Electron Donor Source Research Articles

Chemical vs. Biological Crystals, all the Same?

Using microorganisms to mediate crystallisation of metals and minerals in open-culture bioreactors has potential to recover recyclable materials from dilute aqueous streams, but also to prevent their emission to the environment. Although this potential is already exploited in practice to some extent, biological crystallization for metal recovery is still largely a black box technology with limited understanding of the role of the microorganisms in the crystallization, and the differences with chemical crystallisation. Using biocrystallisation of scorodite (FeAsO4.2H2O) and sphalerite (ZnS) as examples we propose that the role of microorganisms strongly depends on established saturation state of the solution. For scorodite, microorganisms are used to exert control over the crystallization as their ferrous iron-oxidizing activity keeps the solution slightly oversaturated. Also, the oversaturation level is kept homogeneous because of continuous biological formation of the reactant ferrous iron throughout the solution. In continuous bioreactor experiments on which we reported previously, scorodite crystal sizes still increased after 72 days of bioreactor operation indicating that indeed crystal growth was favored over nucleation. On the other hand, in our experiments with zinc sulfide, crystallization proceeded in highly oversaturated solutions in a continuous sulfate reducing bioreactor fed with a zinc sulfate solution and H2/CO2 as electron donor and carbon source. The high oversaturation likely resulted in dominant primary nucleation in the bulk solution, with little or no control over crystal growth, even though agglomeration may still have occurred. This was exemplified by particle sizes which decreased in the bioreactor experiment and remained stable after already about 2 weeks of operation.

Isolation and characterization of molybdenum-reducing and PEG-degrading Enterobacter cloacae strain KIK-14 in agricultural soil from Nigeria

Today, numerous researches have demonstrated the cost-effectiveness of bioremediation to waste removal from agricultural and industrial sectors particularly at lower levels of the toxicants, where other physicochemical techniques are ineffective. Multiple toxicant remediation by a single microorganism is important for remediation of sites contaminated with numerous toxicants. In this work, a molybdenum-reducing bacterium was screened for its ability to use the xenobiotic polyethylene glycol (PEG) as the sole source of carbon for growth and as electron donor source for molybdate reduction. Biochemical analysis results in the tentative identification of the isolate as Enterobacter cloacae strain KIK-14. The use of PEGs as an electron donor in this bacterium did not support molybdenum-blue production, even though the bacterium grew well on PEGs 200, 300, 600 and 1000 independent of molybdate reduction. Reduction of molybdate to Mo-blue was optimal at pH between 6.0 and 6.3, the temperature between 25 and 37 oC, molybdate and phosphate concentrations between 15 and 20 mM and between 5.0 and 7.5 mM respectively. The best electron donor source supporting the reduction process was glucose. The Mo-blue absorption spectrum resembles reduced phosphomolybdate and is similar to that of the previous Mo-reducing bacterium. At 2 ppm of silver, mercury and copper, molybdenum reduction was inhibited by 41.5, 57.1 and 40.5%, respectively. The ability of this bacterium to detoxify mixed toxicants makes it an important tool for bioremediation.

English

Excessive use of pesticides has been known to be hazardous to the environment, affect soil fertility as well may impart toxicity in living beings. Presently there have been physical, chemical, biological and enzymatic approaches implicated to reduce pesticides. Although aimed to eradicate, physical and chemical methods are inefficient. Curiously, microbial pesticide remediation has been cost effective and thermodynamically more affordable, which may use any physical mater soiled with pesticide. Under favourable conditions microbes have been reported to use pesticides as source of carbon, sulphur and electron donor. Microbes; bacteria, actinomycetes and fungi have been found to help remove or detoxify chlorinated pesticides; polychlorinated diphenyl, polycyclic aromatic hydrocarbons, organophosphorus. Major bacterial genera includes; Bacillus, Pseudomonas, Flavobacterium, Moraxalla, Acinetobacter, Arthrobacter, Paracoccus, Aerobacter, Alkaligens, Burkholderia and Sphingomonas. Fungi with pesticide degradation potential includes;Fusarium, Aspergilus niger, Penicillium, Lentinulaedodes, Lecanicillium, Oxysporum. Among the Actinomycetes theStreptomycetes have been found to successfully detoxify pesticides. Persistent organic pollutants in the form of pesticides have also been reported to be taken care by the microbial enzymes viz-a-viz; dehydrogenase, ligninase, oxygenase, peroxidises, phosphotriesterase, hydrolases, dehalogenase, laccase and organophosphorus acid anhydrolase. Microbial strategies and tools; enzymes and genes involved in pesticide catabolism are reviewed. Key words: Chlorinated pesticides, organophosphors pesticides, bacteria, fungi, enzyme.

