Organic Electron Donors Research Articles

Metagenomic analysis revealed the sulfur- and iron- oxidation capabilities of heterotrophic denitrifying sludge.

Heterotrophic denitrification is widely applied in wastewater treatment processes to remove nitrate. However, the ability of the heterotrophic denitrifying sludge to use inorganic matter as electron donors to perform autotrophic denitrification has rarely been investigated. In this study, we enriched heterotrophic denitrifying sludge and demonstrated its sulfur- and iron- oxidizing abilities and denitrification performance with batch experiments. Based on high-throughput sequencing of 16S rRNA genes, high diversity and abundance of sulfur-oxidizing bacteria (SOB) (e.g., Sulfuritalea, Thiobacillus, and Thiothrix) and iron (II)-oxidizing bacteria (FeOB) (e.g., Azospira and Thiobacillus) were observed. Metagenomic sequencing and genome binning results further suggested that the SOB in the heterotrophic denitrifying sludge were mainly Alphaproteobacteria and Betaproteobacteria instead of Gammaproteobacteria and Epsilonproteobacteria. The similarities of potential iron-oxidizing genes with known sequences were very low (32-51%), indicating potentially novel FeOB species in this system. The findings of this study suggested that the heterotrophic denitrifying sludge harbors diverse mixotrophic denitrifying bacterial species, and based on this finding, we proposed that organic carbon and inorganic electron donors (e.g., sulfur, thiosulfate, and iron) could be jointly used in engineering practices according to the quality and quantity of wastewater to balance the cost and efficiency of the denitrification process.

VV Reduction by Polaromonas spp. in Vanadium Mine Tailings.

Vanadium (V) is an important metal with critical industrial and medical applications. Elevated V contamination, however, can be a threat to the environment and human health. Microorganisms can reduce the more toxic and mobile VV to the less toxic and immobile VIV, which could be a detoxification and energy metabolism strategy adopted by V-reducing bacteria (VRB). The limited understanding of microbial responses to V contamination and the mechanisms for VV reduction, however, hamper our capability to attenuate V contamination. This study focused on determining the microbial responses to elevated V concentration and the mechanisms of VV reduction in V tailings. The bacterial communities were characterized and compared between the V tailings and the less contaminated adjacent mineral soils. Further, VV-reducing enrichments indicated that bacteria associated with Polaromonas, a genus belonging to the family Burkholderiaceae, were potentially responsible for VV reduction. Retrieved metagenome-assembled genomes (MAGs) suggested that the Polaromonas spp. encoded genes (cymA, omcA, and narG) were responsible for VV reduction. Additionally, Polaromonas spp. was metabolically versatile and could use both organic and inorganic electron donors. The metabolic versatility of Polaromonas spp. may be important for its ability to flourish in the V tailings.

Enhanced low-temperature denitrification by microbial consortium using solid-phase humin

Reducing the use of liquid organic carbon electron donors during biostimulation of heterotrophic denitrification is critical for sustainable groundwater remediation. Solid-phase humin isolated from natural sources can provide a cost-effective alternative to classical electron donors. In this study, the low-temperature denitrification capacity of an acetate-fed microbial community was enhanced using humin at 20 °C and 10 °C. These enhancements were not caused by faster acetate consumption and greater bacterial growth with the addition of humin. Estimation of the electron balance and first-order kinetics suggested that the enhancement in denitrification occurred mainly after acetate exhaustion. Humin may therefore have acted as an additional electron donor for the denitrifying microbial community, with the reduced quinone group in humin potentially responsible for electron donation. The addition of humin increased the richness and diversity of the denitrifying microbial community, in which Dechloromonas spp. played a critical role. Given the prevalence of humin and denitrifiers using humic substances, our results have important implications in the bioremediation of nitrate-contaminated groundwater using less liquid organic carbon electron donors.

