Electron transfer process in microbial electrochemical technologies: The role of cell-surface exposed conductive proteins

Inhibition of zinc ions in sulfur-driven autotrophic denitrification process: What is the behavior of extracellular polymeric substances?

Sulfur-driven autotrophic denitrification (SAD) process is a cost-effective and sustainable method for nitrogen removal from wastewater. However, a higher concentration of zinc ions (Zn(II)) flowing into wastewater treatment plants poses a potential threat to the SAD process. This study examined that a half maximal inhibitory concentration (IC50) of Zn(II) was 7 mg·L-1 in the SAD process. Additionally, the addition of 20 mg·L-1 Zn(II) resulted in a severe accumulation of nitrite to 150.20 ± 6.00 mg·L-1 when the initial concentration of nitrate was 500 mg·L-1. Moreover, the activities of nitrate reductase, nitrite reductase, dehydrogenase and electron transport system were significantly inhibited under Zn(II) stress. The addition of Zn(II) inhibited EPS secretion and worsened electrochemical properties. The result was attributed to the spontaneous binding between EPS and Zn(II), with a ΔG of -17.50 KJ·mol-1 and a binding constant of 1.77 × 104 M-1, respectively. Meanwhile, the protein, fulvic acid, and humic-like substances occurred static quenching after Zn(II) addition, with -OH and -C=O groups providing binding sites. The binding sequence was fulvic acid→protein→humic acid and -OH → -C=O. Zn(II) also reduced the content of α-helix, which was unfavorable for electron transfer. Additionally, the Zn(II) loosened protein structure, resulting in a 50 % decrease in α-helix/(β-sheet+random coil). This study reveals the effect of Zn(II) on the SAD process and enhances our understanding of EPS behavior under metal ions stress.

Integrated tea polyphenols and polydopamine functionalized graphene anode for improved bioelectricity generation and Cr(VI) reduction in microbial fuel cells

Microbial fuel cells (MFCs) have the dual advantage of mitigating Cr(Ⅵ) wastewater ecological threats while generating electricity. However, the low electron transfer efficiency and the limited enrichment of active electrogens are barriers to MFCs advancement. This study describes the synthesis of the TP-PDA-RGO@CC negative electrode using tea polyphenol as a reducing agent and polydopamine-doped graphene, significantly enhances the roughness and hydrophilicity of the anode. The charge transfer resistance was reduced by 94%, and the peak MFC power was 1375.80 mW m-2. Under acidic conditions, the Cr(Ⅵ) reduction rate reached 92% within 24 h, with a 52% increase in coulombic efficiency. Biodiversity analysis shows that the TP-PDA-RGO@CC anode could enrich electrogens, thereby boosting the electron generation mechanism at the anode and enhancing the reduction efficiency of Cr(Ⅵ) in the cathode chamber. This work emphasizes high-performance anode materials for efficient pollutant removal, energy conversion, and biomass reuse.

Evidence for autotrophic growth of purple sulfur bacteria using pyrite as electron and sulfur source

Purple sulfur bacteria (PSB) are capable of anoxygenic photosynthesis via oxidizing reduced sulfur compounds and are considered key drivers of the sulfur cycle in a range of anoxic environments. In this study, we show that Allochromatium vinosum (a PSB species) is capable of autotrophic growth using pyrite as the electron and sulfur source. Comparative growth profile, substrate characterization, and transcriptomic sequencing data provided valuable insight into the molecular mechanisms underlying the bacterial utilization of pyrite and autotrophic growth. Specifically, the pyrite-supported cell cultures ("py"') demonstrated robust but much slower growth rates and distinct patterns from their sodium sulfide-amended positive controls. Up to ~200-fold upregulation of genes encoding various c- and b-type cytochromes was observed in "py," pointing to the high relevance of these molecules in scavenging and relaying electrons from pyrite to cytoplasmic metabolisms. Conversely, extensive downregulation of genes related to LH and RC complex components indicates that the electron source may have direct control over the bacterial cells' photosynthetic activity. In terms of sulfur metabolism, genes encoding periplasmic or membrane-bound proteins (e.g., FccAB and SoxYZ) were largely upregulated, whereas those encoding cytoplasmic proteins (e.g., Dsr and Apr groups) are extensively suppressed. Other notable differentially expressed genes are related to flagella/fimbriae/pilin(+), metal efflux(+), ferrienterochelin(-), and [NiFe] hydrogenases(+). Characterization of the biologically reacted pyrite indicates the presence of polymeric sulfur. These results have, for the first time, put the interplay of PSB and transition metal sulfide chemistry under the spotlight, with the potential to advance multiple fields, including metal and sulfur biogeochemistry, bacterial extracellular electron transfer, and artificial photosynthesis.

IMPORTANCE

Microbial utilization of solid-phase substrates constitutes a critical area of focus in environmental microbiology, offering valuable insights into microbial metabolic processes and adaptability. Recent advancements in this field have profoundly deepened our knowledge of microbial physiology pertinent to these scenarios and spurred innovations in biosynthesis and energy production. Furthermore, research into interactions between microbes and solid-phase substrates has directly linked microbial activities to the surrounding mineralogical environments, thereby enhancing our understanding of the relevant biogeochemical cycles. Our study represents a significant step forward in this field by demonstrating, for the first time, the autotrophic growth of purple sulfur bacteria using insoluble pyrite (FeS2) as both the electron and sulfur source. The presented comparative growth profiles, substrate characterizations, and transcriptomic sequencing data shed light on the relationships between electron donor types, photosynthetic reaction center activities, and potential extracellular electron transfer in these organisms capable of anoxygenic photosynthesis. Furthermore, the findings of our study may provide new insights into early-Earth biogeochemical evolutions, offering valuable constraints for understanding the environmental conditions and microbial processes that shaped our planet's history.