Rock magnetic and geochemical evidence for authigenic magnetite formation via iron reduction in coal‐bearing sediments offshore Shimokita Peninsula, Japan (IODP Site C0020)

AbstractSediments recovered at Integrated Ocean Drilling Program (IODP) Site C0020, in a fore‐arc basin offshore Shimokita Peninsula, Japan, include numerous coal beds (0.3–7 m thick) that are associated with a transition from a terrestrial to marine depositional environment. Within the primary coal‐bearing unit (∼2 km depth below seafloor) there are sharp increases in magnetic susceptibility in close proximity to the coal beds, superimposed on a background of consistently low magnetic susceptibility throughout the remainder of the recovered stratigraphic sequence. We investigate the source of the magnetic susceptibility variability and characterize the dominant magnetic assemblage throughout the entire cored record, using isothermal remanent magnetization (IRM), thermal demagnetization, anhysteretic remanent magnetization (ARM), iron speciation, and iron isotopes. Magnetic mineral assemblages in all samples are dominated by very low‐coercivity minerals with unblocking temperatures between 350 and 580°C that are interpreted to be magnetite. Samples with lower unblocking temperatures (300–400°C), higher ARM, higher‐frequency dependence, and isotopically heavy δ56Fe across a range of lithologies in the coal‐bearing unit (between 1925 and 1995 mbsf) indicate the presence of fine‐grained authigenic magnetite. We suggest that iron‐reducing bacteria facilitated the production of fine‐grained magnetite within the coal‐bearing unit during burial and interaction with pore waters. The coal/peat acted as a source of electron donors during burial, mediated by humic acids, to supply iron‐reducing bacteria in the surrounding siliciclastic sediments. These results indicate that coal‐bearing sediments may play an important role in iron cycling in subsiding peat environments and if buried deeply through time, within the subsequent deep biosphere.

Desulfamplus magnetovallimortis gen. nov., sp. nov., a magnetotactic bacterium from a brackish desert spring able to biomineralize greigite and magnetite, that represents a novel lineage in the Desulfobacteraceae

A magnetotactic bacterium, designated strain BW-1T, was isolated from a brackish spring in Death Valley National Park (California, USA) and cultivated in axenic culture. The Gram-negative cells of strain BW-1T are relatively large and rod-shaped and possess a single polar flagellum (monotrichous). This strain is the first magnetotactic bacterium isolated in axenic culture capable of producing greigite and/or magnetite nanocrystals aligned in one or more chains per cell. Strain BW-1T is an obligate anaerobe that grows chemoorganoheterotrophically while reducing sulfate as a terminal electron acceptor. Optimal growth occurred at pH 7.0 and 28°C with fumarate as electron donor and carbon source. Based on its genome sequence, the G+C content is 40.72mol %. Phylogenomic and phylogenetic analyses indicate that strain BW-1T belongs to the Desulfobacteraceae family within the Deltaproteobacteria class. Based on average amino acid identity, strain BW-1T can be considered as a novel species of a new genus, for which the name Desulfamplus magnetovallimortis is proposed. The type strain of D. magnetovallimortis is BW-1T (JCM 18010T–DSM 103535T).

