Current production by bacterial communities in microbial fuel cells enriched from wastewater sludge with different electron donors

Enriching electroactive microorganisms from ferruginous lake waters - Mind the sulfate reducers!

Electroactive microorganisms are pivotal players in mineral transformation within redox interfaces characterized by pronounced oxygen and dissolved metal gradients. Yet, their systematic cultivation from such environments remains elusive. Here, we conducted an anodic enrichment using anoxic ferruginous waters from a post-mining lake as inoculum. Weak electrogenicity (j = ∼5 µA cm-2) depended on electroactive planktonic cells rather than anodic biofilms, with a preference for formate as electron donor. Addition of yeast extract decreased the lag phase but did not increase current densities. The enriched bacterial community varied depending on the substrate composition but mainly comprised of sulfate- and nitrate-reducing bacteria (e.g., Desulfatomaculum spp. and Stenotrophomonas spp.). A secondary enrichment strategy resulted in different bacterial communities composed of iron-reducing (e.g., Klebsiella spp.) and fermentative bacteria (e.g., Paeniclostridium spp.). Secondary electron microscopy and energy-dispersive X-ray spectroscopy results indicate the precipitation of sulfur- and iron-rich organomineral aggregates at the anode surface, presumably impeding current production. Our findings indicate that (i) anoxic waters containing geogenically derived metals can be used to enrich weak electricigens, and (ii) it is necessary to specifically inhibit sulfate reducers. Otherwise, sulfate reducers tend to dominate over EAM during cultivation, which can lead to anode passivation due to biomineralization.

Super-fast Charging Biohybrid Batteries through a Power-to-formate-to-bioelectricity Process by Combining Microbial Electrochemistry and CO2 Electrolysis

Extensive study on renewable energy storage has been sparked by the growing worries regarding global warming. In this study, incorporating the latest advancements in microbial electrochemistry and electrochemical CO2 reduction, a super-fast charging biohybrid battery was introduced by using pure formic acid as an energy carrier. CO2 electrolyser with a slim-catholyte layer and a solid electrolyte layer was built, which made it possible to use affordable anion exchange membranes and electrocatalysts that are readily accessible. The biohybrid battery only required a 3-minute charging to accomplish an astounding 25-hour discharging phase. In the power-to-formate-to-bioelectricity process, bioconversion played a vital role in restricting both the overall Faradaic efficiency and Energy efficiency. The CO2 electrolyser was able to operate continuously for an impressive total duration of 164 hours under Gas Stand-By model, by storing N2 gas in the extraction chamber during stand-by periods. Additionally, the electric signal generated during the discharging phase was utilized for monitoring water biotoxicity. Functional genes related to formate metabolism were identified in the bioanode and electrochemically active bacteria were discovered. On the other hand, Paracoccus was predominantly found in the used air cathode. These results advance our current knowledge of exploiting biohybrid technology.

Structure and core taxa of bacterial communities involved in extracellular electron transfer in paddy soils across China

Microbial communities with extracellular electron transfer (EET) activity are capable of driving geochemical changes and cycles, but a comprehensive understanding of the key microbiota responsible for EET in complex soil matrices is still lacking. Herein, the EET activities, in terms of maximum current density (j_max) and accumulated charge output (C_out), of 41 paddy soils across China were evaluated from the exoelectrogenic properties with a conventional bioelectrochemical system (BES). The j_max with a range of 8.85 × 10^-4 to 0.41 A/m² and C_out with a range of 0.27 to 172.21C were obtained from these soil-based BESs. The bacterial community analyses revealed that the most abundant phylum, order, and genus were Firmicutes, Clostridiales, and Clostridum-sensus-stricto 10, respectively. Bacterial network analysis displayed the positive correlations between the majority of electroactive bacteria-containing genera and multiple other genera, indicating their underlying cooperation for the EET. Partial least squares regression (PLSR) model showed remarkable performance in describing the EET activity with 75 most abundant genera as input variables, identified that 32 genera were very important for governing the EET activities. Multiple linear regression (MLR) analyses further prioritized that the genera norank-c-Berkelbacteria and Fonticella were the key contributors, while the genus Paenibacillus was the key competitor against bacterial exoelectrogenesis in paddy soils. Moreover, the spearman analysis showed that the abundance of these keystone taxa was mainly influenced by the carbon content and pH. This approach provides a promising avenue to monitor the microbial activities in paddy soils as well as the links between microbial community composition and ecological function.

Acetate and electricity generation from methane in conductive fiber membrane- microbial fuel cells

Microbial conversion of methane to electricity, fuels, and liquid chemicals has attracted much attention. However, due to the low solubility of methane, it is not considered a suitable substrate for microbial fuel cells (MFCs). In this study, a conductive fiber membrane (CFM) module was constructed as the bioanode of methane-driven MFCs, directly delivering methane. After biofilm formation on the CFM surface, a steady voltage output of 0.6 to 0.7 V was recorded, and the CFM-MFCs obtained a maximum power density of 64 ± 2 mW/m². Moreover, methane oxidation produced a high concentration of intermediate acetate (up to 7.1 mM). High-throughput 16S rRNA gene sequencing suggests that the microbial community was significantly changed after electricity generation. Methane-related archaea formed a symbiotic consortium with characterized electroactive bacteria and fermentative bacteria, suggesting a combination of three types of microorganisms for methane conversion into acetate and electricity.

