Testing biological actions of medicinal plants from northern Vietnam on zebrafish embryos and larvae: Developmental, behavioral, and putative therapeutical effects.

Evaluating the risks and benefits of using traditional medicinal plants is of utmost importance for a huge fraction of the human population, in particular in Northern Vietnam. Zebrafish are increasingly used as a simple vertebrate model for testing toxic and physiological effects of compounds, especially on development. Here, we tested 12 ethanolic extracts from popular medicinal plants collected in northern Vietnam for their effects on zebrafish survival and development during the first 4 days after fertilization. We characterized more in detail their effects on epiboly, hatching, growth, necrosis, body curvature, angiogenesis, skeletal development and mostly increased movement behavior. Finally, we confirm the effect on epiboly caused by the Mahonia bealei extract by staining the actin filaments and performing whole genome gene expression analysis. Further, we show that this extract also inhibits cell migration of mouse embryo fibroblasts. Finally, we analyzed the chemical composition of the Mahonia bealei extract and test the effects of its major components. In conclusion, we show that traditional medicinal plant extracts are able to affect zebrafish early life stage development to various degrees. In addition, we show that an extract causing delay in epiboly also inhibits mammalian cell migration, suggesting that this effect may serve as a preliminary test for identifying extracts that inhibit cancer metastasis.

Rapid Surface Modification of Stainless Steel 304L Electrodes for Microbial Electrochemical Sensor Application.

Electrogenic microorganisms serve as important biocatalysts for microbial electrochemical sensors (MESes). The electrical signal produced is based on the rate of electron transfer between the microbes and electrodes, which represents the biotoxicity of water. However, existing MESes require complex and sophisticated fabrication methods. Here, several low-cost and rapid surface modification strategies (carbon powder-coated, flame-oxidized, and acid-bleached) have been demonstrated and studied for biosensing purposes. Surface-modified MESe bioanodes were successfully applied to detect multiple model pollutants including sodium acetate, ethanol, thinner, and palm oil mill effluent under three different testing sequences, namely, pollutant incremental, pollutant dumping, and water dilution tests. The carbon powder-coated bioanode showed the most responsive signal profile for all the three tests, which is in line with the average roughness values (Ra) when tested with atomic force microscopy. The carbon powder-coated electrode possessed a Ra value of 0.844, while flame-oxidized, acid-bleached, and control samples recorded 0.323, 0.336, and 0.264, respectively. The higher roughness was caused by the carbon coating and provided adhesive sites for microbial attachment and growth. The accuracy of MESe was also verified by correlating with chemical oxygen demand (COD) results. Similar to the sensitivity test, the carbon powder-coated bioanode obtained the highest R2 value of 0.9754 when correlated with COD results, indicating a high potential of replacing conventional water quality analysis methods. The reported work is of great significance to showcase facile surface modification techniques for MESes, which are cost-effective and sustainable while retaining the biocompatibility toward the microbial community with carbon-based coatings.

A novel finding of intra-genus inhibition of quorum sensing in Vibrio bacteria.

Quorum sensing is the process by which microbial cells sense and respond to the co-presence of others in their surrounding, through the detection of their autoinducers associated with gene expression regulation and thereby controlling many physiological processes, such as biofilm formation and/or bioluminescence, etc. In Vibrio bacteria, where quorum sensing is relatively well understood with three commonly known autoinducers (HAI-1, AI-2 and CAI-1), both intra-species and inter-species cell-cell communications occur but no inter-Vibrio-species quorum sensing inhibition has been reported. In this study, by screening bacterial isolated from soil and mud samples in a northern province in Vietnam, we discovered a strain that reduced more than 75% of the bioluminescence of a Vibrio harveyi, with evidence showing that such an inhibition might be associated with quorum sensing inhibition. The strain, designated as XTS1.2.9, was identified to be a Vibrio parahaemolyticus bacterium based on its morphological, physiological, biochemical and phylogenetic characteristics. We also tested XTS1.2.9 for its bioluminescence inhibition against different mutants lacking different quorum sensing autoinducers by using plate assays. The results showed that XTS1.2.9 inhibited the bioluminescence of the mutants having sensor 1, especially the one detecting CAI-1, and lacking sensor for AI-2; while it did not inhibit the mutants having only sensor for AI-2 and lacking sensor 1. Therefore, we propose an intra-genus quorum sensing inhibition mechanism involving CAI-1 to explain for such interactions between Vibrio parahaemolyticus and Vibrio harveyi. This phenomenon is reported for the first time and may have certain scientific and application implications.

