Novel Strategy for Tracking the Microbial Degradation of Azo Dyes with Different Polarities in Living Cells

Fluorescent chemosensors facilitate the visualization of plant health and their living environment in sustainable agriculture

Globally, 91% of plant production encounters diverse environmental stresses that adversely affect their growth, leading to severe yield losses of 50-60%. In this case, monitoring the connection between the environment and plant health can balance population demands with environmental protection and resource distribution. Fluorescent chemosensors have shown great progress in monitoring the health and environment of plants due to their high sensitivity and biocompatibility. However, to date, no comprehensive analysis and systematic summary of fluorescent chemosensors used in monitoring the correlation between plant health and their environment have been reported. Thus, herein, we summarize the current fluorescent chemosensors ranging from their design strategies to applications in monitoring plant-environment interaction processes. First, we highlight the types of fluorescent chemosensors with design strategies to resolve the bottlenecks encountered in monitoring the health and living environment of plants. In addition, the applications of fluorescent small-molecule, nano and supramolecular chemosensors in the visualization of the health and living environment of plants are discussed. Finally, the major challenges and perspectives in this field are presented. This work will provide guidance for the design of efficient fluorescent chemosensors to monitor plant health, and then promote sustainable agricultural development.

Kinetic study of electron transfer process in methyl orange decolorization by shewanella in MFCs with covalent organic frameworks modified anode

As a new electrode material for electrochemical systems, covalent organic framework (COF) materials have been gradually applied to bioelectrochemical systems. In our previous study, the COFBTA-DPPD-rGO composite was synthesized via Schiff-base coupling between benzene-1,3,5-tricarbaldehyde (BTA) and 3,8-diamino-6-phenylphenanthridine (DPPD) on reduced graphene oxide (rGO) at room temperature. Here, COFBTA-DPPD-rGO modified MFC anode was used to assist microorganisms to decolorize methyl orange (MO), and the properties of MFCs were studied. The results showed that compared to the unmodified electrode MFC (28 mA m-2, 4.20 mW m-2) the current density and maximum power density of the anode MFC modified by COFBTA-DPPD-rGO (134.5 mA m-2, 21.78 mW m-2) were increased by 380.3% and 423.6%, respectively. The transferred electron number n and charge transfer coefficient α of the modified COFBTA-DPPD-rGO anode (4 and 0.43) compared to the unmodified electrode (2.4 and 0.38) were increased by 67% and 13%, respectively. The decolorization ratio of MO could reach 90.3% at 10 h. Compared with the unmodified electrode MFC (53.0%), the decolorization ratio and kinetic constant of decolorization process were enhanced by 26% and 372%, respectively. Therefore, COFBTA-DPPD-rGO could be a new choice for applying to the MFCs.

Electrogenic engineered flow through tri-phasic wetland system for azo dye treatment: Microbial dynamics and functional metagenomics

Electrogenic engineered flow through tri-phasic wetland (EEFW) system based on nature-based ecological principles was studied by integrating successive biological microenvironments. The potential mechanism of the plant root-based microbial community and its functional diversity with the influence of plant-microbe-electrode synergism towards dye degradation was evaluated. The EEFW system was operated at three varied dye loads of 10, 25 and 50 mg L-1, where the results from the cumulative outlets revealed a maximum dye removal efficiency of 96%, 96.5% and 93%, respectively. Microbial community analysis depicted synergistic dependence on the plant-microbe-electrode interactions, influencing their functional diversity and metabolism towards detoxification of pollutants. The core microbial taxa enriched against the microenvironment variation were mostly associated with carbon and dye removal viz., Desulfomonile tiedjei and Rhodopseudomonas palustris in Tank 1 and Chloroflexi bacterium and Steroidobacter denitrificans in Tank 2. The degradation of polycyclic aromatic hydrocarbons, chloroalkane/chloroalkene, nitrotoluene, bisphenol, caprolactam and 1,1,1-trichloro-2,2-bis(4-chlorophenyl) ethane (DDT) were observed to be predominant in Tank 1. EEFW system could be one of the option for utilizing nature-based processes for the treatment of wastewater by self-induced bioelectrogenesis to augment process efficiency.

Enhanced Biodegradation of Methyl Orange Through Immobilization of Shewanella oneidensis MR-1 by Polyvinyl Alcohol and Sodium Alginate

Shewanella oneidensis MR-1 has great potential for use in remediating azo dye pollution. Here, a new high-efficiency biodegradation method was developed utilizing S. oneidensis MR-1 immobilized by polyvinyl alcohol (PVA) and sodium alginate (SA). After determining the optimal immobilization conditions, the effects of various environmental factors on methyl orange (MO) degradation were analyzed. The biodegradation activity of the immobilized pellets was evaluated by analyzing the MO removal efficiency, and characterization was performed using scanning electron microscopy. The MO adsorption kinetics can be described using pseudo-second-order kinetics. Compared with free bacteria, the MO degradation rate of the immobilized S. oneidensis MR-1 increased from 41% to 92.6% after 21 days, suggesting that the immobilized bacteria performed substantially better and had more stable removal rates. These factors indicate the superiority of bacteria entrapment in addition to its easy application. This study demonstrates that the application of immobilized S. oneidensis MR-1 entrapped by PVA-SA can be used to establish a reactor with stable and high MO removal rates.

