Cathodic supply of electrons to living microbial cells via cytocompatible redox-active polymers

TEMPO-Modified Polymethacrylates as Mediators in Electrosynthesis: Influence of the Molecular Weight on Redox Properties and Electrocatalytic Activity

Homogeneous catalysts ("mediators") are frequently employed in organic electrosynthesis to control selectivity. Despite their advantages, they can have a negative influence on the overall energy and mass balance if used only once or recycled inefficiently. Polymediators are soluble redox-active polymers applicable as electrocatalysts, enabling recovery by dialysis or membrane filtration. Using anodic alcohol oxidation as an example, we have demonstrated that TEMPO-modified polymethacrylates (TPMA) can act as efficient and recyclable catalysts. In the present work, the influence of the molecular size on the redox properties and the catalytic activity was carefully elaborated using a series of TPMAs with well-defined molecular weight distributions. Cyclic voltammetry studies show that the polymer chain length has a pronounced impact on the key-properties. Together with preparative-scale electrolysis experiments, an optimum size range was identified for polymediator-guided sustainable reaction control.

Electrochemically Driven Photosynthetic Electron Transport in Cyanobacteria Lacking Photosystem II

Light-activated photosystem II (PSII) carries out the critical step of splitting water in photosynthesis. However, PSII is susceptible to light-induced damage. Here, results are presented from a novel microbial electro-photosynthetic system (MEPS) that uses redox mediators in conjunction with an electrode to drive electron transport in live Synechocystis (ΔpsbB) cells lacking PSII. MEPS-generated, light-dependent current increased with light intensity up to 2050 μmol photons m^-2 s^-1, which yielded a delivery rate of 113 μmol electrons h^-1 mg-chl^-1 and an average current density of 150 A m^-2 s^-1 mg-chl^-1. P700⁺ re-reduction kinetics demonstrated that initial rates exceeded wildtype PSII-driven electron delivery. The electron delivery occurs ahead of the cytochrome b₆f complex to enable both NADPH and ATP production. This work demonstrates an electrochemical system that can drive photosynthetic electron transport, provides a platform for photosynthetic foundational studies, and has the potential for improving photosynthetic performance at high light intensities.

Engineering microbial metabolic energy homeostasis for improved bioproduction

Metabolic energy (ME) homeostasis is essential for the survival and proper functioning of microbial cell factories. However, it is often disrupted during bioproduction because of inefficient ME supply and excessive ME consumption. In this review, we propose strategies, including reinforcement of the capacity of ME-harvesting systems in autotrophic microorganisms; enhancement of the efficiency of ME-supplying pathways in heterotrophic microorganisms; and reduction of unessential ME consumption by microbial cells, to address these issues. This review highlights the potential of biotechnology in the engineering of microbial ME homeostasis and provides guidance for the higher efficient bioproduction of microbial cell factories.

100th Anniversary of Macromolecular Science Viewpoint: Soft Materials for Microbial Bioelectronics

Bioelectronics brings together the fields of biology and microelectronics to create multifunctional devices with the potential to address longstanding technological challenges and change our way of life. Microbial electrochemical devices are a growing subset of bioelectronic devices that incorporate naturally occurring or synthetically engineered microbes into electronic devices and have broad applications including energy harvesting, chemical production, water remediation, and environmental and health monitoring. The goal of this Viewpoint is to highlight recent advances and ongoing challenges in the rapidly developing field of microbial bioelectronic devices, with an emphasis on materials challenges. We provide an overview of microbial bioelectronic devices, discuss the biotic-abiotic interface in these devices, and then present recent advances and ongoing challenges in materials related to electron transfer across the abiotic-biotic interface, microbial adhesion, redox signaling, electronic amplification, and device miniaturization. We conclude with a summary and perspective of the field of microbial bioelectronics.

Internalization Mechanisms of Pyridinium Sulfobetaine Polymers Evaluated by Induced Protic Perturbations on Cell Surfaces

Understanding the interactions of soft nanomatters with cell membranes is particularly important for research into nanocarrier-based drug delivery systems, cell engineering, and subcellular imaging. Most nanoparticles, vesicles, micelles, and polymeric aggregates are internalized into endosomes and, eventually, lysosomes in the cytosol because of energy-dependent endocytic processes. Endocytic uptake substantially limits the access to the cytoplasm where a cargo agent acts. Bypassing the endocytic pathways by direct penetration into plasma membrane barriers would enhance the efficacy of nanomedicines. Some zwitterionic polymer nanoaggregates have been shown to permeate into the cell interior in an energy-independent manner. We have elucidated this phenomenon by observing changes in the biomembrane barrier functions against protons as the smallest indicator and have used these results to further design and develop poly(betaines). In this work, we investigated the translocation mechanisms for a series of zwitterionic poly(methacrylamide) and poly(methacrylate) species bearing a pyridinium propane sulfonate moiety in the monomers. Minor differences in the monomer structures and functional groups were observed to have dramatic effects on the interaction with plasma membranes during translocation. The ability to cross the plasma membrane involves a balance among the betaine dipole-dipole interaction, NH-π interaction, π-π interaction, cation-π interaction, and amide hydrogen bonding. We found that the cell-penetrating polysulfobetaines had limited or no detrimental effect on cell proliferation. Our findings enhance the opportunity to design and synthesize soft nanomatters for cell manipulations by passing across biomembrane partitions.

