinogenes is the fermentation in bioelectrochemical systems (BES), where mediated extracellular electron transfer (MEET) plays a key role. In this context, the choice of redox mediator (RM) is important. However, current studies show a limited selection of RMs, which have only rarely been investigated for A. succinogenes. This study therefore analyses different RMs regarding their efficiency and compatibility with A. succinogenes in cathodic systems. In addition to key parameters such as toxicity, stability and redox potential, the total turnover number (TTN) was used as a performance indicator. Among the RMs tested, neutral red proved to be the most efficient mediator with a TTN value of 160.39 at a concentration of 0.1 mM and 23.32 ± 3.34 at 0.5 mM over 72 h. In contrast, riboflavin, safranin O, resazurin and methylene blue showed far poorer performance due to low TTN values, high toxicity or low stability. In addition, active secretion of endogenous RMs could most likely be excluded. The results prove that neutral red is currently the most suitable RM for the process and at the same time illustrate the considerable potential for optimisation in the development of ideal redox mediators for cathodic electro-fermentation.