Making quantitative sense of electromicrobial production

Abstract

The integration of electrochemical and microbial processes offers a unique opportunity to displace fossil carbon with CO2 and renewable energy as the primary feedstocks for carbon-based chemicals. Yet, it is unclear which strategy for CO2 activation and electron transfer to microbes has the capacity to transform the chemical industry. Here, we systematically survey experimental data for microbial growth on compounds that can be produced electrochemically, either directly or indirectly. We show that only a few strategies can support efficient electromicrobial production, where formate and methanol seem the best electron mediators in terms of energetic efficiency of feedstock bio-conversion under both anaerobic and aerobic conditions. We further show that direct attachment of microbes to the cathode is highly constrained due to an inherent discrepancy between the rates of the electrochemical and biological processes. Our quantitative perspective provides a data-driven roadmap towards economically and environmentally viable realization of electromicrobial production.

Bioelectrochemical engineering analysis of formate-mediated microbial electrosynthesis

“…[1] Among the many strategies to fix CO2 using renewable energy, so-called "mediated" or "coupled" microbial electrosynthesis (MES) has received significant attention. [2][3][4][5] In this scheme, electrons (ideally from a renewable source) are used to electrochemically reduce a mediator molecule that is then oxidized by planktonic microbes as a growth substrate.…”

“…The choice of redox mediator requires careful consideration: the ideal one should be abundantly available or easily produced electrochemically (eliminating complex organic molecules and inorganic ions), electropositive enough to directly reduce NAD(P)H for efficient energy transfer to cellular metabolism, and highly soluble in liquid water (eliminating H2). [3,5] Formate/ic acid stands out as an especially promising redox mediator because it is readily and specifically produced from CO2 [18][19][20] and multiple natural and engineered formatotrophic growth mechanisms exist in workhorse bacteria. [21][22][23][24][25] Initial scale-up, [26] component integration, [27] and media optimization [28] studies have been performed for MES systems, demonstrating the need for careful attention to process parameters including the gas/liquid mass transfer coefficient (kLa).…”

Engineering Escherichia Coli for the Utilization of Ethylene Glycol

“…While CO 2 has itself been considered as a feedstock, other alternative feedstocks typically consist of compounds derived from CO 2 or from industrial waste materials (1,3). These are commonly one-carbon (C1) compounds such as methane (CH 4 ), methanol (CH 3 OH), formaldehyde (C 2 HO), formate (HCOO − ), carbon monoxide (CO), and syngas (predominantly CO and H 2 ) (1,(3)(4)(5)(6). Notably, methane and syngas fermentations are currently under intense study and are the focus of commercial development (7)(8)(9).…”

“…From these, the electrochemical conversion of CO 2 has been identi ed as promising, owing to several advantages (10). Such advantages include its ambient operating conditions, selectivity and scalability, as well as its potential to integrate with bioprocesses and its ability to couple with renewable energy sources (3,6,10). Furthermore, while other CO 2 conversion technologies are limited in their product scope, electrochemical conversion can be used to produce a wide range of one-carbon (C1), two-carbon (C2) and three-carbon (C3) compounds.…”

“…In particular, formate, glucose, and xylose were selected as the comparative feedstocks, while succinate, ethanol, glycolate and 2,3-butanediol, were selected as the products of interest. Formate was selected as a feedstock as it is another product of electrochemical CO 2 reduction (eCO 2 R) that has been well-studied, and has already been successfully employed within biological systems (3,6,(40)(41)(42). Meanwhile, glucose and xylose were selected as typical renewable sugar feedstocks, considering that they comprise the largest fraction of sugars in lignocellulosic biomass (43).…”