Recent advances in enrichment, isolation, and bio-electrochemical activity evaluation of exoelectrogenic microorganisms

Exoelectrogenic microorganisms (EEMs) catalyzed the conversion of chemical energy to electrical energy via extracellular electron transfer (EET) mechanisms, which underlay diverse bio-electrochemical systems (BES) applications in clean energy development, environment and health monitoring, wearable/implantable devices powering, and sustainable chemicals production, thereby attracting increasing attentions from academic and industrial communities in the recent decades. However, knowledge of EEMs is still in its infancy as only ~100 EEMs of bacteria, archaea, and eukaryotes have been identified, motivating the screening and capture of new EEMs. This review presents a systematic summarization on EEM screening technologies in terms of enrichment, isolation, and bio-electrochemical activity evaluation. We first generalize the distribution characteristics of known EEMs, which provide a basis for EEM screening. Then, we summarize EET mechanisms and the principles underlying various technological approaches to the enrichment, isolation, and bio-electrochemical activity of EEMs, in which a comprehensive analysis of the applicability, accuracy, and efficiency of each technology is reviewed. Finally, we provide a future perspective on EEM screening and bio-electrochemical activity evaluation by focusing on (i) novel EET mechanisms for developing the next-generation EEM screening technologies, and (ii) integration of meta-omics approaches and bioinformatics analyses to explore nonculturable EEMs. This review promotes the development of advanced technologies to capture new EEMs.

Evidence for the feasibility of transmembrane proton gradient regulating oxytetracycline extracellular biodegradation mediated by biosynthesized palladium nanoparticles

Extracellular biodegradation is a promising technology for removing antibiotics and repressing the spread of resistance genes, but the strategy is limited by the low extracellular electron transfer (EET) efficiency of microorganisms. In this work, biogenic Pd⁰ nanoparticles (bio-Pd⁰) were introduced in cells in situ to enhance oxytetracycline (OTC) extracellular degradation and the effects of transmembrane proton gradient (TPG) on EET and energy metabolism mediated by bio-Pd⁰ were investigated. The results indicated that the intracellular OTC concentration gradually decreased with increase in pH due to the simultaneous decreases of OTC adsorption and TPG-dependent OTC uptake. On the contrary, the efficiency of OTC biodegradation mediated by bio-Pd⁰@B. megaterium showed a pH-dependent increase. The negligible intracellular OTC degradation, the high dependence of OTC biodegradation on respiration chain and the results on enzyme activity and respiratory chain inhibition experiments showed that NADH-dependent (rather than FADH₂-dependent) EET process mediated by substrate-level phosphorylation modulated OTC biodegradation due to high energy storage and proton translocation capacity. Moreover, the results showed that altering TPG is an efficient approach to improve EET efficiency, which can be attributed to the increased NADH generation by the TCA cycle, enhanced transmembrane electron output efficiency (as evidenced by increased intracellular electron transfer system (IETS) activity, the negative shift of onset potential, and enhanced one-electron transfer through bound flavin) and stimulation of substrate-level phosphorylation energy metabolism catalyzed by succinic thiokinase (STH) under low TPG conditions. The results of structural equation model that OTC biodegradation was directly and positively modulated by the net outward proton flux as well as STH activity, and indirectly regulated by TPG through NADH level and IETS activity confirmed the previous findings. This study provides a new perspective for engineering microbial EET and application of bioelectrochemistry processes in bioremediation.

Versatile mechanisms and enhanced strategies of pollutants removal mediated by Shewanella oneidensis: A review

The removal of environmental pollutants is important for a sustainable ecosystem and human health. Shewanella oneidensis (S. oneidensis) has diverse electron transfer pathways and can use a variety of contaminants as electron acceptors or electron donors. This paper reviews S. oneidensis's function in removing environmental pollutants, including heavy metals, inorganic non-metallic ions (INMIs), and toxic organic pollutants. S. oneidensis can mineralize o-xylene (OX), phenanthrene (PHE), and pyridine (Py) as electron donors, and also reduce azo dyes, nitro aromatic compounds (NACs), heavy metals, and iodate by extracellular electron transfer (EET). For azo dyes, NACs, Cr(VI), nitrite, nitrate, thiosulfate, and sulfite that can cross the membrane, S. oneidensis transfers electrons to intracellular reductases to catalyze their reduction. However, most organic pollutants cannot be directly degraded by S. oneidensis, but S. oneidensis can remove these pollutants by self-synthesizing catalysts or photocatalysts, constructing bio-photocatalytic systems, driving Fenton reactions, forming microbial consortia, and genetic engineering. However, the industrial-scale application of S. oneidensis is insufficient. Future research on the metabolism of S. oneidensis and interfacial reactions with other materials needs to be deepened, and large-scale reactors should be developed that can be used for practical engineering applications.

Molecular response of Anoxybacillus sp. PDR2 under azo dye stress: An integrated analysis of proteomics and metabolomics

Treating azo dye wastewater using thermophilic bacteria is considered a more efficient bioremediation strategy. In this study, a thermophilic bacterial strain, Anoxybacillus sp. PDR2, was regarded as the research target. This strain was characterized at different stages of azo dye degradation by using TMT quantitative proteomic and non-targeted metabolome technology. A total of 165 differentially expressed proteins (DEPs) and 439 differentially metabolites (DMs) were detected in comparisons between bacteria with and without azo dye. It was found that Anoxybacillus sp. PDR2 can degrade azo dye Direct Black G (DBG) through extracellular electron transfer with glucose serving as electron donors. Most proteins related to carbohydrate metabolism, including acetoacetate synthase, and malate synthase G, were overexpressed to provide energy. The bacterium can also self-synthesize riboflavin as a redox mediator of in vitro electron transport. These results lay a theoretical basis for industrial bioremediation of azo dye wastewater.