Redox-Active Polymers Connecting Living Microbial Cells to an Extracellular Electrical Circuit

Microbial electrochemical systems in which metabolic electrons in living microbes have been extracted to or injected from an extracellular electrical circuit have attracted considerable attention as environmentally-friendly energy conversion systems. Since general microbes cannot exchange electrons with extracellular solids, electron mediators are needed to connect living cells to an extracellular electrode. Although hydrophobic small molecules that can penetrate cell membranes are commonly used as electron mediators, they cannot be dissolved at high concentrations in aqueous media. The use of hydrophobic mediators in combination with small hydrophilic redox molecules can substantially increase the efficiency of the extracellular electron transfer process, but this method has side effects, in some cases, such as cytotoxicity and environmental pollution. In this Review, recently-developed redox-active polymers are highlighted as a new type of electron mediator that has less cytotoxicity than many conventional electron mediators. Owing to the design flexibility of polymer structures, important parameters that affect electron transport properties, such as redox potential, the balance of hydrophobicity and hydrophilicity, and electron conductivity, can be systematically regulated.

Extracellular electron transfer mediated by a cytocompatible redox polymer to study the crosstalk among the mammalian circadian clock, cellular metabolism, and cellular redox state

The circadian clock is an endogenous biological timekeeping system that controls various physiological and cellular processes with a 24 h rhythm. The crosstalk among the circadian clock, cellular metabolism, and cellular redox state has attracted much attention. To elucidate this crosstalk, chemical compounds have been used to perturb cellular metabolism and the redox state. However, an electron mediator that facilitates extracellular electron transfer (EET) has not been used to study the mammalian circadian clock due to potential cytotoxic effects of the mediator. Here, we report evidence that a cytocompatible redox polymer pMFc (2-methacryloyloxyethyl phosphorylcholine-co-vinyl ferrocene) can be used as the mediator to study the mammalian circadian clock. EET mediated by oxidized pMFc (ox-pMFc) extracted intracellular electrons from human U2OS cells, resulting in a longer circadian period. Analyses of the metabolome and intracellular redox species imply that ox-pMFc receives an electron from glutathione, thereby inducing pentose phosphate pathway activation. These results suggest novel crosstalk among the circadian clock, metabolism, and redox state. We anticipate that EET mediated by a redox cytocompatible polymer will provide new insights into the mammalian circadian clock system, which may lead to the development of new treatments for circadian clock disorders.

Cytocompatible redox polymer pMFc altered the cellular redox state and metabolism, resulting in a longer circadian period.

Extracellular electron transfer features of Gram-positive bacteria

Electroactive microorganisms possess the unique ability to transfer electrons to or from solid phase electron conductors, e.g., electrodes or minerals, through various physiological mechanisms. The processes are commonly known as extracellular electron transfer and broadly harnessed in microbial electrochemical systems, such as microbial biosensors, microbial electrosynthesis, or microbial fuel cells. Apart from a few model microorganisms, the nature of the microbe-electrode conductive interaction is poorly understood for most of the electroactive species. The interaction determines the efficiency and a potential scaling up of bioelectrochemical systems. Gram-positive bacteria generally have a thick electron non-conductive cell wall and are believed to exhibit weak extracellular electron shuttling activity. This review highlights reported research accomplishments on electroactive Gram-positive bacteria. The use of electron-conducting polymers as mediators is considered as one promising strategy to enhance the electron transfer efficiency up to application scale. In view of the recent progress in understanding the molecular aspects of the extracellular electron transfer mechanisms of Enterococcus faecalis, the electron transfer properties of this bacterium are especially focused on. Fundamental knowledge on the nature of microbial extracellular electron transfer and its possibilities can provide insight in interspecies electron transfer and biogeochemical cycling of elements in nature. Additionally, a comprehensive understanding of cell-electrode interactions may help in overcoming insufficient electron transfer and restricted operational performance of various bioelectrochemical systems and facilitate their practical applications.

Extracellular electron transfer in acetogenic bacteria and its application for conversion of carbon dioxide into organic compounds

Acetogenic bacteria (i.e., acetogens) produce acetate from CO₂ during anaerobic chemoautotrophic growth. Because acetogens fix CO₂ with high energy efficiency, they have been investigated as biocatalysts of CO₂ conversion into valuable chemicals. Recent studies revealed that some acetogens are capable of extracellular electron transfer (EET), which enables electron exchange between microbial cells and extracellular solid materials. Thus, acetogens are promising candidates as biocatalysts in recently developed bioelectrochemical technologies, including microbial electrosynthesis (MES), in which useful chemicals are biologically produced from CO₂ using electricity as the energy source. In microbial photoelectrosynthesis, a variant of MES technology, the conversion of CO₂ into organic compounds is achieved using light as the sole energy source without an external power supply. In this mini-review, we introduce the general features of bioproduction and EET of acetogens and describe recent progress and future prospects of MES technologies based on the EET capability of acetogens.

Engineering of Microbial Electrodes

This chapter provides an overview of the current state-of-the-art in the engineering of microbial electrodes for application in microbial electrosynthesis. First, important functional aspects and requirements of basic materials for microbial electrodes are introduced, including the meaningful benchmarking of electrode performance, a comparison of electrode materials, and methods to improve microbe-electrode interaction. Suitable current collectors and composite materials that combine different functionalities are also discussed. Subsequently, the chapter focuses on the design of macroscopic electrode structures. Aspects such as mass transfer and electrode topology are touched upon, and a comparison of the performance of microbial electrodes relevant for practical application is provided. The chapter closes with an overall conclusion and outlook, highlighting the future prospects and challenges for the engineering of microbial electrodes toward practical application in the field of microbial electrosynthesis. Graphical Abstract.