Ex-situ electrochemical characterisation of fixed-bed denitrification biocathodes: A promising strategy to improve bioelectrochemical denitrification

The worldwide issue of nitrate-contaminated groundwater requires practical solutions, and electro-bioremediation offers a promising and sustainable treatment. While it has shown potential benefits, there is room for improvement in treatment rates, which is crucial for its further and effective implementation. In this field, electrochemical characterisation is a valuable tool for providing the foundation for optimising bioelectrochemical reactors, but applying it in fixed-bed reactors is challenging due to its high intrinsic electrical resistance. To overcome these challenges, this study employed the easy and swift eClamp methodology to screen different process parameters and their influence on the performance of fixed-bed denitrifying biocathodes composed of granular graphite. Granules were extracted and studied ex-situ under controlled conditions while varying key operational parameters (such as pH, temperature, and nitrate concentration). In the studied biocathode, the extracellular electron transfer associated with denitrification was identified as the primary limiting step with a formal potential of -0.225 ± 0.007 V vs. Ag/AgCl sat. KCl at pH 7 and 25 °C. By varying the nitrate concentration, it was revealed that the biocathode exhibits a strong affinity for nitrate (KMapp of 0.7 ± 0.2 mg N-NO3- L-1). The maximum denitrification rate was observed at a pH of 6 and a temperature of 35 °C. Furthermore, the findings highlight a 2e-/1H+ transfer, which holds considerable implications for the energy metabolism of bioelectrochemical denitrifiers. These compiled results provide valuable insights into the understanding of denitrifying biocathodes and enable the improvement and prediction of their performance.

Electron Transfer Mechanism at the Interface of Multi-Heme Cytochromes and Metal Oxide

Electroactive microbial cells have evolved unique extracellular electron transfer to conduct the reactions via redox outer-membrane (OM) proteins. However, the electron transfer mechanism at the interface of OM proteins and nanomaterial remains unclear. In this study, the mechanism for the electron transfer at biological/inorganic interface is investigated by integrating molecular modeling with electrochemical and spectroscopic measurements. For this purpose, a model system composed of OmcA, a typical OM protein, and the hexagonal tungsten trioxide (h-WO3 ) with good biocompatibility is selected. The interfacial electron transfer is dependent mainly on the special molecular configuration of OmcA and the microenvironment of the solvent exposed active center. Also, the apparent electron transfer rate can be tuned by site-directed mutagenesis at the axial ligand of the active center. Furthermore, the equilibrium state of the OmcA/h-WO3 systems suggests that their attachment is attributed to the limited number of residues. The electrochemical analysis of OmcA and its variants reveals that the wild type exhibits the fastest electron transfer rate, and the transient absorption spectroscopy further shows that the axial histidine plays an important role in the interfacial electron transfer process. This study provides a useful approach to promote the site-directed mutagenesis and nanomaterial design for bioelectrocatalytic applications.

Protein Interactions in Rhodopseudomonas palustris TIE-1 Reveal the Molecular Basis for Resilient Photoferrotrophic Iron Oxidation

Rhodopseudomonas palustris is an alphaproteobacterium with impressive metabolic versatility, capable of oxidizing ferrous iron to fix carbon dioxide using light energy. Photoferrotrophic iron oxidation is one of the most ancient metabolisms, sustained by the pio operon coding for three proteins: PioB and PioA, which form an outer-membrane porin-cytochrome complex that oxidizes iron outside of the cell and transfers the electrons to the periplasmic high potential iron-sulfur protein (HIPIP) PioC, which delivers them to the light-harvesting reaction center (LH-RC). Previous studies have shown that PioA deletion is the most detrimental for iron oxidation, while, the deletion of PioC resulted in only a partial loss. The expression of another periplasmic HiPIP, designated Rpal_4085, is strongly upregulated in photoferrotrophic conditions, making it a strong candidate for a PioC substitute. However, it is unable to reduce the LH-RC. In this work we used NMR spectroscopy to map the interactions between PioC, PioA, and the LH-RC, identifying the key amino acid residues involved. We also observed that PioA directly reduces the LH-RC, and this is the most likely substitute upon PioC deletion. By contrast, Rpal_4085 demontrated significant electronic and structural differences from PioC. These differences likely explain its inability to reduce the LH-RC and highlight its distinct functional role. Overall, this work reveals the functional resilience of the pio operon pathway and further highlights the use of paramagnetic NMR for understanding key biological processes.

In Situ Enhanced Yields of Microbial Nanowires: The Key Role of Environmental Stress

The conductive microbial nanowires of Geobacter sulfurreducens serve as a model for long-range extracellular electron transfer (EET), which is considered a revolutionary "green" nanomaterial in the fields of bioelectronics, renewable energy, and bioremediation. However, there is no efficient pathway to induce microorganisms to express a large amount of microbial nanowires. Here, several strategies have been used to successfully induce the expression of microbial nanowires. Microbial nanowire expression was closely related to the concentration of electron acceptors. The microbial nanowire was around 17.02 μm in length, more than 3 times compared to its own length. The graphite electrode was used as an alternative electron acceptor by G. sulfurreducens, which obtained a fast start-up time of 44 h in microbial fuel cells (MFCs). Meanwhile, Fe(III) citrate-coated sugarcane carbon and biochar were prepared to test the applicability of these strategies in the actual microbial community. The unsatisfied EET efficiency between c-type cytochrome and extracellular insoluble electron receptors promoted the expression of microbial nanowires. Hence, microbial nanowires were proposed to be an effective survival strategy for G. sulfurreducens to cope with various environmental stresses. Based on this top-down strategy of artificially constructed microbial environmental stress, this study is of great significance for exploring more efficient methods to induce microbial nanowires expression.