Penicillium digitatum as a Model Fungus for Detecting Antifungal Activity of Botanicals: An Evaluation on Vietnamese Medicinal Plant Extracts.

Medicinal plants play important roles in traditional medicine, and numerous compounds among them have been recognized for their antimicrobial activity. However, little is known about the potential of Vietnamese medicinal plants for antifungal activity. In this study, we examined the antagonistic activity of twelve medicinal plant species collected in Northern Vietnam against Penicillium digitatum, Aspergillus flavus, Aspergillus fumigatus, and Candida albicans. The results showed that the antifungal activities of the crude extracts from Mahonia bealei, Ficus semicordata, and Gnetum montanum were clearly detected with the citrus postharvest pathogen P. digitatum. These extracts could fully inhibit the growth of P. digitatum on the agar medium, and on the infected citrus fruits at concentrations of 300-1000 µg/mL. Meanwhile, the other tested fungi were less sensitive to the antagonistic activity of the plant extracts. In particular, we found that the ethanolic extract of M. bealei displayed a broad-spectrum antifungal activity against all four pathogenic fungi. Analysis of this crude extract by enrichment coupled with high-performance liquid chromatography revealed that berberine and palmatine are major metabolites. Additional inspections indicated berberine as the key compound responsible for the antifungal activity of the M. bealei ethanolic extract. Our study provides a better understanding of the potential of Vietnamese medicinal plant resources for combating fungal pathogens. This work also highlights that the citrus pathogen P. digitatum can be employed as a model fungus for screening the antifungal activity of botanicals.

Notes on Culturable Endophytic Microorganisms Isolated from 14 Medicinal Plants in Vietnam: A Diversity Analysis to Predict the Host-Microbe Correlations.

Endophytes can generate a cornucopia of marvelous bioactive secondary metabolites useful for mankind but their biodiversity and associations with host plants are still elusive. In this study, we explored the culturable endophytic microorganisms associated with 14 medicinal plants that are of high socio-economic value and/or reportedly endemic to northern Vietnam. Specifically, we isolated the endophytic microorganisms by applying surface sterilization methods and identified them based on morphological and rDNA sequence analyses. Agglomerative Hierarchical Clustering (AHC) and Principal Component Analysis (PCA) were used to analyze the correlations between the taxonomic affiliations of the culturable endophytes and the characteristics of their hosts. Most of the culturable endophytes obtained were bacteria (80), and few of those were actinomycetes (15) and fungi (8). Many of them are reported to be endophytes of medicinal plants for the first time. A number of plants (5) are also reported for the first time to contain microbial endophytes, while some plants with powerful pharmaceutical potential harbor unique endophytes. Furthermore, our results reveal a strikingly close relation between the compositions of bacterial and fungal isolates from plants having anti-bacterial activity and those from plants having anti-inflammatory activity, or between the compositions of the microbial endophytic isolates from plants having anti-cancer activity and those from plants having antioxidant activity. Altogether, the results provide new findings which can be inspiring for further in-depth studies to explore and exploit the relationships between medicinal plants and their associated endophytes in northern Vietnam and world-wide.

Water quality and climate variability of urban and tourism areas in Thailand and northern Vietnam: correlations and possible influences on nearby coastal shrimp farms.

This research explored the long-term relationships of climate parameters and coastal water quality nearby shrimp farms in two countries. In Thailand, two sampling points in canals located in Bangkok and Trang provinces were selected as the urban and tourism areas, respectively. In northern Vietnam, the canals located in Thai Binh and Quang Ninh provinces were used as the urban and tourism areas, respectively. The diurnal monitoring of water quality and weather was performed at each site to evaluate the relationships between climate and water quality, pollution load, and risk analysis. A questionnaire was also used to assess the climate and water exposure, vulnerability, and adaptability of each site. All data were ranked on a scale of 1 to 5 to integrate each factor. It was determined that the main water quality problem was fecal pollution. Notably, aquaculture farming in northern Vietnam is more vulnerable than in Thailand; however, Vietnam farmers were adaptable for climate variability.

