Axenic Biofilm Formation and Aggregation by Synechocystis sp. Strain PCC 6803 Are Induced by Changes in Nutrient Concentration and Require Cell Surface Structures.

Phototrophic biofilms are key to nutrient cycling in natural environments and bioremediation technologies, but few studies describe biofilm formation by pure (axenic) cultures of a phototrophic microbe. The cyanobacterium Synechocystis sp. strain PCC 6803 (here Synechocystis) is a model microorganism for the study of oxygenic photosynthesis and biofuel production. We report here that wild-type (WT) Synechocystis caused extensive biofilm formation in a 2,000-liter outdoor nonaxenic photobioreactor under conditions attributed to nutrient limitation. We developed a biofilm assay and found that axenic Synechocystis forms biofilms of cells and extracellular material but only when cells are induced by an environmental signal, such as a reduction in the concentration of growth medium BG11. Mutants lacking cell surface structures, namely type IV pili and the S-layer, do not form biofilms. To further characterize the molecular mechanisms of cell-cell binding by Synechocystis, we also developed a rapid (8-h) axenic aggregation assay. Mutants lacking type IV pili were unable to aggregate, but mutants lacking a homolog to Wza, a protein required for type 1 exopolysaccharide export in Escherichia coli, had a superbinding phenotype. In WT cultures, 1.2× BG11 medium induced aggregation to the same degree as 0.8× BG11 medium. Overall, our data support that Wza-dependent exopolysaccharide is essential to maintain stable, uniform suspensions of WT Synechocystis cells in unmodified growth medium and that this mechanism is counteracted in a pilus-dependent manner under altered BG11 concentrations.IMPORTANCE Microbes can exist as suspensions of individual cells in liquids and also commonly form multicellular communities attached to surfaces. Surface-attached communities, called biofilms, can confer antibiotic resistance to pathogenic bacteria during infections and establish food webs for global nutrient cycling in the environment. Phototrophic biofilm formation is one of the earliest phenotypes visible in the fossil record, dating back over 3 billion years. Despite the importance and ubiquity of phototrophic biofilms, most of what we know about the molecular mechanisms, genetic regulation, and environmental signals of biofilm formation comes from studies of heterotrophic bacteria. We aim to help bridge this knowledge gap by developing new assays for Synechocystis, a phototrophic cyanobacterium used to study oxygenic photosynthesis and biofuel production. With the aid of these new assays, we contribute to the development of Synechocystis as a model organism for the study of axenic phototrophic biofilm formation.

Electron-acceptor loadings affect chloroform dechlorination in a hydrogen-based membrane biofilm reactor.

Chloroform (CF) can undergo reductive dechlorination to dichloromethane, chloromethane, and methane. However, competition for hydrogen (H2 ), the electron-donor substrate, may cause poor dechlorination when multiple electron acceptors are present. Common acceptors in anaerobic environments are nitrate (NO3- ), sulfate (SO42- ), and bicarbonate (HCO3- ). We evaluated CF dechlorination in the presence of HCO3- at 1.56 e- Eq/m2 -day, then NO3- at 0.04-0.15 e- Eq/m2 -day, and finally NO3- (0.04 e- Eq/m2 -day) along with SO42- at 0.33 e- Eq/m2 -day in an H2 -based membrane biofilm reactor (MBfR). When the biofilm was initiated with CF-dechlorination conditions (no NO3- or SO42- ), it yielded a CF flux of 0.14 e- Eq/m2 -day and acetate production via homoacetogenesis up to 0.26 e- eq/m2 -day. Subsequent addition of NO3- at 0.05 e- Eq/m2 -day maintained full CF dechlorination and homoacetogenesis, but NO3- input at 0.15 e- Eq/m2 -day caused CF to remain in the reactor's effluent and led to negligible acetate production. The addition of SO42- did not affect CF reduction, but SO42- reduction significantly altered the microbial community by introducing sulfate-reducing Desulfovibrio and more sulfur-oxidizing Arcobacter. Dechloromonas appeared to carry out CF dechlorination and denitrification, whereas Acetobacterium (homoacetogen) may have been involved with hydrolytic dechlorination. Modifications to the electron acceptors fed to the MBfR caused the microbial community to undergo changes in structure that reflected changes in the removal fluxes.

Bromate and Nitrate Bioreduction Coupled with Poly-ß-hydroxybutyrate Production in a Methane-Based Membrane Biofilm Reactor.