Moving towards the enhancement of extracellular electron transfer in electrogens

Electrogens are very common in nature and becoming a contemporary theme for research as they can be exploited for extracellular electron transfer. Extracellular electron transfer is the key mechanism behind bioelectricity generation and bioremediation of pollutants via microbes. Extracellular electron transfer mechanisms for electrogens other than Shewanella and Geobacter are less explored. An efficient extracellular electron transfer system is crucial for the sustainable future of bioelectrochemical systems. At present, the poor extracellular electron transfer efficiency remains a decisive factor in limiting the development of efficient bioelectrochemical systems. In this review article, the EET mechanisms in different electrogens (bacteria and yeast) have been focused. Apart from the well-known electron transfer mechanisms of Shewanella oneidensis and Geobacter metallireducens, a brief introduction of the EET pathway in Rhodopseudomonas palustris TIE-1, Sideroxydans lithotrophicus ES-1, Thermincola potens JR, Lysinibacillus varians GY32, Carboxydothermus ferrireducens, Enterococcus faecalis and Saccharomyces cerevisiae have been included. In addition to this, the article discusses the several approaches to anode modification and genetic engineering that may be used in order to increase the rate of extracellular electron transfer. In the side lines, this review includes the engagement of the electrogens for different applications followed by the future perspective of efficient extracellular electron transfer.

Extracellular Fe(III) reductase structure reveals a modular organization enabling S-layer insertion and electron transfer to insoluble substrates

The thermophilic anaerobic Gram-positive bacterium Carboxydothermus ferrireducens utilizes insoluble Fe(III) oxides as electron acceptors in respiratory processes using an extracellular 11-heme cytochrome c OmhA as a terminal reductase. OmhA is able to transfer electrons to soluble and insoluble Fe(III) compounds, substrates of multiheme oxidoreductases, and soluble electron shuttles. The crystal structure of OmhA at 2.5 Å resolution shows that it consists of two functionally distinct parts: the cytochrome с electron transfer and the S-layer binding domains. Nonaheme C-terminal subdomain of the cytochrome с domain is structurally similar to the extracellular multiheme cytochrome OcwA from the metal-reducing Gram-positive bacterium "Thermincola potens." S-layer binding domain of OmhA is responsible for interaction with the S-layer that surrounds the Carboxydothermus ferrireducens cell envelope. The structural foundations enabling the embedding of extracellular multiheme cytochromes to the S-layer of a Gram-positive-type cell wall and putative electron transfer pathways to insoluble minerals are discussed.

Biochar Facilitated Direct Interspecies Electron Transfer in Anaerobic Digestion to Alleviate Antibiotics Inhibition and Enhance Methanogenesis: A Review

Efficient conversion of organic waste into low-carbon biofuels such as methane through anaerobic digestion (AD) is a promising technology to alleviate energy shortages. However, issues such as inefficient methane production and poor system stability remain for AD technology. Biochar-facilitated direct interspecies electron transfer (DIET) has recently been recognized as an important strategy to improve AD performance. Nonetheless, the underlying mechanisms of biochar-facilitated DIET are still largely unknown. For this reason, this review evaluated the role of biochar-facilitated DIET mechanism in enhancing AD performance. First, the evolution of DIET was introduced. Then, applications of biochar-facilitated DIET for alleviating antibiotic inhibition and enhancing methanogenesis were summarized. Next, the electrochemical mechanism of biochar-facilitated DIET including electrical conductivity, redox-active characteristics, and electron transfer system activity was discussed. It can be concluded that biochar increased the abundance of potential DIET microorganisms, facilitated microbial aggregation, and regulated DIET-associated gene expression as a microbial mechanism. Finally, we also discussed the challenges of biochar in practical application. This review elucidated the role of DIET facilitated by biochar in the AD system, which would advance our understanding of the DIET mechanism underpinning the interaction of biochar and anaerobic microorganisms. However, direct evidence for the occurrence of biochar-facilitated DIET still requires further investigation.

Efficient bioelectricity generation and carbazole biodegradation using an electrochemically active bacterium Sphingobium yanoikuyae XLDN2-5

Carbazole and its derivatives are polycyclic aromatic heterocycles with unusual toxicity and mutagenicity. However, disposal of these polycyclic aromatic heterocycles remains a significant challenge. This study focused on efficient resource recovery from carbazole using an obligate aerobe, Sphingobium yanoikuyae XLDN2-5, in microbial fuel cells (MFCs). S. yanoikuyae XLDN2-5 successfully achieved carbazole degradation and simultaneously electricity generation in MFCs with a maximum power density of 496.8 mW m^-2 and carbazole degradation rate of 100%. It is the first time that S. yanoikuyae XLDN2-5 was discovered as an electrochemically active bacterium with high extracellular electron transfer (EET) capability. Redox mediator analysis indicated that no self-produced redox mediators were found for S. yanoikuyae XLDN2-5 under analysis conditions, and the exogenous redox mediators used in this study did not promote its EET. The nanowires produced by S. yanoikuyae XLDN2-5 cells were found in the biofilm by morphology characterization and the growth process of the nanowires was consistent with the discharge process of the MFC. Conductivity determination further verified that the nanowires produced by S. yanoikuyae XLDN2-5 cells were electrically conductive. Based on these results, it is speculated that S. yanoikuyae XLDN2-5 may mainly utilize conductive nanowires produced by itself rather than redox mediators to meet the requirements of normal energy metabolism when it grows in the low dissolved oxygen zone of the anodic biofilm. These novel findings on the EET mechanism of S. yanoikuyae XLDN2-5 lay a foundation for further exploration of polycyclic aromatic heterocyclic pollutants treatment in electrochemical devices, which may create new biotechnology processes for these pollutants control.

Recent advances in electro-fermentation technology: A novel approach towards balanced fermentation

Biotransformation of organic substrates via acidogenic fermentation (AF) to high-value products such as C1-C6 carboxylic acids and alcohol serves as platform chemicals for various industrial applications. However, the AF technology suffers from low product titers due to thermodynamic constraints. Recent studies suggest that augmenting AF redox potential can regulate the metabolic pathway and provide seamless electron flow by lowering the activation energy barrier, thus positively influencing the substrate utilization rate, product yield, and speciation. Hence, the augmented AF system with an exogenous electricity supply is termed as electro-fermentation (EF), which has enormous potential to strengthen the fermentation technology domain. Therefore, this critical review systematically discusses the current understanding of EF with a special focus on the extracellular electron transfer mechanism of electroactive bacteria and provides perspectives and research gaps to further improve the technology for green chemical synthesis, sustainable waste management, and circular bio-economy.

