Facultative nitrate reduction by electrode-respiring Geobacter metallireducens biofilms as a competitive reaction to electrode reduction in a bioelectrochemical system

Multiple pathways of vanadate reduction and denitrification mediated by denitrifying bacterium Acidovorax sp. strain BoFeN1

Contamination of aquifers by a combination of vanadate [V(V)] and nitrate (NO3-) is widespread nowadays. Although bioremediation of V(V)- and nitrate-contaminated environments is possible, only a limited number of functional species have been identified to date. The present study demonstrates the effectiveness of V(V) reduction and denitrification by a denitrifying bacterium Acidovorax sp. strain BoFeN1. The V(V) removal efficiency was 76.5 ± 5.41 % during 120 h incubation, with complete removal of NO3- within 48 h. Inhibitor experiments confirmed the involvement of electron transport substances and denitrifying enzymes in the bioreduction of V(V) and NO3-. Cyt c and riboflavin were important for extracellular V(V) reduction, with quinone and EPS more significant for NO3- removal. Intracellular reductive compounds including glutathione and NADH directly reduce V(V) and NO3-. Reverse transcription quantitative PCR confirmed the important roles of nirK and napA genes in regulating V(V) reduction and denitrification. Bioaugmentation by strain BoFeN1 increased V(V) and NO3- removal efficiency by 55.3 % ± 2.78 % and 42.1 % ± 1.04 % for samples from a contaminated aquifer. This study proposes new microbial resources for the bioremediation of V(V) and NO3-contaminated aquifers, and contributes to our understanding of coupled vanadium, nitrogen, and carbon biogeochemical processes.

Sulfur bacteria-reinforced microbial electrochemical denitrification

Two limiting factors of microbial electrochemical denitrification (MED) are the abundance and efficiency of the functional microorganisms. To supply these microorganisms, MED systems are inoculated with denitrifying sludge, but such method has much room for improvement. This study compared MED inoculated with autotrophic denitrifying inoculum (ADI) versus with heterotrophic denitrifying inoculum (HDI). ADI exhibited electroactivity for 50% less of timethan HDI. The denitrification efficiency of the ADI biocathode was42% higherthan that of the HDI biocathode. The HDI biocathode had high levels of polysaccharides while the ADI biocathode was rich in proteins, suggesting that two biocathodes may achieveMED but via differentpathways. Microbial communities of two biocathodes indicated MED of HDI biocathode may rely on interspecies electron transfer, whereas sulfur bacteria of ADI biocathode take electrons directly from the cathode to achieve MED. Utilizing autotrophic sulfur-oxidizing denitrifiers, this study offers a strategy for enhancing MED.

Electron transport bifurcation in bioanode with the metabolic shift to nitrate reduction

Electron transport bifurcation in bioanode determines the performance of microbial electrochemical technologies with the presence of an alternative electron acceptor. Here, the bioanode responses including electron transfer efficiency, microbial community, and microbial structure are investigated with the metabolic shift from current production to denitrification. Electrochemical measurements including cyclic voltammetry and electrochemical impedance spectra are performed to identify the change of electron transfer pathways in bioanode. Electron transfer efficiency for electrode reduction decreases ∼17 % with nitrate reduction. Biofilm resistance and charge transfer resistance increase from 23.3 Ω and 22.5 Ω to 36.6 Ω and 61.4 Ω with the metabolic shift, respectively. These results are mainly due to the loss of exoelectrogens inhabited in bioanode. Confocal imaging results indicate the elevated proportion of inactive cells in bioanode as the denitrification. Our results propose a possible mechanism for electron transfer bifurcation in bioanode with the metabolic shift from electrode reduction to soluble electron acceptor reduction.

Effects of chemical oxygen demand/nitrogen on electrochemical performances and denitrification efficiency in single-chamber microbial fuel cells: Insights from electron transfer and bacterial communities

The electrochemical performances and denitrification efficiency of microbial fuel cells (MFCs) are often limited by chemical oxygen demand/nitrogen (COD/N) of wastewater. To overcome this limitation, single-chamber air cathode MFCs with varying COD/N (16/1, 8/1, and 4/1) were established to investigate their electrochemical performances, denitrification efficiency, and bacterial communities. The optimal COD/N for maximizing electricity generation and denitrification efficiency was 8/1, as supported by the greatest corrected coulomb efficiency (13.6%) and electron transfer rate (2.36 C/h for electricity generation, 39.77 C/h for denitrification). As COD/N decreased, the electrochemically active genus Geobacter was replaced by the denitrifying genera Un._f_Burkholderiaceae, Dechlorosoma, and Petrimonas. These results indicated that the efficiency of electricity generation and denitrification was not solely determined by the abundance of electrochemically active and denitrifying bacteria. The presence of a faster electron transfer pathway, possibly direct interspecies electron transfer, enhanced simultaneous electricity generation and denitrification in MFCs with COD/N of 8/1.