Assessing the influence of the carbon source on the abatement of industrial N2O emissions coupled with the synthesis of added-value bioproducts

The continuous abatement of a synthetic N2O emission from a nitric acid plant coupled with the simultaneously production of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) copolymer and the coenzyme Q10 (CoQ10) in a bubble column reactor (BCR) was tested using methanol, glycerol and a mixture of sodium acetate-acetic acid (Ac-HAc) as a carbon and electron donor source. The BCRs were inoculated with Paracoccus denitrificans and supplied with the carbon/electron donor at a loading rate of 139gCm−3d−1. High N2O removal efficiencies (81–91%) were achieved, with glycerol supporting the highest abatement. The PHBV cell content ranged from 25 to 53%, with highest accumulation in the culture obtained with methanol and Ac-HAc. However, the greatest PHBV productivities were observed in the BCRs operated with glycerol and Ac-HAc (21.7 and 33.5gPHBVm−3d−1, respectively). Glycerol supply induced the highest molar ratio (23%) of the homopolymer 3-hydroxyvalerate in the composition of PHBV. In addition, the specific cell content of CoQ10 ranged from 0.4 to 1mgg−1. This work constitutes, to the best of our knowledge, the first study combining N2O abatement with the simultaneous production of multiple bioproducts, which pave the way to the development of greenhouse gas biorefineries for climate change mitigation.

Methanogenic Biocathode Microbial Community Development and the Role of Bacteria.

The cathode microbial community of a methanogenic bioelectrochemical system (BES) is key to the efficient conversion of carbon dioxide (CO2) to methane (CH4) with application to biogas upgrading. The objective of this study was to compare the performance and microbial community composition of a biocathode inoculated with a mixed methanogenic (MM) culture to a biocathode inoculated with an enriched hydrogenotrophic methanogenic (EHM) culture, developed from the MM culture following pre-enrichment with H2 and CO2 as the only externally supplied electron donor and carbon source, respectively. Using an adjacent Ag/AgCl reference electrode, biocathode potential was poised at -0.8 V (versus SHE) using a potentiostat, with the bioanode acting as the counter electrode. When normalized to cathode biofilm biomass, the methane production in the MM- and EHM-biocathode was 0.153 ± 0.010 and 0.586 ± 0.029 mmol CH4/mg biomass-day, respectively. This study showed that H2/CO2 pre-enriched inoculum enhanced biocathode CH4 production, although the archaeal communities in both biocathodes converged primarily (86-100%) on a phylotype closely related to Methanobrevibacter arboriphilus. The bacterial community of the MM-biocathode was similar to that of the MM inoculum but was enriched in Spirochaetes and other nonexoelectrogenic, fermentative Bacteria. In contrast, the EHM-biocathode bacterial community was enriched in Proteobacteria, exoelectrogens, and putative producers of electron shuttle mediators. Similar biomass levels were detected in the MM- and EHM-biocathodes. Thus, although the archaeal communities were similar in the two biocathodes, the difference in bacterial community composition was likely responsible for the 3.8-fold larger CH4 production rate observed in the EHM-biocathode. Roles for abundant OTUs identified in the biofilm and inoculum cultures were highlighted on the basis of previous reports.

Application of Commercial Coffee for the Remediation of Hexavalent Chromium-Contaminated Water

The potential application of commercial coffee as a source of electron donors for detoxifying hexavalent chromium [Cr(VI)]-contaminated water was investigated. Various amounts of coffee were reacted with 50 mg/L of artificially prepared Cr(VI)-contaminated water, and the Cr(VI) concentration was monitored as a function of the reaction time using the diphenylcarbazide colorimetric method with an Aquamate 8000 UV-Vis spectrophotometer at a 540-nm wavelength. When the ratio of the coffee mass applied to the volume of Cr(VI) solution was 75 g/L, more than 80% of the initial Cr(VI) disappeared within 5 min of reaction time, and the Cr(VI) concentration became lower than the detection limit of 1 mg/L within 20 min. More Cr(VI) disappeared as more coffee was introduced. In general, smaller particles of coffee were more effective at Cr(VI) reduction, but the advantage that particle size conferred disappeared once the coffee particle size was smaller than 125 μm. As a result, the reduction of the Cr(VI) in the solution was not considered to result from the surface catalytic reduction but by the electron transfer from the electron donors released from the applied coffee.

