Different methods used to form oxygen reducing biocathodes lead to different biomass quantities, bacterial communities, and electrochemical kinetics

Enhancing the microbial advanced oxidation of P-nitrophenol in sediment through accelerating extracellular respiration with electrical stimulation

Microbial advanced oxidation, a fundamental process for pollutant degradation in nature, is limited in efficiency by the weak respiration of indigenous microorganisms. In this study, an electric field was employed to enhance microbial respiration and facilitate the microbial advanced oxidation of p-nitrophenol (PNP) in simulated wetlands with alternation of anaerobic and aerobic conditions. With intermittent air aeration, an electric field of 0.8 V promoted extracellular electron transfer to increase Fe2+ generation through dissimilatory iron reduction and the production of hydroxyl radicals (•OH) through Fenton-like reactions. As a result, the PNP removal rate of the electrically-stimulated group was higher than that of the control (72.15 % vs 46.88 %). Multiple lines of evidence demonstrated that the electrically-induced polarization of respiratory enzymes expedited proton-coupled electron transfer within the respiratory chain to accelerate microbial advanced oxidation of PNP. The polarization of respiratory enzymes with the electric field hastened proton outflow to increase cell membrane potential for adenosine triphosphate (ATP) generation, which enhanced intracellular electron transportation to benefit reactive oxygen species generation. This study provided a new method to enhance microelectrochemical remediation of the contaminant in wetlands via the combination of intermittent air aeration.

Two polarity reversal modes lead to different nitrate reduction pathways in bioelectrochemical systems

Polarity reversal is one of the effective strategies to rapidly start up denitrifying BESs，but the long-term performances of the denitrifying BESs operated under polarity reversal receive little attention. This study investigated the effects of periodic polarity reversal (PPR) and polarity reversal once only (PRO) on the long-term performances of denitrifying BESs. Repeatable oxidative and reductive currents were observed in the BESs obtained by PPR (PPR-BESs). The peak reductive currents of the PPR-BESs reached 0.95 A/m², and nitrate was mainly removed by dissimilatory nitrate reduction to ammonium pathway with removal rates higher than 95 %. In contrast, the peak reductive currents of the BESs obtained by PRO (PRO-BESs) progressively decreased from 1.01 A/m² to 0.12 A/m². The nitrate removal rates of the PRO-BESs were <50 %, and the product of nitrate reduction turned to N₂ instead of ammonium. 16S rDNA sequencing and metatranscriptomic analysis revealed that Geobacter capable of bidirectional extracellular electron transfer (EET) and Afipia capable of autotrophic growth were the dominant genera in the two types of BESs. Outer membrane cytochrome c and formate dehydrogenase were potentially involved in the cathodic electron uptake. These findings contribute to a better understanding of the EET mechanisms of electroautotrophic denitrifiers.

Kinetics and scale up of oxygen reducing cathodic biofilms

The goals of this work were to study the kinetics and investigate the factors controlling the scale up of oxygen reducing mixed culture cathodic biofilms. Cathodic biofilms were enriched on different electrode sizes (14.5 cm2, 40.3 cm2, 131 cm2 and 466 cm2). Biofilm enrichment shifted the oxygen reduction onset potential from -0.1 VAg/AgCl to 0.3 VAg/AgCl, indicating the biofilm catalyzed oxygen reduction. The kinetics of oxygen reduction were studied by varying the bulk dissolved oxygen concentration. Oxygen reduction followed a Michaelis-Menten kinetics on all electrode sizes. The maximum current density decreased with increasing electrode surface area (-97.0 ± 10.6 μA/cm2, -76.0 ± 8.2 μA/cm2, -66.3 ± 3.0 μA/cm2 and -43.5 ± 10.5 μA/cm2, respectively). Cyclic voltammograms suggest that scale up was limited by ohmic resistance, likely due to the low ionic conductivity in the wastewater medium. Mathematical modeling using combined Michaelis-Menten and Butler-Volmer model supports that the decrease in current density with increasing electrode surface area is caused by ohmic losses. Analysis of the microbial community structure in different size electrodes and in multiple regions on the same electrode showed low variability, suggesting that the microbial community does not control the scale up of cathodic biofilms.

Hypersaline microbial fuel cell equipped with an oxygen-reducing microbial cathode

Microbial anodes and oxygen reducing microbial cathodes were designed separately under constant polarization at + 0.1 V/SCE in a hypersaline medium (NaCl 45 g/L). They were then associated to design two-compartment microbial fuel cells (MFCs). These MFCs produced up to 209 ± 24 mW m^-2 during a week. This was the first demonstration that hypersaline MFCs equipped with microbial cathodes can produce power density at this level. Desulfuromonas sp. were confirmed to be key species of the anodes. The efficiency of the cathodes was linked to the development of a redox system centred at + 0.2 V/SCE and to the presence of Gammaproteobacteria (Alteromonadales and Oceanospirillales), especially an unclassified order phylogenetically linked to the genus Thioalobacter. Comparing the different performance of the four MFCs with the population analyses suggested that polarization at + 0.1 V/SCE should be maintained longer to promote the growth of Thioalobacter on the cathode and thus increase the MFC performance.