A. Jorand F, Barrière F, Etienne M. Challenges and Applications of Nitrate-Reducing Microbial Biocathodes

Bioelectrochemical systems which employ microbes as electrode catalysts to convert chemical energy into electrical energy (or conversely), have emerged in recent years for water sanitation and energy recovery. Microbial biocathodes, and especially those reducing nitrate are gaining more and more attention. The nitrate-reducing biocathodes can efficiently treat nitrate-polluted wastewater. However, they require specific conditions and they have not yet been applied on a large scale. In this review, the current knowledge on nitrate-reducing biocathodes will be summarized. The fundamentals of microbial biocathodes will be discussed, as well as the progress towards applications for nitrate reduction in the context of water treatment. Nitrate-reducing biocathodes will be compared with other nitrate-removal techniques and the challenges and opportunities of this approach will be identified.

Heterotrophic anodic denitrification coupled with cathodic metals recovery from on-site smelting wastewater with a bioelectrochemical system inoculated with mixed Castellaniella species

Although Castellaniella species are crucial for denitrification, there is no report on their capacity to carry out denitrification and anode respiration simultaneously in a bioelectrochemical system (BES). Herein, the ability of a mixed inoculum of electricigenic Castellaniella species to perform simultaneous denitrification and anode respiration coupled with cathodic metals recovery was investigated in a BES. Results showed that 500 mg/L NO₃^--N significantly decreased power generation, whereas 100 and 250 mg/L NO₃^--N had a lesser impact. The single-chamber MFCs (SCMFCs) fed with 100 and 250 mg/L NO₃^--N concentrations achieved a removal efficiency higher than 90% in all cycles. In contrast, the removal efficiency in the SCMFCs declined dramatically at 500 mg/L NO₃^--N, which might be attributable to decreased microbial viability as revealed by SEM and CLSM. EPS protein content and enzymatic activities of the biofilms decreased significantly at this concentration. Cyclic voltammetry results revealed that the 500 mg/L NO₃^--N concentration decreased the redox activities of anodic biofilms, while electrochemical impedance spectroscopy showed that the internal resistance of the SCMFCs at this concentration increased significantly. In addition, BES inoculated with the Castellaniella species was able to simultaneously perform heterotrophic anodic denitrification and cathodic metals recovery from real wastewater. The BES attained Cu²⁺, Hg²⁺, Pb²⁺, and Zn²⁺ removal efficiencies of 99.86 ± 0.10%, 99.98 ± 0.014%, 99.98 ± 0.01%, and 99.17 ± 0.30%, respectively, from the real wastewater. Cu²⁺ was bio-electrochemically reduced to Cu⁰ and Cu₂O, whereas Hg⁰ and HgO constituted the Hg species recovered via bioelectrochemical reduction and chemical deposition, respectively. Furthermore, Pb²⁺ and Zn²⁺ were bio-electrochemically reduced to Pb⁰ and Zn⁰, respectively. Over 89% of NO₃^--N was removed from the BES anolyte during the recovery of the metals. This research reveals promising denitrifying exoelectrogens for enhanced power generation, NO₃^--N removal, and heavy metals recovery in BES.

Coupling mixotrophic denitrification and electroactive anodic nitrification by nitrate addition for promoting current generation and nitrogen removal

Nitrate promotes anodic denitrification and fasts organic matter removal in microbial fuel cells (MFCs). However, it suffers from poor total nitrogen (TN) removal and current recovery. In this study, some novel electroactive nitrifying/denitrifying bacteria (ENDB) were introduced in a single chambered air-cathode MFC to investigate the performance of this device and the microbial community shift by adding nitrate. Results showed a similar disturbance in current output by adding nitrate during a short-term operation. However, a stable and reproducible current increase was achieved in the continuous experiment. A maximum current of 0.76 A m^-3 and a maximum TN removal of >99 % were accomplished. The corresponding corrected coulombic efficiency was approximately 18 %. Under repeatable batches, a sharp decrease in chemical oxygen demand (COD) with feeding nitrate confirmed the temporary competition on electron donors through heterotrophic denitrification. The later current increase and nitrite detection occurring without metabolized COD could be considered evidence of electroactive anodic nitrification. The ENDB biofilm successfully coupled mixotrophic denitrification and electroactive anodic nitrification. It eventually promoted TN removal. In the process, genera Pseudoxanthomonas, Thauera, and Pseudomonas were enriched in the anodic ENDB biofilms. Cyclic voltammetry data confirmed the promotion of the electron transfer process by biofilms. The bacterial function predication revealed that the genes related to nitrogen removal and electron transfer were upregulated. Therefore, mixotrophic denitrification and electroactive anodic nitrification processes facilitated power recovery with the high efficiency of pollutant removal, finally ensuring water body security.

