How to Sustainably Feed a Microbe: Strategies for Biological Production of Carbon-Based Commodities with Renewable Electricity

Abstract

As interest and application of renewable energy grows, strategies are needed to align the asynchronous supply and demand. Microbial metabolisms are a potentially sustainable mechanism for transforming renewable electrical energy into biocommodities that are easily stored and transported. Acetogens and methanogens can reduce carbon dioxide to organic products including methane, acetic acid, and ethanol. The library of biocommodities is expanded when engineered metabolisms of acetogens are included. Typically, electrochemical systems are employed to integrate renewable energy sources with biological systems for production of carbon-based commodities. Within these systems, there are three prevailing mechanisms for delivering electrons to microorganisms for the conversion of carbon dioxide to reduce organic compounds: (1) electrons can be delivered to microorganisms via H₂ produced separately in a electrolyzer, (2) H₂ produced at a cathode can convey electrons to microorganisms supported on the cathode surface, and (3) a cathode can directly feed electrons to microorganisms. Each of these strategies has advantages and disadvantages that must be considered in designing full-scale processes. This review considers the evolving understanding of each of these approaches and the state of design for advancing these strategies toward viability.

Keywords: CO2 sequestration; artificial photosynthesis; biocommodities; microbial electrosynthesis; renewable energy storage.

Figure 1

Reduced carbon products produced by acetogens and methanogens. Through genetic engineering, the catalog of feasible products can be expanded. Further detail regarding these products is reviewed in Liew et al. (2016).

Figure 2

A schematic of three electron delivery schemes. (A) An electrolyzer coupled with a bioreactor where hydrogen is produced separately from the microorganisms consuming it. (B) On-demand hydrogen generation at the cathode where microorganism consume hydrogen at the point of generation. (C) Direct electron transfer from the cathode to microorganisms.

Figure 3

(A) A schematic of a reactor with a membrane partition. (B) A schematic of a membrane-less reactor for electrobiosynthesis of reduce carbon compounds (previously described in Giddings et al., 2015).