Recent progress on visible active nanostructured energy materials for water split generated hydrogen

Solar-driven water splitting is one amongst the fastest growing areas of research for hydrogen evolution. The technology employs application of nanostructured energy materials with featured charge-separation and light absorption characteristics. The high charge-carrier recombination and wide band-gap of energy materials are major material-related bottlenecks that restricted the large-scale implementation. Besides it, the co-catalyst, electron mediator and sacrificial electron donor (SED) were the other major components for further improving the H2 yield. A water splitting reactor with well-optimized photon and mass transfer properties is mandatory for studying hydrogen evolution at large scale. The present review compiles various strategies that were presented for reducing charge-carrier recombination and extending visible light absorption of materials, insights for loading earth-abundant co-catalysts on photocatalyst surface, recently developed reversible redox and non-noble metal-based solid-state electron mediators and advantage of applying organic contaminant and biomass-derived waste as SEDs. The discussions on several engineered reactor designs for the photocatalytic water splitting technology were also presented. A cost-effective way of designing a photocatalytic water splitting system with functionalized nanostructured energy materials, the earth-abundant co-catalysts, organic waste-based electron donors and efficient electron mediators for achieving improved hydrogen evolution rate is comprehensively discussed.

Isolation and characterization of a novel acidophilic zero-valent sulfur- and ferric iron-respiring Firmicute

A novel, obligately anaerobic, acidophilic bacterium (strain I2511), isolated from sediment in an abandoned copper mine, was shown to couple the oxidation of organic electron donors to the reduction of both zero-valent sulfur and ferric iron in acidic media. The isolate was an obligate heterotroph that used a variety of organic compounds as electron donors and required yeast extract for growth. Alternative electron acceptors (sulfate, tetrathionate, thiosulfate and nitrate) were not used by the novel isolate. The strain grew as motile, endospore-forming rods, and was mesophilic and moderately acidophilic, with a growth rate of 0.01 h−1 at optimum pH (3.7) and temperature (35 °C). Analysis of its 16S rRNA gene sequence placed strain I2511 within the phylum Firmicutes, distantly related to validated species. Phylogenetic analysis and physiological traits indicate that the novel strain represents a species of a candidate novel genus. Strain I2511 was included in a microbial consortium in a low pH “hybrid” sulfidogenic bioreactor designed to remove chalcophilic metals from metal-contaminated liquors and was present in >50% relative abundance when bioreactor was operated at pH ∼ 2.0. Results indicate that the novel isolate could be applied in biotechnologies to treat acidic and neutral pH, metal-rich effluents.

Polymer Acceptors Containing B←N Units for Organic Photovoltaics.

ConspectusOrganic photovoltaics (OPVs), in which blend films of organic or polymer electron donor and electron acceptor are used as the active layer, are a promising photovoltaic technology with the great advantages of solution processing, low cost, and flexibility. The development of small molecular or polymer electron acceptors has boosted power conversion efficiency (PCE) of OPVs from 10% to 18%. Among them, polymer acceptors have the merits of superior morphology stability and excellent mechanical properties. However, owing to the key requirement of very low-lying LUMO/HOMO energy levels for polymer acceptors, very few conjugated polymers can work as polymer acceptors in OPVs. The majority of polymer electron acceptors are based on strong electron-withdrawing imide units or cyano substituents. Since 2015, conjugated polymers containing the boron-nitrogen coordination bond (B←N) have emerged as a new kind of polymer electron acceptor with excellent photovoltaic performance in various kinds of organic photovoltaic devices. In this Account, we summarize our research progress on polymer acceptors containing B←N units.At first, we introduce the principle of B←N to greatly down shift LUMO/HOMO energy levels, which enables B←N to be used to design polymer acceptors. Then we describe the two molecular design strategies for polymer acceptors containing B←N units. For high-efficiency OPVs, polymer acceptors should have wide absorption spectra, proper LUMO/HOMO energy levels, high electron mobility, and good donor/acceptor blend morphology. We discuss how to use molecular design to finely tune the absorption spectra, energy levels, and electron mobility of the B←N-containing polymer acceptors. We also discuss how to improve the phase separation morphology of the blends of these polymer acceptors with small molecular donors or polymer donors. These improvements lead to excellent performance of the polymer acceptors containing B←N units in three kinds of organic photovoltaic devices. The small molecular donor/polymer acceptor type organic solar cells show excellent thermal stability and PCE of 8.0%, which is the highest value reported so far. The all-polymer solar cells exhibit PCE of 10.1%. The all-polymer indoor photovoltaics show PCE as high as 27.4% under fluorescent lamp illumination at 2000 lx. This PCE is fairly comparable to those of the best organic or inorganic indoor photovoltaics. These results provide a solid foundation for future advances. Finally, we propose that great attention should be paid to further PCE enhancement of OPVs and indoor photovoltaic applications of this new emerging kind of polymer acceptor.