Improvement of Direct Interspecies Electron Transfer via Adding Conductive Materials in Anaerobic Digestion: Mechanisms, Performances, and Challenges

Anaerobic digestion is an effective and sustainable technology for resource utilization of organic wastes. Recently, adding conductive materials in anaerobic digestion to promote direct interspecies electron transfer (DIET) has become a hot topic, which enhances the syntrophic conversion of various organics to methane. This review comprehensively summarizes the recent findings of DIET mechanisms with different mediating ways. Meanwhile, the influence of DIET on anaerobic digestion performance and the underlying mechanisms of how DIET mediated by conductive materials influences the lag phase, methane production, and system stability are systematically explored. Furthermore, current challenges such as the unclear biological mechanisms, influences of non-DIET mechanisms, limitations of organic matters syntrophically oxidized by way of DIET, and problems in practical application of DIET mediated by conductive materials are discussed in detail. Finally, the future research directions for practical application of DIET are outlined.

Let's chat: Communication between electroactive microorganisms

Electroactive microorganisms can exchange electrons with other cells or conductive interfaces in their extracellular environment. This property opens the way to a broad range of practical biotechnological applications, from manufacturing sustainable chemicals via electrosynthesis, to bioenergy, bioelectronics or improved, low-energy demanding wastewater treatments. Besides, electroactive microorganisms play key roles in environmental bioremediation, significantly impacting process efficiencies. This review highlights our present knowledge on microbial interactions promoting the communication between electroactive microorganisms in a biofilm on an electrode in bioelectrochemical systems (BES). Furthermore, the immediate knowledge gaps that must be closed to develop novel technologies will also be acknowledged.

Development of Whole Genome-Scale Base Editing Toolbox to Promote Efficiency of Extracellular Electron Transfer in Shewanella oneidensis MR-1

Shewanella oneidensis MR-1, as a model electroactive microorganism (EAM) for extracellular electron transfer (EET) study, plays a key role in advancing practical applications of bio-electrochemical systems (BES). Efficient genome-level manipulation tools are vital to promote EET efficiency; thus, a powerful and rapid base editing toolbox in S. oneidensis MR-1 is developed. Firstly a CRISPR/dCas9-AID base editor that shows a relatively narrow editing window restricted to the "-20 to -16" range upstream of the protospacer adjacent motif (PAM) is constructed. Cas9 is also confined by its native PAM requirement, NGG. Then to expand the editable scope, the sgRNA and the Cas-protein to broaden the editing window to "-22 to -9" upstream of the PAM are engineered, and the PAM field to NNN is opened up. Consequently, the coverage of the editable gene is expanded from 89% to nearly 100% in S. oneidensis MR-1. This whole genome-scale cytidine deaminase-based base editing toolbox (WGcBE) is applied to regulate the cell length and the biofilm morphology, which enhances the EET efficiency by 6.7-fold. WGcBE enables an efficient deactivation of genes with full genome coverage, which would contribute to the in-depth and multi-faceted EET study in Shewanella.

High current density via direct electron transfer by hyperthermophilic archaeon, Geoglobus acetivorans, in microbial electrolysis cells operated at 80 °C

Utilization of hyperthermophilic electro-active microorganisms in microbial electrolysis cells (MECs) that are used for hydrogen production from organic wastes offers significant advantages, such as increased reaction rate and enhanced degradation of insoluble materials. However, only a limited number of hyperthermophilic bioelectrochemical systems have been investigated so far. This study is the first to illustrate hydrogen production in hyperthermophilic MECs with a maximum rate of 0.57 ± 0.06 m³ H₂/m³d, where an iron reducing archaeon, Geoglobus acetivorans, was used as inoculum. In fact, this is the first study to report that G. acetivorans, as the fourth hyperthermophilic electro-active archaeon. In single chamber MECs operated at 80 °C with a set potential of 0.7 V, a peak current density of 1.53 ± 0.24 A/m² has been attained and this is the highest record of current produced by pure culture hyperthermophilic microorganisms. Turnover cyclic voltammetry curve illustrated a sigmoidal shape (midpoint of -0.40 V vs. Ag/AgCl), and together with linear relation of scan rate and peak anodic current, proves the biofilm attachment to the anode and its capability of direct electron transfer. Along with simple substrate (acetate), G. acetivorans effectively utilized dark fermentation effluent for hydrogen production in MECs.

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.

Empower C1: Combination of Electrochemistry and Biology to Convert C1 Compounds

The idea to somehow combine electrical current and biological systems is not new. It was subject of research as well as of science fiction literature for decades. Nowadays, in times of limited resources and the need to capture greenhouse gases like CO₂, this combination gains increasing interest, since it might allow to use C1 compounds and highly oxidized compounds as substrate for microbial production by "activating" them with additional electrons. In this chapter, different possibilities to combine electrochemistry and biology to convert C1 compounds into useful products will be discussed. The chapter first shows electrochemical conversion of C1 compounds, allowing the use of the product as substrate for a subsequent biosynthesis in uncoupled systems, further leads to coupled systems of biology and electrochemical conversion, and finally reaches the discipline of bioelectrosynthesis, where electrical current and C1 compounds are directly converted by microorganisms or enzymes. This overview will give an idea about the potentials and challenges of combining electrochemistry and biology to convert C1 molecules.