Autotrophic nitrate reduction to ammonium via reverse electron transfer in Geobacter dominated biofilm

Geobacter dominated electroactive biofilms (EABs) have been demonstrated to perform bidirectional extracellular electron transfer (EET) in bioelectrochemical systems, but it is largely unknown when nitrate is the electron acceptor at the cathode. If reverse EET occurs on biocathode, this EAB has to perform dissimilatory nitrate reduction to ammonia (DNRA) rather than denitrification according to genomes. Here, we have proven the feasibility of reverse bioelectron transfer in EAB, achieving a DNRA efficiency up to 93 ± 3% and high Faraday efficiency of 74 ± 1%. Constant current was found to be more effective than constant potential to maintain Geobacter on the cathode, which highly determines this electrotrophic respiration. The prevalent DNRA at constant current surpassed denitrification, demonstrated by the reverse tendencies of DNRA (nrfA) and denitrification (nirS and nirK) gene transcription. Metatranscriptomics further revealed the possible electron uptake mechanisms by which the outer membrane (OmcZ and OmcB) and periplasmic cytochromes (PpcB and PpcD) may be involved. These findings extend our understanding of the bidirectional electron transfer and advance the applications of EABs.

Electrochemical Control of Biofilm Formation and Approaches to Biofilm Removal

This review deals with microbial adhesion to metal-based surfaces and the subsequent biofilm formation, showing that both processes are a serious problem in the food industry, where pathogenic microorganisms released from the biofilm structure may pollute food and related material during their production. Biofilm exhibits an increased resistance toward sanitizers and disinfectants, which complicates the removal or inactivation of microorganisms in these products. In the existing traditional techniques and modern approaches for clean-in-place, electrochemical biofilm control offers promising technology, where surface properties or the reactions taking place on the surface are controlled to delay or prevent cell attachment or to remove microbial cells from the surface. In this overview, biofilm characterization, the classification of bacteria-forming biofilms, the influence of environmental conditions for bacterial attachment to material surfaces, and the evaluation of the role of biofilm morphology are described in detail. Health aspects, biofilm control methods in the food industry, and conventional approaches to biofilm removal are included as well, in order to consider the possibilities and limitations of various electrochemical approaches to biofilm control with respect to potential applications in the food industry.

Flavin mononucleotide-stimulated microbial fuel cell for efficient gaseous toluene abatement

Chemical park is regarded as a major contributor of VOCs emissions in China. Currently, a green and safe technology, microbial fuel cells (MFCs), is being developed for the VOCs abatement. Noting that effective electron transfer is critical to the MFC performance. In this work, flavin mononucleotide (FMN) was dosed as an electron shuttle to improve the removal of the typical toxic VOCs, toluene. The experimental results revealed that the performance of toluene removal and power generation were accelerated with the dosage of 0.2-2 μM FMN. With the addition of 1 μM FMN, the removal efficiency, the maximum output voltage and the coulombic efficiency of MFC were increased by 18.4%, 64.4% and 56.3%, respectively. However, a further increase in FMN concentration to 2 μM caused a reduction in the removal efficiency and coulombic efficiency. The images of scanning electron microscopy and confocal laser scanning microscopy showed that the presence of FMN greatly promoted the microbial growth and its activity. Furthermore, microbial community analysis also implied that the moderate dosage of FMN (0.2-1 μM) was beneficial for the growth of the typical exoelectrogens, Geobacter sp., and thus the coulombic efficiency was increased. In addition, an electron transfer pathway involving in cytochrome b, OMCs, cytochrome c, and MtrA was proposed based on the cyclic voltammetry analysis. This work will provide a fundamental theoretical support for its application of toxic VOCs abatement from the chemical park.

Mixotrophic bacteria for environmental detoxification of contaminated waste and wastewater

