Methane oxidation coupled to perchlorate reduction in a membrane biofilm batch reactor

Biochar modulates intracellular electron transfer for nitrate reduction in denitrifying anaerobic methane oxidizing archaea

Denitrifying anaerobic methane oxidizing (DAMO) archaea plays a significant role in simultaneously nitrogen removal and methane mitigation, yet its limited metabolic activity hinders engineering applications. This study employed biochar to explore its potential for enhancing the metabolic activity and nitrate reduction capacity of DAMO microorganisms. Sawdust biochar (7 g/L) was found to increase the nitrate reduction rate by 2.85 times, although it did not affect the nitrite reduction rate individually. Scanning electron microscopy (SEM) and fluorescence excitation-emission matrix (EEM) analyses revealed that biochar promoted microbial aggregation, and stimulated the secretion of extracellular polymeric substances (EPS). Moreover, biochar bolstered the redox capacity and conductivity of the biofilm, notably enhancing the activity of the electron transfer system by 1.65 times. Key genes involved in intracellular electron transport (Hdr, MHC, Rnf) and membrane transport proteins (BBP, ABC, NDH) of archaea were significantly up-regulated. These findings suggest that biochar regulates electrons generated by reverse methanogenesis to the membrane for nitrate reduction.

Efficient perchlorate reduction in microaerobic environment facilitated by partner methane oxidizers

The conventional perchlorate (ClO4-) reduction typically necessitates anaerobic conditions. However, in this study, we observed efficient ClO4- reduction using CH4 as the electron donor in a microaerobic environment. The maximum ClO4- removal flux of 2.18 g/m2·d was achieved in CH4-based biofilm. The kinetics of ClO4- reduction showed significant differences, with trace oxygen increasing the reduction rate of ClO4-, whereas oxygen levels exceeding 2 mg/L decelerated the ClO4- reduction. In the absence of exogenous oxygen, anaerobic methanotrophic (ANME) archaea contribute more than 80% electrons through the reverse methanogenesis pathway for ClO4- reduction. Simultaneously, microorganisms activate CH4 by utilizing oxygen generated from chlorite (ClO2-) disproportionation. In the presence of exogenous oxygen, methane oxidizers predominantly consume oxygen to drive the aerobic oxidation of methane. It is indicated that methane oxidizers and perchlorate reducing bacteria can form aggregates to resist external oxygen shocks and achieve efficient ClO4- reduction under microaerobic condition. These findings provide new insights into biological CH4 mitigation and ClO4- removal in hypoxic environment.

Microbial stratification protects denitrifying anaerobic methane oxidation archaea and bacteria from external oxygen shock in membrane biofilm reactor

Different gradients of dissolved oxygen (DO) regulate the microbial community and nitrogen removal pathways of denitrifying anaerobic methane oxidation (DAMO) and anaerobic ammonium oxidation (Anammox) coupled process in a batch biofilm reactor. Under completely anaerobic condition, approximately 72 mg NO3--N/L was removed at a daily rate of 6.55 mg N/L, whereas a peak accumulation of 95 mg NO3--N/L was observed during DO reached 0.5 mg/L. There is a decrease in the abundance of Candidatus Methylomirabilis (24.1%), Candidatus Methanoperedens (23.3%), and Candidatus Kuenenia (22.6%) to below 5% when DO levels reached 0.2 mg/L. Moreover, key genes associated with the reverse methanogenesis (mcrA) and anaerobic ammonium oxidase (hzo) decreased. These findings indicate that during oxygen shock, methanotrophs and denitrifiers replace Anammox bacteria on the outer sphere of the biofilm, whereas DAMO bacteria and archaea are protected from external oxygen shock due to the microbial stratification of biofilm.

The ionic liquids upon perchlorate to promote the C-C/C-O bonds cleavage in alkali lignin under photothermal synergism

Transforming lignin into aromatic monomers is critically attractive to develop green and sustainable energy supplies. However, the usage of the additional catalysts like metal or base/acid is commonly limited by the caused repolymerized and environmental issues. The key step is to mediate electron transfer in lignin to trigger lignin C-C/C-O bonds cleavage without the catalysts mentioned above. Here, we report that the ionic liquids [BMim][ClO4] was found to trigger lignin electron transfer to cleave the C-C/C-O bonds for aromatic monomers without any additional catalyst. The proton transfer from [BMim]+ to [ClO4]- could polarize the anion and decrease its structure stability, upon which the active hydroxyl radical generated and induced lignin C-C/C-O bonds fragmentation via free radical-mediated routes with the assistance of photothermal synergism. About 4.4 wt% yields of aromatic monomers, mainly composed of vanillin and acetosyringone, are afforded in [BMim][ClO4] under UV-light irradiation in the air at 80 °C. This work opens the way to produce value-added aromatic monomers from lignin using an eco-friendly, energy-efficient, and simple route that may contribute to the sustainable utilization of renewable natural resources.

