Anode-respiring Bacteria Research Articles

Anodic microbial community analysis of microbial fuel cells based on enriched inoculum from freshwater sediment.

The characterization of anodic microbial communities is of great importance in the study of microbial fuel cells (MFCs). These kinds of devices mainly require a high abundance of anode respiring bacteria (ARB) in the anode chamber for optimal performance. This study evaluated the effect of different enrichments of environmental freshwater sediment samples used as inocula on microbial community structures in MFCs. Two enrichment media were compared: ferric citrate (FeC) enrichment, with the purpose of increasing the ARB percentage, and general enrichment (Gen). The microbial community dynamics were evaluated by polymerase chain reactionfollowed by denaturing gradient gel electrophoresis (PCR-DGGE) and real time polymerase chain reaction (qPCR). The enrichment effect was visible on the microbial community composition both during precultures and in anode MFCs. Both enrichment approaches affected microbial communities. Shannon diversity as well as β-Proteobacteria and γ-Proteobacteria percentages decreased during the enrichment steps, especially for FeC (p < 0.01). Our data suggest that FeC enrichment excessively reduced the diversity of the anode community, rather than promoting the proliferation of ARB, causing a condition that did not produce advantages in terms of system performance.

Electro-selective fermentation enhances lipid extraction and biohydrogenation of Scenedesmus acutus biomass

Extraction of lipids from microalgae has been economically and energy prohibitive due to the requirement for pretreatment, and the use of toxic solvents in the process has exacerbated the problem. Selective Fermentation (SF) helps overcome these problems by selectively biodegrading carbohydrate and protein, while conserving lipids. We introduced and evaluated electro-selective fermentation (ESF), in which anode respiration in a microbial electrolysis cell (MEC) altered fermentation performance in ways that enhanced lipid wet-extraction. Using Scenedesmus acutus biomass, we evaluated ESF and compared it to SF. Compared to SF, ESF increased protein degradation by 4-fold, even though anode respiration represented <1% of the total electron input. Although ESF led to greater loss of total lipids, it improved lipid wet-extraction efficiency using a non-toxic solvent by 3-fold. Lipid loss was due to β-oxidation linked to biohydrogenation, and the long-chain fatty acid (LCFA) profile shifted from C18:1 to C16:0 and C14:0. Microbial community analysis emphasized the presence of protein-degrading bacteria and biohydrogenators in the ESF suspension and anode-respiring bacteria (ARB) on the ESF anode. Overall, ESF improved lipid extractability and biofuel quality despite current being a small component in the overall COD balance.

Impact of antimicrobial silver nanoparticles on anode respiring bacteria in a microbial electrolysis cell

This study assessed the impact of antimicrobial AgNPs (50 mg L−1, 30–50 nm) on the electrocatalytic activity of a mixed-culture anode biofilm enriched with Geobacter species. The current densities and electrochemical kinetics were maintained after exposure to AgNPs in consecutive fed-batch cycles, despite significant changes in morphological structures and bacterial communities. Bacterial community analysis showed a substantial increase in the Geobacter population in response to AgNPs exposure, indicating their higher tolerance to AgNPs. In contrast, the population of other anode respiring bacteria (ARB) belongs to Acinetobacter, Dysgonomonas, and Cloacibacillus genera appeared to be very sensitive to AgNPs toxicity as their relative abundance significantly decreased. Microscopic imaging showed that AgNPs were accumulated within anode biofilm matrix without penetration inside the cells. Moreover, the anode biofilm became denser because of enhanced extracellular polymeric substances (EPSs) production by ARB after exposure of AgNPs, implying that EPS could protect ARB against AgNPs toxicity.

Electrokinetic behaviour of conducting polymer modified stainless steel anodes during the enrichment phase in microbial fuel cells

Two of the major bottlenecks in achieving large-scale power generation with MFCs to date are the low power output and the usually long start-up time, both of which are mainly associated with poor bacterial kinetics and inefficient anode electrode materials. We have demonstrated in this study that the electron transfer kinetics of stainless steel (SS) can be significantly improved by modification with polyaniline (PANi) and polypyrrole (PPy). Furthermore, we have demonstrated that the kinetics of the bacterial growth can be significantly enhanced by the application of a carefully selected external resistance (Rext), resulting in significantly shorter start-up time. The half-cell reactors used for the investigations were enriched under different conditions including without Rext (Open circuit mode), with Rext = Rint (Ohmic region), and with very low Rext (mass transfer region). The MFC anodes enriched under Rext = Rint gave maximum exchange current density (j0max) on the 4th day of operation. The calculated j0max for SS wool, PANi-wool, and PPy-wool anodes were 0.3 ± 0.2 A m−2, 10.5 ± 0.4 A m−2 and 5.0 ± 0.4 A m−2, respectively. The lowest charge transfer resistance (Rct) of 0.23 Ω cm−2 was obtained with SS/PANi-wool anode which exhibited the highest electron transfer kinetics and better compatibility than SS/PPy-wool. The high current drawn from the system during the biofilm establishment phase did not support electroactive biofilm formation because it prevented the growing anode-respiring bacteria (ABR) from providing sufficient electron flow to the counter electrode.