Core-shell ZVI@carbon composites reduce phosphate inhibition of ZVI dissolution and enhance methane production in an anaerobic sewage treatment

Inhibitory effects of phosphate on zero-valent iron (ZVI) dissolution have been studied mainly focusing on a single chemical system. Little is known about inhibitory effects and the mechanism of phosphate on ZVI dissolution within a bioreactor during long-term operation. This study demonstrates the feasibility of achieving energy recovery from phosphate-containing domestic sewage using an efficient anaerobic reactor with micro-sized or nano-sized ZVI addition. The results indicate that the chemical oxygen demand (COD) removal and methane production are enhanced by ZVI addition. A maximum COD removal efficiency of 89% and methane content of 60% was achieved. However, the strengthening effect of ZVI on methane production is weakened by the presence of phosphate in domestic sewage. Analyzing the variations of Fe²⁺ ions and phosphate concentrations and characterizing the micro-morphology of corroded ZVI proved that the generated Fe²⁺ ions reacts with phosphate and forms a passivation layer on the ZVI surface, inhibiting further dissolution of ZVI. As an improved alternative, we chose the double layered core-shell structured ZVI@carbon composite as an excellent candidate to reduce the inhibitory effects of phosphate on ZVI dissolution. In this way, the direct formation of precipitates on the ZVI surface can be avoided due to the protective carbon layer which adjusts the ion transfer. Adding ZVI@carbon composites accelerate the methane content by 16%. To our knowledge, this is the first report on adding ZVI@carbon composites to promote the anaerobic metabolism in studies, which are focusing on reducing the inhibition of ZVI by phosphate.

Microbial fuel cells: a comprehensive review for beginners

Microbial fuel cells (MFCs) have shown immense potential as a one-stop solution for three major sustainability issues confronting the world today-energy security, global warming and wastewater management. MFCs represent a cross-disciplinary platform for research at the confluence of the natural and engineering sciences. The diversity of variables influencing performance of MFCs has garnered research interest across varied scientific disciplines since the beginning of this century. The increasing number of research publications has made it necessary to keep track of work being carried out by research groups across the globe and consolidate significant findings on a regular basis. Review articles are often the nodal points for beginners who are required to undertake an exploratory survey of literature to identify a suitable research problem. This 'review of reviews' is a ready-reckoner that directs readers to relevant reviews and research articles reporting significant developments in MFC research in the last two decades. The article also highlights the areas needing research attention which when addressed could take this technology a few more steps closer to practical implementation.

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.

Microbial extracellular electron transfer and strategies for engineering electroactive microorganisms

Electroactive microorganisms (EAMs) are ubiquitous in nature and have attracted considerable attention as they can be used for energy recovery and environmental remediation via their extracellular electron transfer (EET) capabilities. Although the EET mechanisms of Shewanella and Geobacter have been rigorously investigated and are well characterized, much less is known about the EET mechanisms of other microorganisms. For EAMs, efficient EET is crucial for the sustainable economic development of bioelectrochemical systems (BESs). Currently, the low efficiency of EET remains a key factor in limiting the development of BESs. In this review, we focus on the EET mechanisms of different microorganisms, (i.e., bacteria, fungi, and archaea). In addition, we describe in detail three engineering strategies for improving the EET ability of EAMs: (1) enhancing transmembrane electron transport via cytochrome protein channels; (2) accelerating electron transport via electron shuttle synthesis and transmission; and (3) promoting the microbe-electrode interface reaction via regulating biofilm formation. At the end of this review, we look to the future, with an emphasis on the cross-disciplinary integration of systems biology and synthetic biology to build high-performance EAM systems.

Microbial electrosynthesis: Towards sustainable biorefineries for production of green chemicals from CO2 emissions

Decarbonisation of the economy has become a priority at the global level, and the resulting legislative pressure is pushing the chemical and energy industries away from fossil fuels. Microbial electrosynthesis (MES) has emerged as a promising technology to promote this transition, which will further benefit from the decreasing cost of renewable energy. However, several technological challenges need to be addressed before the MES technology can reach its maturity. The aim of this review is to critically discuss the bottlenecks hampering the industrial adoption of MES, considering the whole production process (from the CO₂ source to the marketable products), and indicate future directions. A flexible stack design, with flat or tubular MES modules and direct CO₂ supply, is required for site-specific decentralised applications. The experience gained for scaling-up electrochemical cells (e.g. electrolysers) can serve as a guideline for realising pilot MES stacks to be technologically and economically evaluated in industrially relevant conditions. Maximising CO₂ abatement rate by targeting high-rate production of acetate can promote adoption of MES technology in the short term. However, the development of a replicable and robust strategy for production and in-line extraction of higher-value products (e.g. caproic acid and hexanol) at the cathode, and meaningful exploitation of the currently overlooked anodic reactions, can further boost MES cost-effectiveness. Furthermore, the use of energy storage and smart electronics can alleviate the fluctuations of renewable energy supply. Despite the unresolved challenges, the flexible MES technology can be applied to decarbonise flue gas from different sources, to upgrade industrial and wastewater treatment plants, and to produce a wide array of green and sustainable chemicals. The combination of these benefits can support the industrial adoption of MES over competing technologies.

Multicellular and unicellular responses of microbial biofilms to stress

Biofilms are a ubiquitous mode of microbial life and display an increased tolerance to different stresses. Inside biofilms, cells may experience both externally applied stresses and internal stresses that emerge as a result of growth in spatially structured communities. In this review, we discuss the spatial scales of different stresses in the context of biofilms, and if cells in biofilms respond to these stresses as a collection of individual cells, or if there are multicellular properties associated with the response. Understanding the organizational level of stress responses in microbial communities can help to clarify multicellular functions of biofilms.

Accelerated Electro-Fermentation of Acetoin in Escherichia coli by Identifying Physiological Limitations of the Electron Transfer Kinetics and the Central Metabolism

Anode-assisted fermentations offer the benefit of an anoxic fermentation routine that can be applied to produce end-products with an oxidation state independent from the substrate. The whole cell biocatalyst transfers the surplus of electrons to an electrode that can be used as a non-depletable electron acceptor. So far, anode-assisted fermentations were shown to provide high carbon efficiencies but low space-time yields. This study aimed at increasing space-time yields of an Escherichia coli-based anode-assisted fermentation of glucose to acetoin. The experiments build on an obligate respiratory strain, that was advanced using selective adaptation and targeted strain development. Several transfers under respiratory conditions led to point mutations in the pfl, aceF and rpoC gene. These mutations increased anoxic growth by three-fold. Furthermore, overexpression of genes encoding a synthetic electron transport chain to methylene blue increased the electron transfer rate by 2.45-fold. Overall, these measures and a medium optimization increased the space-time yield in an electrode-assisted fermentation by 3.6-fold.