Soil Amoebae Affect Iron and Chromium Reduction through Preferential Predation between Two Metal-Reducing Bacteria

Soil protists are essential but often overlooked in soil and could impact microbially driven element cycling in natural ecosystems. However, how protists influence heavy metal cycling in soil remains poorly understood. In this study, we used a model protist, Dictyostelium discoideum, to explore the effect of interactions between soil amoeba and metal-reducing bacteria on the reduction of soil Fe(III) and Cr(VI). We found that D. discoideum could preferentially prey on the Fe(III)-reducing bacterium Shewanella decolorationis S12 and significantly decrease its biomass. Surprisingly, this predation pressure also stimulated the activity of a single S. decolorationis S12 bacterium to reduce Fe(III) by enhancing the content of electron-transfer protein cyt c, intracellular ATP synthesis, and reactive oxygen species (e.g., H₂O₂). We also found that D. discoideum could not prey on the Cr(VI)-reducing bacterium Brevibacillus laterosporus. In contrast, B. laterosporus became edible to amoebae in the presence of S. decolorationis S12, and their Cr(VI) reduction ability decreased under amoeba predation pressure. This study provides direct evidence that protists can affect the Cr and Fe cycling via the elective predation pressure on the metal-reducing bacteria, broadening our horizons of predation of protists on soil metal cycling.

Mutual interaction between the secreted flavins and immobilized quinone in anaerobic removal of high-polarity aromatic compounds containing nitrogen by Shewanella sp. RQs-106

The immobilized anthraquinone-2-sulfonate (iAQS) could significantly promote anaerobic biotransformation of the contaminants. During this process, the role of flavins secreted by bacteria remains unclear. In the present study, mutual interaction between extracellular flavins and AQS-modified polyurethane foam (AQS-PUF) during the reduction of azo dye Acid Red 18 and 3-nitrobenzenesulfonate (3-NBS) was investigated. Results showed that the amount of extracellular flavins secreted by Shewanella sp. RQs-106 was positively correlated with the concentration of iAQS ranging from 10 to 100 μM. The presence of iAQS resulted in the increased concentration of extracellular and intracellular flavins, implying that iAQS could induce the synthesis and secretion of flavins. The deletion of gene bfe encoding the flavin adenine dinucleotide exporter resulted in approximately 63.8% decrease in the amount of extracellular flavins. Further analysis showed that the decreased amount of extracellular flavins could contribute to around 50.8% reduction of iAQS. Moreover, around 23.2% and 34.0% decreases were observed in AQS-PUF-mediated removal rates of AR 18 and 3-NBS by mutant lacking bfe gene, respectively, compared with that by wild type strain RQs-106. These results indicated that the secreted flavins played an important role in the bio-reduction of AQS-PUF, resulting in their contribution to AQS-PUF-mediated removals of high-polarity aromatic compounds containing nitrogen.

Engineering Shewanella carassii, a newly isolated exoelectrogen from activated sludge, to enhance methyl orange degradation and bioelectricity harvest

Electroactive microorganisms (EAMs) play important roles in biogeochemical redox processes and have been of great interest in the fields of energy recovery, waste treatment, and environmental remediation. However, the currently identified EAMs are difficult to be widely used in complex and diverse environments, due to the existence of poor electron transfer capability, weak environmental adaptability, and difficulty with engineering modifications, etc. Therefore, rapid and efficient screening of high performance EAMs from environments is an effective strategy to facilitate applications of microbial fuel cells (MFCs). In this study, to achieve efficient degradation of methyl orange (MO) by MFC and electricity harvest, a more efficient exoelectrogen Shewanella carassii-D₅ that belongs to Shewanella spp. was first isolated from activated sludge by WO₃ nanocluster probe technique. Physiological properties experiments confirmed that S. carassii-D₅ is a Gram-negative strain with rounded colonies and smooth, slightly reddish surface, which could survive in media containing lactate at 30 °C. Moreover, we found that S. carassii-D₅ exhibited remarkable MO degradation ability, which could degrade 66% of MO within 72 h, 1.7 times higher than that of Shewanella oneidensis MR-1. Electrochemical measurements showed that MFCs inoculated with S. carassii-D₅ could generate a maximum power density of 704.6 mW/m², which was 5.6 times higher than that of S. oneidensis MR-1. Further investigation of the extracellular electron transfer (EET) mechanism found that S. carassii-D₅ strain had high level of c-type cytochromes and strong biofilm formation ability compared with S. oneidensis MR-1, thus facilitating direct EET. Therefore, to enhance indirect electron transfer and MO degradation capacity, a synthetic gene cluster ribADEHC encoding riboflavin synthesis pathway from Bacillus subtilis was heterologously expressed in S. carassii-D₅, increasing riboflavin yield from 1.9 to 9.0 mg/g DCW with 1286.3 mW/m² power density output in lactate fed-MFCs. Furthermore, results showed that the high EET rate endowed a faster degradation efficient of MO from 66% to 86% with a maximum power density of 192.3 mW/m², which was 1.3 and 1.6 times higher than that of S. carassii-D₅, respectively. Our research suggests that screening and engineering high-efficient EAMs from sludge is a feasible strategy in treating organic pollutants.