Mixotrophic bacteria provide a desirable alternative to the use of classical heterotrophic or chemolithoautotrophic bacteria in environmental technology, particularly under limiting nutrients conditions. Their bi-modal ability of adapting to inorganic or organic carbon feed and sulfur, nitrogen, or even heavy metal stress conditions are attractive features to achieve efficient bacterial activity and favorable operation conditions for the environmental detoxification or remediation of contaminated waste and wastewater. This review provides an overview on the state of the art and summarizes the metabolic traits of the most promising and emerging non-model mixotrophic bacteria for the environmental detoxification of contaminated wastewater and waste containing excess amounts of limiting nutrients. Although mixotrophic bacteria usually function with low organic carbon sources, the unusual capabilities of mixotrophic electroactive exoelectrogens and electrotrophs in bioelectrochemical systems and in microbial electrosynthesis for accelerating simultaneous metabolism of inorganic or organic C and N, S or heavy metals are reviewed. The identification of the mixotrophic properties of electroactive bacteria and their capability to drive mono- or bidirectional electron transfer processes are highly exciting and promising aspects. These aspects provide an appealing potential for unearthing new mixotrophic exoelectrogens and electrotrophs, and thus inspire the next generation of microbial electrochemical technology and mixotrophic bacterial metabolic engineering. KEY POINTS: • Mixotrophic bacteria efficiently and simultaneously remove C and N, S or heavy metals. • Exoelectrogens and electrotrophs accelerate metabolism of C and N, S or heavy metals. • New mixotrophic exoelectrogens and electrotrophs should be discovered and exploited.

Microbial fuel cell-upflow biofilter coupling system for deep denitrification and power recovery: Efficiencies, bacterial succession and interactions

The excess organic carbon is often added to meet denitrification requirements during municipal wastewater treatment, resulting in the carbon waste and increased risk of secondary pollution. In this study, microbial fuel cell (MFC) was coupled with an up-flow denitrification biofilter (BF), and the long-term performances of denitrification and power output were investigated under the different carbon source concentration. With sodium acetate (NaAc) of 600 mg/L and 300 mg/L, the favorable denitrification efficiencies were obtained (98.60%) and the stable current output was maintained (0.44 mÃ0.48 mA). By supplying NaAc of 150 mg/L, the high denitrification efficiency remained in a high range (89.31%) and the current output maintained at 0.12 mA, while, the denitrification efficiency dropped to 71.34% without coupling MFC. Electron balance analysis indicated that both nitrate removal and electron recovery efficiencies were higher in MFC-BF than that in BF, verifying the improved denitrification and carbon utilization performance. Coupling MFC significantly altered the bacterial community structure and composition, and while, the diversified abundance and distribution of bacterial genera were observed at the different locations. Compared with BF, the more exoelectrogenic genera (Desulfobacterium, Trichococcus) and genera holding both denitrifying and electrogenic functions (Dechloromonas, Geobacter) were found dominated in MFC-BF. Instead, the dominating genera in BF were Dechloromonas, Desulfomicrobium, Acidovorax and etc. By coupling MFC, the more complex and diversified network and the closer interaction relationships between the dominant potential functional genera were found. The study provides a feasible approach to effectively improve the denitrification efficiency and organic carbon recovery for deep denitrification process.

Sulfur transformation in sulfur autotrophic denitrification using thiosulfate as electron donor

Thiosulfate is frequently used as an energy source and electron donor in autotrophic denitrification (AD) for removing nitrate from wastewater. However, transforming pathways of S₂O₃^2- in this process is unclear. Herein, the aim of this study is to explore possible transforming pathways of sulfur compounds in thiosulfate-based AD process. After measuring the variation of NO₃^-, NO₂^-, and various sulfur compounds such as S⁰, SO₄^2-, S₂O₃^2-, acid volatile sulfide (AVS), and S^2- in the presence and absence of S₂O₃^2-, the variation process of S₂O₃^2- and the contribution of various sulfur compounds were analyzed. The results indicated that S⁰, AVS, and S^2- were the intermediate products when S₂O₃^2- was applied as an electron donor. All S₂O₃^2-, S⁰, AVS, and S^2- could act as electron donors in the nitrate removal process with the final products of SO₄^2-. The utilization priority of these four sulfur sources was presumed in the following order: S^2- > S₂O₃^2- > AVS ≈ S⁰. Furthermore, sulfur transformation and balance in nitrate removal process was also investigated. This suggests the transforming pathways of sulfur compounds in denitrification process. Nitrogen removal and sulfur conversion process are dependent on the presence of microorganisms in the sludge.

Nitric oxide reduction by microbial fuel cell with carbon based gas diffusion cathode for power generation and gas purification

Nitric oxide (NO) from anthropogenic emission is one of the main air contaminants and induces many environmental problems. Microbial fuel cells (MFCs) with gas diffusion cathode provide an alternative technology for NO reduction. In this work, pure NO as the sole electron acceptor of MFCs with gas diffusion cathode (NO-MFCs) was verified. The NO-MFCs obtained a maximum power density of 489 ± 50 mW/m². Compared with MFCs using O₂ in air as electron acceptor (Air-MFCs), the columbic efficiency increased from 23.2% ± 4.3% (Air-MFCs) to 55.7% ± 4.6% (NO-MFCs). The NO removal rate was 12.33 ± 0.14 mg/L/h and N₂ was the main reduction product. Cathode reduction was the dominant pathway of NO conversion in NO-MFCs, including abiotic electrochemical reduction and microbial denitrification process. The predominant genera in anodic microbial community changed from exoelectrogenic bacteria in Air-MFCs to denitrifying bacteria in NO-MFCs and effected the power generation.