Evaluation of plant microbial fuel cells for urban green roofs in a subtropical metropolis

Plant microbial fuel cells (PMFCs) is a sustainable technology that can convert sunlight to electricity through the integration of plants, microorganism and electrode systems. Urban greening, such as green roofs, is considered as one of the measures to resolve the urban heat island effect caused by the increasing urbanization. In this study, PMFCs were installed as green roofs in a subtropical metropolis. During the operation, the biomass of Chinese pennisetum, Dwarf rotala, and Narrowleaf cattail increased from spring to summer. Furthermore, the maximum daily average output voltage of Chinese pennisetum and Narrowleaf cattail PMFCs was 667.94 ± 128.65 mV in March and 451.12 ± 94.37 mV in June, respectively. For no plant conditions, the maximum daily average output voltage of soil MFCs was 243.70 ± 128.93 mV in March and 100.16 ± 23.43 mV in June. However, little output voltage of Dwarf rotala PMFCs indicated different plant species in PMFC systems would result in varied efficiencies of electricity generation. The trends of electricity generation in Chinese pennisetum and Narrowleaf cattail PMFCs were influenced by net solar radiation and air temperature, respectively according to the results of correlation analysis. The PMFCs based green roofs could lower the temperature of underneath floor slabs as many as 24.81 °C and 29.37 °C compared with bare slabs at noon in March and June. Vegetation of the PMFCs could relieve soil heat flux, and simulated results showed Chinese pennisetum PMFCs with higher vegetation had lower U-value for energy savings of air conditioning. Microbial community analysis showed Geobacter was among the dominant genera and had higher relative abundance in anode soils than cathode soils in Chinese pennisetum and Narrowleaf cattail PMFCs, which generated higher output voltage. Our roof-top research demonstrated that using PMFCs based green roofs for urban greening is promising and warrants the potential for future application.

Microorganisms in sediment microbial fuel cells: Ecological niche, microbial response, and environmental function

A sediment microbial fuel cell (SMFC) is a device that harvests electrical energy from sediments rich in organic matter. SMFCs have been attracting increasing amounts of interest in environmental remediation, since they are capable of providing a clean and inexhaustible source of electron donors or acceptors and can be easily controlled by adjusting the electrochemical parameters. The microorganisms inhabiting sediments and the overlying water play a pivotal role in SMFCs. Since the SMFC is applied in an open environment rather than in an enclosed chamber, the effects of the environment on the microbes should be intense and the microbial community succession should be extremely complex. Thus, this review aims to provide an overview of the microorganisms in SMFCs, which few previous review papers have reported. In this study, the anodic and cathodic niches for the microorganisms in SMFCs are summarized, how the microbial population and community interact with the SMFC environment is discussed, a new microbial succession strategy called the electrode stimulation succession is proposed, and recent developments in the environmental functions of SMFCs are discussed from the perspective of microorganisms. Future studies are needed to investigate the electrode stimulation succession, the environmental function and the electron transfer mechanism in order to boost the application of SMFCs for power generation and environmental remediation.

Methane-fueled microbial fuel cells with the formate-acclimating electroactive culture

Methane gas is widespread in natural environments and anaerobic wastewater treatment sites, bringing the risk of the greenhouse effect and energy loss if left unmanaged. A methane-fueled microbial fuel cell (MFC) can convert methane to electricity under mild condition, but faced difficulties in startup. In this study, the new startup strategy and operation performance for methane-fueled MFCs were investigated. After the pre-cultivation of formate-acclimating electroactive culture, the methane-fueled MFC was successfully started up in a short time of 53 d. Increasing concentrations of molybdenum and tungsten in medium facilitated both methane consumption and electricity generation. Under the optimal condition (batch duration of 11 h, 30 °C, pH 7 buffered by phosphate buffer solution), the methane-fueled MFC achieved the maximum power density of 166 mW/m³, a coulomb production of 6.58 ± 0.07C/batch, a CE of 27.4 ± 0.4% and a methane consumption of 31.2 ± 0.3 μmol/batch. This work explored a suitable inoculum (formate-acclimating electroactive culture) for methane-fueled MFCs.

Microbial fuel cell-based biosensor for online monitoring wastewater quality: A critical review

Recently, the application of the microbial fuel cell (MFC)-based biosensor for rapid and real-time monitoring wastewater quality is very innovative due to its simple compact design, disposability, and cost-effectiveness. This review represents recent advances in this emerging technology for the management of wastewater quality, where the emphasis is on biochemical oxygen demand, toxicity, and other environmental applications. In addition, the main challenges of this technology are discussed, followed by proposing possible solutions to those challenges based on the existing knowledge of detection principles and signal processing. Potential future research of MFC-based biosensor has been demonstrated in this review.