Phase Separation and Nanostructure Formation in Binary and Ternary Blends of Spiro-Linked Molecular Glasses.

Understanding phase separation phenomena in blends of organic electron acceptor and donor materials is of special interest in the context of organic optoelectronic applications. In this study, we focus on the phase behavior of a special class of spiro-linked compounds, which serve as model systems for morphological control in phase-separated small-molecule electron donor-acceptor blends. Thermal analysis and quantitative image analysis were the key techniques for developing a suitable approach for modeling the phase diagram with minimal material consumption. We report an uncommon miscibility gap in the liquid and glassy phase and show that the phase diagram can be modified by addition of a third, ambipolar compound in analogy to ternary A/B/AB polymeric blends. For an exemplary ternary system, a bicontinuous morphology with a pattern length scale of a few tens of nanometers was realized in the bulk that verifies the applicability of this approach to morphology control.

Metagenomic evidence reveals denitrifying community diversity rather than abundance drives nitrate removal in stormwater biofilters amended with different organic and inorganic electron donors

Various sole and mixed electron donors were tested to promote the denitrification rate and nitrate removal efficiency in biofilter systems with high phosphate and ammonia removal efficiency (92.6% and 95.3% respectively). Compared to sole electron donors, complex organic carbon (bits of wood and straw) substantially improved the denitrification rate and nitrate removal efficiency (from 6.3%-18.5% to35.4%) by shifting the denitrifying microbial community composition, even though the relative abundance of functional genes mediating denitrification decreased. The mixed electron donor combining complex organic carbon with sulfur, iron and CH4 further promoted nitrate removal efficiency by 37.2%. The significantly higher abundance and diversity of bacteria mediating organic carbon decomposition in the treatments with complex organic carbon indicated the continuous production of organic carbon with small molecular weights, which provided sustainable and effective electron donor for denitrification. However, sole sulfur or iron did not effectively promote the denitrification rate and nitrogen removal efficiency, even though the related microbial community had been formed.

Photoinduced monooxygenation involving NAD(P)H-FAD sequential single-electron transfer

Light-dependent or light-stimulated catalysis provides a multitude of perspectives for implementation in technological or biomedical applications. Despite substantial progress made in the field of photobiocatalysis, the number of usable light-responsive enzymes is still very limited. Flavoproteins have exceptional potential for photocatalytic applications because the name-giving cofactor intrinsically features light-dependent reactivity, undergoing photoreduction with a variety of organic electron donors. However, in the vast majority of these enzymes, photoreactivity of the enzyme-bound flavin is limited or even suppressed. Here, we present a flavoprotein monooxygenase in which catalytic activity is controllable by blue light illumination. The reaction depends on the presence of nicotinamide nucleotide-type electron donors, which do not support the reaction in the absence of light. Employing various experimental approaches, we demonstrate that catalysis depends on a protein-mediated photoreduction of the flavin cofactor, which proceeds via a radical mechanism and a transient semiquinone intermediate.

Anoxic Microbial Community Robustness Under Variation of Hydraulic Retention Time and Availability of Endogenous Electron Donors.

The ADNMED (Anaerobic Digestion, Nitrification, and Mixotrophic Endogenous Denitrification) system comprises a triple chamber configuration that was shown to provide high-quality effluent regarding carbon, nitrogen, and sulfide. Hydraulic retention time (HRT) was 7h in the anaerobic and anoxic chambers, and 5h in the aerobic chamber (stage A). Sewage was directly added to the anoxic chamber to provide extra organic electron donors for denitrification (stage B) to improve the nitrogen removal efficiency (stage A 47 ± 19%). The addition of sewage at a flow rate equivalent to 10% of the feed flow increased nitrogen removal efficiency to 61 ± 12%. Illumina® sequencing revealed a restructuring of the microbial community in the anoxic chamber, according to the availability of the endogenous electron donors for denitrification. At stage A, denitrification was related to the decay of biomass, while the addition of sewage during stage B stimulated the establishment of fermentative bacteria.