Acetylene Fuels TCE Reductive Dechlorination by Defined Dehalococcoides/Pelobacter Consortia.

Acetylene (C2H2) can be generated in contaminated groundwater sites as a consequence of chemical degradation of trichloroethene (TCE) by in situ minerals, and C2H2 is known to inhibit bacterial dechlorination. In this study, we show that while high C2H2 (1.3 mM) concentrations reversibly inhibit reductive dechlorination of TCE by Dehalococcoides mccartyi isolates as well as enrichment cultures containing D. mccartyi sp., low C2H2 (0.4 mM) concentrations do not inhibit growth or metabolism of D. mccartyi. Cocultures of Pelobacter SFB93, a C2H2-fermenting bacterium, with D. mccartyi strain 195 or with D. mccartyi strain BAV1 were actively sustained by providing acetylene as the electron donor and carbon source while TCE or cis-DCE served as the electron acceptor. Inhibition by acetylene of reductive dechlorination and methanogenesis in the enrichment culture ANAS was observed, and the inhibition was removed by adding Pelobacter SFB93 into the consortium. Transcriptomic analysis of D. mccartyi strain 195 showed genes encoding for reductive dehalogenases (e.g., tceA) were not affected during the C2H2-inhibition, while genes encoding for ATP synthase, biosynthesis, and Hym hydrogenase were down-regulated during C2H2 inhibition, consistent with the physiological observation of lower cell yields and reduced dechlorination rates in strain 195. These results will help facilitate the optimization of TCE-bioremediation at contaminated sites containing both TCE and C2H2.

Metatranscriptomic Analysis Reveals Unexpectedly Diverse Microbial Metabolism in a Biogeochemical Hot Spot in an Alluvial Aquifer.

Organic matter deposits in alluvial aquifers have been shown to result in the formation of naturally reduced zones (NRZs), which can modulate aquifer redox status and influence the speciation and mobility of metals, affecting groundwater geochemistry. In this study, we sought to better understand how natural organic matter fuels microbial communities within anoxic biogeochemical hot spots (NRZs) in a shallow alluvial aquifer at the Rifle (CO) site. We conducted a 20-day microcosm experiment in which NRZ sediments, which were enriched in buried woody plant material, served as the sole source of electron donors and microorganisms. The microcosms were constructed and incubated under anaerobic conditions in serum bottles with an initial N2 headspace and were sampled every 5 days for metagenome and metatranscriptome profiles in combination with biogeochemical measurements. Biogeochemical data indicated that the decomposition of native organic matter occurred in different phases, beginning with mineralization of dissolved organic matter (DOM) to CO2 during the first week of incubation, followed by a pulse of acetogenesis that dominated carbon flux after 2 weeks. A pulse of methanogenesis co-occurred with acetogenesis, but only accounted for a small fraction of carbon flux. The depletion of DOM over time was strongly correlated with increases in expression of many genes associated with heterotrophy (e.g., amino acid, fatty acid, and carbohydrate metabolism) belonging to a Hydrogenophaga strain that accounted for a relatively large percentage (~8%) of the metatranscriptome. This Hydrogenophaga strain also expressed genes indicative of chemolithoautotrophy, including CO2 fixation, H2 oxidation, S-compound oxidation, and denitrification. The pulse of acetogenesis appears to have been collectively catalyzed by a number of different organisms and metabolisms, most prominently pyruvate:ferredoxin oxidoreductase. Unexpected genes were identified among the most highly expressed (>98th percentile) transcripts, including acetone carboxylase and cell-wall-associated hydrolases with unknown substrates (numerous lesser expressed cell-wall-associated hydrolases targeted peptidoglycan). Many of the most highly expressed hydrolases belonged to a Ca. Bathyarchaeota strain and may have been associated with recycling of bacterial biomass. Overall, these results highlight the complex nature of organic matter transformation in NRZs and the microbial metabolic pathways that interact to mediate redox status and elemental cycling.