Research Tales from the Clardy Laboratory: Function-Driven Natural Product Discovery

Over the past 70 years, the search for small molecules from nature has transformed biomedical research: natural products are the basis for half of all pharmaceuticals; the quest for total synthesis of natural products fueled development of methodologies for organic synthesis; and their biosynthesis presented unprecedented biochemical transformations, expanding our chemo-enzymatic toolkit. Initially, the discovery of small molecules was driven by bioactivity-guided fractionation. However, this approach yielded the frequent rediscovery of already known metabolites. As a result, focus shifted to identifying novel scaffolds through either structure-first methods or genome mining, relegating function as a secondary concern. Over the past two decades, the laboratory of Jon Clardy has taken an alternative route and focused on an ecology-driven, function-first approach in pursuit of uncovering bacterial small molecules with biological activity. In this review, we highlight several examples that showcase this ecology-first approach. Though the highlighted systems are diverse, unifying themes are (1) to understand how microbes interact with their host or environment, (2) to gain insights into the environmental roles of microbial metabolites, and (3) to explore pharmaceutical potential from these ecologically relevant metabolites.

A Review on Microbial Electrocatalysis Systems Coupled with Membrane Bioreactor to Improve Wastewater Treatment

Microbial electrocatalysis is an electro reaction that uses microorganisms as a biocatalyst, mainly including microbial electrolytic cells (MEC) and microbial fuel cells (MFC), which has been used for wastewater treatment. However, the low processing efficiency is the main drawback for its practical application and the additional energy input of MEC system results in high costs. Recently, MFC/MEC coupled with other treatment processes, especially membrane bioreactors (MBR), has been used for high efficiency and low-cost wastewater treatment. In these systems, the wastewater treatment efficiency can be improved after two units are operated and the membrane fouling of MBR can also be alleviated by the electric energy that was generated in the MFC. In addition, the power output of MFC can also reduce the energy consumption of microbial electrocatalysis systems. This review summarizes the recent studies about microbial electrocatalysis systems coupled with MBR, describing the combination types and microorganism distribution, the advantages and limitations of the systems, and also addresses several suggestions for the future development and practical applications.

Novel Gas Diffusion Cloth Bioanodes for High-Performance Methane-Powered Microbial Fuel Cells

Microbial fuel cells (MFCs) are a promising technology that converts chemical energy into electricity. However, up to now only few MFCs have been powered by gas fuels, such as methane, and their limited performance is still challenged by the low solubility and bioavailability of gases. Here, we developed a gas diffusion cloth (GDC) anode to significantly enhance the performance of methane-powered MFCs. The GDC anode was constructed by simply coating waterproof GORE-TEX cloth with conductive carbon cloth in one step. After biofilm enrichment, the GDC anodes obtained a methane-dependent current up to 1130.2 mA m^-2, which was 165.2 times higher than conventional carbon cloth (CC) anodes. Moreover, MFCs equipped with GDC anodes generated a maximum power density of 419.5 mW m^-2. Illumina high-throughput sequencing revealed that the GDC anode biofilm was dominated mainly by Geobacter, in contrast with the most abundant Methanobacterium in planktonic cells. It is hypothesized that Methanobacterium reversed the methanogenesis process by transferring electrons to the anodes, and Geobacter generated electricity via the intermediates (e.g., acetate) of anaerobic methane oxidation. Overall, this work provides an effective route in preparing facile and cost-effective anodes for high-performance methane MFCs.

Geobacter Dominates the Inner Layers of a Stratified Biofilm on a Fluidized Anode During Brewery Wastewater Treatment

In this study, we designed a microbial electrochemical fluidized bed reactor (ME-FBR), with an electroconductive anodic bed made of activated carbon particles for treating a brewery wastewater. Under a batch operating mode, acetate and propionate consumption rates were 13-fold and 2.4-fold higher, respectively, when the fluidized anode was polarized (0.2 V) with respect to open circuit conditions. Operating in a continuous mode, this system could effectively treat the brewery effluent at organic loading rates (OLR) over 1.7 kg m^-3_NRV d^-1 and with removal efficiencies of 95 ± 1.4% (hydraulic retention time of 1 day and an influent of 1.7 g-COD L^-1). The coulombic efficiency values highly depended upon the OLR applied, and varied from a 56 ± 15% to 10 ± 1%. Fluorescence in situ hybridization (FISH) analysis revealed a relative high abundance of Geobacter species (ca. 20%), and clearly showed a natural microbial stratification. Interestingly, the Geobacter cluster was highly enriched in the innermost layers of the biofilm (thickness of 10 μm), which were in contact with the electroconductive particles of bed, whereas the rest of bacteria were located in the outermost layers. To our knowledge, this is the first time that such a clear microbial stratification has been observed on an anode-respiring biofilm. Our results revealed the relevant role of Geobacter in switching between the electrode and other microbial communities performing metabolic reactions in the outermost environment of the biofilm.

High tolerance of and removal of cefazolin sodium in single-chamber microbial fuel cells operation

Single-chamber microbial fuel cells (MFCs) have been shown to be a promising approach for cefazolin sodium (CFZS)-contaminated wastewater treatment, in terms of electricity production, high CFZS tolerance and effective CFZS removal. MFCs exposed to CFZS loadings up to 100 mg L^-1, produced stable power of 18.2 ± 1.1 W m^-3 and a maximum power of 30.4 ± 2.1 W m^-3, similar to that of CFZS-free MFCs (stable power 19.4 ± 0.8 W m^-3 and maximum power 32.5 ± 1.6 W m^-3), notwithstanding a longer acclimitisation MFC activation. More anodophilic genera (i.e. Acinetobacter, Stenotrophomonas, Lysinibacillus) and antibiotic-resisting genera (i.e. Dysgonomonas) were enriched in CFZS acclimitised anodes. Both the thickness of biofilms and the duration of CFZS acclimitisation were essential for the development of high CFZS tolerance (e.g. 450 mg L^-1). The inhibition of MFCs by CFZS was reversible. The present MFCs generated a CFZS removal rate of 1.2-6.8 mg L^-1 h^-1 without any apparent inhibition of electricity production.