Full-scale application of ELS® microemulsion Technology for the Treatment of an Aquifer Contaminated with perchloroethylene and trichloroethylene via Ehnanced Reductive Dechlorination

Mixed plumes of chlorinated organics and oxidized metals are a common contaminant at many sites. The oxidized metals can be mediated by the establishment of moderately reducing conditions. The chlorinated organics have been demonstrated to be degradable by specific dechlorinating microrganisms in anaerobic environment such as Dehalococcoides sp. Enhanced biological dechlorination requires the presence of an effective electron donor to provide molecular hydrogen (H2) to completely degrade chlorinated ethenes. Distribution of the electron donor results in the biologically mediated establishment of highly reducing conditions in the treatment zone. This process also results in the reduction and precipitation of the oxidized metals via sulphate-reducing conditions. Peroxychem LLC has developed an innovative electron donor, ELS® Microemulsion Reagent (ELS) for in situ treatment of chlorinated organics and metals. This substrate has been successfully applied at numerous sites to address a variety of contaminants. ELS® is an organic electron donor composed of an easily fermentable organic substrate based on lecithin, and designed to enhance in situ anaerobic bioremediation aquifers contaminated by organochlorine compounds and heavy metals such as hexavalent chromium Cr[VI]. The product is easy to mix, dilute and inject into the subsurface. Once in the groundwater, indigenous microorganisms utilize ELS to rapidly generate highly reducing conditions, favoring biotic dechlorination reactions and the reduction of oxidized metals such as Cr[VI]. This innovative technology was successfully applied to a former manufacturing site in the center of Italy, where groundwater was historically contaminated with Tetrachloroethylene (PCE > 5.5 milligrams per Liter; mg/L), Trichloroethylene (TCE > 2 mg/L), 1,2-Dichloroethene (1,2-DCE > 1 mg/L) and, to a lesser extent, Vinyl Chloride (VC) and 1,2-Dichloropropane (DP). A pump-and-treat system (P&T) installed in the source was active as a source containment measure and to speed up the overall groundwater remediation. However, there was concern that the pumping could affect the ELS treatment effectiveness because of the increased groundwater flow velocity and the potential for removal of the injected bioremediation substrate. To mitigate this potential some wells were switched off the flow rates of others was adjusted to ensure compatibility with the planned product injection. In particular, an upstream low-flow-rate pump and treat system was maintained over the ELS® treatment period, primarily to delay the fast-downstream diffusion of the amendments in the aquifer, thus enhancing the source treatment. Following the calibration of the P&T system, approximately 4,900 kg of ELS® concentration was injected under high pressure at 51 locations into the source area. In about 12 months from injection of ELS® Microemulsion into the groundwater in the main source area, concentrations of PCE, TCE and the recognized catabolites, such as DCE and VC, rapidly reduced, compared to the pre-treatment concentrations, until they reached the statutory national limits (CSC D.lgs 152/06) in the main monitoring piezometers of the area, also highlighting the establishment of clear and enhanced biotic reducing conditions. No rebound effects have been observed in the next three years of monitoring.

Chemolithoautotropic Diazotrophy Dominates the Nitrogen Fixation Process in Mine Tailings.

