Phototrophic N2 and CO2 Fixation Using a Rhodopseudomonas palustris-H2 Mediated Electrochemical System With Infrared Photons.

Mathangi Soundararajan,Lance C Seefeldt,Paul Kusuma,Paul Ludden,Bruce Bugbee,Scott A Ensign,Shuyang Zhen,Rhesa Ledbetter

doi:10.3389/fmicb.2019.01817

Abstract

A promising approach for the synthesis of high value reduced compounds is to couple bacteria to the cathode of an electrochemical cell, with delivery of electrons from the electrode driving reductive biosynthesis in the bacteria. Such systems have been used to reduce CO2 to acetate and other C-based compounds. Here, we report an electrosynthetic system that couples a diazotrophic, photoautotrophic bacterium, Rhodopseudomonas palustris TIE-1, to the cathode of an electrochemical cell through the mediator H2 that allows reductive capture of both CO2 and N2 with all of the energy coming from the electrode and infrared (IR) photons. R. palustris TIE-1 was shown to utilize a narrow band of IR radiation centered around 850 nm to support growth under both photoheterotrophic, non-diazotrophic and photoautotrophic, diazotrophic conditions with growth rates similar to those achieved using broad spectrum incandescent light. The bacteria were also successfully cultured in the cathodic compartment of an electrochemical cell with the sole source of electrons coming from electrochemically generated H2, supporting reduction of both CO2 and N2 using 850 nm photons as an energy source. Growth rates were similar to non-electrochemical conditions, revealing that the electrochemical system can fully support bacterial growth. Faradaic efficiencies for N2 and CO2 reduction were 8.5 and 47%, respectively. These results demonstrate that a microbial-electrode hybrid system can be used to achieve reduction and capture of both CO2 and N2 using low energy IR radiation and electrons provided by an electrode.

Highlights

Coupling microbes to electrodes is a frontier area for specialty chemical production
In order to identify which wavelengths could potentially be used by the bacteria, but would not be used by plants, absorption spectra of leaves and of R. palustris TIE-1 were recorded (Figure 1)
While the absorption of photons by leaves drops to zero beyond ∼750 nm, the bacteria still show some absorbance in the 750–900 nm region

Summary

Introduction

Coupling microbes to electrodes is a frontier area for specialty chemical production. A number of microbial electrosynthetic systems have been used to fix CO2 (autotrophy) and upgrade the C to an array of value added compounds such as acetate, precursors for polymers, and precursors for pharmaceuticals (Rabaey and Rozendal, 2010; Liu et al, 2015). While these bacteria can obtain all C from CO2 reduction, a source of reduced N is required to sustain life. A number of bacteria and archaea contain nitrogenase, the enzyme that catalyzes the ATP-dependent reduction of N2 to ammonia (NH3) according to the minimal reaction stoichiometry for the Mo-dependent enzyme of: N2 + 8 H+ + 16 MgATP + 8 e−

Methods

Results

Conclusion