Redox Sensing within the Genus Shewanella

A novel bacterial behavior called congregation was recently described in Shewanella oneidensis MR-1 as the accumulation of cells around insoluble electron acceptors (IEA). It is the result of a series of "run-and-reversal" events enabled by modulation of swimming speed and direction. The model proposed that the swimming cells constantly sense their surroundings with specialized outer membrane cytochromes capable of extracellular electron transport (EET). Up to this point, neither the congregation nor attachment behavior have been studied in any other strains. In this study, the wild type of S. oneidensis MR-1 and several deletion mutants as well as eight other Shewanella strains (Shewanella putrefaciens CN32, S. sp. ANA-3, S. sp. W3-18-1, Shewanella amazonensis SB2B, Shewanella loihica PV-4, Shewanella denitrificans OS217, Shewanella baltica OS155, and Shewanella frigidimarina NCIMB400) were screened for the ability to congregate. To monitor congregation and attachment, specialized cell-tracking techniques, as well as a novel cell accumulation after photo-bleaching (CAAP) confocal microscopy technique were utilized in this study. We found a strong correlation between the ability of strain MR-1 to accumulate on mineral surface and the presence of key EET genes such as mtrBC/omcA (SO_1778, SO_1776, and SO_1779) and gene coding for methyl-accepting protein (MCPs) with Ca+ channel chemotaxis receptor (Cache) domain (SO_2240). These EET and taxis genes were previously identified as essential for characteristic run and reversal swimming around IEA surfaces. CN32, ANA-3, and PV-4 congregated around both Fe(OH)3 and MnO2. Two other Shewanella spp. showed preferences for one oxide over the other: preferences that correlated with the metal content of the environments from which the strains were isolated: e.g., W3-18-1, which was isolated from an iron-rich habitat congregated and attached preferentially to Fe(OH)3, while SB2B, which was isolated from a MnO2-rich environment, preferred MnO2.

Electrochemistry and microbiology of microbial fuel cells treating marine sediments polluted with heavy metals

Novel laboratory-designed aerated and non-aerated sediment microbial fuel cell (SMFC) models were constructed for power generation and heavy metal bioremediation.

Microbiome involved in microbial electrochemical systems (MESs): A review

Microbial electrochemical systems (MESs) are an attracting technology for the disposal of wastewater treatment and simultaneous energy production. In MESs, at the anode microorganisms through the catalytic activity generates electrons that can be converted into electricity or other valuable chemical compounds. Microorganisms those having ability to donate and accept electrons to and from anode and cathode electrodes, respectively are recognized as 'exoelectrogens'. In the MESs, it renders an important function for its performance. In the present mini-review, we have discussed the role of microbiome including pure culture, enriched culture and mixed culture in different BESs application. The effects of operational and biological factors on microbiome development have been discussed. Further discussion about the molecular techniques for the evaluation of microbial community analysis is addressed. In addition different electrochemical techniques for extracellular electron transfer (EET) mechanism of electroactive biofilms have been discussed. This review highlights the importance of microbiome in the development of MESs, effective operational factors for exo-electrogens activities as well their key challenges and future technological aspects are also briefly discussed.

Conversion of orange peel waste biomass to bioelectricity using a mediator-less microbial fuel cell

Microorganisms have the potential to become a game-changer in sustainable energy production in the coming generations. Microbial fuel cells (MFCs) as an alternative renewable technology can capture bioenergy (electricity) from carbon-based sources by utilizing microorganisms as biocatalysts. This study demonstrated that MFC technology can be explored for bioelectricity production from orange peel waste (OPW), an agricultural byproduct and an organic substrate, without any chemical pretreatment or the addition of extra mediators. A maximum voltage generation of 0.59 ± 0.02 V (at 500 Ω) was achieved in a dual chamber MFC during stable voltage generation stages. The maximum power density and current density obtained were 358.8 ± 15.6 mW/m(2) and 847 ± 18.4 mA/m(2), respectively. Key components of OPW, namely pectin and cellulose, were also tested in their pure form, with pectin giving a stable current, while no significant current generation was achieved using cellulose alone as the substrate, thus demonstrating the absence of cellulose-degrading bacteria. Maximum pectinase and polygalacturonase enzyme activities of 18.55 U/g and 9.04 U/g (per gram of substrate), respectively were achieved during orange peel degradation in MFCs. Bacterial identification using 16S rRNA analysis of the initial inoculum fed to the MFC, the biofilm attached to the anode, and the anode suspension, showed significant diversity in community composition. A well-known exoelectrogen, Pseudomonas, was present among the predominant genera in the anode biofilm.