Electron Flow Shifts from Anode Respiration to Nitrate Reduction During Electroactive Biofilm Thickening

As electrons generated through substrate oxidation compete with electrodes, dissimilatory nitrate reduction to ammonium (DNRA), denitrification in bioelectrochemical systems in the presence of nitrate, and nitrate reduction through an electroactive biofilm (EAB) are unpredictable. We find that pathways of nitrate reduction are related to EAB thickness and that 76 ± 2 μm is the critical thickness of a biofilm at which both the inner and outer layers simultaneously include DNRA, leading to a maximum level of DNRA efficiency of 42%. Fractions of electrons flowing during nitrate reduction are relatively stable, but their distributions between DNRA and denitrification vary with biofilm thickness. Electrons prefer denitrification in an EAB that is 66 ± 2 μm, while DNRA reversely surpasses denitrification when the thickness increases in the range of 76 ± 2 to 210 ± 2 μm. Biofilm thickening enhances the DNRA of all biofilms close to solution, where nirK remains constant and nrfA is significantly upregulated. However, nrfA is downregulated in layers close to the electrode when the biofilm is thicker than 76 ± 2 μm. These findings reveal the spatially heterogeneous reduction of nitrate in thick EABs, highlighting the importance of biofilm thickness to the regulation of end products of nitrate reduction.

Bioelectrochemical Ammoniation Coupled with Microbial Electrolysis for Nitrogen Recovery from Nitrate in Wastewater

Nitrate-N in wastewaters is hard to be recovered because it is difficult to volatilize with an opposite charge to ammonium. Here, we have proved the feasibility of dissimilatory nitrate reduction to ammonia (DNRA) by the easy-acclimated mixed electroactive bacteria, achieving the highest DNRA efficiency of 44%. It was then coupled with microbial electrolysis to concentrate ammonium by a factor of 4 in the catholyte for recovery. The abundance of electroactive bacteria in the biofilm before nitrate addition, especially Geobacter spp., was found to determine the DNRA efficiency. As the main competitors of DNRA bacteria, the growth of denitrifiers was more sensitive to C/N ratios. The DNRA microbial community contrarily showed a stable and recoverable ammoniation performance over C/N ratios ranging from 0.5 to 8.0. A strong competition of the electrode and nitrate on electron donors was observed at the early stage (15 d) of electroactive biofilm formation, which can be weakened when the biofilm was mature on 40 d. Quantitative PCR showed a significant increase in nirS and nrfA transcripts in the ammoniation process. nirS was inhibited significantly after nitrate depletion while nrfA was still upregulated. These findings provided a novel way to recover nitrate-N using organic wastes as both electron donor and power, which has broader implications on the sustainable wastewater treatment and the ecology of nitrogen cycling.

Enhancement of the denitrification activity by exoelectrogens in single-chamber air cathode microbial fuel cells

Single-chamber microbial fuel cells (MFCs) can efficiently treat wastewater containing nitrate, probably because the interaction between exoelectrogens and denitrifying bacteria may enhance the denitrification activity of MFCs. In this study, the denitrification of nitrate with a wide range of concentrations was investigated by using single-chamber air cathode MFCs. The maximum average denitrification rate of the MFCs inoculated and operated under closed-circuit conditions (Group N-CC) was up to 12.2 ± 0.6 kg NO₃^--N m^-3 d^-1 at a high nitrate concentration of 2000 mg NO₃-N L^-1, which was 74.3% higher than that of the MFCs inoculated and operated under open-circuit conditions and which was significantly higher than those of other MFC systems and many traditional bioreactors. The high denitrification activity of the MFCs of Group N-CC was attributed to the significant reduction of nitrite accumulation through the possible bioelectrochemical nitrite reduction by exoelectrogens that were only enriched at the anodes of the MFCs of Group N-CC. In addition, the MFCs of Group N-CC showed good stability (over 3.5 years) and low apparent activation energy (34.0 kJ mol^-1) of the denitrification, indicating the good coexistence of exoelectrogens (Geobacter) and denitrifying bacteria (Thauera) with high performance on denitrification during the long-term operation.