Polyhydroxyalkanoate as a slow-release carbon source for in situ bioremediation of contaminated aquifers: From laboratory investigation to pilot-scale testing in the field

A pilot-scale study aiming to evaluate the potential use of poly-3-hydroxy-butyrate (PHB) as an electron donor source for in situ bioremediation of chlorinated hydrocarbons in groundwater was conducted. Compared with commercially available electron donors, PHB offers a restricted fermentation pathway (i.e., through acetic acid and molecular hydrogen) by avoiding the formation of any residual carbon that could potentially spoil groundwater quality. The pilot study was carried out at an industrial site in Italy, heavily contaminated by different chlorinated aliphatic hydrocarbons (CAHs). Prior to field testing, PHB was experimentally verified as a suitable electron donor for biological reductive dechlorination processes at the investigated site by microcosm studies carried out on site aquifer material and measuring the quantitative transformation of detected CAHs to ethene. Owing to the complex geological characteristics of the aquifer, the use of a groundwater circulation well (GCW) was identified as a potential strategy to enable effective delivery and distribution of electron donors in less permeable layers and to mobilise contaminants. A 3-screened, 30-m-deep GCW coupled with an external treatment unit was installed at the site. The effect of PHB fermentation products on the in situ reductive dechlorination processes were evaluated by quantitative real-time polymerase chain reaction (qPCR). The results from the first 4 months of operation clearly demonstrated that the PHB fermentation products were effectively delivered to the aquifer and positively influenced the biological dechlorination activity. Indeed, an increased abundance of Dehalococcoides mccartyi (up to 6.6 fold) and reduced CAH concentrations at the installed monitoring wells were observed.

Performance and Influencing Factors of Dissimilatory Nitrate Reduction to Ammonium Process by the Strain Desulfovibrio sp. CMX

The process of dissimilatory nitrate reduction to ammonium (DNRA) plays an important role in the presence and conversion of ammonium ions in nature. However, the influencing factors and mechanisms of DNRA process in sulfate-reducing bacteria (SRB) have not yet been identified. The ability, influencing factors and mechanisms of DNRA was investigated in Desulfovibrio sp. CMX isolated previously in our laboratory. The results showed that the production rate of ammonium reached 85.8% and 97.3% respectively with NO3- and NO2- (10 mmol·L-1) as the sole electron donor and nitrogen source. Meanwhile, there was no by-products such as N2 and N2O. Then the effects of different extra nitrogen sources and initial concentrations of sulfate and sulfide on DNRA process were also investigated. As a result, extra nitrogen, yeast extract, could promote DNRA process by improving both growth and metabolism of the strain. In another study, sulfate promoted the nitrate reduction stage while inhibited the nitrite reduction. Overall, the DNRA process by Desulfovibrio sp. CMX was inhibited by sulfate. In addition, both the growth and two stages of the DNRA process were inhibited in the presence of sulfide. Dose effect of sulfide was observed. Inhibition was enhanced with the increase of sulfide concentration. It's noticeable that the inhibition on nitrate reduction was stronger than the inhibition on nitrite reduction with 6 mmol·L-1 of sulfide, leading to the lower reduction rate of nitrate than that of nitrite. In this condition, no accumulation of nitrite was observed in the system.

Effects of salinity on simultaneous reduction of perchlorate and nitrate in a methane-based membrane biofilm reactor.