In vitro antiviral activities of ethanol and aqueous extracts of Vietnamese traditional medicinal plants against Porcine Epidemic Diarrhea virus: a coronavirus family member.

Porcine epidemic diarrhea virus (PEDV) causes diarrhea in pigs leading to severe illnesses and high mortality rates. The development of medicinal agents to treat PEDV infection is therefore crucial. In this study, antiviral activities against PEDV of ethanol and aqueous extracts of 17 Vietnamese traditional medicinal plants were evaluated using the cytopathic effect-based assay. The results showed that 14 out of 17 medicinal plants could inhibit the cytopathic effect of PEDV. The ethanol extract of Stixis scandens was identified as the most active extract with its MIC (minimum inhibitory concentration) being 0.15 µg/mL. Other plant extracts also displayed strong antiviral activity against PEDV, including Anisomeles indica, Pericampylus glaucus and Croton kongensis. The results demonstrate that certain medicinal plants have a high antiviral potential and may serve as a lead to develop novel pharmaceutical agents to cure PED as well as the diseases caused by other coronaviruses.

Zinc removal and recovery from industrial wastewater with a microbial fuel cell: Experimental investigation and theoretical prediction.

Microbial fuel cells (MFCs) that simultaneously remove organic contaminants and recovering metals provide a potential route for industry to adopt clean technologies. In this work, two goals were set: to study the feasibility of zinc removal from industrial effluents using MFCs and to understand the removal process by using reaction rate models. The removal of Zn2+ in MFC was over 96% for synthetic and industrial samples with initial Zn2+ concentrations less than 2.0 mM after 22 h of operation. However, only 83 and 42% of the zinc recovered from synthetic and industrial samples, respectively, was attached on the cathode surface of the MFCs. The results marked the domination of electroprecipitation rather than the electrodeposition process in the industrial samples. Energy dispersive X-ray (EDX) analysis showed that the recovered compound contained not only Zn but also O, evidence that Zn(OH)2 could be formed. The removal of Zn2+ in the MFC followed a mechanism where oxygen was reduced to hydroxide before reacting with Zn2+. Nernst equations and rate law expressions were derived to understand the mechanism and used to estimate the Zn2+ concentration and removal efficiency. The zero-, first- and second-order rate equations successfully fitted the data, predicted the final Zn2+ removal efficiency, and suggested that possible mechanistic reactions occurred in the electrolysis cell (direct reduction), MFC (O2 reduction), and control (chemisorption) modes. The half-life, t1/2, of the Zn2+ removal reaction using synthetic and industrial samples was estimated to be 7.0 and 2.7 h, respectively. The t1/2 values of the controls (without the power input from the MFC bioanode) were much slower and were recorded as 21.5 and 7.3 h for synthetic and industrial samples, respectively. The study suggests that MFCs can act as a sustainable and environmentally friendly technology for heavy metal removal without electrical energy input or the addition of chemicals.

Ammonium and organic carbon co-removal under feammox-coupled-with-heterotrophy condition as an efficient approach for nitrogen treatment.

Nitrification is the rate limiting step in the nitrogen removal processes since nitrifiers have high oxygen demand, but poorly compete with aerobic heterotrophs. In a laboratory-scaled system, we investigated a process of ammonium oxidation under ferric-iron reducing condition (feammox) in the presence of organic carbon using influents with high NH4+ and COD contents, and ferrihydrite as the only electron acceptor. Batch incubations testing influents with different NH4+ and COD concentrations revealed that the [COD]/[NH4+] ratio of 1.4 and the influent redox potential ranging from - 20 to + 20 mV led to the highest removal efficiencies, i.e. 98.3% for NH4+ and 58.8% for COD. N2 was detected as the only product of NH4+ conversion, whereas NO2- and NO3- were not detected. While operating continuously with influent having a [COD]/[NH4+] ratio of 1.4, the system efficiently removed NH4+ (> 91%) and COD (> 54%) within 6 day retention time. Fluorescence in situ hybridization analyses using Cy3-labeled 16S rRNA oligonucleotide probes revealed that gamma-proteobacteria dominated in the microbial community attaching to the matrix bed of the system. The iron-reduction dependent NH4+ and COD co-removal with a thorough conversion of NH4+ to N2 demonstrated in this study would be a novel approach for nitrogen treatment.

