Applications of Electromicrobiology

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This entry is adapted from the peer-reviewed paper 10.3390/su141710676

Electromicrobiology is concerned with the interplay of microbes with electronic equipment and the electrical characteristics of microorganisms. Bioelectrochemical systems are those in which an organism provides electrons. Bioelectrochemical systems are pretty similar to electrochemical methods in their fundamentals. Electrochemical systems generally use electricity to begin a chemical reaction or create power via a chemical reaction.

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1. Bioelectrochemical Systems

Bioelectrochemical systems are those in which an organism provides electrons ^[1]. Bioelectrochemical systems are pretty similar to electrochemical systems in their fundamentals. Electrochemical systems generally use electricity to begin a chemical reaction or create power via a chemical reaction ^[2]. Fuel cells are considered a subcategory of the latter category of electrochemical devices. The first fuel cell of its kind was designed in the nineteenth century by Sir William Grove to conserve electrical energy ^[3]. Before that, the Bagdad battery is claimed to be one of the earliest known fuel cell systems ^[4]. A fuel cell is an enclosed device that converts energy generated by chemical processes to electrical energy. An anode, a cathode, a fuel supply, and an energy acceptor make up a fuel cell. The anode and cathode are coupled to facilitate electron passage. The anode chemically degrades the fuel by contributing an electron, which travels to the cathode and is absorbed by an acceptor undergoing a chemical process. For instance, hydrogen gas might be utilized as a fuel in which the anode chemically breaks down hydrogen gas to H+ ions, which are then coupled with oxygen that has been chemically split after the cathode accepts the hydrogen ions. Fuel cells are typically classified according to the electrolytes utilized, the temperature at which the reaction occurs, and the kind of fuel employed. Alkaline cells, for example, are placed beneath low-temperature cells, whereas molten carbonate cells are placed beneath high-temperature cells.

On the other hand, Electrochemical cells are more adaptable than fuel cells since they may generate or consume electricity. The potential governs the mechanism of action and whether it is generated externally or by the cell’s own reaction. The difference in cell potential between the two electrodes determines the direction of the response within the cell. This point is defined in contrast to a standard hydrogen electrode and equals 0 volts. A salt bridge measures the link between two half cells, tanks with electrolytes, and an electrode so that ions can enter and exit each half cell separately without mingling, as well as linking the electrodes with each other and evaluating the cell potential between the electrodes.

1.1. Mechanism of Bioelectrochemical System of Microbial Fuel Cell

Bioelectrochemical systems utilize microorganisms’ electroactive characteristics to create power from organic materials. These devices operate similarly to batteries but are powered by bacteria. Initially, this technique was primarily employed to construct microbial fuel cells as wastewater treatment reactors, as bacteria can utilize organic substances found in wastewater to produce electricity. However, as the diversity of electroactive bacteria became clear, fundamental study into the nature of electroactivity accelerated. Various microbial electrochemical cells are already in use, including microbial fuel cells, microbial three-electrode cells, microbial electrolysis cells, microbial electrosynthesis cells, and even microbial solar cells. These devices, mainly presented as microbial electrochemical technologies (MXCs), where X specifies the reactor’s functionality, have advanced rapidly in several environmental, technical, and medicinal applications. Microbial fuel cells (MFCs) based on mediators are critical for instructional reasons and the examination of the processes underlying the electron transfer activities of microorganisms. However, the diminished outcome of the fuel cell (current density is typically 50 mA cm⁻²) and the high cost and environmental concerns associated with using artificial mediators render mediator-based MFCs technologically impracticable and economically impracticable unviable, which is why this strategy was later discarded. The last several years have seen the discovery and application of a variety of electron transport approaches that do not rely on artificial redox mediators to power MFCs. The term “mediatorless” is frequently used to describe these fuel cells, even though inlying electron commuting molecules are frequently engaged in the current generation process. These latest proposals propose to leverage anaerobic microbes’ naturally developed pathways and mechanisms for disposing of electrons generated during their metabolic operations.

To characterize electron transfer processes, it is crucial to distinguish between those in which soluble entities assist in electron transfer from the cell membrane to the electrode and those that do not. Electron transfer processes are classified into direct electron transfer (DET) and mediated electron transfer (MET).

