pH Dependency in Anode Biofilms of Thermincola ferriacetica Suggests a Proton-Dependent Electrochemical Response

Active pH regulation facilitates Bacillus subtilis biofilm development in a minimally buffered environment

Biofilms provide individual bacteria with many advantages, yet dense cellular proliferation can also create intrinsic metabolic challenges including excessive acidification. Because such pH stress can be masked in buffered laboratory media-such as MSgg commonly used to study Bacillus subtilis biofilms-it is not always clear how such biofilms cope with minimally buffered natural environments. Here, we report how B. subtilis biofilms overcome this intrinsic metabolic challenge through an active pH regulation mechanism. Specifically, we find that these biofilms can modulate their extracellular pH to the preferred neutrophile range, even when starting from acidic and alkaline initial conditions, while planktonic cells cannot. We associate this behavior with dynamic interplay between acetate and acetoin biosynthesis and show that this mechanism is required to buffer against biofilm acidification. Furthermore, we find that buffering-deficient biofilms exhibit dysregulated biofilm development when grown in minimally buffered conditions. Our findings reveal an active pH regulation mechanism in B. subtilis biofilms that could lead to new targets to control unwanted biofilm growth.IMPORTANCEpH is known to influence microbial growth and community dynamics in multiple bacterial species and environmental contexts. Furthermore, in many bacterial species, rapid cellular proliferation demands the use of overflow metabolism, which can often result in excessive acidification. However, in the case of bacterial communities known as biofilms, these acidification challenges can be masked when buffered laboratory media are employed to stabilize the pH environment for optimal growth. Our study reveals that B. subtilis biofilms use an active pH regulation mechanism to mitigate both growth-associated acidification and external pH challenges. This discovery provides new opportunities for understanding microbial communities and could lead to new methods for controlling biofilm growth outside of buffered laboratory conditions.

Intracytoplasmic membranes develop in Geobacter sulfurreducens under thermodynamically limiting conditions

Geobacter sulfurreducens is an electroactive bacterium capable of reducing metal oxides in the environment and electrodes in engineered systems1,2. Geobacter sp. are the keystone organisms in electrogenic biofilms, as their respiration consumes fermentation products produced by other organisms and reduces a terminal electron acceptor e.g. iron oxide or an electrode. To respire extracellular electron acceptors with a wide range of redox potentials, G. sulfurreducens has a complex network of respiratory proteins, many of which are membrane-bound3-5. We have identified intracytoplasmic membrane (ICM) structures in G. sulfurreducens. This ICM is an invagination of the inner membrane that has folded and organized by an unknown mechanism, often but not always located near the tip of a cell. Using confocal microscopy, we can identify that at least half of the cells contain an ICM when grown on low potential anode surfaces, whereas cells grown at higher potential anode surfaces or using fumarate as electron acceptor had significantly lower ICM frequency. 3D models developed from cryo-electron tomograms show the ICM to be a continuous extension of the inner membrane in contact with the cytoplasmic and periplasmic space. The differential abundance of ICM in cells grown under different thermodynamic conditions supports the hypothesis that it is an adaptation to limited energy availability, as an increase in membrane-bound respiratory proteins could increase electron flux. Thus, the ICM provides extra inner-membrane surface to increase the abundance of these proteins. G. sulfurreducens is the first Thermodesulfobacterium or metal-oxide reducer found to produce ICMs.

Enhancement of Cr(VI) reduction by indigenous bacterial consortia using natural pyrite: A detailed study to elucidate the mechanisms involved in the highly efficient and possible sustainable system

Pyrite was applied to Cr(VI) bioremediation as an inorganic electron donor due to the ability to provide electrons, while the role of pyrite in Cr(VI) bioremediation where organics as electron donors remains unknown. Herein a pyrite-based Cr(VI) bioreduction process in the sediment system containing lactate was demonstrated to be effective to detoxify Cr(VI): over 2200 mg L^-1 Cr(VI) was continuously removed within 210 h with high reactivity (10.5 mg/(L·h)) all along. High-throughput 16S rDNA gene sequencing indicated that the pyrite could shape a functioning community that electrochemically active bacteria dominated (such as Fusibacter sp. and Rhodobacteraceae) instead of iron-oxidizing bacteria and sulfur-oxidizing bacteria. Mineralogy analysis results indicated that Fe(III), S₂^2- and S⁰ formed on the pyrite surface after the oxidation of Cr(VI) might serve as the electron acceptor of microflora, then the S^2- and Fe(II) with strong Cr(VI) reduction ability were formed by microbial reduction to enhance the removal of Cr(VI). This study provides new insights into thoroughly understanding the role of pyrite in the practical application of Cr(VI) bioreduction.