Bioelectrochemical system accelerates reductive dechlorination through extracellular electron transfer networks

Bioelectrochemical system is considered as a promising approach for enhanced bio-dechlorination. However, the mechanism of extracellular electron transfer in the dechlorinating consortium is still a controversial issue. In this study, bioelectrochemical systems were established with cathode potential settings at -0.30 V (vs. SHE) for trichloroethylene reduction. The average dechlorination rate (102.0 μM Cl·d-1) of biocathode was 1.36 times higher than that of open circuit (74.7 μM Cl·d-1). Electrochemical characterization via cyclic voltammetry illustrated that electrostimulation promoted electrochemical activity for redox reactions. Moreover, bacterial community structure analyses indicated electrical stimulation facilitated the enrichment of electroactive and dechlorinating populations on cathode. Metagenomic and quantitative polymerase chain reaction (qPCR) analyses revealed that direct electron transfer (via electrically conductive pili, multi-heme c-type cytochromes) between Axonexus and Desulfovibrio/cathode and indirect electron transfer (via riboflavin) for Dehalococcoides enhanced dechlorination process in BES. Overall, this study verifies the effectiveness of electrostimulated bio-dechlorination and provides novel insights into the mechanisms of dechlorination process enhancement in bioelectrochemical systems through electron transfer networks.

Molecular Mechanisms Determining the Role of Bacteria from the Genus Azospirillum in Plant Adaptation to Damaging Environmental Factors

Agricultural plants are continuously exposed to environmental stressors, which can lead to a significant reduction in yield and even the death of plants. One of the ways to mitigate stress impacts is the inoculation of plant growth-promoting rhizobacteria (PGPR), including bacteria from the genus Azospirillum, into the rhizosphere of plants. Different representatives of this genus have different sensitivities or resistances to osmotic stress, pesticides, heavy metals, hydrocarbons, and perchlorate and also have the ability to mitigate the consequences of such stresses for plants. Bacteria from the genus Azospirillum contribute to the bioremediation of polluted soils and induce systemic resistance and have a positive effect on plants under stress by synthesizing siderophores and polysaccharides and modulating the levels of phytohormones, osmolytes, and volatile organic compounds in plants, as well as altering the efficiency of photosynthesis and the antioxidant defense system. In this review, we focus on molecular genetic features that provide bacterial resistance to various stress factors as well as on Azospirillum-related pathways for increasing plant resistance to unfavorable anthropogenic and natural factors.

Isolation and identification of the molybdenum-resistant strain Raoultella ornithinolytica A1 and its effect on MoO42- in the environment

The mining and leakage of molybdenum (Mo) can cause environmental contamination which has not been realized until recently. Bacteria that can mitigate Mo-contamination was enriched and isolated. The low temperature and different pH conditions were considered to analysis its feasibility in Northern China which suffers from a long time of low temperatures every year. The result showed that the removal rate of MoO₄^2- by Raoultella ornithinolytica A1 reached 30.46% at 25 °C and pH 7.0 in Luria-Bertani medium (LB). Meanwhile, A1 also showed some efficiency in the reduction of MoO₄^2- in low phosphate molybdate medium (LPM), which reached optimum at the MoO₄^2- concentration of 10 mM. The results of FTIR indicated that the cell wall performed an essential role in the MoO₄^2- removal process, which was illustrated by the distribution of Mo in A1 (Mo bound to cell wall accounted for 92.29% of the total MoO₄^2- removed). In addition, low temperature (10 °C) effect the removal rate of MoO₄^2- by - 8.38 to 11.66%, indicating the potential for the in-situ microbial remediation of Mo-contaminated environments in low temperature areas.