Substrate and electrode potential affect electrotrophic activity of inverted bioanodes

Electricity-consuming microbial communities can serve as biocathodic catalysts in microbial electrochemical technologies. Initiating their functionality, however, remains a challenge. One promising approach is the polarity inversion of bioanodes. The objective of this study was to examine the impact of bioanode substrate and electrode potentials on inverted electrotrophic activity. Bioanodes derived from domestic wastewater were operated at -0.15V or +0.15V (vs. standard hydrogen electrode) with either acetate or formate as the sole carbon source. After this enrichment phase, cathodic linear sweep voltammetry and polarization revealed that formate-enriched cultures consumed almost 20 times the current (-3.0±0.78mA; -100±26A/m(3)) than those established with acetate (-0.16±0.09mA; -5.2±2.9A/m(3)). The enrichment electrode potential had an appreciable impact for formate, but not acetate, adapted cultures, with the +0.15V enrichment generating twice the cathodic current of the -0.15V enrichment. The total charge consumed during cathodic polarization was comparable to the charge released during subsequent anodic polarization for the formate-adapted cultures, suggesting that these communities accumulated charge or generated reduced products that could be rapidly oxidized. These findings imply that it may be possible to optimize electrotrophic activity through specific bioanodic enrichment procedures.

Influence of inoculum and anode surface properties on the selection of Geobacter-dominated biofilms

This study evaluated the impact of inoculum source and anode surface modification (carboxylate -COO(-) and sulfonamide -SO2NH2 groups) on the microbial composition of anode-respiring biofilms. These two factors have not previously been considered in detail. Three different inoculum sources were investigated, a dry aerobic soil, brackish estuarine mud and freshwater sediment. The biofilms were selected using a poised anode (-0.36 V vs Ag/AgCl) and acetate as the electron donor in a three-electrode configuration microbial fuel cell (MFC). Population profiling and cloning showed that all biofilms selected were dominated by Geobacter sp., although their electrochemical properties varied depending on the source inoculum and electrode surface modification. These findings suggest that Geobacter sp. are widespread in soils, even those that do not provide a continuously anaerobic environment, and are better at growing in the MFC conditions than other bacteria.

Stimulation of oxygen to bioanode for energy recovery from recalcitrant organic matter aniline in microbial fuel cells (MFCs)

The challenge of energy generation from biodegradation of recalcitrant organics in microbial fuel cells (MFCs) is mainly attributed to their persistence to degradation under anaerobic condition in anode chamber of MFCs. In this work, we demonstrated that electricity generation from aniline, a typical recalcitrant organic matter under anaerobic condition was remarkably facilitated by employing oxygen into bioanode of MFCs. By exposing bioanode to air, electrons of 47.2 ± 6.9 C were recovered with aniline removal efficiency of 91.2 ± 2.2% in 144 h. Limited oxygen supply (the anodic headspace was initially filled with air and then closed) resulted in the decrease of electrons recovery and aniline removal efficiency by 52.5 ± 9.4% and 74.2 ± 2.1%, respectively, and further decline by respective 64.3 ± 4.5% and 82.7 ± 1.0% occurred under anaerobic condition. Community analysis showed that anode biofilm was predominated by several aerobic aniline degrading bacteria (AADB) and anode-respiration bacteria (ARB), which likely cooperated with each other and finally featured the energy recovery from aniline. Cyclic voltammetry indicated that anodic bacteria transferred electrons to anode mainly through electron shuttle. This study provided a new sight to acquaint us with the positive role of oxygen in biodegradation of recalcitrant organics on anode as well as electricity generation.

Microbial community structure in a dual chamber microbial fuel cell fed with brewery waste for azo dye degradation and electricity generation

The expansion in knowledge of the microbial community structure can play a vital role in the electrochemical features and operation of microbial fuel cells (MFCs). In this study, bacterial community composition in a dual chamber MFC fed with brewery waste was investigated for simultaneous electricity generation and azo dye degradation. A stable voltage was generated with a maximum power density of 305 and 269 mW m(-2) for brewery waste alone (2000 mg L(-1)) and after the azo dye (200 mg L(-1)) addition, respectively. Azo dye degradation was confirmed by Fourier transform infrared spectroscopy (FT-IR) as peak corresponding to -N=N- (azo) bond disappeared in the dye metabolites. Microbial communities attached to the anode were analyzed by high-throughput 454 pyrosequencing of the 16S rRNA gene. Microbial community composition analysis revealed that Proteobacteria (67.3 %), Betaproteobacteria (30.8 %), and Desulfovibrio (18.3 %) were the most dominant communities at phylum, class, and genus level, respectively. Among the classified genera, Desulfovibrio most likely plays a major role in electron transfer to the anode since its outer membrane contains c-type cytochromes. At the genus level, 62.3 % of all sequences belonged to the unclassified category indicating a high level of diversity of microbial groups in MFCs fed with brewery waste and azo dye.

HIGHLIGHTS

• Azo dye degradation and stable bioelectricity generation was achieved in the MFC. • Anodic biofilm was analyzed by high-throughput pyrosequencing of the 16S rRNA gene. • Desulfovibrio (18.3 %) was the dominant genus in the classified genera. • Of the genus, 62.3 % were unclassified, thereby indicating highly diverse microbes. Graphical Abstract A schematic diagram of a dual chamber microbial fuel cell for azo dye degradation and current generation (with microbial communities at anode electrode).