Self-standing Electrochemical Set-up to Enrich Anode-respiring Bacteria On-site

Self-standing Electrochemical Set-up to Enrich Anode-respiring Bacteria On-site

Self-standing Electrochemical Set-up to Enrich Anode-respiring Bacteria On-site.

Anaerobic respiration coupled with electron transport to insoluble minerals (referred to as extracellular electron transport [EET]) is thought to be critical for microbial energy production and persistence in many subsurface environments, especially those lacking soluble terminal electron acceptors. While EET-capable microbes have been successfully isolated from various environments, the diversity of bacteria capable of EET is still poorly understood, especially in difficult-to-sample, low energy or extreme environments, such as many subsurface ecosystems. Here, we describe an on-site electrochemical system to enrich EET-capable bacteria using an anode as a respiratory terminal electron acceptor. This anode is connectedto a cathode capable ofcatalyzing abiotic oxygen reduction. Comparing this approach with electrocultivation methods that use a potentiostat for poising the electrode potential, the two-electrode system does not require an external power source. We present an example of our on-site enrichment utilized in an alkaline pond at the Cedars, a terrestrial serpentinization site in Northern California. Prior attempts to cultivate mineral reducing bacteria were unsuccessful, which is likely due to the low-biomass nature of this site and/or the low relative abundance of metal reducing microbes. Prior to implementing our two-electrode enrichment, we measured the vertical profile of dissolved oxygen concentration. This allowed us to place the carbon felt anode and platinum-electroplated carbon felt cathode at depths that would support anaerobic and aerobic processes, respectively. Following on-site incubation, we further enriched the anodic electrode in the laboratory and confirmeda distinct microbial community compared to the surface-attached or biofilm communities normally observed at the Cedars. This enrichment subsequently led to the isolation of the first electrogenic microbe from the Cedars. This method of on-site microbial enrichment has the potential to greatly enhance the isolation of EET-capable bacteria from low biomass or difficult to sample habitats.

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

Monitoring the electrochemical response of anode respiring bacteria (ARB) helps elucidate the fundamental processes of anode respiration and their rate limitations. Understanding these limitations provides insights on how ARB create the complex interfacing of biochemical metabolic processes with insoluble electron acceptors and electronics. In this study, anode biofilms of the thermophilic (60 °C) Gram-positive ARB Thermincola ferriacetica were studied to determine the presence of a proton-dependent electron transfer response. The effects of pH, the presence of an electron donor (acetate), and biofilm growth were varied to determine their influence on the electrochemical midpoint potential ( EKA) and formal redox potential ( E°') under nonturnover conditions. The EKA and E°' are associated with an enzymatic process within ARB's metabolism that controls the rate and energetic state of their respiration. Results for all conditions indicate that pH was the major contributor to altering the energetics of T.ferriacetica anode biofilms. Electrochemical responses measured in the absence of an electron donor and with a minimal proton gradient within the anode biofilms resulted in a 48 ± 7 mV/pH unit shift in the E°', suggesting a proton-dependent rate-limiting process. Given the limited energy available for anode respiration (<200 mV when using acetate as electron donor), our results provide a new perspective in understanding proton-transport limitations in ARB biofilms, one in which ARB are thermodynamically limited by pH gradients. Since the anode biofilms of all ARB that perform direct extracellular electron transfer (EET) investigated thus far exhibit an n = 1 Nernstian behavior, and because this behavior is affected by changes in pH, we hypothesize that the Nernstian response is associated with membrane proteins responsible for proton translocation. Finally, this study shows that the EKA and E°' are a function of pH within the physiological range of ARB, and thus, given the significant effect pH has on this parameter, we recommend reporting the EKA and E°' of ARB biofilms at a specific bulk pH.