1.1.1. Direct Electron Transfer (DET)

A DET is an electron transfer from the biocatalyst to the fuel cell anode that takes place in the absence of any intermediary entity other than the bacteria. It has relied on physical interaction between the membrane of a bacterial cell or a membrane organelle and the anode of a fuel cell. Authors in ^[5] define an indirect biofuel cell as one in which the biological reaction generates a secondary fuel for the electrode. In contrast, direct biofuel cells involve either electron-shuttling reversible mediators or direct electron transfer between the biological component and the electrode. Because the great majority of microbes are electrically non-conducting, such transmission processes were deemed impossible for a long time. The existence of membrane-bound electron transport proteins like cytochromes, which allow electrons to travel from the inside of the cell to its outside, is required for direct electron transport from microbial cells to the anode of a fuel cell. These membrane characteristics have been particularly adapted by sediment-dwelling metal reducing bacteria such as Shewanella, Rhodoferax, and Geobacter, which employ solid iron (III) oxides as final electron acceptors. In this circumstance, the fuel cell’s anode may immediately reinstate its job as an electron acceptor. Fuel cell anode-to-bacterial-cell contact (adherence) is required for direct electron transfer; as a result, only bacteria within the first monolayer exhibit electrochemical activity. Thus, the MFC’s productivity is constrained by the maximal cell density in the bacterial monolayer. For example, an MFC powered by Rhodoferax ferrireducens can generate an optimum current density of 3 mA cm⁻².

Because of recent evolution, certain Shewanella and Geobacter strains now possess molecular pili (nanowires) that conduct electricity, enabling them to access and utilize remote solid electron acceptors. As an additional benefit, these pili allow the microbes to accept electrons from an electrode that is not directly in touch with the cell. The pili are associated with membrane-bound cytochromes, which are vital for electron transport outside the cell. The fabrication of such nanowires permits the development of denser electrochemically active biofilms, increasing anode performance. To date, a growing number of microorganisms have been discovered that are capable of effectively transporting electrons to an electrode. However, many of these species cannot generate energy from complicated substrates. Thus, Geobacter species, for example, are limited to low molecular weight substrates, e.g., acetate or butyrate, and must rely on the availability of fermenting species to disintegrate complex organic materials.

1.1.2. Mediated Electron Transfer (MET)

Another successful method of connecting microbial metabolism to an electrode is by mediated electron transfer. Three main transmission pathways are well recognized–all dependent on microbial production of reduced metabolic intermediates, which are then released by microbial cells. These compounds are classified as primary and secondary metabolites based on their function in microbial metabolism. Primary metabolites are substances fundamentally involved in the metabolic processes of microorganisms (substrate decomposition) and are frequently significant metabolic byproducts. Secondary metabolites are generally unrelated to basic metabolic activities. Microorganisms, on the other hand, can produce them to aid in the disposal of electrons, e.g., to a remote solid electron acceptor. Pseudomonas aeruginosa and Shewanella oenidensis produce bacterial phenazines and quinine, respectively, in the presence of a positively polarized electrode or a solid electron acceptor such as iron (III) oxide and the absence of competing soluble electron acceptors. These electron shuttle chemicals may be oxidized at solid electron acceptors like electrodes, allowing many redox cycles between cells and acceptors. Compared to secondary metabolite biosynthesis, diminished primary metabolite exudation is strictly related to substrate oxidation. Evidently, the total number of reduction counterparts produced must equal the total amount of oxidized metabolites. A metabolite must fulfill specified conditions to be advantageous as a reductant in anodic oxidation. Its redox potential should be as minimal as possible and must be oxidizable electrochemically under MFC parameters. Fermentation and anaerobic respiration are the two basic anaerobic metabolic pathways from which vital reduced metabolites can be generated. The utilization of fermentation for MFC functioning has attracted more attention than anaerobic respiration. Numerous fermentative and photoheterotrophic activities generate energy-dense reduced metabolites, such as ethanol, hydrogen, and formate. These substances can be oxidized in the microbiological medium if electrocatalytic anodes are utilized to help the oxidation reaction.