Catalysis of the electrochemical oxygen reduction reaction (ORR) by animal and human cells

Animal cells from the Vero lineage and MRC5 human cells were checked for their capacity to catalyse the electrochemical oxygen reduction reaction (ORR). The Vero cells needed 72 hours’ incubation to induce ORR catalysis. The cyclic voltammetry curves were clearly modified by the presence of the cells with a shift of ORR of 50 mV towards positive potentials and the appearance of a limiting current (59 μA.cm^-2). The MRC5 cells induced considerable ORR catalysis after only 4 h of incubation with a potential shift of 110 mV but with large experimental deviation. A longer incubation time, of 24 h, made the results more reproducible with a potential shift of 90 mV. The presence of carbon nanotubes on the electrode surface or pre-treatment with foetal bovine serum or poly-D-lysine did not change the results. These data are the first demonstrations of the capability of animal and human cells to catalyse electrochemical ORR. The discussion of the possible mechanisms suggests that these pioneering observations could pave the way for electrochemical biosensors able to characterize the protective system of cells against oxidative stress and its sensitivity to external agents.

Electrochemical and phylogenetic comparisons of oxygen-reducing electroautotrophic communities

The mechanisms of extracellular electron transfer and the microbial taxa associated with the observed electroactivity are fundamental to oxygen-reducing microbial cathodes. Here we confirmed the apparent 'electroautotrophic' behavior of electroactive biofilms (EABs) grown on carbon electrodes at + 0.20V vs. Ag/AgCl under air. The EABs catalyzed O₂ electroreduction into water ─ as demonstrated by a rotating ring disc experiment ─ and performed quasi-reversible heterogeneous electron transfer (HET). By using electrodes of low surface capacitance, we report for the first time nonturnover redox peaks that are very likely intrinsic to the redox protein(s) performing the HET. Because the formal potential of redox proteins is pH-dependent, we investigated the evolution of characteristic potentials of the EABs with the solution pH: (i) open circuit potential, (ii) half-wave potential, and (iii) averaged peak potential of nonturnover cyclic voltammograms, which is presumably the formal potential of the primary electron acceptor(s) for the community. In addition to describing the redox thermodynamics behind HET, we suggest that the corresponding data provides an electrochemical fingerprint that could help in comparing the electroactivity of diverse microbial communities. The taxon with the highest relative abundance in our EABs was an unclassified member of the Gammaproteobacteria that was phylogenetically closely related to most other abundant unclassified Gammaproteobacteria commonly reported in EABs reducing O₂ at high potentials, further suggesting that those taxa are responsible for the bioelectroactivity. Phylogenetic and electrochemical similarities between reported EABs jointly support the hypothesis that similar biomolecular mechanisms may be responsible for this highly probable electroautotrophic metabolism.

Oxygen-reducing microbial cathodes in hypersaline electrolyte

Hypersaline electrolytes offer a way to boost the development of microbial fuel cells by overcoming the issue due to the low conductivity of the usual media. Efficient halotolerant bioanodes have already been designed but O₂-reducing cathodes remain a strong bottleneck. Here, O₂-reducing biocathodes were designed by using salt marsh sediment as the inoculum and a hypersaline media (45 g/L NaCl) of high conductivity (10.4 S m^-1). Current density up to 2.2 A m^-2 was reached from potential of +0.2 V/SCE. The efficiency of the biocathodes was correlated to the presence of Gammaproteobacteria strain(s) related to Thiohalobacter thiocyanaticus, which were considerably enriched in the best performing biocathodes. This work opens up new perspectives to overcome the O₂ reduction issue in hypersaline MFCs by designing efficient halotolerant microbial cathodes and pointing out the strains that should now be focused to improve them.

High-efficiency salt, sulfate and nitrogen removal and microbial community in biocathode microbial desalination cell for mustard tuber wastewater treatment

Considering there is no study involving simultaneous salt, sulfate and nitrogen removal from high-salinity mustard tuber wastewater (MTWW), biocathode microbial desalination cell (BMDC) was first constructed and used to treat MTWW. The results showed that 97.4% of salt, 99.7% of sulfate and 99.8% of nitrogen could be removed from MTWW. The relative abundances of electrgenic bacteria in anode and cathode were 15.95% and 15.10%, respectively, which greatly promoted the electricity generation and desalination. The bacteria involved in sulfate reduction in anode were the dominant population, with relative abundance of 13.94%. Microbial community analysis of cathode biofilm indicated that autotrophic nitrification-anaerobic denitrification, electrochemical reduction and anaerobic ammonium oxidation might coexist for high-efficiency nitrogen removal. Besides, the BMDC showed stable power output for 150 days. These findings provide a promising approach for efficient treatment of MTWW.