Developmental toxicity of Clerodendrum cyrtophyllum turcz ethanol extract in zebrafish embryo.

ETHNOPHARMACOLOGICAL RELEVANCE: Clerodendrum cyrtophyllum Turcz has been used in traditional medicine for the treatment of various diseases. In spite of its therapeutic applications, research on its toxicity and teratogenicity is still limited. AIM OF THE STUDY: The study aimed to investigate the developmental toxicity of the ethanol extract of C. cyrtophyllum (EE) in zebrafish embryo model. MATERIAL AND METHODS: Major compounds from crude ethanol extract of Clerodendron cyrtophyllum Turcz leaves were determined using HPLC-DAD-Orbitrap-MS analysis. The developmental toxicity of EE were investigated using zebrafish embryo model. Zebrafish embryos at 6 h post-fertilization (hpf) were treated with EE at different concentrations. Egg coagulation, mortality, hatching, yolk sac edema, pericardial edema and teratogenicity were recorded each day for during a 5-day exposure. At time point 120 hpf, body length, pericardial area, heartbeat and yolk sac area were assessed. In order to elucidate molecular mechanisms for the developmental toxicity of EE, we further evaluated the effects of the EE on the expression of genes involved on signaling pathways affecting fish embryo's development such as heart development (gata5, myl7, myh6, has2, hand2, nkx 2.5), oxidative stress (cat, sod1, gpx4, gstp2), wnt pathway (ß-catenin, wnt3a, wnt5, wnt8a, wnt11), or cell apoptosis (p53, bax, bcl2, casp3, casp8, casp9, apaf-1, gadd45bb) using qRT-PCR analysis. RESULTS: Our results demonstrated that three major components including acteoside, cirsilineol and cirsilineol-4'-O-ß-D-glucopyranoside were identified from EE. EE exposure during 6-96 h post-fertilization (hpf) at doses ranging from 80 to 200 µg/mL increased embryo mortality and reduced hatching rate. EE exposure at 20 and 40 µg/mL until 72-120 hpf induced a series of malformations, including yolk sac edema, pericardial edema, spine deformation, shorter body length. Based on two prediction models using a teratogenic index (TI), a 25% lethality concentration (LD25) and the no observed-adverse-effect level (NOAEL), EE is considered as teratogenic for zebrafish embryos with TI (LC50/EC50) and LD25/NOAEC values at 96 hpf reaching 3.87 and 15.73 respectively. The mRNA expression levels of p53, casp8, bax/bcl2, gstp2, nkx2.5, wnt3a, wnt11, gadd45bb and gata5 were significantly upregulated by EE exposure at 20 and 40 µg/mL while the expression of wnt5, hand2 and bcl2 were downregulated. CONCLUSIONS: These results provide evidence for toxicity effects of EE to embryo stages and provide an insight into the potential toxicity mechanisms on embryonic development.

A Laboratory-Scale Study of the Applicability of a Halophilic Sediment Bioelectrochemical System for in situ Reclamation of Water and Sediment in Brackish Aquaculture Ponds: Effects of Operational Conditions on Performance.

