Amping Up the World of Electromicrobiology
The microbial world exhibits extraordinary morphological, biochemical and metabolic diversity. Whether surviving in extreme environments, adapting to stress conditions or altering the landscape of our planet, microbes perform astonishing feats. Diversity in metabolic functioning enables microbes to utilize a wide range of chemicals as energy sources. Some bacteria even possess the remarkable ability to generate electricity. These bacteria are referred to by a variety of names, including electricigens, exoelectrogens, electroactive bacteria or anode-respiring bacteria. The mechanisms behind this unique ability are rooted in electrochemistry, which involves transfer of electrons from one molecule to another and underpins respiration in all living organisms. What makes electrogenic bacteria unique is that they possess special redox circuitry that can extend outside the cell to transfer electrons to solid, conducting surfaces, like electrodes, and generate current.
Bioelectrochemical Systems for a Sustainable World
Bioelectrochemical systems harness the electroactive properties of microorganisms to generate electricity from organic matter. These systems function similarly to batteries, but rely on microbes to generate current. Early developments using this technology mainly focused on using microbial fuel cells as reactors for wastewater treatment, since microbes can use organic matter present in wastewater to generate electricity. However, as the breadth of electroactive microbes was gradually uncovered, fundamental studies into the nature of electroactivity gained traction. Microbial fuel cells, microbial three-electrode cells, microbial electrolysis cells, microbial electrosynthesis cells and microbial solar cells are a few examples of the different types of microbial electrochemical cells being used today. These devices, commonly abbreviated as MXCs, where X denotes the functionality of the reactor, have made rapid strides for a variety of environmental, technological and medical applications.
Waste Treatment and Resource Recovery
Integration of microbial fuel cells with wastewater treatment plants has led to energy recovery and an improvement in the wastewater treatment process. A pilot plant in India that incorporated a bioelectrochemical reactor was used to develop a self-sustainable, electric toilet that used human waste to light up a bulb. A similar project in England integrated ceramic microbial fuel cells with electronic interfaces into toilets for powering an electronic faucet. Such projects treat wastes effectively, offer a sustainable energy generation system from waste and improve the hygiene and efficiency of public toilets.
The potential of bioelectrochemical systems for carbon capture from the environment has also been recognized. Microbial electrosynthesis, which involves electron uptake by microbes in order to reduce carbon dioxide to useful chemicals, has already attracted significant interest and research funding. The use of electroactive bacteria in biorefineries (analogous to petroleum refineries) is touted as an important technology that can recover energy, chemicals and water from biomass and wastewater.
Microbial fuel cells have also been used in the treatment of xenobiotics, organic pollutants and recovery of heavy metals. In remote regions, bioelectrochemical systems can be deployed as effective biosensors to detect the presence of toxicants like heavy metals and pesticides. The electroactive biofilm acts as the sensing element, the current generated constitutes the signal and the electrode itself functions as the transducer. Such sensors can provide a non-invasive, rapid and effective method to further monitor water quality.
While it is not directly apparent, electromicrobiology can have significant relevance in medicine as well. The discovery of electroactivity in the Gram-positive, opportunistic pathogen Enterococcus faecalis under iron-rich conditions raises the possibility of electron transfer-linked pathogenicity. Some other pathogens demonstrating electroactivity include Listeria monocytogenes, Corynebacterium matruchotii and Streptococcus mutans. A deeper understanding of the electroactivity of these pathogens could potentially lead to the development of novel drug targets to treat infections. Several gut bacteria are also thought to be electroactive, and further studies could elucidate the role of electroactivity toward maintaining a healthy gut microbiome. Furthermore, abiotic electrochemical fuel cells powered autonomously via body fluids have the potential to be developed into wearable sensors that are small, easy-to-handle and can provide real-time data about diseases, leading to effective management of chronic diseases.