Nutrient deficiency, especially bio-available nitrogen deficiency, often impedes the bioremediation efforts of mining generated tailings. Biological nitrogen fixation is a critical process necessary for the initial nitrogen buildup in tailings. Current knowledge regarding the diazotrophs that inhabit tailings is still in its infancy. Therefore, in this study, a comprehensive investigation combining geochemical characterization, sequence analyses, molecular techniques, and activity measurements was conducted to characterize the diazotrophic community residing in tailing environments. Significant differences between tailings and their adjacent soils in prokaryotic and diazotrophic communities were detected. Meanwhile, strong and significant correlations between the absolute abundance of the nitrogen fixation (nifH), carbon fixation (cbbL), sulfur oxidation (soxB), and arsenite oxidation (aioA) genes were observed in the tailings but not in the soils. The reconstructed nif-containing metagenome-assembled genomes (MAGs) suggest that the carbon fixation and sulfur oxidation pathways were important for potential diazotrophs inhabiting the tailings. Activity measurements further confirmed that diazotrophs inhabiting tailings preferentially use inorganic electron donors (e.g., elemental sulfur) compared to organic electron donors (e.g., sucrose), while diazotrophs inhabiting soils preferred organic carbon sources. Collectively, these findings suggest that chemolithoautotrophic diazotrophs may play essential roles in acquiring nutrients and facilitating ecological succession in tailings.

Determining the exciton diffusion length of copper phthalocyanine in operating planar-heterojunction organic solar cells

Organic solar cells based on planar copper phthalocyanine (CuPc)/C60 heterojunction have been characterized, in which a 2 nm-thick layer of bathocuproine (BCP) is inserted into the CuPc layer. The thin layer of BCP allows hole current to tunnel it through but blocks the exciton diffusion, thereby altering the steady-state exciton profile in the CuPc zone (zone 1) sandwiched between BCP and C60. The short-circuit current density (JSC) of device is limited by the hole-exciton scattering effect at the BCP/CuPc (zone 1) interface. Based on the variation of JSC with the width of zone 1, the exciton diffusion length of CuPc is deduced to be 12.5–15 nm. The current research provides an easy and helpful method to determine the exciton diffusion lengths of organic electron donors.

Stereoselective Biotransformations of Cyclic Imines in Recombinant Cells of Synechocystis sp. PCC 6803

AbstractLight‐driven biotransformations in recombinant cyanobacteria allow to employ photosynthetic water‐splitting for cofactor‐regeneration and thus, to save the use of organic electron donors. The genes of three recombinant imine reductases (IREDs) were expressed in the cyanobacterium Synechocystis sp. PCC 6803 and eight cyclic imine substrates were screened in whole‐cell biotransformations. While initial reactions showed low to moderate rates, optimization of the reaction conditions in combination with promoter engineering allowed to alleviate toxicity effects and achieve full conversion of prochiral imines with initial rates of up to 6.3 mM h−1. The high specific activity of up to 22 U gCDW−1 demonstrates that recombinant cyanobacteria can provide large amounts of NADPH during whole cell reactions. The excellent optical purity of the products with up to >99 %ee underlines the usefulness of cyanobacteria for the stereoselective synthesis of amines.

Operational Performance and Microbiological Characteristics of an Iron-Salt Denitrification Reactor in Co-substrate Mode

When iron salt is used as an autotrophic denitrification electron donor, the high iron yield generated by oxidation is easy to precipitate, resulting in "iron encrustation" on the surface of denitrifying microorganisms, which inhibits their activity and even leads to their death. In order to solve the degradation of the efficiency of the autotrophic ferric denitrification reactor caused by the "iron encrustation" coating, this paper adopted the co-substrate mode to cultivate the ferric denitrification reactor; that is, a small amount of sodium acetate was added into the water of the reactor as an organic electron donor, to realize the efficient and stable operation of the ferric denitrification reactor. The results showed that adding an appropriate amount of organic matter could make the iron salt denitrification reactor run efficiently and stably, with an efficiency of up to 0.51 kg·(m3·d)-1, for more than 30 days. Heterotrophic bacteria could always be detected during the operation of the reactor in the co-substrate mode. Combined with the transmission electron microscopy (TEM) test results of the sludge, it was found that during the stable operation of the iron-salt denitrification reactor, heterotrophic bacteria were the main cause of iron-salt denitrification, and their unique iron-salt metabolism mode could effectively avoid the formation of iron encrustation. This study effectively solved the problem of microbial "iron encrustation" coating in the process of iron-salt denitrification, and will contribute to the development and application of autotrophic denitrification technology.