The catalyst must meet several parameters that drastically restrict the spectrum of electrocatalytic materials accessible. It must be (1) biocompatible (non-toxic to microorganisms), (2) be extremely active in electrocatalytic oxidation of a variety of metabolites, (3) have strong electrocatalytic activity in the pH range of 5–7 and at relatively low temperature (10–40 °C), (4) stable in terms of both chemical and electrochemical stability and biofouling resistance, and (5) invulnerable to biological product poisoning. This classification imposes severe constraints on the selection of acceptable catalyst systems. Thus, nickel, copper, and cobalt-based metal oxide and transition metal catalysts must be ruled out due to their low consistency at the appropriate pH and antibacterial properties (nickel, cobalt, copper, silver, and others). Authors in ^[6] published the first investigation on the direct creation of power through the use of these metabolites, using immobilized hydrogen-producing cultures as biocatalysts and platinum as an electrocatalyst for hydrogen oxidation. Due to the platinum electrodes’ susceptibility to poisoning and deactivation, the claimed power densities were somewhat low.

Authors in ^[7] made significant progress by developing platinum sandwich electrodes shielded from poisoning reactions by applying conductive polymers such as polyaniline or its fluorinated derivatives. These electrodes significantly enhanced the efficiency of MFCs with current densities as high as 1.5 mA cm⁻². Additionally, they enabled the use of an extensive range of microorganisms that are heterotrophic, photoheterotrophic, or even photosynthetic, as well as the availability of complex polysaccharides such as cellulose and starch for production process in MFCs. The costly noble metal electrocatalyst was also switched with tungsten carbide (WC). A very affordable but highly effective and resilient electrocatalyst. However, tungsten carbide looks to be a potential anodic electrocatalyst for MFCs. It has a high level of electrocatalytic oxidation performance at a low cost. Additionally, unlike platinum, the poisonous platinum poisons hydrogen sulphide and carbon monoxide did not affect tungsten carbide. In terms of current MFC technology, the greatest 3 mA cm⁻² current density and 586 mW cm⁻² optimum power density is the greatest ever achieved.

2. Applications of Electromicrobiology

Electromicrobiology investigation has accelerated significantly throughout the preceding decade. This has been sparked by a variety of factors, including a rise in research into alternative energy sources, as well as advances in waste management, reclaimed water, and sensors. Recently, new efforts have been made to understand how extraneous electron transfer happens in microorganisms and how it might facilitate bacteria’ behaviors in the surrounding environment, even at relatively long distances. While these techniques will undoubtedly impact the advancement of the technologies mentioned above, an additional field of electromicrobiology may rely on extrinsic electron transfer: microbial electron synthesis of valuable fuels and chemicals. Biocathodes are crucial to electrosynthesis because they need bacteria to act as an electron source and catalyze chemical production. Using carbon dioxide waste as a carbon source in organic molecule synthesis reduces the demand for arable land. If a sustainable energy source is employed, it is carbon neutral.

2.1. Bioelectronics

Electrically active microbes might make significant enhancements in the expanding field of bioelectronics. For instance, microbes’ capacity to monitor various substances and environmental circumstances indicates several opportunities to create biosensors and biocomputers ^[8]. Biological oxygen demand (BOD) and toxicity may be detected in water using biosensors. These metrics can be coupled to bacterial metabolic activity measurement. Using living materials to grow or build electronics has potential advantages, as they can be obtained from less expensive feedstocks, resulting in less waste and avoiding using harmful substances. Self-repair and replication are possible if the electronic application incorporates microbes and their subassemblies. There are several varieties of bioelectrical gadgets, but one of the most common is the so-called mud battery, which comprises an electrode embedded in organic-rich silt and linked to a cathode submerged in aerobic water. When bacteria capable of respiration in the presence of EET instantly start respiring when the electrode is introduced, this decomposes the organic matter in the sediment, creating current. This is an eye-opening experiment for research scientists, and it marks the start of our knowledge of sediment batteries, equipment capable of generating very low currents, which can power sensor devices or other limited power gadgets on the darker sea bottom or in other regions where sunshine or wind are scarce ^[9]^[10].