Electroactivity across the cell wall of Gram-positive bacteria

The growing interest on sustainable biotechnological processes for the production of energy and industrial relevant organic compounds have increased the discovery of electroactive organisms (i.e. organisms that are able to exchange electrons with an electrode) and the characterization of their extracellular electron transfer mechanisms. While most of the knowledge on extracellular electron transfer processes came from studies on Gram-negative bacteria, less is known about the processes performed by Gram-positive bacteria. In contrast to Gram-negative bacteria, Gram-positive bacteria lack an outer-membrane and contain a thick cell wall, which were thought to prevent extracellular electron transfer. However, in the last decade, an increased number of Gram-positive bacteria have been found to perform extracellular electron transfer, and exchange electrons with an electrode. In this mini-review the current knowledge on the extracellular electron transfer processes performed by Gram-positive bacteria is introduced, emphasising their electroactive role in bioelectrochemical systems. Also, the existent information of the molecular processes by which these bacteria exchange electrons with an electrode is highlighted. This understanding is fundamental to advance the implementation of these organisms in sustainable biotechnological processes, either through modification of the systems or through genetic engineering, where the organisms can be optimized to become better catalysts.

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.

His/Met heme ligation in the PioA outer membrane cytochrome enabling light-driven extracellular electron transfer by Rhodopseudomonas palustris TIE-1

A growing number of bacterial species are known to move electrons across their cell envelopes. Naturally this occurs in support of energy conservation and carbon-fixation. For biotechnology it allows electron exchange between bacteria and electrodes in microbial fuel cells and during microbial electrosynthesis. In this context Rhodopseudomonas palustris TIE-1 is of much interest. These bacteria respond to light by taking electrons from their external environment, including electrodes, to drive CO₂-fixation. The PioA cytochrome, that spans the bacterial outer membrane, is essential for this electron transfer and yet little is known about its structure and electron transfer properties. Here we reveal the ten c-type hemes of PioA are redox active across the window +250 to -400 mV versus Standard Hydrogen Electrode and that the hemes with most positive reduction potentials have His/Met and His/H₂O ligation. These chemical and redox properties distinguish PioA from the more widely studied family of MtrA outer membrane decaheme cytochromes with ten His/His ligated hemes. We predict a structure for PioA in which the hemes form a chain spanning the longest dimension of the protein, from Heme 1 to Heme 10. Hemes 2, 3 and 7 are identified as those most likely to have His/Met and/or His/H₂O ligation. Sequence analysis suggests His/Met ligation of Heme 2 and/or 7 is a defining feature of decaheme PioA homologs from over 30 different bacterial genera. His/Met ligation of Heme 3 appears to be less common and primarily associated with PioA homologs from purple non-sulphur bacteria belonging to the alphaproteobacteria class.

Exploring the Effects of bolA in Biofilm Formation and Current Generation by Shewanella oneidensis MR-1

Microbial electrochemical technologies (METs) have emerged in recent years as a promising alternative green source of energy, with microbes consuming organic matter to produce energy or valuable byproducts. It is the ability of performing extracellular electron transfer that allows these microbes to exchange electrons with an electrode in these systems. The low levels of current achieved have been the limiting factor for the large-scale application of METs. Shewanella oneidensis MR-1 is one of the most studied electroactive organisms regarding extracellular electron transfer, and it has been shown that biofilm formation is a key factor for current generation. The transcription factor bolA has been identified as a central player in biofilm formation in other organisms, with its overexpression leading to increased biofilm. In this work we explore the effect of this gene in biofilm formation and current production by S. oneidensis MR-1. Our results demonstrate that an increased biofilm formation and consequent current generation was achieved by the overexpression of this gene. This information is crucial to optimize electroactive organisms toward their practical application in METs.

Cultivating electroactive microbes-from field to bench

Electromicrobiology is an emerging field investigating and exploiting the interaction of microorganisms with insoluble electron donors or acceptors. Some of the most recently categorized electroactive microorganisms became of interest to sustainable bioengineering practices. However, laboratories worldwide typically maintain electroactive microorganisms on soluble substrates, which often leads to a decrease or loss of the ability to effectively exchange electrons with solid electrode surfaces. In order to develop future sustainable technologies, we cannot rely solely on existing lab-isolates. Therefore, we must develop isolation strategies for environmental strains with electroactive properties superior to strains in culture collections. In this article, we provide an overview of the studies that isolated or enriched electroactive microorganisms from the environment using an anode as the sole electron acceptor (electricity-generating microorganisms) or a cathode as the sole electron donor (electricity-consuming microorganisms). Next, we recommend a selective strategy for the isolation of electroactive microorganisms. Furthermore, we provide a practical guide for setting up electrochemical reactors and highlight crucial electrochemical techniques to determine electroactivity and the mode of electron transfer in novel organisms.

Cultivation of Exoelectrogenic Bacteria in Conductive DNA Nanocomposite Hydrogels Yields a Programmable Biohybrid Materials System

The use of living microorganisms integrated within electrochemical devices is an expanding field of research, with applications in microbial fuel cells, microbial biosensors or bioreactors. We describe the use of porous nanocomposite materials prepared by DNA polymerization of carbon nanotubes (CNTs) and silica nanoparticles (SiNPs) for the construction of a programmable biohybrid system containing the exoelectrogenic bacterium Shewanella oneidensis. We initially demonstrate the electrical conductivity of the CNT-containing DNA composite by employment of chronopotentiometry, electrochemical impedance spectroscopy, and cyclic voltammetry. Cultivation of Shewanella oneidensis in the conductive materials shows that the exoelectrogenic bacteria populate the matrix of the conductive composite, while nonexoelectrogenic Escherichia coli remain on its surface. Moreover, the ability to use extracellular electron transfer pathways is positively correlated with the number of cells within the conductive synthetic biofilm matrix. The Shewanella-containing composite remains stable for several days and shows electrochemical activity, indicating that the conductive backbone is capable of extracting the metabolic electrons produced by the bacteria under strictly anoxic conditions and conducting them to the anode. Programmability of this biohybrid material system is demonstrated by on-demand release and degradation induced by a short-term enzymatic stimulus. We believe that the application possibilities of such biohybrid materials could even go beyond microbial biosensors, bioreactors, and fuel cell systems.