In Silico Metabolic Design of Two-Strain Biofilm Systems Predicts Enhanced Biomass Production and Biochemical Synthesis

Engineered biofilm consortia have the potential to solve important biotechnological problems that have proved difficult for monoculture biofilms and planktonic consortia, such as conversion of lignocellulosic material to useful biochemicals. While considerable experimental progress has been reported for engineering and characterizing biofilm consortia, the field still lacks in silico tools for simulation, design, and optimization of stable, robust, and productive designed consortia. We developed biofilm consortia metabolic models for two coculture systems centered around the ecological design motif of a primary cell type that utilizes a supplied electron donor and secretes acetate as a byproduct and a secondary cell type that consumes the acetate, relieving byproduct inhibition on the primary cell type and enhancing overall system biomass. The models presented in this paper predict that distinct metabolic niches for the two cell types could be established by supplying electron donors and acceptors at opposite ends of the biofilm and that acetate consumption by the secondary cell type could increase total biomass accumulation and the synthesis of valuable biochemicals, such as isobutanol, by the primary cell type. System tunability is enhanced when each cell type is supplied with a unique terminal electron acceptor at opposite ends of the biofilm rather than competing for a common electron acceptor. Our model provides good qualitative agreement with data for a synthetic Escherichia coli coculture system, suggesting that the proposed design rules may have wide applicability to engineered biofilm consortia.

Mobile Incubator for Iron(III) Reduction in the Gut of the Soil-Feeding Earthworm Pheretima guillelmi and Interaction with Denitrification

Diets of soil-feeding earthworms contain abundant nitrate and iron(III) oxides, which are potential electron acceptors for mineralization of organic compounds. The earthworm gut provides an ideal habitat for ingested iron(III)-reducing microorganisms. However, little is known about iron(III) reduction and its interaction with other processes in the guts of earthworms. Here, we determined the dynamics of iron(III) and revealed its interaction with the turnover of organic acids and nitrate in the gut of the earthworm Pheretima guillelmi. Samples from gut contents combined with anoxic incubation were used for chemical analysis and 16S rRNA based Illumina sequencing. Chemical analysis showed that higher ratios of iron(II)/iron(III), nitrite/nitrate, and more abundant organic acids were contained in the in vivo gut of the earthworm P. guillelmi than those in the in situ soil. A higher rate of iron(III) reduction was detected in treatments of microcosmic incubation with gut contents (IG gut) than that with soil (IG soil), and nitrate reduction occurred earlier than iron(III) reduction in both treatments. Potential iron(III) reducers were dominated by fermentative genera Clostridium, Bacillus, and Desulfotomaculum in the treatment of IG gut, while they were dominated by dissimilatory iron(III)-reducing genera Geobacter in the treatment of IG soil. The iron(III)-reducing microbial community shared several genera with denitrifers in the treatment of IG gut, revealing a close link between iron(III) reduction and denitrification in the gut of earthworms. Collectively, our findings demonstrated that iron(III) reduction occurred along the gut and provided novel insights into the great contribution of earthworm gut microbiota on Fe and the associated C and N cycling in soil environments.

Electricity production and the analysis of the anode microbial community in a constructed wetland-microbial fuel cell

The objective of this study is to assess bioelectricity generation, pollutant removal (COD, ammonium, nitrate) and the bacterial communities on anodes in constructed wetlands coupled with microbial fuel cells (CW-MFCs), through feeding the systems with three different types of synthetic wastewater (system 1: normal wastewater; system 2: ammonium-free wastewater; system 3: nitrate-free wastewater). Three CW-MFCs were operated with different wastewater concentrations and hydraulic retention times (HRTs) over a long time period (6 months). The results indicate that the maximum open circuit voltage (775.63 mV) and maximum power density (0.628 W m⁻³) were observed in system 3 (period 3), and that bioenergy production was inhibited in system 2, when feeding with ammonium-free wastewater continuously. COD removal rates in the three systems were similar during each period and ranged from 82.2 ± 6.8% to 98.3 ± 2.2%. Ammonium removal occurred at the air cathode of the CW-MFCs through nitrification, and a higher level of ammonium removal was found in system 1 (period 3) compared with the others. Meanwhile, denitrification occurred at the anaerobic anode of the CW-MFCs, and a large amount of nitrate was removed effectively. The highest nitrate removal rate was 98.8 ± 0.5% in system 2 (period 3). Additionally, four genera related to electricity generation were detected at the anode: Geothrix; Desulfovibrio; Desulfobulbus; and Geobacter. The relative abundances of Desulfovibrio, Desulfobulbus and Geothrix gradually increased during the three periods in system 3, which might be beneficial for bioelectricity generation. Further investigations are needed to optimize the CW-MFC performance and explain the mechanism behind the pollutant degradation and electron motion in the CW-MFCs.

The objective of this study is to assess bioelectricity generation, pollutant removal and the bacterial communities on anodes in constructed wetlands coupled with microbial fuel cells, through feeding the systems with three different types of synthetic wastewater.