2.2. Bioremediation

Bioelectrochemical processes represent a new technology platform for the treatment of wastewater contamination. The application of microbes as a catalyst in fuel cells for power generation was discovered 40 years ago ^[11]. During aerobic conditions, bacteria create carbon dioxide and water; in anaerobic conditions, they release carbon dioxide, protons, and electrons. Since organic matter is utilized to “fuel” the Microbial Fuel Cell (MFC), MFCs are recommended for wastewater treatment plants ^[12]. Sludge from the water would be consumed by bacteria and provide extra electricity for the plant while utilizing inorganic intermediates to access the cellular electron transport chain and obtain the electrons generated. This procedure must be indulged in fuel cells, which are comprised of organisms with the ability to generate electrical current. These organisms are referred to as Exoelectrogens. The bioremediation technique utilizes Geobacter’s capacity to oxidize organic materials to remove hydrocarbon pollutants from soil and water. Metals are easily oxidized or reduced, which is one of their distinctive features: Mn⁴⁺ oxides are solids that undergo reduction to form soluble Mn²⁺ salts (e.g., MnCl₂). At the same time, oxidized versions of U or Cr are harmful partly because they are very soluble and reduce to insoluble metal hydroxides. As a result, if an S-BED unit is built with this in account and contains the proper microorganisms, such devices might convert soluble uranium or chromium to insoluble forms, collecting them in a cathode chamber and collecting them erasing them effectively ^[13].

2.3. Generation of Electricity from Solar Energy

A new approach for converting solar power into electricity has emerged with microbial fuel cell technology advancement. Photosynthetic MFCs are a new kind of energy harvesting device that uses the sun’s irradiation to generate electricity 24 h a day, seven days a week. Under day-night cycles, photosynthetic MFCs may produce power constantly from the respiratory and photosynthetic activities of microorganisms. When it comes to small-scale biological fuel cells, they have a lot more energy per unit size than bigger ones do ^[14]. Shrinkage of photosynthetic MFCs automatically creates ideal circumstances for increased power density through reduced internal resistance and improved mass transfer. Additionally, small-scale photosynthetic MFCs enable the scaling up of MFCs by using numerous units in a stack arrangement.

In MFCs, organic materials are delivered into the anode chamber. The electrode is tuned to an appropriate voltage for bacteria to use as an electron acceptor. The aerobic cathode compartment receives electrons via a conductive cable, in which they produce water by combining with distributing protons and molecules of oxygen. Well-established methods for producing energy from organic materials, such as sewage or agricultural or industrial runoff, have been employed in the past. Because they are used to digest organic material and create little waste and no methane, they are considered sustainable and environmentally benign energy sources. While this is correct, power densities and current efficiencies are poor, and the total energy expenditure per unit of energy produced is high. Unless you reside in a place where very little or no light is available, and wind energy cannot be generated due to a lack of sufficient air movement, the notion of these gadgets affecting the energy aspect of sustainability is entirely speculative in this era of wind and solar power dominance. There are such areas, and notwithstanding what has been said so far, if applications for bio-electrochemical devices can be developed, they might be both ecologically friendly and self-sufficient. These devices may have considerable environmental and/or human health advantages, particularly in locations where electricity infrastructures are lacking or insecure.

2.4. Wastewater Reclamation

The utilization of bio-electrochemical systems for wastewater treatment has advanced significantly since the initial demonstration of MFCs ^[15]. These advancements entail a shift away from moving from pure cultures to mixed ones of microorganisms that are more resistant to variations in the substrate supply ^[16] and hence provide greater power outputs than pure cultures. When an aerial cathode is used despite a submerged cathode, the increased oxygen level enables a quicker reaction and a larger energy output. If, on the other hand, a water cathode is employed, the clean water generated in the cathode compartment is stored for possible reuse. There have been no large-scale demonstrations of these systems using industrial or municipal waste streams. The latter example demonstrated excellent BOD and COD removal with minimal or no sludge buildup in the sewage system ^[17]. Expanding the lab-scale technologies to the municipal scale remains a ‘work in progress.’ These systems are significant for human health because they might enable water reclamation in areas where electricity networks do not exist, displacing harmful sewage disposal methods. In this case, a low power yield might result in a significant sustainability dividend in terms of energy, water, and waste (i.e., environmental quality). Enhancing inorganic nitrogen removal is critical for the sustainable growth of the mariculture business due to the low carbon to nitrogen (C/N) ratio of effluent and rigorous discharge standards. Authors in ^[18] demonstrated the effective treatment of simulated mariculture wastewater (high salinity, low COD/N ratio of 0.5–1.0) using an integrated self-biased bio-electrochemical system with a catalyst (TiO₂/Co-WO₃/SiC) on the cathode and naturally growing algae in the cathode chamber. The synergy of bacteria, algae, and cathode enhanced pollutant removal and increased the system’s sustainability and efficiency in treating mariculture effluent.

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