Identification, expression pattern, and immune response of Tim-1 and Tim-4 in embryos and adult medaka (Oryzias latipes)

T-cell immunoglobulin (Ig) and mucin domain-containing 1 (Tim-1) and Tim-4 are two members of the Tim family. In mammals, Tim-1 and Tim-4 are proteins mainly expressed in immune cells and are associated with immune response. In the present study, medaka Oryzias latipes' Tim-1 (OlTim-1) and OlTim-4 were identified and characterized using bioinformatics analyses. With the use of reverse-transcription polymerase chain reaction, the expression profiles of OlTim-1 and OlTim-4 were examined in embryos and adult fish and in immune tissues following the intraperitoneal injection of stimulants. The results revealed that OlTim-1 possesses a cytoplasmic region, a transmembrane region, a mucin domain, and an Ig-like domain, while OlTim-4 is composed of two Ig-like domains and a mucin domain, but without the transmembrane region and cytoplasmic region. OlTim-1 and OlTim-4 expressions are detectable from the gastrula stage on, indicating that they are zygotic genes. Furthermore, OlTim-1 and OlTim-4 are expressed ubiquitously in the adult. Administration of immune stimulants, namely lipopolysaccharides and polyinosinic:polycytidylic acid, significantly increased the expression levels of OlTim-1 and OlTim-4 in the liver and intestine within 1 day and in the head, kidney, and spleen within 3 to 4 days postinjection. These results suggest that OlTim-1 and OlTim-4 are possibly involved in both innate and adaptive immunities.

Novel Outlook in Microbial Ecology: Nonmutualistic Interspecies Electron Transfer

Recent advances in microbial electrochemical technologies have revealed the existence of numerous and highly diverse microorganisms able to exchange electrons with electrodes. This diversity could reflect the capacity of microorganisms to release and/or retrieve electrons with each other in natural environments. So far, this interspecies electron transfer has been studied with a special focus on syntrophy and was successfully demonstrated for several couples of species. In this article we argue that electron exchange between microbes exists beyond syntrophy or mutualism and could also promote competitive and even parasitic behaviour. Based on three interesting case studies identified from the literature, we also highlight that such nonmutualistic interactions could be widespread and of particular significance for the survival of pathogens or the shaping of complex microbial communities.

Redox mediators as charge agents for changing electrochemical reactions

This review provides a comprehensive discussion toward understanding the effects of RMs in electrochemical systems, underlying redox mechanisms, and reaction kinetics both experimentally and theoretically.

Soluble versions of outer membrane cytochromes function as exporters for heterologously produced cargo proteins

This study reveals that it is possible to secrete truncated versions of outer membrane cytochromes into the culture supernatant and that these proteins can provide a basis for the export of heterologously produced proteins. Different soluble and truncated versions of the outer membrane cytochrome MtrF were analyzed for their suitability to be secreted. A protein version with a very short truncation of the N-terminus to remove the recognition sequence for the addition of a lipid anchor is secreted efficiently to the culture supernatant, and moreover this protein could be further truncated by a deletion of 160 amino acid and still is detectable in the supernatant. By coupling a cellulase to this soluble outer membrane cytochrome, the export efficiency was measured by means of relative cellulase activity. We conclude that outer membrane cytochromes of S. oneidensis can be applied as transporters for the export of target proteins into the medium using the type II secretion pathway.

Enhancing direct interspecies electron transfer in syntrophic-methanogenic associations with (semi)conductive iron oxides: Effects and mechanisms

Anaerobic digestion is an effective biological treatment process that produces methane by degrading organic compounds in waste/wastewater. It is a complicated microbial process by metabolic interactions among different types of microorganisms. In this process, efficient interspecies electron transfer between secondary fermenting bacteria and methanogens is the critical process for fast and effective methanogenesis. In syntrophic metabolism, hydrogen or formate has been considered as the conventional electron carrier transferring electrons from secondary fermenting bacteria to hydrogenotrophic methanogens. Recently, direct interspecies electron transfer (DIET) without the involvement of dissolved redox mediators is arousing great concerns and has been regarded as a more efficient and thermodynamically favorable interspecies electron transfer pathway for methanogenesis. Interspecies electron exchange through DIET is accomplished via the membrane-bound cytochromes or conductive pili. Several kinds of exogenously-added conductive or semiconductive iron oxides have been discovered to greatly enhance anaerobic methanogenesis through promoting DIET. Different (semi)conductive iron oxides give a boost to DIET through different mechanisms based on the physicochemical properties of the iron oxides and the reciprocal interactions between iron oxides and functional microorganisms. In this review, the current understanding of interspecies electron transfer in syntrophic-methanogenic consortions is summarized, the effects and deep-rooted mechanisms of (semi)conductive iron oxides on methanogenesis and DIET are discussed, and possible future perspectives and development directions are suggested for DIET via (semi)conductive iron oxides in anaerobic digestion.

