Microbial electrosynthesis: anode- and cathode-driven bioproduction of chemicals and biofuels

Abstract

Microbial electrochemical technologies (METs) have great potential to enhance bioproduction of chemicals and biofuels. In particular, microbial electrosynthesis (MES) can drive electrochemically a desired metabolic production route in a cell by empowering microorganisms to use an anode as an electron sink or a cathode as an electron source in a bioelectrochemical system (BES). However, this emerging biotechnology is still in its initial stage and many knowledge gaps need to be addressed before MES can be applied on an industrial scale. Therefore, this thesis highlights the benefits and limitations of anodic and cathodic MES processes and studies them in depth to gain novel knowledge on this technology.In order to optimize a MES process, the underlying mechanisms of electron transfer between microorganism and electrode need to be fully understood. In this light, this work analyses various electron transport mechanisms of different microorganisms and discusses their potential advantages and limitations for use in a BES. In particular, the focus lies on microbial interaction with the electrode via extra- and intracellular electron-carrier molecules (e.g. cytochromes, ferredoxins, quinones, flavins) and the electrical influence on the cellular redox state and energy level.Based on the review of electron transport mechanisms for METs, the industrial important bacterium Corynebacterium glutamicum, was chosen as a microorganism example to demonstrate an anodic process. Growing aerobically, the amino acid producer relies on oxygen as terminal electron acceptor, which limits product yields through substrate loss in form of CO2. Therefore, an anaerobic process was established using an anode in a BES as the final electron acceptor. In order to enable electron transfer between cells and the anode, the mediator ferricyanide was introduced in the BES as an electron-shuffle molecule. In this way, the anoxic character of C. glutamicum was improved by stabilizing its redox and energy state resulting in enabled anaerobic cell growth, faster glucose uptake and enhanced production of organic acids and the amino acid L‑lysine (feed additive) as compared to the anaerobic cultivation conditions without a suitable external electron acceptor.On the other hand, as an example for a cathodic process, this thesis demonstrates the cultivation of an open microbiome enriched with Clostridium spp. in a BES to allow the conversion of the greenhouse gas CO2 into multicarbon compounds using the cathode as the sole electron source. However, the reported product spectrum of MES was so far mainly limited to acetic acid, which production in a BES is economically not very attractive. Therefore, this thesis investigated also into specific cultivation conditions to broaden the MES product spectrum toward products with higher economic value and higher industrial interest as compared to acetate. In particular, mildly acidic pH condition (pH of ~5) triggered a metabolic shift in the microbial community from the production of mostly acetate through acetogenesis, to ethanol through solventogenesis. In turn, the simultaneous presence of acetate and ethanol led to the production of the platform chemicals butyric, isobutyric, and caproic acids and their corresponding alcohols (potential biofuels) via the reverse b‑oxidation pathway and via sequential solventogensis, respectively.However, the synthesis of carboxylic acids and alcohols requires different specific environmental pH conditions to achieve an optimal production process. In fact, a stable neutral pH is required to obtain high titers of carboxylic acids in a bioreactor without an integrated inline extraction. In contrast, a premise for alcohol production via solventogenesis is mildly acidic pH. Hence, providing optimal conditions to favor all production steps simultaneously is challenging, and typically leads to reactors that are operated under suboptimal conditions. This issue of competing pH requirements was addressed by introducing an innovative three-chamber electrochemical system design comprising of two biological cathode chambers and one abiotic anode compartment. This new design achieves the physical separation of acetogenesis/chain elongation from solventogenesis, and allows their operation under optimal conditions without the requirement of acid/base dosing by fine-tuning the pH through a combination of electrochemical control, electromigration, and gas sparging.This work demonstrates the great potential of METs for bioelectrochemical synthesis of industrially relevant chemicals and highlights the benefits as well as the limitations of this emerging biotechnology. In addition, the analysis of the proposed oxidative and reductive MES processes on a reactor‑engineering level and on a cellular level via metabolomics and metagenomics will add an important piece of fundamental and engineering understanding of MES to the research community, thus will support further development of this technology to bring MES one step closer to real-life applications.