This study builds upon prior work showing that methane (CH4) could be utilized as the sole electron donor and carbon source in a membrane biofilm reactor (MBfR) for complete perchlorate (ClO4-) and nitrate (NO3-) removal. Here, we further investigated the effects of salinity on the simultaneous removal of the two contaminants in the reactor. By testing ClO4- and NO3- at different salinities, we found that the reactor performance was very sensitive to salinity. While 0.2% salinity did not significantly affect the hydrogen-based MBfR for ClO4- and NO3- removals, 1% salinity completely inhibited ClO4- reduction and significantly lowered NO3- reduction in the CH4-based MBfR. In salinity-free conditions, NO3- and ClO4- removal fluxes were 0.171g N/m2-day and 0.091g/m2-day, respectively, but NO3- removal fluxes dropped to 0.0085g N/m2-day and ClO4- reduction was completely inhibited when the medium changed to 1% salinity. Scanning electron microscopy (SEM) showed that the salinity dramatically changed the microbial morphology, which led to the development of wire-like cell structures. Quantitative real-time PCR (qPCR) indicated that the total number of microorganisms and abundances of functional genes significantly declined in the presence of NaCl. The relative abundances of Methylomonas (methanogens) decreased from 31.3 to 5.9% and Denitratisoma (denitrifiers) decreased from 10.6 to 4.4% when 1% salinity was introduced.

Biological Nitrous Oxide Abatement by Paracoccus denitrificans in Bubble Column and Airlift Reactors

Nitrous oxide (N2O), with a global warming potential 300 times higher than that of CO2, represents 6.2 % of the total greenhouse gas emission inventory worldwide. Furthermore, N2O is considered the most critical O3- depleting substance emitted in this XXI century. In spite of the environmental relevance of this pollutant, very little research on biotechnologies for the treatment of N2O emissions has been conducted. In this study, the potential of a bubble column (BCR) and an internal loop airlift (ALR) bioreactors of 2.3 L was evaluated for the abatement of N2O from industrial emissions from nitric acid plants along 62 days of operation. The systems were inoculated with a methylotrophic Paracoccus denitrificans strain (DSM 413) and continuously supplied with methanol as a carbon and electron donor source for the anoxic reduction of N2O. The simulated waste gas consisted of a N2 gas stream containing 1 ± 0.1 % of O2 and 3377 ± 312 ppmv of N2O at the inlet of theBCR and 1 ± 0.1 % of O2 with N2O concentration of 3617 ± 342 ppmv at the inlet of the ALR. This N2-laden stream was supplied at a constant flow rate of 110 ml min-1 in each reactor. The performance of the BCR was characterized by a steady state N2O removal efficiency (RE) of 87 ± 3 % with CO2 productions of 308 ± 36 gm-3 d-1 and total suspended solid (TSS) concentrations of 867 ± 109 mg L-1. On the other hand, the ALR showed a N2O RE of 88 ± 2 % with productions of CO2 of 346 ± 28 g m-3 d-1 and TSS concentrations of 874 ±88 mg L-1. This work constitutes, to the best of our knowledge, the first systematic study of a biotechnology for the continuous abatement of N2O from nitric acid plants.

Azo dye decolorization in an up-flow bioelectrochemical reactor with domestic wastewater as a cost-effective yet highly efficient electron donor source

A major challenge of employing bioelectrochemical system (BES) for reductively degrading recalcitrant contaminants in industrial wastewater is lacking sufficient electron donors. In this work, domestic wastewater (DW) was demonstrated to efficiently drive BES for implementing the decolorization of azo dye, acid orange 7 (AO7). Side benefit was the simultaneous treatment of DW. Decolorization efficiency in BES fed with DW (RDW) was found to be comparable with that either fed with glucose (RGlu) or acetate (RAc). Much lower reductant usage ratio was observed in RDW. As a result, when the ratio of electron donors to azo dye decreased to 4.4 mol COD mol−1 AO7, DE of RDW kept over 90% while DEs of RAc and RGlu were significantly dropped due to the insufficient electrons donation. Besides serving as electron donor, DW was proved to also provide some conductivity and buffer capacity. Accordingly, DE of RDW was less deteriorated when fully removing the external buffer slats. This study comprehensively revealed the feasibility and superiority of DW as a cost-effective electron donor source in BES and brings this technology closer to the practice.