Mechanism of stable power generation and nitrogen removal in the ANAMMOX-MFC treating low C/N wastewater

This study investigated the mechanism of enhanced power generation and nitrogen removal in an ANAMMOX-MFC reactor through subsequent acetate addition. Data showed that nearly 99% total nitrogen removal (≤1 mg L^-1) and 1.41 W m^-3 power generation were achieved synchronously under low COD/N (∼1.5) after the subsequent addition of acetate (100 mgCOD·L^-1). The columbic efficiency of the system has increased by 15 times (from 0.64% to 9.48%) after adding acetate. Batch tests showed that the denitrification and ANAMMOX progress occurred synchronously before acetate addition the nitrogen removal rate was accelerated. A distinct shift of bacterial community driven by acetate addition was discovered. The high throughput sequencing analysis indicated acetate addition stimulated the enrichment of denitrifiers, such as Aquimonas, Bradyrhizobium, Thauera, and the potential exoelectrogens changing from Comamonas to Pseudomonas. Functional genes forecasts based on KEGG database and COG database showed that the expressions of TCA functional genes were highly promoted in ANAMMOX-MFC, which demonstrated the enhanced electron transfer pathway driven by acetate addition under low COD/N ratio.

Defined and unknown roles of conductive nanoparticles for the enhancement of microbial current generation: A review

The ability of various bacteria to make use of solid substrates through extracellular electron transfer (EET) or extracellular electron uptake (EEU) has enabled the development of valuable biotechnologies such as microbial fuel cells (MFCs) and microbial electrosynthesis (MES). It is common practice to use metallic and semiconductive nanoparticles (NPs) for microbial current enhancement. However, the effect of NPs is highly variable between systems, and there is no clear guideline for effectively increasing the current generation. In the present review, the proposed mechanisms for enhancing current production in MFCs and MES are summarized, and the critical factors for NPs to enhance microbial current generation are discussed. Implications for microbially induced iron corrosion, where iron sulfide NPs are proposed to enhance the rate of EEU, photochemically driven MES, and several future research directions to further enhance microbial current generation, are also discussed.

Electrografted anthraquinone to monitor pH at the biofilm-anode interface in a wastewater microbial fuel cell

Electrografted anthraquinone on graphite was used as a probe to monitor the pH change at the biofilm-electrode interface at the anode of a microbial fuel cell inoculated with wastewater. The grafting procedure was optimized so that the pH-dependent electrochemical response of the grafted quinone did not overlay with that of the electroactive biofilm. The variation of the formal potential of the grafted quinone as a function of pH was linear over the pH range 1-10 with a slope of - 64 mV. This allowed to monitor the interfacial pH change over three weeks of biofilm colonization of the electrode. During that time the interfacial pH decreased from neutrality to 5.3 while the anolyte only acidified down to pH 6.2. This finding is relevant as local pH change usually leads to alterations of the bioenergetics process of microbial communities and hence on the performance of bioelectrochemical devices.

Progress in microbial biomass conversion into green energy

The increase in global population size over the past 100 decades has doubled the requirements for energy resources. To mitigate the limited fossil fuel available, new clean energy sources being environmental sustainable for replacement of traditional energy sources are explored to supplement the current scarcity. Biomass containing lignin and cellulose is the main raw material to replace fossil energy given its abundance and lower emission of greenhouse gases and NOx when transformed into energy. Bacteria, fungi and algae decompose lignocellulose leading to generation of hydrogen, methane, bioethanol and biodiesel being the clean energy used for heating, power generation and the automobile industry. Microbial Fuel Cell (MFC) uses microorganisms to decompose biomass in wastewater to generate electricity and remove heavy metals in wastewater. Biomass contains cellulose, hemicellulose, lignin and other biomacromolecules which need hydrolyzation for conversion into small molecules by corresponding enzymes in order to be utilized by microorganisms. This paper discusses microbial decomposition of biomass into clean energy and the five major ways of clean energy production, and its economic benefits for future renewable energy security.