Reductive Dehalogenation of Herbicides Catalyzed by Pd0NPs in a H2-Based Membrane Catalyst-Film Reactor

More food production required to feed humans will require intensive use of herbicides to protect against weeds. The widespread application and persistence of herbicides pose environmental risks for nontarget species. Elemental-palladium nanoparticles (Pd⁰NPs) are known to catalyze reductive dehalogenation of halogenated organic pollutants. In this study, the reductive conversion of 2,4-dichlorophenoxyacetic acid (2,4-D) was evaluated in a H₂-based membrane catalyst-film reactor (H₂-MCfR), in which Pd⁰NPs were in situ-synthesized as the catalyst film and used to activate H₂ on the surface of H₂-delivery membranes. Batch kinetic experiments showed that 99% of 2,4-D was removed and converted to phenoxyacetic acid (POA) within 90 min with a Pd⁰ surface loading of 20 mg Pd/m², achieving a catalyst specific activity of 6.6 ± 0.5 L/g-Pd-min. Continuous operation of the H₂-MCfR loaded with 20 mg Pd/m² sustained >99% removal of 50 μM 2,4-D for 20 days. A higher Pd⁰ surface loading, 1030 mg Pd/m², also enabled hydrosaturation and hydrolysis of POA to cyclohexanone and glycolic acid. Density functional theory identified the reaction mechanisms and pathways, which involved reductive hydrodechlorination, hydrosaturation, and hydrolysis. Molecular electrostatic potential calculations and Fukui indices suggested that reductive dehalogenation could increase the bioavailability of herbicides. Furthermore, three other halogenated herbicides─atrazine, dicamba, and bromoxynil─were reductively dehalogenated in the H₂-MCfR. This study documents a promising method for the removal and detoxification of halogenated herbicides in aqueous environments.

Nitrate/nitrite-dependent anaerobic oxidation of methane (N-AOM) as a technology platform for greenhouse gas abatement in wastewater treatment plants: State-of-the-art and challenges

Nitrate/nitrite-dependent anaerobic oxidation of methane (N-AOM) is a metabolic process recently discovered and partially characterized in terms of the microorganisms and pathways involved. The N-AOM process can be a powerful tool for mitigating the impacts of greenhouse gas emissions from wastewater treatment plants by coupling the reduction of nitrate or nitrite with the oxidation of residual dissolved methane. Besides specific anaerobic methanotrophs such as bacteria members of the phylum NC10 and archaea belonging to the lineage ANME-2d, recent reports suggested that other methane-oxidizing bacteria in syntrophy with denitrifiers can also perform the N-AOM process, which facilitates the application of this metabolic process for the oxidation of residual methane under realistic scenarios. This work constitutes a state-of-art review that includes the fundamentals of the N-AOM process, new information on process microbiology, bioreactor configurations, and operating conditions for process implementation in WWTP. Potential advantages of the N-AOM process over aerobic methanotrophic biotechnologies are presented, including the potential interrelation of the N-AOM with other nitrogen removal processes within the WWTP, such as the anaerobic ammonium oxidation. This work also addressed the challenges of this biotechnology towards its application at full scale, identifying and discussing critical research niches.

Metabolic potential of anaerobic methane oxidizing archaea for a broad spectrum of electron acceptors

Methane (CH₄) is a potent greenhouse gas significantly contributing to the climate warming we are currently facing. Microorganisms play an important role in the global CH₄ cycle that is controlled by the balance between anaerobic production via methanogenesis and CH₄ removal via methanotrophic oxidation. Research in recent decades advanced our understanding of CH₄ oxidation, which until 1976 was believed to be a strictly aerobic process. Anaerobic oxidation of methane (AOM) coupled to sulfate reduction is now known to be an important sink of CH₄ in marine ecosystems. Furthermore, in 2006 it was discovered that anaerobic CH₄ oxidation can also be coupled to nitrate reduction (N-DAMO), demonstrating that AOM may be much more versatile than previously thought and linked to other electron acceptors. In consequence, an increasing number of studies in recent years showed or suggested that alternative electron acceptors can be used in the AOM process including Fe^III, Mn^IV, As^V, Cr^VI, Se^VI, Sb^V, V^V, and Br^V. In addition, humic substances as well as biochar and perchlorate (ClO₄^-) were suggested to mediate AOM. Anaerobic methanotrophic archaea, the so-called ANME archaea, are key players in the AOM process, yet we are still lacking deeper understanding of their metabolism, electron acceptor preferences and their interaction with other microbial community members. It is still not clear whether ANME archaea can oxidize CH₄ and reduce metallic electron acceptors independently or via electron transfer to syntrophic partners, interspecies electron transfer, nanowires or conductive pili. Therefore, the aim of this review is to summarize and discuss the current state of knowledge about ANME archaea, focusing on their physiology, metabolic flexibility and potential to use various electron acceptors.