Investigating microbial activities of electrode-associated microorganisms in real-time

Electrode-associated microbial biofilms are essential to the function of bioelectrochemical systems (BESs). These systems exist in a number of different configurations but all rely on electroactive microorganisms utilizing an electrode as either an electron acceptor or an electron donor to catalyze biological processes. Investigations of the structure and function of electrode-associated biofilms are critical to further the understanding of how microbial communities are able to reduce and oxidize electrodes. The community structure of electrode-reducing biofilms is diverse and often dominated by Geobacter spp. whereas electrode-oxidizing biofilms are often dominated by other microorganisms. The application of a wide range of tools, such as high-throughput sequencing and metagenomic data analyses, provide insight into the structure and possible function of microbial communities on electrode surfaces. However, the development and application of techniques that monitor gene expression profiles in real-time are required for a more definite spatial and temporal understanding of the diversity and biological activities of these dynamic communities. This mini review summarizes the key gene expression techniques used in BESs research, which have led to a better understanding of population dynamics, cell–cell communication and molecule-surface interactions in mixed and pure BES communities.

Methods for understanding microbial community structures and functions in microbial fuel cells: a review

Microbial fuel cells (MFCs) employ microorganisms to recover electric energy from organic matter. However, fundamental knowledge of electrochemically active bacteria is still required to maximize MFCs power output for practical applications. This review presents microbiological and electrochemical techniques to help researchers choose the appropriate methods for the MFCs study. Pre-genomic and genomic techniques such as 16S rRNA based phylogeny and metagenomics have provided important information in the structure and genetic potential of electrode-colonizing microbial communities. Post-genomic techniques such as metatranscriptomics allow functional characterizations of electrode biofilm communities by quantifying gene expression levels. Isotope-assisted phylogenetic analysis can further link taxonomic information to microbial metabolisms. A combination of electrochemical, phylogenetic, metagenomic, and post-metagenomic techniques offers opportunities to a better understanding of the extracellular electron transfer process, which in turn can lead to process optimization for power output.

Impact of tobramycin on the performance of microbial fuel cell

Background

The release of antibiotics into aquatic environments has made the treatment of wastewater containing antibiotics a world-wide public health problem. The ability of microbial fuel cells (MFCs) to harvest electricity from organic waste and renewable biomass is attracting increased interest in wastewater treatment. In this paper we investigated the bioelectrochemical response of an electroactive mixed-culture biofilm in MFC to different tobramycin concentrations.

Results

The electroactive biofilms showed a high degree of robustness against tobramycin at the level of μg/L. The current generation responses of the biofilms were affected by the presence of tobramycin. The inhibition ratio of the MFC increased exponentially with the tobramycin concentrations in the range of 0.1-1.9 g/L. The bacterial communities of the biofilms vary with the concentrations of tobramycin, the equilibrium of which is critical for the stability of electroactive biofilms based-MFC.

Conclusions

Experimental results demonstrate that the electroactive biofilm-based MFC is robust against antibiotics at the level of μg/L, but sensitive to changes in antibiotic concentration at the level of g/L. These results could provide significant information about the effects of antibiotics on the performance MFC as a waste-treatment technology.

Feasibility Analysis of Anaerobic Biocathode Enhancing Biological Degradation of Recalcitrant Chlorinated Nitroaromatic Compounds (CNAs)

Anaerobic biological technology and bioelectrochemical technology are regarded as promising sustainable wastes treatment processes. With biocatalysis in BESs anode or cathode, various pollutants can be removed. The pollutants range from nitrogen and sulfur to complex compounds. However, the investigation on recalcitrant wastes removal with biocathode has only been reported recently. Recalcitrant wastes, especially chlorinated nitroaromatic compounds, are highly persistent and toxic environmental pollutions which should be removed before discharging to environment. This paper provides a review on anaerobic biocathode BESs for recalcitrant wastes treatment and the feasibility of this system for CANs transformation. It is expected that anaerobic biocathode BESs can provide an appropriate condition for these compounds to transform to easily degradable forms. The technical challenges for future research are also discussed.

Inoculum selection to enhance performance of a microbial fuel cell for electricity generation during wastewater treatment

Experiments were designed to evaluate the influence of various anaerobic inoculums to enhance microbial fuel cell (MFC) performance utilizing tannery wastewater as substrate. Three bacterial electrogenic strains, tolerant to tannery environment, were isolated from soil contaminated with tannery waste and tannery wastewater was inoculated with these monotypes and mixed consortia of three bacterial strains in different MFCs. Comparative analysis was made by treating the tannery wastewater with foreign microbial consortia (activated sludge inoculum) and with only natural habitat microbes already present in plain wastewater. It was observed that inoculum contributes great effect on the MFC performance. Among the studied inoculation strategies, mixed electrogenic strain inocula enabled higher current yield along with concurrent substrate removal efficiency. On the contrary, plain wastewater resulted in relatively low efficiency.

