Microbial electrosynthesis: opportunities for microbial pure cultures

Abstract

Microbial electrosynthesis (MES) is an emerging technology that couples renewable electricity to microbial production processes. Although advances in MES performance have been driven largely by microbial mixed cultures, we see a great limitation in the diversity, and hence value, of products that can be achieved in undefined mixed cultures. By contrast, metabolic control of pure cultures and genetic engineering could greatly expand the scope of MES, and even of broader electrobiotechnology, to include targeted high-value products. To leverage this potential, we advocate for more efforts and activities to develop engineered electroactive microbes for synthesis, and we highlight the need for a standardized electrobioreactor infrastructure that allows the establishment and engineering of electrobioprocesses with these novel biocatalysts.

Keywords: electrobioproduction; electrobioreactor; electrobiotechnology; microbial electromethanogenesis; microbial electrosynthesis; microbial pure culture.

Figure I

Illustration of the different modes of extracellular electron transfer to an anode (unbroken blue bar, pink arrows) or from a cathode (shaded blue bar, blue arrows). (A) Direct electron transfer via specialized redox proteins in the cell wall, for example, cytochromes. (B) Direct electron transfer via conductive nanowires. (C) Electron transfer via diffusible redox mediators.

Figure 1

Metabolic driving forces in microbial electrosynthesis. (A) With undefined mixed cultures, thermodynamic constraints drive the overall metabolic balance of cathodic reactions (shaded left electrode) to stable fermentation products such as methane or acetate. (B) A defined microbial pure culture catalyst enables the tailored production of a high-value target product at the cathode.

Figure 2

The promise of pure culture electrobioproduction. Schematic of potential electrobioproduction with pure cultures for both anodic and cathodic reactions, and the expected advantages (blue) of physiological knowledge and synthetic biology approaches for strain development. The graphic also emphasizes the possible disadvantages (pink) of using pure cultures. Abbreviations: Fd_ox, oxidized ferredoxin; Fdred, reduced ferredoxin.

Figure 3

An envisaged off-the-shelf electrobioreactor in which the main components are labeled. Based on a 2023 online survey among industrial and academic stakeholders regarding >100 physical, chemical, microbial, and other process parameters for bioelectrosynthesis, the mind-mapping list shows the most important desired design (upper panel) and process control (lower panel) features to guide off-the shelf electrobioreactor development. Please note that the listed design specifications are only exemplary and many are optional. One key design element is the flexible choice between a single-chamber electrochemical set-up with working and counter electrodes in the same electrolyte solution, and a two-chamber set-up where the working and counter electrodes are spatially separated by a membrane. The drawing proposes a single design solution to accommodate this task: the application of a counter electrode chamber with flexible installation options for a membrane.