This work demonstrates bromate (BrO3-) reduction in a methane (CH4)-based membrane biofilm reactor (MBfR), and it documents contrasting impacts of nitrate (NO3-) on BrO3- reduction, as well as formation of poly-ß-hydroxybutyrate (PHB), an internal C- and electron-storage material. When the electron donor, CH4, was in ample supply, NO3- enhanced BrO3- reduction by stimulating the growth of denitrifying bacteria ( Meiothermus, Comamonadaceae, and Anaerolineaceae) able to reduce BrO3- and NO3- simultaneously. This was supported by increases in denitrifying enzymes (e.g., nitrate reductase, nitrite reductase, nitrous-oxide reductase, and nitric-oxide reductase) through quantitative polymerase chain reaction (qPCR) analysis and metagenomic prediction of these functional genes. When the electron donor was in limited supply, NO3- was the preferred electron acceptor over BrO3- due to competition for the common electron donor; this was supported by the significant oxidation of stored PHB when NO3- was high enough to cause electron-donor limitation. Methanotrophs (e.g., Methylocystis, Methylomonas, and genera within Comamonadaceae) were implicated as the main PHB producers in the biofilms, and their ability to oxidize PHB mitigated the impacts of competition for CH4.

Contribution of Liquid/Gas Mass-Transfer Limitations to Dissolved Methane Oversaturation in Anaerobic Treatment of Dilute Wastewater.

The mechanisms controlling the accumulation of dissolved methane in anaerobic membrane bioreactors (AnMBRs) treating a synthetic dilute wastewater (a glucose medium) were assessed experimentally and theoretically. The AnMBR was maintained at a temperature of 24-26 °C as the organic loading rate increased from 0.39 to 1.1 kg COD/m(3)-d. The measured concentration of dissolved methane was consistently 2.2- to 2.5-fold larger than the concentration of dissolved methane at thermodynamic equilibrium with the measured CH4 partial pressure, and the fraction of dissolved methane was as high as 76% of the total methane produced. The low gas production rate in the AnMBR significantly slowed the mass transport of dissolved methane to the gas phase. Although the production rate of total methane increased linearly with the COD loading rate, the concentration of dissolved methane only slightly increased with an increasing organic loading rate, because the mass-transfer rate increased by almost 5-fold as the COD loading increased from 0.39 to 1.1 kg COD/m(3)-d. Thus, slow mass transport kinetics exacerbated the situation in which dissolved methane accounted for a substantial fraction of the total methane generated from the AnMBR.

Complete perchlorate reduction using methane as the sole electron donor and carbon source.

Using a CH4-based membrane biofilm reactor (MBfR), we studied perchlorate (ClO4(-)) reduction by a biofilm performing anaerobic methane oxidation coupled to denitrification (ANMO-D). We focused on the effects of nitrate (NO3(-)) and nitrite (NO2(-)) surface loadings on ClO4(-) reduction and on the biofilm community's mechanism for ClO4(-) reduction. The ANMO-D biofilm reduced up to 5 mg/L of ClO4(-) to a nondetectable level using CH4 as the only electron donor and carbon source when CH4 delivery was not limiting; NO3(-) was completely reduced as well when its surface loading was ≤ 0.32 g N/m(2)-d. When CH4 delivery was limiting, NO3(-) inhibited ClO4(-) reduction by competing for the scarce electron donor. NO2(-) inhibited ClO4(-) reduction when its surface loading was ≥ 0.10 g N/m(2)-d, probably because of cellular toxicity. Although Archaea were present through all stages, Bacteria dominated the ClO4(-)-reducing ANMO-D biofilm, and gene copies of the particulate methane mono-oxygenase (pMMO) correlated to the increase of respiratory gene copies. These pieces of evidence support that ClO4(-) reduction by the MBfR biofilm involved chlorite (ClO2(-)) dismutation to generate the O2 needed as a cosubstrate for the mono-oxygenation of CH4.

Pyrosequencing analysis yields comprehensive assessment of microbial communities in pilot-scale two-stage membrane biofilm reactors.