Atomic and Electronic Structure of Pilus from Geobacter sulfurreducens through QM/MM Calculations: Evidence for Hole Transfer in Aromatic Residues

Long-range electron transport has been widely and experimentally reported in Geobacter sulfurreducens pilus protein. However, a better understanding of the still undefined molecular arrangement can bring to light the role of key residues in this phenomenon. We propose a theoretical investigation of the electronic structure of aromatic residue groups in the protein through a classical molecular dynamics (MD) simulation, followed by a quantum mechanics/molecular mechanics (QM/MM) electronic study of different frames sampled from MD trajectories, an electrostatic potential and electron density analysis, an analysis of the density of states, and an investigation of hole formation through Dyson orbital calculations. We observe a highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) energy gap in the ranges of 1.4-2.3 eV and 2.9-3.3 eV and a less intense dipole moment along the aromatic residues in the presence of water in comparison to the system in vacuum. HOMO and LUMO electron densities highlight the occupation of one tyrosine residue in every representation for HOMO and a delocalization along two to three rings for LUMO. The results show how the electronic structure of the aromatic residues is sensitive to the ring arrangement and the surrounding environment. In our study, we observe that slight rearrangements in the fiber geometry can create temporary conditions for hole transfer.

A modelling study of the spatially heterogeneous mutualism between electroactive biofilm and planktonic bacteria

Microbial cooperation widely exists in anaerobic reactors degrading complex pollutants, conventionally studied separately inside the biofilm or the planktonic community. Recent experiments discovered the mutualism between the planktonic bacteria and electroactive biofilm treating propionate, an end-product usually accumulated in anaerobic digesters. Here, a one-dimensional multispecies model found the preference on acetate-based pathway over the hydrogen-based in such community, evidenced by the fact that acetate-originated current takes 66% of the total value and acetate-consuming anode-respiring bacteria takes over 80% of the biofilm. Acetate-based anodic respiration most apparently influences biofilm function while propionate fermentation is the dominant planktonic bio-reaction. Additionally, initial planktonic propionate level shows the ability of coordinating the balance between these two extracellular electron transfer pathways. Increasing the propionate concentration from 2 to 50 mM would increase the steady hydrogen-originated current by 210% but decrease the acetate-originated by 26%, suggesting a vital influence of the planktonic microbial process to the metabolic balance in biofilm. Best strategy to promote the biofilm activity is to increase the biomass density and biofilm conductivity simultaneously, which would increase the current density by 875% without thickening the biofilm thickness or prolonging the growth apparently.

Crossing the Wall: Characterization of the Multiheme Cytochromes Involved in the Extracellular Electron Transfer Pathway of Thermincola ferriacetica

Bioelectrochemical systems (BES) are emerging as a suite of versatile sustainable technologies to produce electricity and added-value compounds from renewable and carbon-neutral sources using electroactive organisms. The incomplete knowledge on the molecular processes that allow electroactive organisms to exchange electrons with electrodes has prevented their real-world implementation. In this manuscript we investigate the extracellular electron transfer processes performed by the thermophilic Gram-positive bacteria belonging to the Thermincola genus, which were found to produce higher levels of current and tolerate higher temperatures in BES than mesophilic Gram-negative bacteria. In our study, three multiheme c-type cytochromes, Tfer_0070, Tfer_0075, and Tfer_1887, proposed to be involved in the extracellular electron transfer pathway of T. ferriacetica, were cloned and over-expressed in E. coli. Tfer_0070 (ImdcA) and Tfer_1887 (PdcA) were purified and biochemically characterized. The electrochemical characterization of these proteins supports a pathway of extracellular electron transfer via these two proteins. By contrast, Tfer_0075 (CwcA) could not be stabilized in solution, in agreement with its proposed insertion in the peptidoglycan wall. However, based on the homology with the outer-membrane cytochrome OmcS, a structural model for CwcA was developed, providing a molecular perspective into the mechanisms of electron transfer across the peptidoglycan layer in Thermincola.

Modeling of interspecies electron transfer in anaerobic microbial communities

Interspecies electron transfer (IET) is a key phenomenon in anaerobic ecosystems, which is traditionally modeled as hydrogen transfer. Recently discovered alternative mediated IET (MIET) or direct IET (DIET) offer exciting alternative mechanisms of microbial partnerships that could lead to new strategies for the improvement of biotechnologies. Here, we analyze mathematical modeling of DIET and MIET in anaerobic ecosystems. Bioenergetics approaches already enable the evaluation of different energy sharing scenarios between microorganisms and give interesting clues on redox mediators and on possible ways of driving microbial communities relying on IET. The modeling of DIET kinetics however is currently only in its infancy. Recent concepts introduced for the modeling of electroactive biofilms should be further exploited. Recent modeling examples confirms the potential of DIET to increase the IET rates compared to H₂-MIET, but also point out the need for additional characterizations of biological components supporting IET to improve predictions.