Sediment bioelectrochemical systems (SBESs) can be integrated into brackish aquaculture ponds for in-situ bioremediation of the pond water and sediment. Such an in-situ system offers advantages including reduced treatment cost, reusability and simple handling. In order to realize such an application potential of the SBES, in this laboratory-scale study we investigated the effect of several controllable and uncontrollable operational factors on the in-situ bioremediation performance of a tank model of a brackish aquaculture pond, into which a SBES was integrated, in comparison with a natural degradation control model. The performance was evaluated in terms of electricity generation by the SBES, Chemical oxygen demand (COD) removal and nitrogen removal of both the tank water and the tank sediment. Real-life conditions of the operational parameters were also experimented to understand the most close-to-practice responses of the system to their changes. Predictable effects of controllable parameters including external resistance and electrode spacing, similar to those reported previously for the BESs, were shown by the results but exceptions were observed. Accordingly, while increasing the electrode spacing reduced the current densities but generally improved COD and nitrogen removal, increasing the external resistance could result in decreased COD removal but also increased nitrogen removal and decreased current densities. However, maximum electricity generation and COD removal efficiency difference of the SBES (versus the control) could be reached with an external resistance of 100 Ω, not with the lowest one of 10 Ω. The effects of uncontrollable parameters such as ambient temperature, salinity and pH of the pond (tank) water were rather unpredictable. Temperatures higher than 35°C seemed to have more accelaration effect on natural degradation than on bioelectrochemical processes. Changing salinity seriously changed the electricity generation but did not clearly affect the bioremediation performance of the SBES, although at 2.5% salinity the SBES displayed a significantly more efficient removal of nitrogen in the water, compared to the control. Variation of pH to practically extreme levels (5.5 and 8.8) led to increased electricity generations but poorer performances of the SBES (vs. the control) in removing COD and nitrogen. Altogether, the results suggest some distinct responses of the SBES under brackish conditions and imply that COD removal and nitrogen removal in the system are not completely linked to bioelectrochemical processes but electrochemically enriched bacteria can still perform nonbioelectrochemical COD and nitrogen removals more efficiently than natural ones. The results confirm the application potential of the SBES in brackish aquaculture bioremediation and help propose efficient practices to warrant the success of such application in real-life scenarios.

A Laboratory-Scale Study of the Applicability of a Halophilic Sediment Bioelectrochemical System for in situ Reclamation of Water and Sediment in Brackish Aquaculture Ponds: Establishment, Bacterial Community and Performance Evaluation.

In this study, we investigated the potential of using sediment bioelectrochemical systems (SBESs) for in situ treatment of the water and sediment in brackish aquaculture ponds polluted with uneaten feed. An SBES integrated into a laboratory-scale tank simulating a brackish aquaculture pond was established. This test tank and the control (not containing the SBES) were fed with shrimp feed in a scheme that mimics a situation where 50% of feed is uneaten. After the SBES was inoculated with microbial sources from actual shrimp pond sediments, electricity generation was well observed from the first experimental week, indicating successful enrichment of electrochemically active bacteria in the test tank sediment. The electricity generation became steady after 3 weeks of operation, with an average current density of 2.3 mA/m2 anode surface and an average power density of 0.05 mW/m2 anode surface. The SBES removed 20-30% more COD of the tank water, compared to the control. After 1 year, the SBES also reduced the amount of sediment in the tank by 40% and thus could remove approximately 40% more COD and approximately 52% more nitrogen from the sediment, compared to the control. Insignificant amounts of nitrite and nitrate were detected, suggesting complete removal of nitrogen by the system. PCR-DGGE-based analyses revealed the dominant presence of Methylophilus rhizosphaerae, Desulfatitalea tepidiphila and Thiothrix eikelboomii, which have not been found in bioelectrochemical systems before, in the bacterial community in the sediment of the SBES-containing tank. The results of this research demonstrate the potential application of SBESs in helping to reduce water pollution threats, fish and shrimp disease risks, and thus farmers' losses.

Biosensors based on lithotrophic microbial fuel cells in relation to heterotrophic counterparts: research progress, challenges, and opportunities.

Biosensors based on the microbial fuel cell (MFC) platform have been receiving increasing attention from researchers owing to their unique properties. The lithotrophic MFC, operated with a neutrophilic iron-oxidizing bacterial community, has recently been developed and proposed to be used as a biosensor to detect iron, and likely metals in general, in water samples. Therefore, in this review, important aspects of the lithotrophic MFC-based biosensor, including its configuration, fabrication, microbiology, electron transfer mechanism, sensing performance, etc. were carefully discussed in comparison with those of heterotrophic (organotrophic) counterparts. Particularly, the challenges for the realization of the practical application of the device were determined. Furthermore, the application potentials of the device were also considered and positioned in the context of technologies for metal monitoring and bioremediation.