Recent advancements in azo dye decolorization in bio-electrochemical systems (BESs): Insights into decolorization mechanism and practical application

Recent advances in bio-electrochemical systems (BESs) for azo dye removal are gaining momentum due to having electrode biocarrier and electro-active bacteria that could stimulate decolorization via extracellular electron transfer. Enhanced decolorization performance is observed in most laboratory studies, indicating the great potential of BESs as an alternative to the traditional biological processes or serving as a pre-/post-processing unit to improve the performance of biological processes. It is proven more competitive in environmental friendly than physicochemical methods. While, the successful application of BESs to azo dye-containing wastewater remediation requires a deeper evaluation of its performance, mechanism and typical attributes, and a comprehensive potential evaluation of BESs practical application in terms of economic analysis and technical optimizations. This review is organized to address BESs as a practical option for azo dye removal by analyzing the decolorization mechanisms and involved functional microorganisms, followed by the comparisons of device configurations, operational conditions, and economic evaluation. It further highlights the current hurdles and prospects for the abatement of azo dyes via BES related techniques.

Design and synthesis of an azobenzene-betaine surfactant for photo-rheological fluids

HYPOTHESIS

Morphology of surfactant self-assemblies are governed by the intermolecular interactions and packing constraints of the constituent molecules. Therefore, rational design of surfactant structure should allow targeting of the specific self-assembly modes, such as wormlike micelles (WLMs). By inclusion of an appropriate photo-responsive functionality to a surfactant molecule, light-based control of formulation properties without the need for additives can be achieved.

EXPERIMENTS

A novel azobenzene-containing surfactant was synthesised with the intention of producing photo-responsive wormlike micelles. Aggregation of the molecule in its cis and trans isomers, and its concomitant flow properties, were characterised using UV-vis spectroscopy, small-angle neutron scattering, and rheological measurements. Finally, the fluids capacity for mediating particle diffusion was assessed using dynamic light scattering.

FINDINGS

The trans isomer of the novel azo-surfactant was found to form a viscoelastic WLM network, which transitioned to inviscid ellipsoidal aggregates upon photo-switching to the cis isomer. This was accompanied by changes in zero-shear viscosity up to 16,000×. UV-vis spectroscopic and rheo-SANS analysis revealed π-π interactions of the trans azobenzene chromophore within the micelles, influencing aggregate structure and contributing to micellar rigidity. Particles dispersed in a 1 wt% surfactant solution showed a fivefold increase in apparent diffusion coefficient after UV-irradiation of the mixture.

Production of bioelectricity may play an important role for the survival of Xanthomonas campestris pv. campestris (Xcc) under anaerobic conditions

The plant pathogen Xanthomonas is commonly found in biocontaminated bioreactors; however, few studies have evaluated the growth and impacts of this microorganism on bioreactors. In this study, we examined the characteristics of Xanthomonas campestris pv. campestris (Xcc). Our results showed that Xcc could reduce metal Fe (III) and decolorise methyl orange in vitro. Moreover, I-t and cyclic voltammetry curves showed that Xcc could generate bioelectricity and had two extracellular electron transfer pathways, similar to that of Shewanella. Based on the spectral analysis of intact cells and scanning electron microscopy analysis, one pathway was speculated to involve cytochrome C by direct contact with the pili or cell surface. The other pathway may involve indirect mediators, such as redox substrates, among extracellular polymeric substances. For the direct extracellular electron transfer process, the charge transfer coefficient α, electron number n, and the electron transfer rate constant k_s were determined to be 0.49, 2.6, and 2.2 × 10^-3 s^-1, respectively. In the indirect extracellular electron transfer processes, the values of α, n, and k_s were 0.52, 4, and 1.21 s^-1, respectively. Of these two transfer methods, indirect electron transfer is dominant and faster than direct electron transfer. Moreover, after mutation of the dsbD gene, which is important for indirect electron transfer, the electrochemical parameters α, n, and k_s decreased. Our findings reveal a new anaerobic mechanism mediating the survival of Xcc during wastewater treatment, and may help develop new strategies for preventing Xcc growth during wastewater treatment.

Anaerobic reduction of high-polarity nitroaromatic compounds by electrochemically active bacteria: Roles of Mtr respiratory pathway, molecular polarity, mediator and membrane permeability

Electrochemically active bacteria (EAB) are effective for the bioreduction of nitroaromatic compounds (NACs), but the exact reduction mechanisms are unclear yet. Therefore, 3-nitrobenzenesulfonate (NBS) was used to explore the biodegradation mechanism of NACs by EAB. Results show that NBS could be anaerobically degraded by Shewanella oneidensis MR-1. The generation of aminoaromatic compounds was accompanied with the NBS reduction, indicating that NBS was biodegraded via reductive approach by S. oneidensis MR-1. The impacts of NBS concentration and cell density on the NBS reduction were evaluated. The removal of NBS depends mainly on the transmembrane electron transfer of S. oneidensis MR-1. Impairment of Mtr respiratory pathway was found to mitigate the reduction of NBS, suggesting that the anaerobic biodegradation of NBS occurred extracellularly. Knocking out cymA severely impaired the extracellular reduction ability of S. oneidensis MR-1. However, the phenotype of ΔcymA mutant could be compensated by the exogenous electron mediators, implying the trans-outer membrane diffusion of mediators into the periplasmic space. This work provides a new insight into the anaerobic reduction of aromatic contaminants by EAB.