Enhanced performance of hexavalent chromium reducing cathodes in the presence of Shewanella oneidensis MR-1 and lactate

Biocathodes for the reduction of the highly toxic hexavalent chromium (Cr(VI)) were investigated using Shewanella oneidensis MR-1 (MR-1) as a biocatalyst and performance was assessed in terms of current production and Cr(VI) reduction. Potentiostatically controlled experiments (-500 mV vs Ag/AgCl) showed that a mediatorless MR-1 biocathode started up under aerated conditions in the presence of lactate, received 5.5 and 1.7 times more electrons for Cr(VI) reduction over a 4 h operating period than controls without lactate and with lactate but without MR-1, respectively. Cr(VI) reduction was also enhanced, with a decrease in concentration over the 4 h operating period of 9 mg/L Cr(VI), compared to only 1 and 3 mg/L, respectively, in the controls. Riboflavin, an electron shuttle mediator naturally produced by MR-1, was also found to have a positive impact in potentiostatically controlled cathodes. Additionally, a microbial fuel cell (MFC) with MR-1 and lactate present in both anode and cathode produced a maximum current density of 32.5 mA/m(2) (1000 Ω external load) after receiving a 10 mg/L Cr(VI) addition in the cathode, and cathodic efficiency increased steadily over an 8 day operation period with successive Cr(VI) additions. In conclusion, effective and continuous Cr(VI) reduction with associated current production were achieved when MR-1 and lactate were both present in the biocathodes.

Acetate oxidation by syntrophic association between Geobacter sulfurreducens and a hydrogen-utilizing exoelectrogen

Anodic microbial communities in acetate-fed microbial fuel cells (MFCs) were analyzed using stable-isotope probing of 16S rRNA genes followed by denaturing gradient gel electrophoresis. The results revealed that Geobacter sulfurreducens and Hydrogenophaga sp. predominated in the anodic biofilm. Although the predominance of Geobacter sp. as acetoclastic exoelectrogens in acetate-fed MFC systems has been often reported, the ecophysiological role of Hydrogenophaga sp. is unknown. Therefore, we isolated and characterized a bacterium closely related to Hydrogenophaga sp. (designated strain AR20). The newly isolated strain AR20 could use molecular hydrogen (H2), but not acetate, with carbon electrode as the electron acceptor, indicating that the strain AR20 was a hydrogenotrophic exoelectrogen. This evidence raises a hypothesis that acetate was oxidized by G. sulfurreducens in syntrophic cooperation with the strain AR20 as a hydrogen-consuming partner in the acetate-fed MFC. To prove this hypothesis, G. sulfurreducens strain PCA was cocultivated with the strain AR20 in the acetate-fed MFC without any dissolved electron acceptors. In the coculture MFC of G. sulfurreducens and strain AR20, current generation and acetate degradation were the highest, and the growth of strain AR20 was observed. No current generation, acetate degradation and cell growth occurred in the strain AR20 pure culture MFC. These results show for the first time that G. sulfurreducens can oxidize acetate in syntrophic cooperation with the isolated Hydrogenophaga sp. strain AR20, with electrode as the electron acceptor.

Integrated photo-bioelectrochemical system for contaminants removal and bioenergy production

An integrated photobioelectrochemical (IPB) system was developed by installing a microbial fuel cell (MFC) inside an algal bioreactor. This system achieves the simultaneous removal from a synthetic solution of organics (in the MFC) and nutrients (in the algal bioreactor), and the production of bioenergy in electricity and algal biomass through bioelectrochemical and microbiological processes. During the one-year operation, the IPB system removed more than 92% of chemical oxygen demand, 98% of ammonium nitrogen, and 82% of phosphate and produced a maximum power density of 2.2 W/m(3) and 128 mg/L of algal biomass. The algal growth provided dissolved oxygen to the cathode reaction of the MFC, whereas electrochemical oxygen reduction on the MFC cathode buffered the pH of the algal growth medium (which was also the catholyte). The system performance was affected by illumination and dissolved oxygen. Initial energy analysis showed that the IPB system could theoretically produce enough energy to cover its consumption; however, further improvement of electricity production is desired. An analysis of the attached and suspended microbes in the cathode revealed diverse bacterial taxa typical of aquatic and soil bacterial communities with functional roles in contaminant degradation and nutrient cycling.

Enrichment and analysis of anode-respiring bacteria from diverse anaerobic inocula

One of the limitations currently faced by microbial electrochemical cell (MXC) technologies lies in the shortage of different organisms capable of forming a biofilm and channeling electrons from substrates to the anode at high current densities. Using a poised anode (-0.30 V vs Ag/AgCl) and acetate as the electron donor in a MXC, we demonstrated the presence of highly efficient anode-respiring bacteria (ARB) able to produce high current densities (>1.5 A/m(2) anode) in seven out of thirteen environmental samples. These included marshes, lake sediments, saline microbial mats, and anaerobic soils obtained from geographically diverse locations. Our microbial ecology analysis, using pyrosequencing, shows that bacteria related to the genus Geobacter, a known and commonly found ARB, dominate only two of the biofilm communities producing high current; other biofilm communities contained different known and/or novel ARB. The presence of ARB in geographically diverse locations indicates that ARB thrive in a wide range of ecosystems. Studying ARB from different environmental conditions will allow us to better understand the ubiquity of anode respiration, compare the capabilities of different ARB consortia, and find ARB with useful metabolic capacities for future applications.

Electricity generation from a floating microbial fuel cell

A floating microbial fuel cell (FMFC) has been designed and its performance has been evaluated for 153 days. The power output gradually increased to a maximum value of 390 mW/m(3) at 125 days. The polarization resistance of the anode (R(p)(a)) changed with operating time reaching a minimum value at 125 days, while the polarization resistance of the cathode (R(p)(c)) was relatively constant and much smaller than R(p)(a). It has been demonstrated that the observed changes of the internal resistance (R(int)) and the maximum power (P(max)) with exposure time were mainly due to the changes of R(p)(a). Compared with sediment MFCs for which the anode is embedded in marine or river sediments, the application of the FMFC, which could be installed in a buoy, is not limited by the depth of the ocean. The FMFC has the potential to supply electricity to low-power consuming electronic devices at remote locations.