Effect of heterotrophic anodic denitrification on anolyte pH control and bioelectricity generation enhancement of bufferless microbial fuel cells

Heterotrophic anodic denitrification (HAD) in the single-chamber microbial fuel cell (MFC) is a promising nitrogen removal technology. In this paper, the benefit (anolyte pH increase) and challenge (substrate consumption) brought by the heterotrophic anodic denitrification process for the electricity generation of bufferless MFCs were studied for the first time. Substrate anaerobic hydrolysis dramatically decreased the anolyte pH to 5.1, which seriously restricted the electric power output of the Control. The anolyte pH of the heterotrophic anodic denitrification MFCs (HADMFCs) with 60 mg/L (HADMFC-60), 90 mg/L (HADMFC-90), and 120 mg/L (HADMFC-120) nitrate nitrogen (NO₃^--N), retained above 6.0, 6.5, and 6.8 in every running cycles, due to the protons (H⁺) consumption by nitrate reduction. In the HADMFC-60 and HADMFC-90, 17.6% and 26.1% of the total organic carbons (TOC) were used for the nitrate reduction, but their electric power output significantly increased. The maximum power densities of the HADMFC-60 and HADMFC-90 were 3.3 and 5.4 times higher than that of the Control. However, when the proportion of TOC consumption for nitrate reduction increased to 35.8%, substrate insufficiency became a serious limitation for the electricity generation. The P_max of the HADMFC-120 dramatically decreased to 17.3 mW/m². Dysgonomonas was the dominant electro-active genus, and Petrimonas, Acidovorax and Devosia appeared as the denitrifying bacteria genera.

Potential-dependent extracellular electron transfer pathways of exoelectrogens

Exoelectrogens are distinct from other bacteria owing to their unique extracellular electron transfer (EET) abilities that allow for anaerobic respiration with various external redox-active surfaces, including electrode and metal oxides. Although the EET process is known to trigger diverse extracellular redox reactions, the reverse impact has been long overlooked. Recent evidences show that exoelectrogens can sense the potential changes of external surfaces and alter their EET strategies accordingly, which imparts them remarkable abilities in adapting to diverse and redox-variable environment. This mini-review provides a condensed overview and critical analysis about the recent discoveries on redox-dependent EET pathways of exoelectrogens, with focus on Geobacter sulfurreducens and Shewanella oneidensis. We summarize the detailed responses of various EET components, analyze the drives and mechanisms of such responses, highlight the diversity of EET dynamics among different bacterial species and under integrated effects of redox potential and surface chemistry, and discusses the future research needs.

Bioenergy recovery from wastewater accelerated by solar power: Intermittent electro-driving regulation and capacitive storage in biomass

Electroactive microorganisms (EAMs) can act as pseudocapacitor to store energy and discharge electrons on need, while electromethanogens acting as receptor are able to utilize electrons, protons and carbon dioxide for methanization. However, external energy is required to overcome thermodynamical barriers for electromethanogenesis. Herein, electro-driving power by solar light was established to accelerate conversion of waste organics to bioenergy. The intermittent power supply modes were elucidated for favourable performances (e.g., current density, methane production rate, energy recovery efficiencies and economic evaluation), compared with the control driven by continuous applied voltage. It was found that natural intermittent solar-powered mode was more beneficial for microorganisms involved in electron transfer and energy recovery than manual sharp on-off mode. Electrochemistry analysis unrevealed that a higher redox current and lower resistance were exhibited under the solar-powered mode. A high charge storage capacity and electron mobility were found through cytochrome c content and live cells ratio in the solar-power assisted bioreactor. The intermittent power driving modes can regulate electron transfer proteins with capacitive storage behavior in biomass, which helps to understand the responses of functional communities on the stress of intermittent electric field. These findings indicate a promising perspective of microbial biotechnology driven by solar power to boost bioenergy recovery from waste/wastewater.