Efficient nitrogen removal from mainstream wastewater through coupling Partial Nitritation, Anammox and Methane-dependent nitrite/nitrate reduction (PNAM)

The application of partial nitritation and anammox to remove nitrogen from mainstream wastewater is of great interest because of the potential to reduce energy cost and carbon dosage. However, this process confronts a dilemma of relatively high effluent nitrogen concentration (>10 mg N/L), owning to the unwanted prevalence of nitrite-oxidizing bacteria (NOB) and the intrinsic nitrate production by anammox bacteria. Here, a novel technology, named the one-stage PNAM, that integrates Partial Nitritation, Anammox and Methane-dependent nitrite/nitrate reduction reactions, was developed in a single membrane biofilm reactor (MBfR). With feeding of synthetic mainstream wastewater containing ∼50 mg NH₄⁺-N/L at a hydraulic retention time of 12 h, more than 95% nitrogen was removed in the established one-stage PNAM process at a practically useful rate of 0.1 kg N/m³/d. Microbial community characterization and in-situ batch tests revealed a sophisticated microbial structure consisting of ammonia-oxidizing bacteria (AOB), anammox bacteria, nitrite/nitrate-dependent anaerobic methane oxidation (n-DAMO) bacteria and archaea, and a small fraction of NOB and aerobic methanotrophs. The role of methane in removing nitrate was confirmed by switching on/off the methane supply, which relaxed the requirement for NOB suppression. In addition, the established system was relatively robust against temperature variations, evidenced by a total nitrogen removal efficiency above 80% at temperature as low as 14 ℃. The results provide a promising alternative for efficient nitrogen removal from domestic wastewater using methane as the sole carbon source.

Microbial reduction of Cr(VI) in the presence of Ni, Cu and Zn by bacterial consortium enriched from an electroplating contaminated site

The bioremediation of Cr(VI) has been intensively reported in recent years, while little information about Cr(VI)-reducing consortium enriched from in-situ contaminated soil has been revealed, specifically the functional genes involved. In this study, we verified a Cr(VI) reduction process by a consortium enriched from in-situ contaminated soil through enzymatic analysis. The chromate reductase gene ChrR has been successfully amplified and further analyzed, provided solid evidence to prove the Cr(VI) bio-reduction was an enzyme-mediated process. Meanwhile, the analysis of metabolic pathways demonstrates that the consortium could detoxicate and resist Cr(VI) and co-existing metals (Ni²⁺, Zn²⁺ and Cu²⁺) through membrane transport and DNA repair process. The co-existing heavy metals Zn and Cu had a relatively significant negative and positive effects on Cr(VI) reduction respectively, which may play important roles in the Cr(VI) contaminated soil bioremediation.

Cross-feeding interactions in short chain gaseous alkane-driven perchlorate and selenate reduction

Short chain gaseous alkanes (SCGAs) mainly consist of methane (CH₄), ethane (C₂H₆), propane (C₃H₈) and butane (C₄H₁₀). The first three SCGAs have been shown to remove perchlorate (ClO₄^-) and selenate (SeO₄^2-), yet it is unknown whether C₄H₁₀ is available to reduce these contaminants. This study demonstrated that C₄H₁₀ fed biofilms were capable of reducing ClO₄^- and SeO₄^2- to chloride (Cl^-) and elemental selenium (Se⁰), respectively, by employing two independent membrane biofilms reactors (MBfRs). Batch tests showed that C₄H₁₀ and oxygen fed biofilms had much higher ClO₄^- and SeO₄^2- reduction rates and enhanced expression levels of bmoX and pcrA than that without C₄H₁₀ or O₂. Polyhydroxyalkanoates (PHA) accumulated in the biofilms when C₄H₁₀ was supplied, and they decomposed for driving ClO₄^- and SeO₄^2- reduction when C₄H₁₀ was absent. Moreover, we revisited the literature and found that a cross-feeding pathway seems to be universal in microaerobic SCGA-driven perchlorate and selenate reduction processes. In the ClO₄^--reducing MBfRs, Mycobacterium primarily conducts C₂H₆ and C₃H₈ oxidation in synergy with Dechloromonas who performs perchlorate reduction, while both Mycobacterium and Rhodococcus carried out C₄H₁₀ oxidation with perchlorate-respiring Azospira as the partner. In the SeO₄^2--reducing MBfRs, Mycobacterium oxidized C₂H₆ solely or oxidized C₃H₈ jointly with Rhodococcus, while Burkholderiaceae likely acted as the selenate-reducing bacterium. When C₄H₁₀ was supplied as the electron donor, both Mycobacterium and Rhodococcus conducted C₄H₁₀ oxidation in synergy with unknow selenate-reducing bacterium. Collectively, we confirm that from CH₄ to C₄H₁₀, all SCGAs could be utilized as electron donors for bio-reduction process. These findings offer insights into SCGA-driven bio-reduction processes, and are helpful in establishing SCGA-based technologies for groundwater remediation.

