(513cv) Prospects for Engineering High-Rate Microbial Electrosynthesis Driven By Direct Electron Uptake

Capture and conversion of CO₂ to fuels, commodity chemicals, and pharmaceutical precursors can help close the anthropogenic carbon cycle. Among the many strategies to fix CO₂, (bio)electrochemical reduction under mild conditions offers several potential benefits including stabilizing the electric grid by storing excess electrical energy and minimizing intermittency issues associated with renewable energy sources. Microbial electrosynthesis (MES) systems that reduce CO₂ can store renewable energy in many-carbon molecules inaccessible to abiotic electrochemistry.

Here, we present a multiphysics model that describes mass transport, electrochemical kinetics, acid-base reactions, and gas-liquid mass transfer to investigate the fundamental and practical limits of MES enabled by direct electron transfer. We develop a physiological mechanism for coupling direct electron uptake to cellular energy carrier regeneration using aerobic or anaerobic nitrate respiration and use this to derive the stoichiometry of half-cell reactions for CO₂ reduction to pyruvate for four major carbon fixation pathways. To determine design guidelines for these systems, we compare hypothetical microbes performing carbon fixation with these pathways under oxic and anoxic conditions. Our results show that although aerobic respiration enables microbes to divert a higher fraction of electrons towards carbon fixation, the pyruvate production rate is limited to <1 μmol/cm²/hr by O₂ transport to and through the biocathode layer. In contrast, anaerobic nitrate respiration enables CO₂ reduction at production rates >17 μmol/cm²/hr for microbes using the reductive tricarboxylic acid cycle and >10 μmol/cm²/hr for microbes using the Calvin cycle. Finally, we identify promising host organisms for the heterologous expression of electron conduits and naturally occurring organisms capable of MES. The presented model, methodology, and proposed electron-uptake physiology provide a complete framework to analyze MES reactor designs and identify promising research avenues that can move MES systems from basic science to technological practice.