Possibility of using a lithotrophic iron-oxidizing microbial fuel cell as a biosensor for detecting iron and manganese in water samples.

Iron-oxidizing bacterial consortia can be enriched in microbial fuel cells (MFCs) operated with ferrous iron as the sole electron donor. In this study, we investigated the possibility of using such lithotrophic iron-oxidizing MFC (LIO-MFC) systems as biosensors to monitor iron and manganese in water samples. When operated with anolytes containing only ferrous iron as the sole electron donor, the experimented LIO-MFCs generated electrical currents in response to the presence of Fe(2+) in the anolytes. For the concentrations of Fe(2+) in the range of 3-20 mM, a linear correlation between the current and the concentration of Fe(2+) could be achieved (r(2) = 0.98). The LIO-MFCs also responded to the presence of Mn(2+) in the anolytes but only when the Mn(2+) concentration was less than 3 mM. The presence of other metal ions such as Ni(2+) or Pb(2+) in the anolytes reduced the Fe(2+)-associated electricity generation of the LIO-MFCs at various levels. Organic compounds, when present at a non-excessive level together with Fe(2+) in the anolytes, did not affect the generation of electricity, although the compounds might serve as alternative electron donors for the anode bacteria. The performance of the LIO-MFCs was also affected to different degrees by operational parameters, including surrounding temperature, pH of the sample, buffer strength and external resistance. The results proved the potential of LIO-MFCs as biosensors sensing Fe(2+) in water samples with a significant specificity. However, the operation of the system should be in compliance with an optimal procedure to ensure reliable performance.

A lithotrophic microbial fuel cell operated with pseudomonads-dominated iron-oxidizing bacteria enriched at the anode.

In this study, we attempted to enrich neutrophilic iron bacteria in a microbial fuel cell (MFC)-type reactor in order to develop a lithotrophic MFC system that can utilize ferrous iron as an inorganic electron donor and operate at neutral pHs. Electrical currents were steadily generated at an average level of 0.6 mA (or 0.024 mA cm(-2) of membrane area) in reactors initially inoculated with microbial sources and operated with 20 mM Fe(2+) as the sole electron donor and 10 ohm external resistance; whereas in an uninoculated reactor (the control), the average current level only reached 0.2 mA (or 0.008 mA cm(-2) of membrane area). In an inoculated MFC, the generation of electrical currents was correlated with increases in cell density of bacteria in the anode suspension and coupled with the oxidation of ferrous iron. Cultivation-based and denaturing gradient gel electrophoresis analyses both show the dominance of some Pseudomonas species in the anode communities of the MFCs. Fluorescent in-situ hybridization results revealed significant increases of neutrophilic iron-oxidizing bacteria in the anode community of an inoculated MFC. The results, altogether, prove the successful development of a lithotrophic MFC system with iron bacteria enriched at its anode and suggest a chemolithotrophic anode reaction involving some Pseudomonas species as key players in such a system. The system potentially offers unique applications, such as accelerated bioremediation or on-site biodetection of iron and/or manganese in water samples.

Phenol sensing by Escherichia coli chemoreceptors: a nonclassical mechanism.