Novel Technologies Based on Electromicrobiolgy
In what may sound like science fiction, a team of researchers, led by Dr. Jun Yao, were able to generate electricity out of thin air! The researchers isolated conductive nanowire films from Geobacter sulfurrreducens and sandwiched them between electrodes to create a device termed “Air-gen.” This device adsorbs water from the surrounding atmosphere, and a combination of electrical conductivity and surface chemistry of the nanowires (filamentous appendages capable facilitating long-range extracellular electron transfer), generates electrical current between the electrodes. Air-gen is a cost-effective and renewable way to generate electricity from ambient humidity and does not require anything besides air. It can function outdoors or indoors, in darkness or light, and requires minimal maintenance, raising possibilities for sustainable, continuous and renewable power generation. In developing countries, such technologies can be especially useful because they present a way to generate off-grid power in an affordable manner.
Studies have also demonstrated that a bio-memristor fabricated with protein nanowires of G. sulfurreducens is capable of facilitating communication between electronic and biological interfaces, leading to interesting possibilities of “e-biologics,” or electronic materials manufactured with microbial components. e-Biologics can not only improve the performance of electronic devices, but can also offer a sustainable way to engineer products that reduce the consumption of costly raw materials. Additionally, these devices are biogedradable, so the problem of waste disposal in conventional electronic devices isn’t an issue. Already, technological advances have enabled large-scale production of nanowires in E. coli cells acting as chassis via genetic engineering. In the not-too-distant future, functional bioelectronics using conductive protein filaments from electroactive bacteria may bring a revolution in “green electronics.”
In order to harness the unique power of microbes to create electircty, a keen understanding of the mechanisms by which they do so is required. Two of the most well-studied electricigens include Shewanella oneidensis and G. sulfurreducens. Extracellular electron transfer in these bacteria (and other electricigens) can be facilitated by either direct electron transfer, where the microbe directly reduces a terminal electron acceptor, or mediated electron transfer, which involves the use of soluble redox shuttles.
Direct electron transfer primarily relies on multiheme cytochromes, which are present on the outer membrane, to establish contact with the terminal electron acceptor and facilitate subsequent electron transfer. In some bacteria, direct electron transfer can also proceed via nanowires, which are capable of establishing direct contact with the terminal electron acceptor. Nanowires are thought to be conductive in nature, thus enabling extracellular electron transfer. However, opposing views suggesting they have more of a secretory role than a conductive one, have also been presented. The debate around the structure and function of nanowires isn’t fully resolved and, as mentioned above, is currently an exciting area of research in electromicrobiology.
Mediated electron transfer relies on soluble electron shuttles, such as flavins, phenazines and quinones. These mediators are often redox molecules and are synthesized endogenously and secreted by the bacteria to aid extracellular electron transfer. They accept electrons from the microbe, and then proceed to transfer them to the electrode, after which they can initiate another round of electron transfer. Bacteria like Shewanella can employ both direct and mediated electron transfer.
In recent years, the discovery of electron transfer mechanisms in cable bacteria has added a new dimension to electromicrobiology. Cable bacteria are multicellular, filamentous bacteria belonging to the group Deltaproteobacteria. A remarkable aspect of these bacteria is that there is division of metabolic labor among the various cells constituting the cable. Some cells oxidize electron donors, while others reduce electron acceptors. Intercellular communication between different cells occurs via electric signals. These bacteria are commonly found in anoxic sediments and can transfer electrons as far as centimeter-scale distances by using gradients in the redox potential along the length of their filaments. In addition to contributing to a fundamental understanding of the molecular mechanisms of electromicrobiology, cable bacteria may have potentially useful applications, including minimizing methane emissions from natural environments.
The Way Forward
The recent discovery of electron transfer in the reverse direction, i.e., from the electrode/minerals to the microbes, has prompted research into how electricigens can generate energy in nutrient-scarce environments. Another interesting development is the idea that electroactivity is not restricted to only electricigens, but rather exists as a spectrum. The importance of weak electricigens (bacteria that demonstrate an unexpected or unreliable current response) is of interest to researchers, and capturing electroactive signals from weak electricigens is warranted. Electromicrobiology is a truly multi-disciplinary field, fusing electrochemistry, microbiology, biochemistry and engineering to present interesting opportunities for real-world applications. Looking ahead, there is much much opportunity for research and development in this field.