Photocatalytic-induced electron transfer via anode-respiring bacteria (ARB) at an anode that intimately couples ARB and a TiO2 photocatalyst

A novel anode that intimately couples anode-respiring bacteria (ARB) with a nitrogen-doped TiO2 photocatalyst on a porous carbon foam electrode (the ICPB-anode) was fabricated and experimentally tested in an electrochemical cell. ARB are well known for obtaining energy by transferring bio-metabolized electrons to the anode, an external solid acceptor. The goal of this work was to identify if and how ARB play a role in transporting photo-generated electrons. When simulated visible-light illuminated an ICPB-anode, the current increased by ∼3 A/m2 (∼30% of the total), due to photocatalytically generated electrons (photo-electrons). In contrast, an abiotic photocatalyst anode (photo-anode) was incapable of transfering photo-electrons. Compared to a non-photocatalytic biofilm-anode, the Coulombic efficiency increased by 15–20% with the ICPB-anode, and the effect was due to the addition of electron flow from photocatalytic water electrolysis, since oxidation of the organic electron donor was unchanged. Electrochemical impedance analyses showed that the Ohmic resistance of the ICPB-anode decreased by ∼98%, compared with the abiotic photo-anode. A redox-potential window typical of ARB’s c-type cytochromes was up-regulated when the ARB transported photo-electrons, supporting that c-type cytochromes were the significant redox proteins for transferring photo-electrons. All the results combine to show that ARB were responsible for the transport of electrons generated by photocatalysis in the ICPB-anode.

Bacterial communities adapted to higher external resistance can reduce the onset potential of anode in microbial fuel cells

We investigated how bacterial communities adapted to external resistances and exhibited the performance of electricity production in microbial fuel cells (MFCs) with external resistance of 10Ω (LR-MFC) and 1000Ω (HR-MFC). The HR-MFC exhibited better performance than the LR-MFC. The power densities of the LR-MFC and the HR-MFC were 5.2±1.6mWm-2 and 28±9.6mWm-2 after day 197, respectively. Low-scan cyclic voltammetry analyses indicated that the onset potential of the HR-MFC was more negative than that of the LR-MFC, suggesting that the higher external resistance led to enrichment of the highly current producing bacteria on the anode surface. All clones of Geobacter retrieved from the LR-MFC and the HR-MFC were members of the Geobacter metallireducens clade. Although the population density of Geobacter decreased from days 366-427 in the HR-MFC, the current density was almost maintained. Multidimensional scaling analyses based on denaturing gradient gel electrophoresis profiles indicated that the dynamics of the biofilm and anolytic communities changed synchronously in the two MFCs, but the dynamics of the bacterial communities in the LR-MFC and the HR-MFC were different from each other, reflecting different processes in adaptation to the different external resistances. The results suggest that the microbial community structure was formed by adapting to higher external resistance, exhibiting more negative onset potential and higher performance of the HR-MFC through collaborating with anode-respiring bacteria and fermenters.

Characterization of the Electric Current Generation Potential of the Pseudomonas aeruginosa Using Glucose, Fructose, and Sucrose in Double Chamber Microbial Fuel Cell.

Background: Different concentrations of the simple carbon substrates i.e. glucose, fructose, and sucrose were tested to enhance the performance of the mediator-less double chamber microbial fuel cell (MFC). Objectives: The power generation potential of the different electron donors was studied using a mesophilic Fe (III) reducer and non-fermentative bacteria Pseudomonas aeruginosa-isolated from municipal wastewater. Materials and Methods: A double chamber MFC was operated with three different electron donors including glucose, sucrose, and fructose. Substrate utilization pattern was determined through chemical oxygen demand (COD) removal rate and voltage generation. In addition, electrochemical, physicochemical, and microscopic analysis of the anodic biofilm was conducted. Results:P. aeruginosa was proven to effectively utilize hexose and pentose sugars through anode respiration. Higher power density was generated from glucose (136 ± 87 mWm2) lead by fructose (3.6 ± 1.6 mWm2) and sucrose (8.606 ± mWm2). Furthermore, a direct relation was demonstrated between current generation rate and COD removal efficiency. COD removal rates were, 88.5% ± 4.3%, 67.5% ± 2.6%, and 54.2% ± 1.9% with the three respective sugars in MFC. Scanning electron microscopy (SEM) demonstrated that the bacterial attachment was considerably abundant in glucose fed MFC than in the fructose and sucrose operated MFC. Conclusion: This study has revealed that electron donor type in the anodic compartment controls the growth of anodic biofilm or anode-respiring bacteria (ARB).