We studied the microbial community structure of pilot two-stage membrane biofilm reactors (MBfRs) designed to reduce nitrate (NO3(-)) and perchlorate (ClO4(-)) in contaminated groundwater. The groundwater also contained oxygen (O2) and sulfate (SO4(2-)), which became important electron sinks that affected the NO3(-) and ClO4(-) removal rates. Using pyrosequencing, we elucidated how important phylotypes of each "primary" microbial group, i.e., denitrifying bacteria (DB), perchlorate-reducing bacteria (PRB), and sulfate-reducing bacteria (SRB), responded to changes in electron-acceptor loading. UniFrac, principal coordinate analysis (PCoA), and diversity analyses documented that the microbial community of biofilms sampled when the MBfRs had a high acceptor loading were phylogenetically distant from and less diverse than the microbial community of biofilm samples with lower acceptor loadings. Diminished acceptor loading led to SO4(2-) reduction in the lag MBfR, which allowed Desulfovibrionales (an SRB) and Thiothrichales (sulfur-oxidizers) to thrive through S cycling. As a result of this cooperative relationship, they competed effectively with DB/PRB phylotypes such as Xanthomonadales and Rhodobacterales. Thus, pyrosequencing illustrated that while DB, PRB, and SRB responded predictably to changes in acceptor loading, a decrease in total acceptor loading led to important shifts within the "primary" groups, the onset of other members (e.g., Thiothrichales), and overall greater diversity.

Cooperation and competition between denitrification and chromate reduction in a hydrogen-based membrane biofilm reactor.

Competition and cooperation between denitrification and Cr(VI) reduction in a H2-based membrane biofilm reactor (H2-MBfR) were documented over 55 days of continuous operation. When nitrate (5 mg N/L) and chromate (0.5 mg Cr/L) were fed together, the H2-MBfR maintained approximately 100 % nitrate removal and 60 % chromate Cr(VI) removal, which means that nitrate outcompeted Cr(VI) for electrons from H2 oxidation. Removing nitrate from the influent led to an immediate increase in Cr(VI) removal (to 92 %), but Cr(VI) removal gradually deteriorated, with the removal ratio dropping to 14 % after five days. Cr(VI) removal resumed once nitrate was again added to the influent. 16S rDNA analyses showed that bacteria able to carry out H2-based denitrification and Cr(VI) reduction were in similar abundances throughout the experiment, but gene expression for Cr(VI)-reduction and export shifted. Functional genes encoding for energy-consuming chromate export (encoded by ChrA) as a means of bacterial resistance to toxicity were more abundant than genes encoding for the energy producing Cr(VI) respiration via the chromate reductase ChrR-NdFr. Thus, Cr(VI) transport and resistance to Cr(VI) toxicity depended on H2-based denitrification to supply energy. With Cr(VI) being exported from the cells, Cr(VI) reduction to Cr(III) was sustained. Thus, cooperation among H2-based denitrification, Cr(VI) export, and Cr(VI) reduction led to sustained Cr(VI) removal in the presence of nitrate, even though Cr(VI) reduction was at a competitive disadvantage for utilizing electrons from H2 oxidation.

Elemental sulfur biorecovery from phosphogypsum using oxygen-membrane biofilm reactor: Bioreactor parameters optimization, metagenomic analysis and metabolic prediction of the biofilm activity.

This work investigated elemental sulfur (S0) biorecovery from Phosphogypsum (PG) using sulfur-oxidizing bacteria in an O2-based membrane biofilm reactor (MBfR). The system was first optimized using synthetic sulfide medium (SSM) as influent, then switched to biogenic sulfide medium (BSM) generated by biological reduction of PG alkaline leachate. The results using SSM had high sulfide-oxidation efficiency (98 %), sulfide to S0 conversion (∼90 %), and S0 production rate up to 2.7 g S0/(m2.d), when the O2/S ratio was ∼0.5 g O2/g S. With the BSM influent, the system maintained high sulfide-to-S0 conversion rate (97 %), and S0-production rate of 1.6 g S0/(m2.d). Metagenomic analysis revealed that Thauera was the dominant genus in SSM and BSM biofilms. Furthermore, influent composition affected the bacterial community structure and abundances of functional microbial sulfur genes, modifying the sulfur-transformation pathways in the biofilms. Overall, this work shows promise for O2-MBfR usage in S0 biorecovery from PG-leachate and other sulfidogenic effluents.

Degradation of reactive dyes in a photocatalytic circulating-bed biofilm reactor.