Making good use of methane to remove oxidized contaminants from wastewater

Being an energetic fuel, methane is able to support microbial growth and drive the reduction of various electron acceptors. These acceptors include a broad range of oxidized contaminants (e.g., nitrate, nitrite, perchlorate, bromate, selenate, chromate, antimonate and vanadate) that are ubiquitously detected in water environments and pose threats to human and ecological health. Using methane as electron donor to biologically reduce these contaminants into nontoxic forms is a promising solution to remediate polluted water, considering that methane is a widely available and inexpensive electron donor. The understanding of methane-based biological reduction processes and the responsible microorganisms has grown in the past decade. This review summarizes the fundamentals of metabolic pathways and microorganisms mediating microbial methane oxidation. Experimental demonstrations of methane as an electron donor to remove oxidized contaminants are summarized, compared, and evaluated. Finally, the review identifies opportunities and unsolved questions that deserve future explorations for broadening understanding of methane oxidation and promoting its practical applications.

Enrichment and characterization of an effective hexavalent chromium-reducing microbial community YEM001

Chromium (Cr) is one of the most widely used heavy metals in industrial processes, resulting in water and soil pollution that seriously threaten environmental safety. In this paper, we have directionally enriched a Cr(VI)-reducing bacterial community YEM001 from no-Cr(VI) polluted pond sedimental sludge by selectively growing it in Cr(VI)-containing media. This community could effectively reduce Cr(VI) in laboratory rich media containing different concentrations of Cr(VI), such as 61% reduction at 435 mg/L Cr(VI), 85% reduction at 355 mg/L Cr(VI), and complete reduction at 269 mg/L Cr(VI) in 93.5 h. It was also able to completely reduce 100 mg/L and 300 mg/L Cr(VI) in landfill leachate and natural sludge in 48 h, respectively. Optimal pH for Cr(VI) reduction of the YEM001 is between 7 and 8 and the best efficiency for Cr(VI) reduction occurs at 30 °C. Metagenomic data demonstrated that the YEM001 community was composed of multiple bacteria, including well-known Cr(VI)-reducing bacteria and non-Cr(VI)-reducing bacteria. Delftia, Comamonas, Alicycliphilus, Acidovorax, Bacillus, and Clostridioides account for 83% of total community abundance. The stability of the composition of the YEM001 community and its Cr(VI)-reducing activity allows for its application in bioremediation of environmental Cr(VI) pollution.

Antimonate removal by diatomite modified with Fe-Mn oxides: application and mechanism study

In this study, diatomite coated with Fe-Mn oxides (DFMO) was synthesized through calcination. The adsorption of antimonate (Sb(V)) by DFMO was studied, and environmental factors affecting the adsorption were investigated. The components of DFMO were identified as γ-Fe₂O₃, γ-MnO₂, and SiO₂, in the presence of diatomite covered with nanoscale metal oxides. Batch experiments were carried out to evaluate the antimonate adsorption performance in aqueous solution. Results showed that maximum Sb(V) adsorption capacity of DFMO reached 10.7 mg/g at pH 4, corresponding to 22.2 mg/g per unit metal oxides. Antimonate adsorption occurred on heterogenous surface, following the Freundlich and Pseudo-second order model. Overall, antimonate adsorption was favored at acidic condition due to low point of zero charge. However, when treating electroplating wastewater, neutral pH condition exhibited a higher efficiency than acidic pH, because co-existing ions in electroplating wastewater significantly affects antimony adsorption. Further investigation showed that among different potential co-existing ions, fluoride can strongly inhibit the adsorption of antimonate at 5 mg/L under pH 4. Density functional theory (DFT) analysis confirmed that adsorption energy on DFMO follows: HF < F^- < Sb(OH)₆^-, indicating that fluoride is easier to bind with DFMO compared to antimonate, especially under pH 3.5 at which fluoride exists as HF. Moreover, the competitive adsorption of fluoride toward antimonate indicated the necessity of pre-treatment like neutralization and precipitation before adsorption process.