Biofilms, active substrata, and me.

Having worked with biofilms since the 1970s, I know that they are ubiquitous in nature, of great value in water technology, and scientifically fascinating. Biofilms are naturally able to remove BOD, transform N, generate methane, and biodegrade micropollutants. What I also discovered is that biofilms can do a lot more for us in terms of providing environmental services if we give them a bit of help. Here, I explore how we can use active substrata to enable our biofilm partners to provide particularly challenging environmental services. In particular, I delve into three examples in which an active substratum makes it possible for a biofilm to accomplish a task that otherwise seems impossible. The first example is the delivery of hydrogen gas (H2) as an electron donor to drive the reduction and detoxification of the rising number of oxidized contaminant: e.g., perchlorate, selenate, chromate, chlorinated solvents, and more. The active substratum is a gas-transfer membrane that delivers H2 directly to the biofilm in a membrane biofilm reactor (MBfR), which makes it possible to deliver a low-solubility gaseous substrate with 100% efficiency. The second example is the biofilm anode of a microbial electrochemical cell (MxC). Here, the anode is the electron acceptor for anode-respiring bacteria, which "liberate" electrons from organic compounds and send them ultimately to a cathode, where we can harvest valuable products or services. The anode's potential is a sensitive tool for managing the microbial ecology and reaction kinetics of the biofilm anode. The third example is intimately coupled photobiocatalysis (ICPB), in which we use photocatalysis to enable the biodegradation of intrinsically recalcitrant organic pollutants. Photocatalysis transforms the recalcitrant organics just enough so that the products are rapidly biodegradable substrates for bacteria in a nearby biofilm. The macroporous substratum, which houses the photocatalyst on its exterior, actively provides donor substrate and protects the biofilm from UV light and free radicals in its interior. These three well-developed topics illustrate how and why an active substratum expands the scope of what biofilms can do to enhance water sustainability.

Changes in Glucose Fermentation Pathways as a Response to the Free Ammonia Concentration in Microbial Electrolysis Cells.

When a mixed-culture microbial electrolysis cell (MEC) is fed with a fermentable substrate, such as glucose, a significant fraction of the substrate's electrons ends up as methane (CH4) through hydrogenotrophic methanogenesis, an outcome that is undesired. Here, we show that free ammonia-nitrogen (FAN, which is NH3) altered the glucose fermentation pathways in batch MECs, minimizing the production of H2, the "fuel" for hydrogenotrophic methanogens. Consequently, the Coulombic efficiency (CE) increased: 57% for 0.02 g of FAN/L of fed-MEC, compared to 76% for 0.18 g of FAN/L of fed-MECs and 62% for 0.37 g of FAN/L of fed-MECs. Increasing the FAN concentration was associated with the accumulation of higher organic acids (e.g., lactate, iso-butyrate, and propionate), which was accompanied by increasing relative abundances of phylotypes that are most closely related to anode respiration (Geobacteraceae), lactic-acid production (Lactobacillales), and syntrophic acetate oxidation (Clostridiaceae). Thus, the microbial community established syntrophic relationships among glucose fermenters, acetogens, and anode-respiring bacteria (ARB). The archaeal population of the MEC fed 0.02 g FAN/L was dominated by Methanobacterium, but 0.18 and 0.37 g FAN/L led to Methanobrevibacter becoming the most abundant species. Our results provide insight into a way to decrease CH4 production and increase CE using FAN to control the fermentation step, instead of inhibiting methanogens using expensive or toxic chemical inhibitors, such as 2-bromoethanesulfonic acid.

The impact of electron donors and anode potentials on the anode-respiring bacteria community.

Both anode potentials and substrates can affect the process of biofilm formation in bioelectrochemical systems, but it is unclear who primarily determine the anode-respiring bacteria (ARB) community structure and composition. To address this issue, we divided microbial electrolysis cells (MECs) into groups, feeding them with different substrates and culturing them at various potentials. Non-turnover cyclic voltammetry indicated that the extracellular electron transfer components were uniform when feeding acetate, because the same oxidation peaks occurred at -0.36±0.01 and -0.17±0.01V (vs. Ag/AgCl). Illumina MiSeq sequencing revealed that the dominating ARB was Geobacter, which did not change with different potentials. When the MECs were cultured with sucrose and mixed substrates, oxidation peak P3 (-0.29±0.015V) occurred at potentials of -0.29 and 0.01V. This may be because of the appearance of Unclassified_AKYG597. In addition, oxidation peak P4 (-0.99±0.01V) occurred at high and low potentials (0.61 and -0.45V, respectively), and the maximum current densities were far below those of the middle potentials. Illumina MiSeq sequencing showed that fermentation microorganisms (Lactococcus and Sphaerochaeta) dominated the biofilms. Consequently, substrate primarily determined the dominating ARB, and Geobacter invariably dominated the acetate-fed biofilms with potentials changed. Conversely, different potentials mainly affected fermentable substrate-fed biofilms, with dominating ARB turning into Unclassified_AKYG59.