Decolorization and mineralization of reactive dyes by intimately coupled TiO2-photocatalysis and biodegradation (ICPB) on a novel TiO2-coated biofilm carrier were investigated in a photocatalytic circulating-bed biofilm reactor (PCBBR). Two typical reactive dyes--Reactive Black 5 (RB5) and Reactive Yellow 86 (RY86)--showed similar first-order kinetics when being photocatalytically decolorized at low pH (~4-5) in batch experiments. Photocatalytic decolorization was inhibited at neutral pH in the presence of phosphate or carbonate buffer, presumably due to electrostatic repulsion from negatively charged surface sites on TiO2, radical scavenging by phosphate or carbonate, or both. Therefore, continuous PCBBR experiments were carried out at a low pH (~4.5) to maintain high photocatalytic efficiency. In the PCBBR, photocatalysis alone with TiO2-coated carriers could remove target compound RB5 and COD by 97% and 47%, respectively. Addition of biofilm inside macroporous carriers maintained a similar RB5 removal efficiency, but COD removal increased to 65%, which is evidence of ICPB despite the low pH. ICPB was further proven by finding microorganisms inside carriers at the end of the PCBBR experiments. A proposed ICPB pathway for RB5 suggests that a major intermediate, a naphthol derivative, was responsible for most of the residual COD, while most of the nitrogen in the azo-bonds (-N=N-) was oxidized to N2.

Effect of NaCl on nitrate removal from ion-exchange spent brine in the membrane biofilm reactor (MBfR).

The H(2)-based membrane biofilm reactor was used to remove nitrate from synthetic ion-exchange brine at NaCl concentrations from ∼3 to 30 g/L. NaCl concentrations below 20 g/L did not affect the nitrate removal flux as long as potassium was available to generate osmotic tolerance for high sodium, the H(2) pressure was adequate, and membrane fouling was eliminated. Operating pHs of 7-8 and periodic citric acid washes controlled membrane fouling and enabled reactor operation for 650 days. At 30 psig H(2) and high nitrate loading rates of 15 to 80 g/m(2) d, nitrate removal fluxes ranged from 2.5 to ∼6 g/m(2) d, which are the highest fluxes observed when treating 30 g/L IX brine. However, percent removals were low, and the H(2) pressure probably limited the removal flux.

Impact of precipitation on the treatment of real ion-exchange brine using the H(2)-based membrane biofilm reactor.

The H(2)-based membrane biofilm reactor (MBfR) was used to remove nitrate and perchlorate from real ion-exchange brine at two different salinities (30- and 50-g/L NaCl). Base production from nitrate reduction to N(2) gas caused the pH to increase, and this exacerbated precipitation of calcium and magnesium carbonates onto the MBfR fibers. The precipitates lowered the H(2) flux to the biofilm and caused a deterioration of denitrification performance that could be reversed by mild citric-acid washing. The addition of acid seems to be the only mechanism to avoid serious precipitation, membrane fouling, and non-optimal pH for denitrification.

Effect of pH on nitrate and selenate reduction in flue gas desulfurization brine using the H2-based membrane biofilm reactor (MBfR).

Increased tightening of air regulations is leading more electric utilities to install flue gas desulfurization (FGD) systems. These systems produce brine containing high concentrations of nitrate, nitrite, and selenate which must be removed before discharge. The H2-based membrane biofilm reactor (MBfR) was shown to consistently remove nitrate, nitrite, and selenate at high efficiencies. The maximum selenate removal flux reached 362 mgSe m(-2)d(-1) and was higher than that observed in earlier research, which shows continual improvement of the biofilm for selenate reduction. A low pH of 6.8 inhibited precipitation when treating actual FGD brine, yet did not inhibit removal. SO4(2-) was not removed and therefore did not compete with nitrate, nitrite, and selenate reduction for the available H2.

Influence of operating conditions on sulfate reduction from real mining process water by membrane biofilm reactors.

Two H2-based membrane biofilm reactor (H2-MBfR) systems, differing in membrane type, were tested for sulfate reduction from a real mining-process water having low alkalinity and high concentrations of dissolved sulfate and calcium. Maximum sulfate reductions were 99%, with an optimum pH range between 8 and 8.5, which minimized any toxic effect of unionized hydrogen sulfide (H2S) on sulfate-reducing bacteria (SRB) and calcite scaling on the fibers and in the biofilm. Although several strategies for control of pH and gas back-diffusion were applied, it was not possible to sustain a high degree of sulfate reduction over the long-term. The most likely cause was precipitation of calcite inside the biofilm and on the surface of fibers, which was shown by scanning electron microscopy coupled with energy dispersive spectroscopy (SEM-EDS) analysis. Another possible cause was a decline in pH, leading to inhibition by H2S. A H2/CO2 mixture in the gas supply was able to temporarily recover the effectiveness of the reactors and stabilize the pH. Biomolecular analysis showed that the biofilm was comprised of 15-20% SRB, but a great variety of autotrophic and heterotrophic genera, including sulfur-oxidizing bacteria, were present. Results also suggest that the MBfR system can be optimized by improving H2 mass transfer using fibers of higher gas permeability and by feeding a H2/CO2 mixture that is automatically adjusted for pH control.