Effect of anaerobic sludge on the bioelectricity generation enhancement of bufferless single-chamber microbial fuel cells

Enhancing the self-buffering capacity is critical in the operation of bufferless microbial fuel cells (BLMFCs). Inorganic carbon (IC) is an ideal endogenous buffer, but its spontaneously accumulated concentration is insufficient to adjust anolyte pH. In this study, BLMFCs were operated with anaerobic sludge to enhance IC accumulation and increase anolyte pH. The accumulated IC concentration during a single running cycle was elevated from 8.3 mM to 12.5 mM, and anolyte pH remained above 7.5. The electric power output was significantly promoted from 332.2 mW·m^-2 to 628.1 mW·m^-2, and the coulombic efficiency (CE) slightly increased from 16.4% to 19.5%. Geobacter was the electro-active genus in the anode biofilms of the MFCs, and its relative abundance in the KCl-S anode biofilm increased from 0.2% to 5.75%. After continuous operation, the predominant genus of the anaerobic sludge had changed from Flavobacterium to Fusibacter.

Effect of pore size on the current produced by 3-dimensional porous microbial anodes: A critical review

Microbial anodes are the cornerstone of most electro-microbial processes. Designing 3-dimensional porous electrodes to increase the surface area of the electroactive biofilm they support is a key challenge in order to boost their performance. In this context, the critical review presented here aims to assess whether an optimal range of pore size may exist for the design of microbial anodes. Pore sizes of a few micrometres can enable microbial cells to penetrate but in conditions that do not favour efficient development of electroactive biofilms. Pores of a few tens of micrometres are subject to clogging. Sizes of a few hundreds of micrometres allow penetration of the biofilm inside the structure, but its development is limited by internal acidification. Consequently, pore sizes of a millimetre or so appear to be the most suitable. In addition, a simple theoretical approach is described to establish basis for porous microbial anode design.

How Thermophilic Gram-Positive Organisms Perform Extracellular Electron Transfer: Characterization of the Cell Surface Terminal Reductase OcwA

Thermophilic Gram-positive organisms were recently shown to be a promising class of organisms to be used in bioelectrochemical systems for the production of electrical energy. These organisms present a thick peptidoglycan layer that was thought to preclude them to perform extracellular electron transfer (i.e., exchange catabolic electrons with solid electron acceptors outside the cell). In this paper, we describe the structure and functional mechanisms of the multiheme cytochrome OcwA, the terminal reductase of the Gram-positive bacterium Thermincola potens JR found at the cell surface of this organism. The results presented here show that this protein can take the role of a respiratory “Swiss Army knife,” allowing this organism to grow in environments with soluble and insoluble substrates. Moreover, it is shown that it is unrelated to terminal reductases found at the cell surface of other electroactive organisms. Instead, OcwA is similar to terminal reductases of soluble electron acceptors. Our data reveal that terminal oxidoreductases of soluble and insoluble substrates are evolutionarily related, providing novel insights into the evolutionary pathway of multiheme cytochromes.

Extracellular electron transfer is the key process underpinning the development of bioelectrochemical systems for the production of energy or added-value compounds. Thermincola potens JR is a promising Gram-positive bacterium to be used in these systems because it is thermophilic. In this paper, we describe the structural and functional properties of the nonaheme cytochrome OcwA, which is the terminal reductase of this organism. The structure of OcwA, determined at 2.2-Å resolution, shows that the overall fold and organization of the hemes are not related to other metal reductases and instead are similar to those of multiheme cytochromes involved in the biogeochemical cycles of nitrogen and sulfur. We show that, in addition to solid electron acceptors, OcwA can also reduce soluble electron shuttles and oxyanions. These data reveal that OcwA can work as a multipurpose respiratory enzyme allowing this organism to grow in environments with rapidly changing availability of terminal electron acceptors without the need for transcriptional regulation and protein synthesis.

Endogenous inorganic carbon buffers accumulation and self-buffering capacity enhancement of air-cathode microbial fuel cells through anolyte recycling

Anolyte acidification is inevitable in the operation of buffer-free microbial fuel cells (MFCs), which restricts the proliferation and metabolism of electroactive bacteria, and results in electric-power deterioration. The anodic metabolic end-products, inorganic carbons (IC), which are composed of H₂CO₃ (dissolved CO₂), HCO₃^-, and CO₃^2-, are ideal endogenous buffers, whereas the naturally accumulated IC are far from enough to prevent anolyte acidification. In this work, different volume ratios of the anolytes (10%, 30%, and 50%) were recycled to increase the IC concentrations of the single-chamber air-cathode buffer-free MFCs. Under anolyte recycling running mode, IC accumulation agreed with the SGompertz model and the fitting IC-asymptotic concentrations (IC_AC) grew exponentially to 18.5 mM, 24.4 mM, and 32.8 mM as the anolyte recycling ratio increased from 10% to 30% and 50%. Self-buffering running can be realized when the anolyte recycling ratio exceeds 50% for the MFC feeding on 1 g·L^-1 of acetate. The electric power for the 50% recycling scenario increased from the baseline control of 272.4 mW·m^-2 to 628.5 mW·m^-2. The coulombic efficiency (CE) was also apparently improved. This paper for the first time clarifies the accumulation law of endogenous IC buffers under anolyte partially recycling mode and their self-buffering effects.