A novel near-infrared fluorescent sensor for zero background nitrite detection via the "covalent-assembly" principle

Different chemical states of nitrogen are present in many freshwater and marine ecosystems, and nitrite ions are one of the most toxic water-soluble nitrogen species. Developing an effective and convenient sensing method to constantly detect the concentration of nitrite has become a wide concern. Here, a novel near-infrared fluorescent probe (AAC) was designed and synthesized via the "covalent assembly" principle, showing excellent selectivity and high sensitivity for nitrite. A new nitrite-quantitative method was established with the help of AAC, and the detection limit of nitrite using the new method was as low as 6.7 nM. AAC was successfully applied for the quantitative detection of nitrite in real-world environmental and food samples (including river water and Chinese sauerkraut), and the detection results were essentially identical to the results obtained from the traditional Griess assay. Moreover, AAC was successfully applied for tracking nitrite in Escherichia coli by fluorescence imaging. Since nitrite can have devastating effects, the method established with AAC allowed us to "see" effectively about the water quality, food quality, etc.

Degradation of estriol (E3) and transformation pathways after applying photochemical removal processes in natural surface water

Steroidal hormones such as estriol (E3), are resistant to biodegradation; hence their removal by conventional treatment systems (aerobic and anaerobic) facilities is limited. These substances are detected in surface water, and present risks to the aquatic ecosystem and humans via potential biological activity. Photochemical treatments can be used to remove E3; however, just a few studies have analyzed the kinetics, intermediates, and E3 degradation pathways in natural surface water. In this study, the behavior of E3 under ultraviolet irradiation associated with H₂O₂, O₃ or TiO₂ was investigated to determine the degradation potential and the transformation pathways in reactions performed with a natural surface water sample. E3 degradation kinetics (200 ppb) fitted well to the pseudo-first-order kinetics model, with kinetic constant k in the following order: k_UV/O3 > k_UV/TiO2 > k_UV/H2O2 > k_UV. The mechanism of degradation using different advanced oxidative processes seemed to be similar and 12 transformation byproducts were identified, with 11 of them being reported here for the first time. The byproducts could be formed by the opening of the aromatic ring and addition of a hydroxyl radical. A possible route of E3 degradation was proposed based on the byproducts identified, and some of the byproducts presented chronic toxicity to aquatic organisms, demonstrating the risks of exposure.

Efficient treatment of alizarin yellow R contained wastewater in an electrostimulated anaerobic-oxic integrated system

An electrostimulated anaerobic-oxic integrated system was constructed for treating alizarin yellow R (AYR) containing wastewater. In electro-stimulated anaerobic unit, AYR decolorization efficiency improved from 51.2% to 96.6%. Two amination metabolites, p-phenylenediamine and 5-aminosalicylic acid, went through oxidation, ammonification and mineralization in oxic unit. Electro-stimulation promoted denitrification and COD removal efficiencies by 15.5% and 8.6%, respectively. A 20% improved nitrification efficiency was observed in oxic unit, due to elimination of AYR toxicity inhibition. No corrosion of heat-treated stainless steel occurred during the 60 days of continuous operation. Electrons sunk in denitrification and decolorization accounted for 34.4-36.8% of those released from COD removal, and 7.3% increase of removed nitrogen in nitrogenous compounds (AYR, nitrate and ammonia) was found. Electro-stimulated anaerobic unit predominated with fermentation and denitrification genera (Propionispira, Rhodocyclus, etc.) and aboundance of electro-active decolorization genus (Desulfovibrio, etc.) increased. Ammonia-oxidizing genus, Comamonas, was the most abundant in aerobic unit. Compared to the suspension, the electrostimulation could increased the abundance of electro-active genera in cathodic biofilm. This study revealed the feasibility of applying electro-stimulation and the conversion laws of nitrogenous organics in secondary bio-treatment system for treating toxic nitrogenous organics-contained wastewater.

Structures of nitroaromatic compounds induce Shewanella oneidensis MR-1 to adopt different electron transport pathways to reduce the contaminants

Nitroaromatic compounds (NACs) are one class of typical refractory biodegradable organic pollutants detected in various environmental media. The reductive transformation of NACs by the electrochemically active bacterium Shewanella oneidensis MR-1 is a possible and cost-effective option for the removal of NACs. However, little information on the respiratory pathway employed by S. oneidensis MR-1 to reduce NACs is available. In the present study, we investigated the reduction of NACs with different nitro or alkyl moieties by S. oneidensis MR-1 and eight constructed mutants with the deletion of the mtrA, mtrB, mtrC/omcA, cymA, napA, napB, nrfA, and nfnB genes to explore these key functional enzymes. The reduction of nitrobenzene, 4-nitrotoluene, 4-ethylnitrobenzene, 1-tert-butyl-4-nitrobenzene, 1,2-dinitrobenzene, 1,3-dinitrobenzene, 1,4-dinitrobenzene, 2,4-dinitrotoluene, 2,6-dinitrotoluene and 2,4,6-trinitrotoluene occurs via both the Mtr respiratory pathway and NfnB related pathway. However, 2,5-ditertbutyl-nitrobenzene, 2-nitrobiphenyl and 2,2'-dinitrobiphenyl are reduced only via the Mtr respiratory pathway. The van der Waals volume of NACs was the key factor in determining the reduction by S. oneidensis MR-1 according the correlation analysis. Our study provides new insights into the environmental adaption of S. oneidensis MR-1 and facilitates the bioremediation of NAC-related contamination.