Systematic evaluation of nitrate and perchlorate bioreduction kinetics in groundwater using a hydrogen-based membrane biofilm reactor.

To evaluate the simultaneous reduction kinetics of the oxidized compounds, we treated nitrate-contaminated groundwater (approximately 9.4 mg-N/L) containing low concentrations of perchlorate (approximately 12.5 microg/L) and saturated with dissolved oxygen (approximately 8 mg/L) in a hydrogen-based membrane biofilm reactor (MBfR). We systematically increased the hydrogen availability and simultaneously varied the surface loading of the oxidized compounds on the biofilm in order to provide a comprehensive, quantitative data set with which to evaluate the relationship between electron donor (H(2)) availability, surface loading of the electron acceptors (oxidized compounds), and simultaneous bioreduction of the electron acceptors. Increasing the H(2) pressure delivered more H(2) gas, and the total H(2) flux increased linearly from approximately 0.04 mg/cm(2)-d for 0.5 psig (0.034 atm) to 0.13 mg/cm(2)-d for 9.5 psig (0.65 atm). This increased rate of H(2) delivery allowed for continued reduction of the acceptors as their surface loading increased. The electron acceptors had a clear hydrogen-utilization order when the availability of hydrogen was limited: oxygen, nitrate, nitrite, and then perchlorate. Spiking the influent with perchlorate or nitrate allowed us to identify the maximum surface loadings that still achieved more than 99.5% reduction of both oxidized contaminants: 0.21 mg NO(3)-N/cm(2)-d and 3.4 microg ClO(4)/cm(2)-d. Both maximum values appear to be controlled by factors other than hydrogen availability.

Simultaneous removal of organic matter and nitrogen compounds by an aerobic/anoxic membrane biofilm reactor.

The hydrogen-based membrane biofilm reactor (MBfR) has been well studied and applied for denitrification of nitrate-containing water and wastewater. Adding an oxygen-based MBfR allows total-nitrogen removal when the input nitrogen is ammonium. However, most wastewaters also contain a significant concentration or organic material, measured as chemical oxygen demand (COD). This study describes experiments to investigate the removal of organic and nitrogenous compounds in the combined Aerobic/Anoxic MBfR, in which an Aerobic MBfR (Aer MBfR) precedes an Anoxic MBfR (An MBfR). The experiments demonstrate that the Aer/An MBfR combination accomplished COD oxidation and nitrogen removal for a total oxygen demand flux (i.e., from COD and NH(4) oxidations) in the range of 1.2-7.2 g O(2)/m(2)-d for 4.5 psi (0.3 atm) oxygen pressure to the Aer MBfR, but was overloaded and did not accomplish nitrification for the total oxygen demand load higher than 14 g O(2)/m(2)-d. Total-nitrogen removal was controlled by nitrification in the Aer MBfR, because the An MBfR denitrified all NO(3)(-) provided to it by the Aer MBfR. The overload of total oxygen demand did not affect COD oxidation in the Aer MBfR, but caused a small increase of COD in the An MBfR due to net release of soluble microbial products (SMP).

Kinetics of nitrate and perchlorate reduction in ion-exchange brine using the membrane biofilm reactor (MBfR).

Several sources of bacterial inocula were tested for their ability to reduce nitrate and perchlorate in synthetic ion-exchange spent brine (30-45 g/L) using a hydrogen-based membrane biofilm reactor (MBfR). Nitrate and perchlorate removal fluxes reached as high as 5.4 g Nm(-2)d(-1) and 5.0 g ClO(4)m(-2)d(-1), respectively, and these values are similar to values obtained with freshwater MBfRs. Nitrate and perchlorate removal fluxes decreased with increasing salinity. The nitrate fluxes were roughly first order in H(2) pressure, but roughly zero-order with nitrate concentration. Perchlorate reduction rates were higher with lower nitrate loadings, compared to high nitrate loadings; this is a sign of competition for H(2). Nitrate and perchlorate reduction rates depended strongly on the inoculum. An inoculum that was well acclimated (years) to nitrate and perchlorate gave markedly faster removal kinetics than cultures that were acclimated for only a few months. These results underscore that the most successful MBfR bioreduction of nitrate and perchlorate in ion-exchange brine demands a well-acclimated inoculum and sufficient hydrogen availability.