Microbial electrochemical technologies: Electronic circuitry and characterization tools

Microbial electrochemistry merges microbiology, electrochemistry and electronics to provide a set of technologies for environmental engineering applications. Understanding the electronic concepts is crucial for effectively adopting these systems, but the importance of electronic circuitry is often overlooked by microbial electrochemistry researchers. This review provides the background on the electronics and electrochemical concepts involved in the study of microorganisms interacting with electricity, and their applications in microbial electrochemical technology (MET). The potentiostat circuitry is described along with its working principles. Electrochemical analyses are presented together with the rational and parameters employed to study MET devices and electroactive microorganisms. Finally, future directions are delineated towards the adoption of MET, and the related electronics, in environmental engineering applications.

Oxygen-containing functional groups on bioelectrode surface enhance expression of c-type cytochromes in biofilm and boost extracellular electron transfer

Introducing oxygen-containing functional groups is a common and convenient method to increase the hydrophilicity of bioelectrodes. In this study, the effect of oxygen-containing functional groups on biofilm was systematically studied to understand how the electron transfer between electrochemically active bacteria (EAB) and bioelectrode was boosted. After electrolysis pretreatment in sulfuric and nitric acid mixture, the oxygen content of the carbon fiber brushes increased from 4.6% to 30.9%. Comparing with the control, the maximum power density increased by 27.7%, while the anode resistance decreased by 21.8%, because charge transfer resistance significantly reduced. The analysis results showed that the content of c-type cytochromes (c-Cyts) in the EAB biofilm was four times higher than that in the control, while the biomass just slightly increased and the bacteria community was similar with that of the control. These findings suggested that the fundamental reason for the enhanced extracellular electron transfer between EAB and electrode was the increased c-Cyts.

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.

Sludge-based biochar-assisted thermophilic anaerobic digestion of waste-activated sludge in microbial electrolysis cell for methane production

The development of microbial electrolysis cells (MECs) for methane production from waste activated sludge (WAS) is arrested due to the limited methane yield and fragile system stability. This study proposed a strategy to accelerate and stabilize MEC via 1.0 g/g DM (dry matter) sludge-based biochar (BC). The results showed that BC clearly accelerated methane production by 24.7% and enhanced VS removal efficiency by 17.9%, compared to control group. Variations of SCOD, proteins, carbohydrates and VFAs indicated biochar promoted hydrolysis and acidogenesis process. Cyclic voltammetry (CV) curves and coulombic efficiency (CE) suggested organic matters degradation and electron generation on anode were enhanced with supplement of biochar. Microbial community analyses revealed that biochar addition could both promote DIET through substituting exoelectrogen (e.g., Thermincola) on anode and enrich hydrogenotrophic methanogens (e.g., Methanothermobacter) on cathode, which is beneficial to development of MEC as to methane recovery from organic matters.

Mechanism study of photo-induced gold nanoparticles formation by Shewanella oneidensis MR-1

Shewanella oneidensis MR-1, a bioelectricity generating bacterium, is broadly used in bioremediation, microbial fuel cell and dissimilatory reduction and recovery of precious metals. Herein, we report for the first time that photo induction as a trigger to stimulate gold nanoparticles (Au@NPs) formation by MR-1, with wavelength and light intensity as two key variables. Results indicated that sigmoidal model is the best fit for Au@NPs formation at various wavelengths (with R² > 0.97). Light intensity in terms of photosynthetic photon flux density (PPFD) critically influences the rate constant in the low-light intensity region (PPFD < 20), while wavelength controls the maximum rate constant in the high-light region (PPFD > 20). By deletion of Mtr pathway genes in MR-1, we proposed the mechanism for light induced Au@NP formation is the excitation effect of light on certain active groups and extracellular polymeric substances (EPS) on the cell surface. Also, the release of electrons from proteins and co-enzyme complexes enhance electron generation. To the best of our knowledge, this is the first-attempt to explore the effect of photo-induction on Au@NPs production by MR-1, which provides an alternative cost-effective and eco-friendly process in green chemical industry.

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.

Neglected Effects of Inoculum Preservation on the Start-Up of Psychrophilic Bioelectrochemical Systems and Shaping Bacterial Communities at Low Temperature

Bioelectrochemical systems (BESs) are capable of simultaneous wastewater treatment and resource recovery at low temperatures. However, the direct enrichment of psychrophilic and electroactive biofilms in BESs at 4°C is difficult due to the lack of understanding in the physioecology of psychrophilic exoelectrogens. Here, we report the start-up and operation of microbial fuel cells (MFCs) at 4°C with pre-acclimated inocula at different temperatures (4°C, 10°C, 25°C, and -20°C) for 7 days and 14 days. MFCs with 7-day-pretreated inocula reached higher peak voltages than did those with 14-day-pretreated inocula. The highest power densities were obtained by MFCs with 25°C - 7-day-, 25°C - 14-day-, and 4°C - 7-day-pretreated inocula (650-700 mW/m2). In contrast, the control MFCs with untreated inocula were stable at 450 mW/m2. The power densities of MFCs with 7-day-pretreated inocula were higher than those obtained by MFCs with 14-day-pretreated inocula. The MFCs with 10°C - 7-day-pretreated inocula and the control MFCs showed higher chemical oxygen demand (COD) removal (90-91%) than other MFCs. Illumina HiSeq sequencing based on 16S rRNA gene amplicons indicated that bacterial communities of the anode biofilms were shaped by pretreated inocula at different temperatures. Compared with the control MFCs with untreated inocula, MFCs with temperature-pretreated inocula demonstrated higher microbial diversity, but did not do so with -20°C-pretreated inocula. Principal components analysis (PCA) revealed an obvious separation between the inocula pretreated at 4°C and those pretreated at 10°C, implying that bacterial community structures could be shaped by pretreated inocula at low temperatures. The pretreatment period also had a diverse impact on the abundance of exoelectrogens and non-exoelectrogens in MFCs with inocula pretreated at different temperatures. The majority of the predominant population was affiliated with Geobacter with a relative abundance of 17-70% at different pre-acclimated temperatures, suggesting that the exoelectrogenic Geobacter could be effectively enriched at 4°C even with inocula pretreated at different temperatures. This study provides a strategy that was previously neglected for fast enrichment of psychrophilic exoelectrogens in BESs at low temperatures.