Mutual effect between electrochemically active bacteria (EAB) and azo dye in bio-electrochemical system (BES)

Herein, the mutual effect between azo dye and the performance of electrochemically active bacteria (EAB) is investigated in detail, which is crucial to understand and control the bio-electrochemical systems (BESs) operation for azo dye containing wastewater treatment. EAB is enriched at controlled potential of -0.2 V vs Ag/AgCl in single-chamber BESs. Over 95% azo dye (alizarin yellow R (AYR)) was decolorized regardless of the initial AYR concentration ranging from 30 to 120 mg/L within 24 h. The fastest decolorization rate was obtained at AYR initial concentration of 70 mg/L, which was 4.25 times greater in the closed circuit BESs than that in the open circuit one. 16S rRNA gene based microbial community analysis showed that Geobacter was dominant in EAB with relative abundance increased from 77.98% (0 mg/L AYR) to 92.22% (70 mg/L AYR), indicating that azo dye selectively boosts the growth of exoelectrogens in electrode biofilm communities. Under electricity stimulation, extracellular process can be mutually conducted by azo dye compounds, which is favorable for accelerating reaction rate and avoiding of significant toxic effect on EAB.

FRET-based fluorescent nanoprobe platform for sorting of active microorganisms by functional properties

Fluorescence-activated cell sorting (FACS) has rarely been applied to screening of microorganisms because of poor detection resolution, which is compromised by poor stability, toxicity, or interference from background fluorescence of the fluorescence sensors used. Here, a fluorescence-based rapid high-throughput cell sorting method was first developed using a fluorescence resonance energy transfer (FRET) fluorescent nanoprobe NP-RA, which was constructed by coating a silica nanoparticle with Rhodamine B and methyl-red (an azo dye). Rhodamine B (inner layer) is the FRET donor and methyl-red (outer layer) is the acceptor. This ready-to-use NP-RA is non-fluorescent, but fluoresces once the outer layer is degraded by microorganisms. In our experiment, NP-RA was ultrasensitive to model strain Shewanella decolorationis S12, showing a broad detection range from 8.0 cfu/mL to 8.7 × 10⁸ cfu/mL under confocal laser scanning microscopy, and from 1.1 × 10⁷ to 9.36 × 10⁸ cfu/mL under a fluorometer. In addition, NP-RA bioimaging can clearly identify other azo-respiring cells in the microbial community, including Bosea thiooxidans DSM 9653 and Lysinibacillus pakistanensis NCCP-54. Furthermore, the fluorescent probe NP-RA is compatible with downstream FACS so that azo-respiring cells can be rapidly sorted out directly from an artificial microbial community. To our knowledge, no fluorescent nanoprobe has yet been designed for tracking and sorting azo-respiration functional microorganisms.

Adaptive Responses of Shewanella decolorationis to Toxic Organic Extracellular Electron Acceptor Azo Dyes in Anaerobic Respiration

Bacterial anaerobic respiration using an extracellular electron acceptor plays a predominant role in global biogeochemical cycles. However, the mechanisms of bacterial adaptation to the toxic organic pollutant as the extracellular electron acceptor during anaerobic respiration are not clear, which limits our ability to optimize the strategies for the bioremediation of a contaminated environment. Here, we report the physiological characteristics and the global gene expression of an ecologically successful bacterium, Shewanella decolorationis S12, when using a typical toxic organic pollutant, amaranth, as the extracellular electron acceptor. Our results revealed that filamentous shift (the cells stretched to fiber-like shapes as long as 18 μm) occurred under amaranth stress. Persistent stress led to a higher filamentous cell rate and decolorization ability in subcultural cells compared to parental strains. In addition, the expression of genes involved in cell division, the chemotaxis system, energy conservation, damage repair, and material transport in filamentous cells was significantly stimulated. The detailed roles of some genes with significantly elevated expressions in filamentous cells, such as the outer membrane porin genes ompA and ompW, the cytochrome c genes arpC and arpD, the global regulatory factor gene rpoS, and the methyl-accepting chemotaxis proteins genes SHD_2793 and SHD_0015, were identified by site-directed mutagenesis. Finally, a conceptual model was proposed to help deepen our insights into both the bacterial survival strategy when toxic organics were present and the mechanisms by which these toxic organics were biodegraded as the extracellular electron acceptors.IMPORTANCE Keeping toxic organic pollutants (TOPs) in tolerable levels is a huge challenge for bacteria in extremely unfavorable environments since TOPs could serve as energy substitutes but also as survival stresses when they are beyond some thresholds. This study focused on the underlying adaptive mechanisms of ecologically successful bacterium Shewanella decolorationis S12 when exposed to amaranth, a typical toxic organic pollutant, as the extracellular electron acceptor. Our results suggest that filamentous shift is a flexible and valid way to solve the dilemma between the energy resource and toxic stress. Filamentous cells regulate gene expression to enhance their degradation and detoxification capabilities, resulting in a strong viability. These novel adaptive responses to TOPs are believed to be an evolutionary achievement to succeed in harsh habitats and thus have great potential to be applied to environment engineering or synthetic biology if we could picture every unknown node in this pathway.