Editorial: Current Challenges and Future Perspectives on Emerging Bioelectrochemical Technologies.

Editorial: Current Challenges and Future Perspectives on Emerging Bioelectrochemical Technologies.

Biomimetic polymeric semiconductor based hybrid nanosystems for artificial photosynthesis towards solar fuels generation via CO2 reduction

In photosynthesis, an intricate polymeric system is constructed by connecting a light-harvesting antenna network, a molecular water oxidation center, and CO2 or proton-reduction machinery in a nanolayered architecture as a basic photosynthetic unit for solar-to-fuels conversion. Herein, we present a prototype basic artificial photosynthetic unit by connecting a typical CO2/proton reduction catalyst, a cocatalyst and an electron mediator as well as CO2 activator into a polymer based nano-architectured system for artificial photosynthesis with water and CO2. Here, g-C3N4 nanosheets, mimicking the nanolayered thylakoids stacks are demonstrated as promising photocatalytic elements with planar configuration and high surface area, which provide an excellent platform for the assembly of other analogous elements. Au NPs are served as a suitable cocatalyst. ZIF-9, as a typical cofactor to illustrate this concept here, is used as a CO2 concentrator and an electron mediator to promote the redox reaction. In artificial photosynthesis, driven by light energy, water and CO2 are served electron donor and carbon source respectively for the generation of H2 and CO. The artificial unit described here as a simple model, provides an important biomimetic step down a path aligned with the low-cost artificial photosynthetic systems manufacturing.

Effect of C/N ratio and support material on heterotrophic denitrification of potable water in bio-filters using sugar as carbon source

This work studied heterotrophic denitrification in packed-bed reactors using sugar as electron donor and carbon source. Two pilot-scale bio-filters, loaded with different types and sizes of support media, were operated under sequencing batch (SBR) mode with recirculation. The materials used were silicic gravel placed in two layers (diameter 2.41 and 4.03 mm) within the first filter (R1 – multilayer), and hollow plastic tubes (diameter 1.6 cm) within the second filter (R2 – monolayer). Feed NO3−-N concentrations ranged between 20 and 400 mg/L, while the C/N ratio was kept constant at 13.5. Higher denitrification rates (4.06–8.76 kg N/(m3d)) and total sugar biodegradation were observed in bio-filter R1 than in R2. However, carbon residues were observed in the treated water of bio-filter R1 for feed concentrations above 70 mgNO3−-N/L. Bio-filter R1 was then used to study the effect of C/N ratio on its performance. Experiments showed that a decrease of the C/N ratio from 13.5 to 6.25 resulted in lower carbon residue in the treated water and high denitrification rates (3.21–6.64 kgN/(m3d)), however, significant NO2−-N accumulation was observed. Further reduction of the C/N ratio (i.e., 5.48 and 4.71) resulted in filter malfunction as insufficient removal of NO3−-N was achieved. Results suggest that under the more favourable conditions of C/N:13.5 and silicic gravel support material, satisfactory NO3−-N and COD removal could be achieved for feed concentrations up to 60 mgNO3−-N/L at a cost of 0.019 €/KgN or 1.13 × 10−3 €/m3, which is much lower than ever reported in the literature and confirms the economic viability of the process. To describe the heterotrophic denitrification process, a mathematical model of microbial growth on nitrate and nitrite treated as substitutable substrates and sugar as the carbon source is proposed. Model simulations could be used as a guide for the optimum design and operation of water treatment systems for nitrate removal.