Enrichment and isolation of acid-tolerant sulfate-reducing microorganisms in the anoxic, acidic hot spring sediments from Copahue volcano, Argentina.

The geothermal Copahue-Caviahue (GCC) system (Argentina) is an extreme acidic environment, dominated by the activity of Copahue volcano. Environments characterised by low pH values, such as volcanic areas, are of particular interest for the search of acidophilic microorganisms with application in biotechnological processes. In this work, sulfate-reducing microorganisms were investigated in geothermal acidic, anaerobic zones from GCC system. Sediment samples from Agua del Limón (AL1), Las Máquinas (LMa2), Las Maquinitas (LMi) and Baño 9 (B9-2, B9-3) were found to be acidic (pH values 2.1-3.0) to moderate acidic (5.1-5.2), containing small total organic carbon values, and ferric iron precipitates. The organic electron donor added to the enrichment was completely oxidised to CO2. Bacteria related to 'Desulfobacillus acidavidus' strain CL4 were found to be dominant (67-83% of the total number of clones) in the enrichment cultures, and their presence was confirmed by their isolation on overlay plates. Other bacteria were also detected with lower abundance (6-20% of the total number of clones), with representatives of the genera Acidithiobacillus, Sulfobacillus, Alicyclobacillus and Athalassotoga/Mesoaciditoga. These enrichment and isolates found at low pH confirm the presence of anaerobic activities in the acidic sediments from the geothermal Copahue-Caviahue system.

The role of organic electron donors in the initiation of BHAS base-induced coupling reactions between haloarenes and arenes

Coupling reactions between haloarenes and arenes (including heteroarenes) that are conducted without added transition metals but in the presence of KOtBu or NaOtBu, have been a topic of great interest since their discovery in 2008. Diverse organic structures act as additives that assist these reactions. These additives are converted into organic electron donors by the butoxide base and this leads to initiation of the coupling reactions, which proceed by radical chain mechanisms. This review provides an overview of the initiation stages of these reactions.

Diverse respiratory capacity among Thermus strains from US Great Basin hot springs.

Thermus species are thermophilic heterotrophs, with most capable of using a variety of organic and inorganic electron donors for respiration. Here, a combined cultivation-independent and -dependent approach was used to explore the diversity of Thermus in Great Boiling Spring (GBS) and Little Hot Creek (LHC) in the US Great Basin. A cultivation-independent 16S rRNA gene survey of ten LHC sites showed that Thermus made up 0-3.5% of sequences and were predominately Thermus thermophilus. 189 Thermus isolates from GBS and LHC were affiliated with T. aquaticus (73.0%), T. oshimai (25.4%), T. sediminis (1.1%), and T. thermophilus (0.5%), with T. aquaticus and T. oshimai forming biogeographic clusters. 22 strains were selected for characterization, including chemolithotrophic oxidation of thiosulfate and arsenite, and reduction of ferric iron, polysulfide, and nitrate, revealing phenotypic diversity and broad respiratory capability within each species. PCR demonstrated the wide distribution of aerobic arsenite oxidase genes. A GBS sediment metaproteome contained sulfite oxidase and Fe3+ ABC transporter permease peptides, suggesting sulfur and iron transformations in situ. This study expands our knowledge of the physiological diversity of Thermus, suggesting widespread chemolithotrophic and anaerobic respiration phenotypes, and providing a foundation for better understanding the ecology of this genus in thermal ecosystems.

Radical Chain Reaction Catalyzed by Organic Super Electron Donors

Radical Chain Reaction Catalyzed by Organic Super Electron Donors

Ground State Cross-Coupling of Haloarenes with Arenes Initiated by Organic Electron Donors, Formed in situ: An Overview

Many reactions have been discovered that lead to coupling of haloarenes to arenes using potassium tert-butoxide as the base, and one of a variety of organic compounds as an additive. The organic additive reacts with the base to form a strong organic electron donor in situ that initiates a base-induced homolytic aromatic substitution (BHAS) coupling reaction, by converting the haloarene into an aryl radical. This brief report presents an overview of the wide range of organic additives that can be used, and the organic electron donors that they form.