Laboratory scale bioreactor designs in the processes of methane bioconversion: Mini-review

Global methane emissions have been steadily increasing over the past few decades, exerting a negative effect on the environment. Biogas from landfills and sewage treatment plants is the main anthropogenic source of methane. This makes methane bioconversion one of the priority areas of biotechnology. This process involves the production of biochemical compounds from non-food sources through microbiological synthesis. Methanotrophic bacteria are a promising tool for methane bioconversion due to their ability to use this greenhouse gas and to produce protein-rich biomass, as well as a broad range of useful organic compounds. Currently, methane is used not only to produce biomass and chemical compounds, but also to increase the efficiency of water and solid waste treatment. However, the use of gaseous substrates in biotechnological processes is associated with some difficulties. The low solubility of methane in water is one of the major problems. Different approaches have been involved to encounter these challenges, including different bioreactor and gas distribution designs, solid carriers and bulk sorbents, as well as varying air/oxygen supply, the ratio of volumetric flow rate of gas mixture to its consumption rate, etc. The aim of this review was to summarize the current data on different bioreactor designs and the aspects of their applications for methane bioconversion and wastewater treatment. The bioreactors used in these processes must meet a number of requirements such as low methane emission, improved gas exchange surface, and controlled substrate supply to the reaction zone.

Microbial Perchlorate Reduction Driven by Ethane and Propane

Previous studies demonstrated that methane can be used as an electron donor to microbially remove various oxidized contaminants in groundwater. Natural gas, which is more widely available and less expensive than purified methane, is potentially an alternative source of methane. However, natural gas commonly contains a considerable amount of ethane (C₂H₆) and propane (C₃H₈), in addition to methane. It is important that these gaseous alkanes are also utilized along with methane to avoid emissions. Here, we demonstrate that perchlorate (ClO₄^-), a frequently reported contaminant in groundwater, can be microbially reduced to chloride (Cl^-) driven by C₂H₆ or C₃H₈ under oxygen-limiting conditions. Two independent membrane biofilm reactors (MBfRs) supplied with C₂H₆ and C₃H₈, respectively, were operated in parallel to biologically reduce ClO₄^-. The continuous ClO₄^- removal during long-term MBfR operation combined with the concurrent C₂H₆/C₃H₈ consumption and ClO₄^- reduction in batch tests confirms that ClO₄^- reduction was associated with C₂H₆ or C₃H₈ oxidation. Polyhydroxyalkanoates (PHAs) were synthesized in the presence of C₂H₆ or C₃H₈ and were subsequently utilized for supporting ClO₄^- bio-reduction in the absence of gaseous alkanes. Analysis by reverse transcription-quantitative polymerase chain reaction (RT-qPCR) showed that transcript abundance of bmoX (encoding alpha hydroxylase subunit of C₂H₆/C₃H₈ monooxygenase) was positively correlated to the consumption rates of C₂H₆/C₃H₈, while pcrA (encoding a catalytic subunit of perchlorate reductase) was positively correlated to the consumption of ClO₄^-. High-throughput sequencing targeting 16S rRNA, bmoX, and pcrA indicated that Mycobacterium was the dominant microorganism oxidizing C₂H₆/C₃H₈, while Dechloromonas may be the major perchlorate-reducing bacterium in the biofilms. These findings shed light on microbial ClO₄^- reduction driven by C₂H₆ and C₃H₈, facilitating the development of cost-effective strategies for ex situ groundwater remediation.

Electro-assisted autohydrogenotrophic reduction of perchlorate and microbial community in a dual-chamber biofilm-electrode reactor

The electro-assisted autohydrogenotrophic reduction of perchlorate (ClO₄^-) was investigated in a dual-chamber biofilm-electrode reactor (BER), in which the microbial community was inoculated from natural sediments. To avoid the effect of extreme pH and direct electron transfer on perchlorate reduction, a novel cathode configuration was designed. The pH of the cathode compartment was successfully controlled in the range of 7.2-8.4 during whole experiment. The effective biological autohydrogenotrophic reduction of perchlorate was achieved using hydrogen generated in-situ on the electrode surface, and the removal rate of 10 mg L^-1 perchlorate reached 98.16% at HRT of 48 h. The highest perchlorate removal flux reached to 1498.420 mg m^-2·d^-1 with a 0.410 kW·h g-perchlorate^-1 energy consumption. The microbial community evolution in the BER was determined by high-throughput sequencing and the results indicated that the Firmicutes and Bacteroidetes were dominant at phylum level when perchlorate concentration was 10 mg L^-1 or lower. And the Proteobacteria became ascendant at the perchlorate concentration of 20 mg L^-1. The functional populations for perchlorate reduction were successfully enriched including Nitrosomonas (30%), Thermomonas (9%), Comamonas (8%) and Hydrogenophaga (3%). Meanwhile, the proportion of functional population in biofilm linked to perchlorate concentration. With the increase of influent perchlorate concentration, the perchlorate-reducing bacteria (PRB) were enriched successfully and became ascendant.