Mineral formation during bacterial sulfate reduction in the presence of different electron donors and carbon sources

Sulfate-reducing bacteria have long been known to promote mineral precipitation. However, the influence of electron donors (energy sources) and carbon sources on the minerals formed during sulfate reduction is less well understood. An investigation was therefore undertaken to determine how these nutrients affect sulfate reduction by the bacterium Desulfovibrio alaskensis G20 in a marine sediment pore water medium. Monohydrocalcite and a small amount of calcite formed during sulfate reduction with formate as the electron donor; Mg-phosphates and calcite precipitated when hydrogen served as the electron donor and when acetate and dissolved inorganic carbon served as carbon sources; and greigite and elemental sulfur were deposited when lactate was used as the electron donor and carbon source. The experimental results were generally consistent with geochemical modeling, suggesting that it may be possible to predict the processes and conditions during formation of these minerals in natural environments.

Electrons from Solid Electrode Alter Phototrophic Mat Morphology and Metabolic Activity

Phototrophic microbial mats are structured microbial communities composed of highly interactive species that have complete energy and elements within them. Aside from being an excellent ecological model that can be used to investigate how microbial populations associate and interact, microbial mats have the potential for energy capture and conversion applications. To use mats for such purpose, it is critical to understand the energy transfer and activity of microbial populations under the existence of external electron donor or acceptor sources. One method to accomplish this is by introducing a solid electrode with a controlled electrochemical potential to act as an electron donor or acceptor to the mat systems. We hypothesized that varying the electrode potential would, in turn, change the structure, activity, and community composition of the mat via regulation of electron transfer processes. In this work, the electrodes were placed under mats to establish a mat electrochemical system for testing our hypothesis. The phototrophic microbial mat was harvested from Hot Lake, a hypersaline, epsomitic lake located near Oroville (Washington, USA). The electrodes in contact with the base of the mat were polarized at -0.7 VAg/AgCl (hereafter referred to as the “cathodic mat)”. Electrodes that served as an electron sink were polarized at +0.3 VAg/AgCl (hereafter referred as the “anodic mat”). We quantified the diel variations of electron transfer rates between the electrode and the cathodic or anodic mats. We observed variations in current over a diel light cycle in which the lowest and highest magnitudes of anodic current occurred simultaneously with those of cathodic current. To understand the factors affecting these variations we examined the influences of temperature, light intensity, and substrate additions, separately, to electron transfer rate. We also used microelectrodes to measure the depth profiles of sulfide and oxygen concentrations and compared them with the variations of electron transfer rates over a diel cycle. To understand how anodic or cathodic electron transfer processes impacted mat structure and activity, we used nuclear magnetic resonance (NMR) imaging for morphology and NMR metabolite analysis to quantify the activity. The results showed that the quantitative porosity and diffusion coefficients were higher in the cathodic mat than in the anodic mat. In addition, cathodic mats produced significantly higher concentrations of osmo-protectants such as betaine and trehalose. The anodic mat had metabolic profiles similar to the control (non-polarized) mat. Finally, we employed amplicon sequencing across the V4 region of the 16S gene to determine the effect of the electrochemical regime upon mat community structure. The community structure data were consistent with variations in metabolite profiles and suggested greater changes in microbial community structure and function in the cathodic vs. the anodic and control mats. Although further work is needed to determine whether the electron transfer to and from the electrode to the mat was mediated by microbial metabolism of selectively-enriched community on the electrode or was caused by abiotic reactions on electrode, our data strongly suggest that donation of electrons via a solid electrode is capable of altering phototrophic mat morphology, community composition, and metabolic activity. It is possible that the addition of electrons from electrode to the mat alleviated the energy limitations allowing the mat to maintain large carbon pools of osmolytes. In addition, some microorganisms in the mat might use electrons from the electrode to drive carbon fixations which resulted in selectively-enriched community on the electrode and the increase of the total carbon available to the mat.