Convergent development of anodic bacterial communities in microbial fuel cells

Microbial fuel cells (MFCs) are often inoculated from a single wastewater source. The extent that the inoculum affects community development or power production is unknown. The stable anodic microbial communities in MFCs were examined using three inocula: a wastewater treatment plant sample known to produce consistent power densities, a second wastewater treatment plant sample, and an anaerobic bog sediment. The bog-inoculated MFCs initially produced higher power densities than the wastewater-inoculated MFCs, but after 20 cycles all MFCs on average converged to similar voltages (470±20 mV) and maximum power densities (590±170 mW m⁻²). The power output from replicate bog-inoculated MFCs was not significantly different, but one wastewater-inoculated MFC (UAJA3 (UAJA, University Area Joint Authority Wastewater Treatment Plant)) produced substantially less power. Denaturing gradient gel electrophoresis profiling showed a stable exoelectrogenic biofilm community in all samples after 11 cycles. After 16 cycles the predominance of Geobacter spp. in anode communities was identified using 16S rRNA gene clone libraries (58±10%), fluorescent in-situ hybridization (FISH) (63±6%) and pyrosequencing (81±4%). While the clone library analysis for the underperforming UAJA3 had a significantly lower percentage of Geobacter spp. sequences (36%), suggesting that a predominance of this microbe was needed for convergent power densities, the lower percentage of this species was not verified by FISH or pyrosequencing analyses. These results show that the predominance of Geobacter spp. in acetate-fed systems was consistent with good MFC performance and independent of the inoculum source.

Electricity generation and microbial community changes in microbial fuel cells packed with different anodic materials

Four materials, carbon felt cube (CFC), granular graphite (GG), granular activated carbon (GAC) and granular semicoke (GS) were tested as packed anodic materials to seek a potentially practical material for microbial fuel cells (MFCs). The microbial community and its correlation with the electricity generation performance of MFCs were explored. The maximum power density was found in GAC, followed by CFC, GG and GS. In GAC and CFC packed MFCs, Geobacter was the dominating genus, while Azospira was the most populous group in GG. Results further indicated that GAC was the most favorable for Geobacter adherence and growth, and the maximum power densities had positive correlation with the total biomass and the relative abundance of Geobacter, but without apparent correlation with the microbial diversity. Due to the low content of Geobacter in GS, power generated in this system may be attributed to other microorganisms such as Synergistes, Bacteroidetes and Castellaniella.

Lactate oxidation coupled to iron or electrode reduction by Geobacter sulfurreducens PCA

Geobacter sulfurreducens PCA completely oxidized lactate and reduced iron or an electrode, producing pyruvate and acetate intermediates. Compared to the current produced by Shewanella oneidensis MR-1, G. sulfurreducens PCA produced 10-times-higher current levels in lactate-fed microbial electrolysis cells. The kinetic and comparative analyses reported here suggest a prominent role of G. sulfurreducens strains in metal- and electrode-reducing communities supplied with lactate.

Comparing photosynthetic and photovoltaic efficiencies and recognizing the potential for improvement

Comparing photosynthetic and photovoltaic efficiencies is not a simple issue. Although both processes harvest the energy in sunlight, they operate in distinctly different ways and produce different types of products: biomass or chemical fuels in the case of natural photosynthesis and nonstored electrical current in the case of photovoltaics. In order to find common ground for evaluating energy-conversion efficiency, we compare natural photosynthesis with present technologies for photovoltaic-driven electrolysis of water to produce hydrogen. Photovoltaic-driven electrolysis is the more efficient process when measured on an annual basis, yet short-term yields for photosynthetic conversion under optimal conditions come within a factor of 2 or 3 of the photovoltaic benchmark. We consider opportunities in which the frontiers of synthetic biology might be used to enhance natural photosynthesis for improved solar energy conversion efficiency.

A rapid fingerprinting approach to distinguish between closely related strains of Shewanella

One of the big operational problems facing laboratories today is the ability to rapidly distinguish between strains of bacteria that, while physiologically distinct, are nearly impossible to separate based on 16S rRNA gene sequence differences. Here we demonstrate that ITS-DGGE provides a convenient approach to distinguishing between closely related strains of Shewanella, some of which were impossible to separate and identify by 16 rRNA gene sequence alone. Examined Shewanella genomes contain 8-11 copies of rrn (ribosomal RNA gene) operons, and variable size and sequence of 16S-23S ITS (intergenic transcribed spacer) regions which result in distinct ITS-DGGE profiles. Phylogenetic constructions based on ITS are congruent with the genomic trees generated from concatenated core genes as well as with those based on conserved indels, suggesting that ITS patterns appear to be linked with evolutionary lineages and physiology. In addition, three new Shewanella strains (MFC 2, MFC 6, and MFC 14) were isolated from microbial fuel cells enriched from wastewater sludge and identified by ITS-DGGE. Subsequent physiological and electrochemical studies of the three isolates confirmed that each strain is phenotypically/genotypically distinct. Thus, this study validates ITS-DGGE as a quick fingerprint approach to identifying and distinguishing between closely related but novel Shewanella ecotypes.