Impact of interfacial charge transfer on the start-up of bioelectrochemical systems

In order to increase the efficiency of microbial fuel cells and related bioelectrochemical systems (BESs), one of the common approaches is to lower the resistance of the anode surface to increase the extracellular electron transfer (EET) of anode respiring bacteria (ARB). As our work demonstrates here, this approach is not ideal when dealing with common species of ARB. Bacteria colonized an electrode surface modified with graphite material doped with nonconductive calcium sulfide (CaS) more favorably than the conductive magnetite (Fe3O4) or semiconductive iron (II) sulfide (FeS). Average anodic current densities of 8.2±0.25Am−2 (Fe3O4), 10.7±0.46Am−2 (FeS) and 21.3±1.12Am−2 (CaS) were achieved as compared to that of non-doped activated carbon (5.04±0.12Am−2). Bioelectrochemical evaluation during growth using simple low-scan (1mVs−1) cycle voltammetry (LSCV) indicated variations in patterns which reflect the variability of the ARBs growth. On the other hand, despite the high affinity of bacteria to grow at a faster rate on Fe3O4-anode and CaS-anode, as indicated by the maximum specific growth rate during the start-up exponential phase, the kinetic scan rate study of derivative cycle voltammetry (DCVs) during growth indicated accumulation of bacteria-produced mediators on iron containing anodes which reduced their electrochemical activity. Thus, irrespective of surface resistance, the CaS doped graphite represented a promising anode material which is suitable for highly efficient BES.

Simultaneous fermentation of cellulose and current production with an enriched mixed culture of thermophilic bacteria in a microbial electrolysis cell.

SummaryAn enriched mixed culture of thermophilic (60°C) bacteria was assembled for the purpose of using cellulose to produce current in thermophilic microbial electrolysis cells (MECs). Cellulose was fermented into sugars and acids before being consumed by anode‐respiring bacteria (ARB) for current production. Current densities (j) were sustained at 6.5 ± 0.2 A m−2 in duplicate reactors with a coulombic efficiency (CE) of 84 ± 0.3%, a coulombic recovery (CR) of 54 ± 11% and without production of CH 4. Low‐scan rate cyclic voltammetry (LSCV) revealed a mid‐point potential (E ka) of −0.17 V versus SHE. Pyrosequencing analysis of the V4 hypervariable region of 16S rDNA and scanning electron microscopy present an enriched thermophilic microbial community consisting mainly of the phylum Firmicutes with the Thermoanaerobacter (46 ± 13%) and Thermincola (28 ± 14%) genera occupying the biofilm anode in high relative abundance and Tepidmicrobium (38 ± 6%) and Moorella (11 ± 8%) genera present in high relative abundance in the bulk medium. The Thermoanaerobacter (15 ± 16%) and Brevibacillus (21 ± 30%) genera were also present in the bulk medium; however, their relative abundance varied by reactor. This study indicates that thermophilic consortia can obtain high CE and CR, while sustaining high current densities from cellulose in MECs.

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.

Response of microbial community structure to pre-acclimation strategies in microbial fuel cells for domestic wastewater treatment

Microbial community structures and performance of air-cathode microbial fuel cells (MFCs) inoculated with activated sludge from domestic wastewater were investigated to evaluate the effects of three substrate pre-acclimation strategies: 1, serial pre-acclimation with acetate and glucose before supplying domestic wastewater; 2, one step pre-acclimation with acetate before supplying domestic wastewater; and 3, direct supply of domestic wastewater without any pre-acclimation. Strategy 1 showed much higher current generation (1.4mA) and Coulombic efficiency (33.5%) than strategies 2 (0.7mA and 9.4%) and 3 (0.9mA and 10.3%). Pyrosequencing showed that microbial communities were significantly affected by pre-acclimation strategy. Although Proteobacteria was the dominant phylum with all strategies, Actinobacteria was abundant when MFCs were pre-acclimated with glucose after acetate. Not only anode-respiring bacteria (ARB) in the genus Geobacter but also non-ARB belonging to the family Anaerolinaceae seemed to play important roles in air-cathode MFCs to produce electricity from domestic wastewater.