Microbial selenate reduction in membrane biofilm reactors using ethane and propane as electron donors

Selenate (Se(VI)) contamination in groundwater is one of major concerns for human health, in particular in shale gas extraction sites. Microbial selenate reduction coupled to methane (CH₄) oxidation has been demonstrated very recently. Little is known whether ethane (C₂H₆) and butane (C₃H₈) are able to drive selenate reduction, although they are also important components in shale gas. In this study, we demonstrated Se(VI) bio-reduction could be achieved using C₂H₆ and C₃H₈ as electron donors and carbon sources. Scanning electron microscopy coupled to energy dispersive X-ray spectroscopy (SEM-EDX) confirmed elemental Se (Se⁰) was the major final product formed from Se(VI) bio-reduction. Polyhydroxyalkanoates (PHAs) were generated in the biofilms as the internal electron-storage materials, which were consumed for sustaining Se(VI) bio-reduction in absence of C₂H₆ and C₃H₈. Microbial community analysis showed that two genera capable of oxidizing gaseous alkanes dominated in the biofilms, including Mycobacterium (in both C₂H₆ and C₃H₈-fed biofilms) and Rhodococcus (in C₃H₈-fed biofilm). In addition, several potential Se(VI) reducers (e.g., Variovorax) were detected in the biofilms. Investigation of Communities by Reconstruction of Unobserved States analysis supported that predictive genes associated with alkanes oxidation, denitrification and PHAs cycle were enriched in the biofilms. These findings offer insights into the process of selenate reduction driven by C₂H₆ and C₃H₈, which ultimately may help to develop a solution to use shale gas for groundwater remediation, especially near shale gas exploitation sites.

NC10 bacteria promoted methane oxidation coupled to chlorate reduction

The strictly anaerobic serum bottles were applied to investigate methane oxidation coupled to chlorate (ClO₃^-) reduction (MO-CR) without exogenous oxygen. 0.35 mM ClO₃^- was consumed within 20 days at the reduction rate of 17.50 μM/d, over three times than that of ClO₄^-. Chlorite (ClO₂^-) was not detected throughout the experiment and the mass recovery of Cl^- was over 89%. Isotope tracing results showed most of ¹³CH₄ was oxided to CO₂, and the electrons recovery reached to 77.6%. Small amounts of ¹³CH₄ was consumed for DOC production probably through aerobic methane oxidation process, with oxygen generated from disproportionation reaction. In pMMO (key enzyme in aerobic oxidation of methane) inhibition tests, ClO₃^- reduction rate was slowed to 7. 0 μmol/d by 2 mM C₂H₂, real-time quantitative PCR also showed the transcript abundance of pMMO and Cld were significantly dropped at the later period of experiment, indicating that the O₂ disproportionated from ClO₂^- was utilized to active CH₄. NC10 bacteria Candidatus Methylomirabilis, related closely to oxygenic denitrifiers M. oxyfera, was detected in the system, and got enriched along with chlorate reduction. Several pieces of evidence supported that NC10 bacteria promoted CH₄ oxidation coupled to ClO₃^- reduction, these oxygenic denitrifiers may perform ClO₂^- disproportionation to produce O₂, and then oxidized methane intracellularly.

A mixed consortium of methanotrophic archaea and bacteria boosts methane-dependent selenate reduction

Though methane-based selenate reduction has been reported, neither the selenate load nor the removal rate could satisfy practical applications, thus limiting this technique to bio-remediate selenate pollution. In the present study, using a membrane biofilm batch reactor (MBBR), we successfully enriched a consortium performing methane-dependent selenate reduction, with enhanced reduction rates from 16.1 to 28.9 μM-day^-1 under a comparable Se concentration to industrial wastewaters (i.e., ~500 μM). During active reduction, 16S rRNA gene copies of Archaea and Bacteria were both increased more than one order of magnitude. Clone library construction and high-throughput sequencing indicated that Methanosarcina and Methylocystis were the only methane-oxidizing microorganisms. The presence of 20 mM bromoethanesulphonate or 0.15 mM acetylene both significantly, but not completely, inhibited methane-dependent selenate reduction, indicating the concurrent contributions of methanotrophic archaea and bacteria. Fluorescence in situ hybridization (FISH) revealed that archaea directly adhered to the surface of the membrane while bacteria were in the outer layer, together forming the mature biofilm. This study highlights the crucial role of both methanotrophic archaea and bacteria in methane-dependent selenate reduction, and lays foundations in applying methane to bio-remediate practical selenate pollution.