Bidirectional extracellular electron transfers of electrode-biofilm: Mechanism and application

The extracellular electron transfer (EET) between microorganisms and electrodes forms the basis for microbial electrochemical technology (MET), which recently have advanced as a flexible platform for applications in energy and environmental science. This review, for the first time, focuses on the electrode-biofilm capable of bidirectional EET, where the electrochemically active bacteria (EAB) can conduct both the outward EET (from EAB to electrodes) and the inward EET (from electrodes to EAB). Only few microorganisms are tested in pure culture with the capability of bidirectional EET, however, the mixed culture based bidirectional EET offers great prospects for biocathode enrichment, pollutant complete mineralization, biotemplated material development, pH stabilization, and bioelectronic device design. Future efforts are necessary to identify more EAB capable of the bidirectional EET, to balance the current density, to evaluate the effectiveness of polarity reversal for biocathode enrichment, and to boost the future research endeavors of such a novel function.

Enhancing the Electricity Generation and Nitrate Removal of Microbial Fuel Cells With a Novel Denitrifying Exoelectrogenic Strain EB-1

Microbial fuel cells (MFCs) have been tentatively applied for wastewater treatment, but the presence of nitrogen, especially nitrate, induces performance instability by changing the composition of functional biofilms. A novel denitrifying exoelectrogenic strain EB-1, capable of simultaneous denitrification and electricity generation and affiliated with Mycobacterium sp., was isolated from the anodic biofilm of MFCs fed with nitrate containing medium. Polarization curves and cyclic voltammetry showed that strain EB-1 could generate electricity through a direct electron transfer mechanism with a maximum power density of 0.84 ± 0.05 W m⁻². Additionally, anodic denitrification, as a concurrent metabolism, was demonstrated with an efficient removal rate of 0.66 ± 0.01 kg N m⁻³ d⁻¹ at a COD/N ratio of 3.5 ± 0.3. Importantly, voltage output was not negatively influenced by nitrate, indicating that the concurrent process of nitrate removal and electricity generation was a limitation of the electron donor rather than an inhibition of the system. Furthermore, various organic materials were successfully utilized as anode donors for strain EB-1, and demonstrated the exciting performances in terms of simultaneous denitrification and electricity generation. Mycobacterium sp. EB-1 thus expands the diversity of exoelectrogens and contributes to the potential applications of MFC for simultaneous energy recovery and wastewater treatment.

Thermodynamic and functional characterization of the periplasmic triheme cytochrome PpcA from Geobacter metallireducens

The Geobacter metallireducens bacterium can couple the oxidation of a wide range of compounds to the reduction of several extracellular electron acceptors, including pollutants or electrode surfaces for current production in microbial fuel cells. For these reasons, G. metallireducens are of interest for practical biotechnological applications. The use of such electron acceptors relies on a mechanism that permits electrons to be transferred to the cell exterior. The cytochrome PpcA from G. metallireducens is a member of a family composed of five periplasmic triheme cytochromes, which are important to bridge the electron transfer from the cytoplasmic donors to the extracellular acceptors. Using NMR and visible spectroscopic techniques, a detailed thermodynamic characterization of PpcA was obtained, including the determination of the heme reduction potentials and their redox and redox-Bohr interactions. These parameters revealed unique features for PpcA from G. metallireducens compared with other triheme cytochromes from different microorganisms, namely the less negative heme reduction potentials and concomitant functional working potential ranges. It was also shown that the order of oxidation of the hemes is pH-independent, but the protein is designed to couple e^-/H⁺ transfer exclusively at physiological pH.

Effects of wastewater constituents and operational conditions on the composition and dynamics of anodic microbial communities in bioelectrochemical systems

Over the last decade, there has been an ever-growing interest in bioelectrochemical systems (BES) as a sustainable technology enabling simultaneous wastewater treatment and biological production of, e.g. electricity, hydrogen, and further commodities. A key component of any BES degrading organic matter is the anode where electric current is biologically generated from the oxidation of organic compounds. The performance of BES depends on the interactions of the anodic microbial communities. To optimize the operational parameters and process design of BES a better comprehension of the microbial community dynamics and interactions at the anode is required. This paper reviews the abundance of different microorganisms in anodic biofilms and discusses their roles and possible side reactions with respect to their implications on the performance of BES utilizing wastewaters. The most important operational parameters affecting anodic microbial communities grown with wastewaters are highlighted and guidelines for controlling the composition of microbial communities are given.