Simultaneous bio-reduction of trichloroethene, trichloroethane, and chloroform using a hydrogen-based membrane biofilm reactor.

The contamination of water by chlorinated solvents is recognized as a serious and widespread problem throughout the industrialized world. Here, we focus on three chlorinated solvents that are among those most commonly detected and that have distinct chemical features: trichloroethene (TCE), trichloroethane (TCA), and chloroform (CF). Because many contaminated waters contain mixtures of the chlorinated solvents, a treatment technology that detoxifies all of them simultaneously is highly desirable. The membrane biofilm reactor (MBfR) is a recent technological advance that makes it possible to deliver H(2) gas to bacteria efficiently and safely, despite hydrogen's low water solubility and risk of forming a combustible atmosphere when mixed with air. The objectives of this work are to document whether or not the three chlorinated compounds can be dechlorinated simultaneously in a H(2)-based MBfR and to determine if competitive or inhibitory interactions affect bio-reduction of any of the solvents. The main finding is a demonstration that directly using H(2) as the electron donor makes it possible to bio-reduce combinations of different chlorinated solvents. This finding supports that the H(2)-based MBfR can treat multiple chlorinated solvents in one step, addressing a common groundwater situation. We saw possible evidence of inhibition by CF at a concentration greater than about 1 muM, competition for H(2) from sulfate and nitrate reductions, and possible inhibition of TCE reduction from the accumulation of chloroethane (CA) or chloromethane (CM).

Complete dechlorination and mineralization of pentachlorophenol (PCP) in a hydrogen-based membrane biofilm reactor (MBfR).

Complete biodegradation and mineralization of pentachlorophenol (PCP), a priority pollutant in water, is challenging for water treatment. In this study, a hydrogen (H2)-based membrane biofilm reactor (MBfR) was applied to treat PCP, along with nitrate and sulfate, which often coexist in contaminated groundwater. Throughout 120-days of continuous operation, almost 100% of up to 10 mg/L PCP was removed with minimal intermediate accumulation and in parallel with complete denitrification of 20 mg-N/L nitrate. PCP initially was reductively dechlorinated to phenol, which was then mineralized to CO2 through pathways that began with aerobic activation via monooxygenation by Xanthobacter and anaerobic activation via carboxylation by Azospira and Thauera. Sulfur cycling induced by SO42- reduction affected the microbial community: The dominant bacteria became sulfate-reducers Desulfomicrobium, sulfur-oxidizers Sulfuritalea and Flavobacterium. This study provides insights and a promising technology for bioremediation of water contaminated with PCP, nitrate, and sulfate.

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

An 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 CH4 . Low-scan rate cyclic voltammetry (LSCV) revealed a mid-point potential (Eka ) 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.

Bio-reduction of soluble chromate using a hydrogen-based membrane biofilm reactor.

Hexavalent chromium (Cr(VI)) is a mutagen and carcinogen that is a significant concern in water and wastewater. A simple and non-hazardous means to remove Cr(VI) is bioreduction to Cr(III), which should precipitate as Cr(OH)3(s). Since Cr(VI)-reducing bacteria can use hydrogen (H2) as an electron donor, we tested the potential of the H2-based membrane biofilm reactor (MBfR) for chromate reduction and removal from water and wastewater. When Cr(VI) was added to a denitrifying MBfR, Cr(VI) reduction was immediate and increased over 11 days. Short-term experiments investigated the effects of Cr(VI) loading, H2 pressure, and nitrate loading on Cr(VI) reduction. Increasing the H2 pressure improved Cr(VI) reduction. Cr(VI) reduction also was sensitive to pH, with an optimum near 7.0, a sharp drop off below 7.0, and a gradual decline to 8.2. Cr(III) precipitated after a small upward adjustment of the pH. These experiments confirm that a denitrifying, H2-based MBfR can be used to reduce Cr(VI) to Cr(III) and remove Cr from water. The research shows that critical operational parameters include the H2 concentration, nitrate concentration, and pH.