Goethite Hinders Azo Dye Bioreduction by Blocking Terminal Reductive Sites on the Outer Membrane of Shewanella decolorationis S12

Iron (hydr)oxides are the most ubiquitous Fe(III)-containing minerals in the near-surface environments and can regulate organic pollutant biotransformation by participating in bacterial extracellular electron transfer under anaerobic conditions. Mechanisms described so far are based on their redox properties in bacterial extracellular respiration. Here, we find that goethite, a typical iron (hydr)oxide, inhibits the bioreduction of different polar azo dyes by Shewanella decolorationis S12 not through electron competition, but by the contact of its surface Fe(III) with the bacterial outer surface. Through the combined results of attenuated total reflectance (ATR) Fourier transform infrared spectroscopy, two-dimensional correlation spectroscopy, and confocal laser scanning microscope, we found that the outer membrane proteins MtrC and OmcA of strain S12 are key binding sites for goethite surface. Meanwhile, they were identified as the important reductive terminals for azo dyes. These results suggest that goethite may block the terminal reductive sites of azo dyes on the bacterial outer membrane to inhibit their bioreduction. This discovered role of goethite in bioreduction provides new insight into the microbial transformation processes of organic pollutants in iron (hydr)oxide-containing environments.

Azoreductase activity of dye-decolorizing bacteria isolated from the human gut microbiota

The gut microbiota enriches the human gene pool and contributes to xenobiotic metabolism. Microbial azoreductases modulate the reduction of azo-bonds, activating produgs and azo polymer-coated dosage forms, or degrading food additives. Here, we aimed to screen the healthy human gut microbiota for food colorant-reducing activity and to characterize factors modulating it. Four representative isolates from screened fecal samples were identified as E. coli (AZO-Ec), E. faecalis (AZO-Ef), E. avium (AZO-Ev) and B. cereus (AZO-Bc). Both AZO-Ef and AZO-Ev decolorized amaranth aerobically and microaerophilically while AZO-Ec and AZO-Bc had higher aerobic reduction rates. The isolates varied in their activities against different dyes, and the azo-reduction activity mostly followed zero-order reaction kinetics, with a few exceptions. Additionally, the isolates had different pH dependence, e.g., AZO-Ec was not affected by pH variation while AZO-Bc exhibited variable degradation kinetics at different pH levels. Cell-free extracts showed NADH-dependent enzymatic activities 14–19 times higher than extracellular fractions. FMN did not affect the reducing activity of AZO-Ef cell-free extract, whereas AZO-Ec, AZO-Ev and AZO-Bc had significantly higher reduction rates in its presence (P values = 0.02, 0.0001 and 0.02, respectively). Using Degenerate primers allowed the amplification of azoreductase genes, whose sequences were 98–99% similar to genes encoding FMN-dependent-NADH azoreductases.

Theoretical study on gas-phase reactions of nitrate radicals with methoxyphenols: Mechanism, kinetic and toxicity assessment

Creosol and 4-ethylguaiacol are two important methoxyphenols, lignin pyrolysis products, which are discharge into the atmosphere in large quantities. In this work, theoretical calculations of the reaction mechanism towards the two compounds with NO₃ radicals was performed using DFT method. The rate constants and toxicity assessment were also investigated. The atmospheric lifetime for creosol and 4-ethylguaiacol were 0.82 and 0.19 h, respectively. A new reaction pathway was proposed for the transformation of methoxyl into hydroxyl, which has not yet been clarified in previous studies. The toxicity of methoxyphenols and their degradation products is closely related to their hydrophobicity. Although most degradation products are less toxic, they also should be pay more attention, especially for nitro-substituents. A new reaction pathway was proposed for the transformation of methoxyl into hydroxyl. The toxicity is closely related to their hydrophobicity.

Combined intra- and extracellular reduction involved in the anaerobic biodecolorization of cationic azo dye by Shewanella oneidensis MR-1

Microbial reduction decolorization is a promising strategy for cationic azo dye pollution remediation, but the reduction mechanism is unclear yet. In this work, the anaerobic reduction decolorization mechanism of cationic red X-GRL (X-GRL) by Shewanella oneidensis MR-1 (MR-1) was investigated from both intracellular and extracellular aspects. The exogenous additional riboflavin treatment test was used to analyze the extracellular reduction mechanism of X-GRL, and the actual role of riboflavin during the reduction of X-GRL was identified by three-dimensional fluorescence analysis for the first time. The proteinase K and the electron competitor treatment tests were used to analyze the intracellular reduction mechanism of X-GRL. Moreover, the effect of external environment on the reduction mechanism of X-GRL was elucidated by the decolorization performance of MR-1 wild type and its mutants, ΔomcA/mtrC, ΔmtrA, ΔmtrB and ΔcymA, under different external pH conditions. The results indicated that X-GRL could be decolorized by MR-1 in both extracellular and intracellular spaces. The extracellular decolorization of X-GRL could be caused by Mtr respiratory pathway or the indirect reduction of riboflavin, while the intracellular decolorization might occur due to the intracellular reduction depending on CymA pathway and a NADH-dependent reduction catalyzed by intracellular azoreductases. Furthermore, the proportion of extracellular decolorization decreased, whereas that of intracellular decolorization increased as the environmental pH rose.