Polarity inversion of bioanode for biocathodic reduction of aromatic pollutants

The enrichment of specific pollutant-reducing consortium is usually required prior to the startup of biocathode bioelectrochemical system (BES) and the whole process is time consuming. To rapidly establish a non-specific functional biocathode, direct polar inversion from bioanode to biocathode is proposed in this study. Based on the diverse reductases and electron transfer related proteins of anode-respiring bacteria (ARB), the acclimated electrochemically active biofilm (EAB) may catalyze reduction of different aromatic pollutants. Within approximately 12 d, the acclimated bioanodes were directly employed as biocathodes for nitroaromatic nitrobenzene (NB) and azo dye acid orange 7 (AO7) reduction. Our results indicated that the established biocathode significantly accelerated the reduction of NB to aniline (AN) and AO7 to discolored products compared with the abiotic cathode and open circuit controls. Several microbes possessing capabilities of nitroaromatic/azo dye reduction and bidirectional electron transfer were maintained or enriched in the biocathode communities. Cyclic voltammetry highlighted the decreased over-potentials and enhanced electron transfer of biocathode as well as demonstrated the ARB Geobacter containing cytochrome c involved in the backward electron transfer from electrode to NB. This study offers new insights into the rapid establishment and modularization of functional biocathodes for the potential treatment of complicated electron acceptors-coexisting wastewaters.

Intimate coupling of an N-doped TiO2 photocatalyst and anode respiring bacteria for enhancing 4-chlorophenol degradation and current generation

Titanium-dioxide (TiO2) semi-conductors are promising for microbial-fuel-cell anodes, because they can accelerate the biodegradation of refractory organic pollutants while recovering electrical current. To make the coupling of TiO2 photocatalysis and biodegradation a success, the anode’s biofilm must be protected from damage from reactive-oxygen species generated by photocatalysis. In this work, we first realized a photocatalytic bioanode using N-doped TiO2 coated on macroporous carbon-foam that accumulated biofilm inside. Photocatalysis occurred on the outer surface, while bacteria were protected inside the foam matrix; this is a unique manifestation of intimately coupled photobiocatalysis (ICPB). Experiments focused on degradation of 4-chlorophenol (4-CP) and electrochemical characterization of the ICPB-anode. The illuminated photo-anode, non-photocatalytic bio-anode, and ICPB-anode achieved ∼10%, ∼28%, and ∼41% 4-CP degradation efficiency, respectively; clearly, the ICPB anode achieved the best performance for 4-CP removal. The corresponding mineralization efficiency of the ICPB-anode also was the highest, and current generation by the ICPB-anode was 50% greater than that of a bio-anode. Cyclic voltammetry showed that photocatalyst and biofilm had to be present together to achieve high current density, and it also suggested that the electron-transport activity of c-type cytochromes of anode-respiring bacteria played an essential role in the transport of electrons.

Electrochemical techniques reveal that total ammonium stress increases electron flow to anode respiration in mixed‐species bacterial anode biofilms

When anode-respiring bacteria (ARB) respire electrons to an anode in microbial electrochemical cells (MXCs), they harvest only a small amount of free energy. This means that ARB must have a high substrate-oxidation rate coupled with a high ratio of electrons used for respiration compared to total electrons removed by substrate utilization. It also means that they are especially susceptible to inhibition that slows anode respiration or lowers their biomass yield. Using several electrochemical techniques, we show that a relatively high total ammonium-nitrogen (TAN) concentration (2.2 g TAN/L) induced significant stress on the ARB biofilms, lowering their true yield and forcing the ARB to boost the ratio of electrons respired per electrons consumed from the substrate. In particular, a higher respiration rate, measured as current density (j), was associated with slower growth and a lower net yield, compared to an ARB biofilm grown with a lower ammonium concentration (0.2 g TAN/L). Further increases in influent TAN (to 3 and then to 4.4 g TAN/L) caused nearly complete inhibition of anode respiration. However, the ARB could recover from high-TAN inhibition after a shift of the MXC's feed to 0.2 g TAN/L. In summary, ARB biofilms were inhibited by a high TAN concentration, but could divert more electron flow toward anode respiration with modest inhibition and recover when severe inhibition was relieved. Biotechnol. Bioeng. 2017;114: 1151-1159. © 2017 Wiley Periodicals, Inc.