Enhancing Lactic Acid Production in Industrial Biocatalysts using Electrofermentation

Abstract

Lactic acid (LA) is an important industrial chemical precursor. Most LA is produced by homofermentative lactic acid bacteria (LAB). LA production in LAB is constrained by a number of redox limitations. Enhancing LA production for industrial biorefinery applications requires overcoming these limitations while maintaining profitability and sustainability. Microbial electrosynthesis systems can be used to overcome energetic limitations but often require high current densities. Electrofermentation (EF) is an electron-efficient form of microbial electrosynthesis in which a small cathodic or anodic current is provided to a culture to alter the oxidation-reduction potential of the medium and, in turn, alter metabolism. Redox balance is a central driver of LA metabolism, but the potential of EF to enhance LA production has not previously been explored.

Here, EF was tested in pure cultures of two industrial, homofermentative LAB: Lactiplantibacillus plantarum and Pediococcus acidilactici. The approach was tested and optimised using a single-chamber apparatus before moving to a two-chamber H-cell apparatus. In the H-cell, the effect of providing electrons to the culture (cathodic EF) and removing them (anodic EF) was tested in aerobic and anaerobic conditions. The effect of adding the chemical redox mediator Neutral Red (NR) was tested in cathodic treatments. Glucose, lactic acid, acetic acid and ethanol concentrations, growth, and pH of cultures were measured. Imaging flow cytometry was conducted to probe for any potential phenotypic changes during EF. Cathodic EF treatment doubled LA production in anaerobic cultures of L. plantarum. This was unaffected by NR addition, improving the techno-economic feasibility of the platform. In aerobic cultures of P. acidilactici where NR was added, ethanol production was doubled. While the effects of electrical current on overall growth were minimal, some phenotypic effects were observed for P. acidilactici. Overall, in every EF treatment, significant changes to metabolism were observed compared to controls. This study is the first demonstration of bioelectrochemical enhancement and fine-tuning of homolactic LA metabolism in industrial biocatalysts, and if scaled up could contribute significantly to carbon mitigation in the bioeconomy.