The four transmembrane chemoreceptors of Escherichia coli sense phenol as either an attractant (Tar) or a repellent (Tap, Trg, and Tsr). In this study, we investigated the Tar determinants that mediate its attractant response to phenol and the Tsr determinants that mediate its repellent response to phenol. Tar molecules with lesions in the aspartate-binding pocket of the periplasmic domain, with a foreign periplasmic domain (from Tsr or from several Pseudomonas chemoreceptors), or lacking nearly the entire periplasmic domain still mediated attractant responses to phenol. Similarly, Tar molecules with the cytoplasmic methylation and kinase control domains of Tsr still sensed phenol as an attractant. Additional hybrid receptors with signaling elements from both Tar and Tsr indicated that the transmembrane (TM) helices and HAMP domain determined the sign of the phenol-sensing response. Several amino acid replacements in the HAMP domain of Tsr, particularly attractant-mimic signaling lesions at residue E248, converted Tsr to an attractant sensor of phenol. These findings suggest that phenol may elicit chemotactic responses by diffusing into the cytoplasmic membrane and perturbing the structural stability or position of the TM bundle helices, in conjunction with structural input from the HAMP domain. We conclude that behavioral responses to phenol, and perhaps to temperature, cytoplasmic pH, and glycerol, as well, occur through a general sensing mechanism in chemoreceptors that detects changes in the structural stability or dynamic behavior of a receptor signaling element. The structurally sensitive target for phenol is probably the TM bundle, but other behaviors could target other receptor elements.

High shear enrichment improves the performance of the anodophilic microbial consortium in a microbial fuel cell.

In many microbial bioreactors, high shear rates result in strong attachment of microbes and dense biofilms. In this study, high shear rates were applied to enrich an anodophilic microbial consortium in a microbial fuel cell (MFC). Enrichment at a shear rate of about 120 s(-1) resulted in the production of a current and power output two to three times higher than those in the case of low shear rates (around 0.3 s(-1)). Biomass and biofilm analyses showed that the anodic biofilm from the MFC enriched under high shear rate conditions, in comparison with that under low shear rate conditions, had a doubled average thickness and the biomass density increased with a factor 5. The microbial community of the former, as analysed by DGGE, was significantly different from that of the latter. The results showed that enrichment by applying high shear rates in an MFC can result in a specific electrochemically active biofilm that is thicker and denser and attaches better, and hence has a better performance.

Microbial fuel cells for sulfide removal.

Thus far, microbial fuel cells (MFCs) have been used to convert carbon-based substrates to electricity. However, sulfur compounds are ubiquitously present in organic waste and wastewater. In this study, a MFC with a hexacyanoferrate cathodic electrolyte was used to convert dissolved sulfide to elemental sulfur. Two types of MFCs were used, a square type closed to the air and a tubular type in which the cathode compartment was open to the air. The square-type MFCs demonstrated a potential-dependent conversion of sulfide to sulfur. In the tubular system, up to 514 mg sulfide L(-1) net anodic compartment (NAC) day(-1) (241 mg L(-1) day(-1) total anodic compartment, TAC) was removed. The sulfide oxidation in the anodic compartment resulted in electricity generation with power outputs up to 101 mW L(-1) NAC (47 W m(-3) TAC). Microbial fuel cells were coupled to an anaerobic upflow anaerobic sludge blanket reactor, providing total removals of up to 98% and 46% of the sulfide and acetate, respectively. The MFCs were capable of simultaneously removing sulfate via sulfide. This demonstrates that digester effluents can be polished by a MFC for both residual carbon and sulfur compounds. The recovery of electrons from sulfides implies a recovery of energy otherwise lost in the methane digester.

Continuous electricity generation at high voltages and currents using stacked microbial fuel cells.

Connecting several microbial fuel cell (MFC) units in series or parallel can increase voltage and current; the effect on the microbial electricity generation was as yet unknown. Six individual continuous MFC units in a stacked configuration produced a maximum hourly averaged power output of 258 W m(-3) using a hexacyanoferrate cathode. The connection of the 6 MFC units in series and parallel enabled an increase of the voltages (2.02 V at 228 W m(-3)) and the currents (255 mA at 248 W m(-3)), while retaining high power outputs. During the connection in series, the individual MFC voltages diverged due to microbial limitations at increasing currents. With time, the initial microbial community decreased in diversity and Gram-positive species became dominant. The shift of the microbial community accompanied a tripling of the short time power output of the individual MFCs from 73 W m(-3) to 275 W m(-3), a decrease of the mass transfer limitations and a lowering of the MFC internal resistance from 6.5 +/- 1.0 to 3.9 +/- 0.5 omega. This study demonstrates a clear relation between the electrochemical performance and the microbial composition of MFCs and further substantiates the potential to generate useful energy by means of MFCs.