Biochemical and functional insights on the triheme cytochrome PpcA from Geobacter metallireducens

G. metallireducens bacterium has highly versatile respiratory pathways that provide the microorganism an enormous potential for many biotechnological applications. However, little is known about the structural and functional properties of its electron transfer components. In this work, the periplasmic cytochrome PpcA from G. metallireducens was studied in detail for the first time using complementary biophysical techniques, including UV-visible, CD and NMR spectroscopy. The results obtained showed that PpcA contains three low-spin c-type heme groups with His-His axial coordination, a feature also observed for its homologue in G. sulfurreducens. However, despite the high sequence homology between the two cytochromes, important structural and functional differences were observed. The comparative analysis of the backbone, side chain and heme substituents NMR signals revealed differences in the relative orientation of the hemes I and III. In addition, redox titrations followed by visible spectroscopy showed that the redox potential values for PpcA from G. metallireducens (-78 and -93 mV at pH 7 and 8, respectively) are considerably less negative. Overall, this study provides biochemical and biophysical data of a key cytochrome from G. metallireducens, paving the way to understand the extracellular electron transfer mechanisms in these bacteria.

Ecological and Transcriptional Responses of Anode-Respiring Communities to Nitrate in a Microbial Fuel Cell

A poorly understood phenomenon with a potentially significant impact on electron recovery is competition in microbial fuel cells (MFC) between anode-respiring bacteria and microorganisms that use other electron acceptors. Nitrate is a constituent of different wastewaters and can act as a competing electron acceptor in the anode. Studies investigating the impact of competition on population dynamics in mixed communities in the anode are lacking. Here, we investigated the impact of nitrate at different C/N ratios of 1.8, 3.7, and 7.4 mg C/mg N on the electrochemical performance and the biofilm community in mixed-culture chemostat MFCs. The electrochemical performance of the MFC was not affected under electron donor non-limiting conditions, 7.4 mg C/mg N. At lower C/N, electron donor limiting and ratio electron recovery were significantly affected. The electrochemical performance recovered upon removal of nitrate at 3.7 mg C/mg N but did not at 1.8 mg C/mg N. Microbial community analysis showed a decrease in Deltaproteobacteria accompanied by an increase in Betaproteobacteria in response to nitrate at low C/N ratios and no significant changes at 7.4 mg C/mg N. Transcriptional analysis showed increased transcription of nirK and nirS genes during nitrate flux, suggesting that denitrification to N₂ and not facultative nitrate reduction by Geobacter spp. might be the primary response to perturbation with nitrate.

Soil biochar amendment shapes the composition of N2O-reducing microbial communities

Soil biochar amendment has been described as a promising tool to improve soil quality, sequester carbon, and mitigate nitrous oxide (N2O) emissions. N2O is a potent greenhouse gas. The main sources of N2O in soils are microbially-mediated nitrogen transformation processes such as nitrification and denitrification. While previous studies have focused on the link between N2O emission mitigation and the abundance and activity of N2O-reducing microorganisms in biochar-amended soils, the impact of biochar on the taxonomic composition of the nosZ gene carrying soil microbial community has not been subject of systematic study to date. We used 454 pyrosequencing in order to study the microbial diversity in biochar-amended and biochar-free soil microcosms. We sequenced bacterial 16S rRNA gene amplicons as well as fragments of common (typical) nosZ genes and the recently described 'atypical' nosZ genes. The aim was to describe biochar-induced shifts in general bacterial community diversity and taxonomic variations among the nosZ gene containing N2O-reducing microbial communities. While soil biochar amendment significantly altered the 16S rRNA gene-based community composition and structure, it also led to the development of distinct functional traits capable of N2O reduction containing typical and atypical nosZ genes related to nosZ genes found in Pseudomonas stutzeri and Pedobacter saltans, respectively. Our results showed that biochar amendment can affect the relative abundance and taxonomic composition of N2O-reducing functional microbial traits in soil. Thus these findings broaden our knowledge on the impact of biochar on soil microbial community composition and nitrogen cycling.

Cross-biome comparison of microbial association networks

Clinical and environmental meta-omics studies are accumulating an ever-growing amount of microbial abundance data over a wide range of ecosystems. With a sufficiently large sample number, these microbial communities can be explored by constructing and analyzing co-occurrence networks, which detect taxon associations from abundance data and can give insights into community structure. Here, we investigate how co-occurrence networks differ across biomes and which other factors influence their properties. For this, we inferred microbial association networks from 20 different 16S rDNA sequencing data sets and observed that soil microbial networks harbor proportionally fewer positive associations and are less densely interconnected than host-associated networks. After excluding sample number, sequencing depth and beta-diversity as possible drivers, we found a negative correlation between community evenness and positive edge percentage. This correlation likely results from a skewed distribution of negative interactions, which take place preferentially between less prevalent taxa. Overall, our results suggest an under-appreciated role of evenness in shaping microbial association networks.