Geometrical Changes in the Hemes of Bacterial Surface c-Type Cytochromes Reveal Flexibility in Their Binding Affinity with Minerals

Microbial extracellular electron transport occurs via the physical and electrical association of outer-membrane c-type cytochromes (OM c-Cyts) with extracellular solid surfaces. However, studies investigating the characteristics of cytochrome binding with solid materials have been limited to the use of purified units of OM c-Cyts dissolved in solution, rather than OM c-Cyts in intact cells, because of the lack of a methodology that specifically allows for the monitoring of OM c-Cyts in whole-cells. Here, we utilized circular dichroism (CD) spectroscopy to examine the molecular mechanisms and binding characteristics of the interaction between MtrC, a unit of OM c-Cyts, in whole Shewanella oneidensis MR-1 cells and hematite nanoparticles. The addition of hematite nanoparticles significantly decreased the intensity of the Soret CD peaks, indicating geometrical changes in the hemes in MtrC associated with their physical contact with hematite. The binding affinity of MtrC estimated using CD spectra changed predominantly depending upon the redox state of MtrC and the concentration of the hematite nanoparticles. In contrast, purified MtrC demonstrated a constant binding affinity following a Langmuir isotherm, with a standard Gibbs free energy of -43 kJ mol^-1, suggesting that the flexibility in the binding affinity of MtrC with hematite was specific in membrane-bound protein complex conditions. Overall, these findings suggest that the binding affinity as well as the heme geometry of OM c-Cyts are flexibly modulated in the membrane complex associated with microbe-mineral interactions.

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.

Thermophiles; or, the Modern Prometheus: The Importance of Extreme Microorganisms for Understanding and Applying Extracellular Electron Transfer

Approximately four billion years ago, the first microorganisms to thrive on earth were anaerobic chemoautotrophic thermophiles, a specific group of extremophiles that survive and operate at temperatures ∼50 - 125°C and do not use molecular oxygen (O2) for respiration. Instead, these microorganisms performed respiration via dissimilatory metal reduction by transferring their electrons extracellularly to insoluble electron acceptors. Genetic evidence suggests that Gram-positive thermophilic bacteria capable of extracellular electron transfer (EET) are positioned close to the root of the Bacteria kingdom on the tree of life. On the contrary, EET in Gram-negative mesophilic bacteria is a relatively new phenomenon that is evolutionarily distinct from Gram-positive bacteria. This suggests that EET evolved separately in Gram-positive thermophiles and Gram-negative mesophiles, and that EET in these bacterial types is a result of a convergent evolutionary process leading to homoplasy. Thus, the study of dissimilatory metal reducing thermophiles provides a glimpse into some of Earth's earliest forms of respiration. This will provide new insights for understanding biogeochemistry and the development of early Earth in addition to providing unique avenues for exploration and discovery in astrobiology. Lastly, the physiological composition of Gram-positive thermophiles, coupled with the kinetic and thermodynamic consequences of surviving at elevated temperatures, makes them ideal candidates for developing new mathematical models and designing innovative next-generation biotechnologies. KEY CONCEPTS Anaerobe: organism that does not require oxygen for growth. Chemoautotroph: organism that obtains energy by oxidizing inorganic electron donors. Convergent Evolution: process in which organisms which are not closely related independently evolve similar traits due to adapting to similar ecological niches and/or environments. Dissimilatory Metal Reduction: reduction of a metal or metalloid that uses electrons from oxidized organic or inorganic electron donors. Exoelectrogen: microorganism that performs dissimilatory metal reduction via extracellular electron transfer. Extremophiles: organisms that thrive in physical or geochemical conditions that are considered detrimental to most life on Earth. Homoplasy: a character shared by a set of species that is not shared by a common ancestor Non-synonymous Substitutions (K a ): a substitution of a nucleotide that changes a codon sequence resulting in a change in the amino acid sequence of a protein. Synonymous Substitutions (K s ): a substitution of a nucleotide that may change a codon sequence, but results in no change in the amino acid sequence of a protein. Thermophiles: a specific group of extremophiles that survive and operate at temperatures ∼50-125°C.