Anaerobic decolorization and detoxification of cationic red X-GRL by Shewanella oneidensis MR-1

The ability of an electrochemically active bacterium, Shewanella oneidensis MR-1, to decolorize azo dye cationic red X-GRL (X-GRL) was investigated. S. oneidensis MR-1 showed a high decolorization capability for X-GRL under anaerobic conditions. The Mtr respiratory pathway was proved to be involved in the extracellular decolorization of X-GRL. The decolorization efficiency of S. oneidensis MR-1 was significantly inhibited when the initial X-GRL concentration was over 200 mg L^-1. Increasing the inoculum volume of S. oneidensis MR-1 could obviously promote the X-GRL decolorization. The 100 mg L^-1 X-GRL and 6% (v/v) inoculum volume were chosen as the optimal parameter. Under such a condition, almost all of X-GRL (100 mg L^-1) could be completely reduced after 12-h incubation at the pH range of 5.5-8.0 and temperature range of 30-40°C. Salinity in the medium also affected X-GRL decolorization. Lactate and citric acid were found to be the suitable electron donors for X-GRL decolorization. Although the genotoxicity increased slightly, the phytotoxicity of X-GRL in the decolorization process was significantly reduced by S. oneidensis MR-1.

A simple method for assaying anaerobic biodegradation of dyes

Anaerobic dye degradation is usually assayed using serum vials, which is time-consuming and costly. In this work, a simple method was established for real-time nondestructive assay of dye biodegradation using 96-well microtiter plates with petrolatum oil to avoid the volatilization and high transmittance transparent tape to prevent the permeation of oxygen. With the anaerobic degradation of methyl red and amaranth by Shewanella oneidensis MR-1, this assay method was verified. Further experiments revealed that blocking Mtr pathway had no substantial effect on the degradation of methyl red and dose of riboflavin also failed to promote the degradation of methyl red. On the contrary, the anaerobic degradation of amaranth depended mainly on the electron transmembrane transfer through Mtr pathway. Our work clearly indicates that Mtr pathway had different effects on intra- and extra-cellular degradation of azo dyes by S. oneidensis MR-1. Such a developed method is helpful for investigating anaerobic dye decolorization.

Exclusive Extracellular Bioreduction of Methyl Orange by Azo Reductase-Free Geobacter sulfurreducens

Azo dyes are a class of recalcitrant organic pollutants causing severe environmental pollution. For their biodecolorization, the azo reductase system was considered as the major molecular basis in bacteria. However, the intracellular localization of azo reductase limits their function for efficient azo dye decolorization. This limitation may be circumvented by electrochemically active bacteria (EAB) which is capable of extracellular respiration. To verify the essential role of extracellular respiration in azo dye decolorization, Geobacter sulfurreducens PCA, a model EAB, was used for the bioreduction of methyl orange (MO), a typical azo dye. G. sulfurreducens PCA efficiently reduced MO into amines. Kinetic results showed that G. sulfurreducens PCA had the highest decolorization efficiency among the currently known MO reducing bacteria. Electrons from acetate oxidization by this strain were transferred by the respiratory chain to MO. The mass and electron balances, fluorescent probing and proteinase K treatment experimental results indicate that the biodecolorization of MO by G. sulfurreducens PCA is an exclusive extracellular process. OmcB, OmcC and OmcE were identified as the key outer-membrane proteins for the extracellular MO reduction. This work deepens our understanding of EAB physiology and is useful for the decontamination of environments polluted with azo dyes. The contribution of extracellular respiration to pollutants reduction will broaden the environmental applications of EAB.

Theoretical investigation on the kinetics and mechanisms of hydroxyl radical-induced transformation of parabens and its consequences for toxicity: Influence of alkyl-chain length

As emerging organic contaminants (EOCs), the ubiquitous presence of preservative parabens in water causes a serious environmental concern. Hydroxyl radical ((•)OH) is a strong oxidant that can degrade EOCs through photochemistry in surface water environments as well as in advanced oxidation processes (AOPs). To better understand the degradation mechanisms, kinetics, and products toxicity of the preservative parabens in aquatic environments and AOPs, the (•)OH-initiated degradation reactions of the four parabens were investigated systematically using a computational approach. The four studied parabens with increase of alkyl-chain length were methylparaben (MPB), ethylparaben (EPB), propylparaben (PPB), and dibutylparaben (BPB). Results showed that the four parabens can be initially attacked by (•)OH through (•)OH-addition and H-abstraction routes. The (•)OH-addition route was more important for the degradation of shorter alkyl-chain parabens like MPB and EPB, while the H-abstraction route was predominant for the degradation of parabens with longer alkyl-chain for example PPB and BPB. In assessing the aquatic toxicity of parabens and their degradation products using the model calculations, the products of the (•)OH-addition route were found to be more toxic to green algae than original parabens. Although all degradation products were less toxic to daphnia and fish than corresponding parental parabens, they could be still harmful to these aquatic organisms. Furthermore, as alkyl-chain length increased, the ecotoxicity of parabens and their degradation products was found to be also increased.