Organotrophic acid-tolerant microorganisms enriched from an acid mine drainage affected environment as inoculum for microbial fuel cells

Exoelectrogenic communities for bioelectrochemical systems such as microbial fuel cells (MFCs) are usually enriched from microbial consortia of municipal wastewater treatment plants and other circumneutral and mesophilic environments. Thus, the study of extreme environments offers an enormous potential to find new exoelectrogens and expand the functionality and applications of MFC technology. In this study, a microbial community previously enriched from acid mine drainage (AMD) sediments was used as inoculum in single-chamber MFCs operated at pH 3.7. The power obtained from the AMD-derived inoculum reached 1 mW m^-2 (27.1 ± 7.8 mV with 1 kΩ external resistance), which compares to previous MFC studies operated under low-pH conditions. Additionally, polarization curves showed power-generation levels of 2.4 ± 0.2 mW m^-2 and 0.4 ± 0.3 mW m^-2, which were associated with the different inoculum sources: MFCs operated with sulfate concentrations of ~2000 and < 25 mg L^-1, respectively. Microbial characterization performed at the end of the operation showed that both anodic and cathodic biofilm communities were highly dominated by the Proteobacteria phylum (>72% of 16S rRNA gene sequences), followed by Firmicutes (4-11%). Furthermore, the anodic microbial communities of the best-performing reactors were dominated by the Delftia genus (phylum Proteobacteria), which was recently identified as a taxon including exoelectrogenic candidates. These findings expand the literature of low-pH operated MFCs and acid-tolerant exoelectrogens, and also represent a starting point to apply this technology to treat acidic organic loads.

Electrochemical Corrosion Prevention in Oilfield Wastewater for Effective Dissolved Oxygen Removal Using a Novel Upflow Bioelectrochemical System

Towards the corrosion issues of oilfield wastewater for water recycling, the dissolved oxygen (DO) is a subsequent corrosive factor after the air desulfurization tower for high-efficiency removal of sulfides. However, an in situ biological technology for efficient DO removal has not been well developed by using organics in oilfield wastewater. A novel upflow bioelectrocatalytic system assembled with three electrodes (cathode-anode-cathode) was designed in this study, in which waste organic matter of oil wastewater was degraded by a bioanode for electron production and dissolved oxygen was efficiently reduced by a biocathode under an assistant external voltage. The results showed that the average current was kept over 6 mA by applying a fixed voltage of 0.8 V to treat oil wastewater with DO as high as 3–5 mg/L. The bottom cathode contributed the largest to DO removal rate, reaching 67%; contribution of the middle anode and the upper cathode for DO removal was 11% and 9%, respectively. The whole DO removal rate by the bioelectrocatalytic system was up to about 90%, and the effluent DO was reduced to below 0.6 mg/L by removing 40–50% COD.

Electroactive Bacteria Associated With Stainless Steel Ennoblement in Seawater

Microorganisms can increase the open-circuit potential of stainless steel immersed in seawater of several hundred millivolts in a phenomenon called ennoblement. It raises the chance of corrosion as the open-circuit potential may go over the pitting corrosion potential. Despite the large impact of the ennoblement, no unifying mechanisms have been described as responsible for the phenomenon. Here we show that the strict electrotroph bacterium "Candidatus Tenderia electrophaga" is detected as an ennoblement biomarker and is only present at temperatures at which we observe ennoblement. This bacterium was previously enriched in biocathode systems. Our results suggest that "Candidatus Tenderia electrophaga," and its previously described extracellular electron transfer metabolism coupled to oxygen reduction activity, could play a central role in modulating stainless steel open-circuit potential and consequently mediating ennoblement.

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.

Metagenomic insights into the ecology and physiology of microbes in bioelectrochemical systems

In bioelectrochemical systems (BESs), electrons are transferred between electrochemically active microbes (EAMs) and conductive materials, such as electrodes, via extracellular electron transfer (EET) pathways, and electrons thus transferred stimulate intracellular catabolic reactions. Catabolic and EET pathways have extensively been studied for several model EAMs, such as Shewanella oneidensis MR-1 and Geobacter sulfurreducens PCA, whereas it is also important to understand the ecophysiology of EAMs in naturally occurring microbiomes, such as those in anode biofilms in microbial fuel cells treating wastewater. Recent studies have exploited metagenomics and metatranscriptomics (meta-omics) approaches to characterize EAMs in BES-associated microbiomes. Here we review recent BES studies that used meta-omics approaches and show that these studies have discovered unexpected features of EAMs and deepened our understanding of functions and behaviors of microbes in BESs. It is desired that more studies will employ meta-omics approaches for advancing our knowledge on microbes in BESs.