A critical review of aerobic denitrification: Insights into the intracellular electron transfer

Aerobic denitrification is a novel biological nitrogen removal technology, which has been widely investigated as an alternative to the conventional denitrification and for its unique advantages. To fully comprehend aerobic denitrification, it is essential to clarify the regulatory mechanisms of intracellular electron transfer during aerobic denitrification. However, reports on intracellular electron transfer during aerobic denitrification are rather limited. Thus, the purpose of this review is to discuss the molecular mechanism of aerobic denitrification from the perspective of electron transfer, by summarizing the advancements in current research on electron transfer based on conventional denitrification. Firstly, the implication of aerobic denitrification is briefly discussed, and the status of current research on aerobic denitrification is summarized. Then, the occurring foundation and significance of aerobic denitrification are discussed based on a brief review of the key components involved in the electron transfer of denitrifying enzymes. Moreover, a strategy for enhancing the efficiency of aerobic denitrification is proposed on the basis of the regulatory mechanisms of denitrification enzymes. Finally, scientific outlooks are given for further investigation on aerobic denitrification in the future. This review could help clarify the mechanism of aerobic denitrification from the perspective of electron transfer.

Enhanced Microbial Chromate Reduction Using Hydrogen and Methane as Joint Electron Donors

Hydrogen and methane commonly co-exist in aquifer. Either hydrogen or methane has been individually utilized as electron donor for bio-reducing chromate. However, little is known whether microbial chromate reduction would be suppressed or promoted when both hydrogen and methane are simultaneously supplied as joint electron donors. This study for the first time demonstrated microbial chromate reduction rate could be accelerated by both hydrogen and methane donating electrons. The maximum chromate reduction rate (4.70 ± 0.03 mg/L·d) with a volume ratio of hydrogen to methane at 1:1 was significantly higher than that with pure hydrogen (2.53 ± 0.02 mg/L·d) or pure methane (2.01 ± 0.02 mg/L·d) as the sole electron donor (p < 0.01). High-throughput 16S rRNA gene amplicon sequencing detected potential chromate reducers (e.g., Spirochaetaceae, Delftia and Azonexus) and hydrogenotrophic bacteria (e.g., Acetoanaerobium) and methane-metabolizing microorganisms (e.g., Methanobacterium), indicating that these microorganisms might play important roles on microbial chromate reduction using both hydrogen and methane as electron donors. Abundant hupL and mcrA genes responsible for hydrogen oxidation and methane conversion were harbored, together with chrA gene for chromate reduction. More abundant extracellular cytochrome c and intracellular NADH were detected with joint electron donors, suggesting more active electron transfers.

Why does sulfate inhibit selenate reduction: Molybdenum deprivation from Mo-dependent selenate reductase

Selenium pollution has become an increasingly serious global concern. Methane-fed selenate reduction has proven to be of great interest for the bioremediation of selenate-contaminated waters even with the coexistence of nitrate and dissolved oxygen. However, it is unclear if the common concurrent sulfate anion affects selenate removal. To address this question, we first introduced selenate (SeO₄^2-) as the sole influent electron acceptor in a CH₄-fed membrane biofilm reactor (CH₄-MBfR); then we added different concentrations of sulfate (SO₄^2-). The initial selenate removal efficiency (∼90%) was decreased by 50% in the presence of 15.6 μM of sulfate and completely inhibited after loading with 171.9 μM of sulfate. 16S rRNA gene sequencing showed that the selenate-reducing bacteria decreased after the addition of sulfate. Metagenomic sequencing showed that the abundance of genes encoding molybdenum (Mo)-dependent selenate reductase reduced by >50% when exposed to high concentrations of sulfate. Furthermore, the decrease in the total genes encoding all Mo-oxidoreductases was much greater than that of the genes encoding molybdate transporters, suggesting that the inhibition of selenate reduction by sulfate was most likely via the direct competition with molybdate for the transport system, leading to a lack of available Mo for Mo-dependent selenate reductases and thus reducing their activities. This result was confirmed by a batch test wherein the supplementation of molybdate mitigated the sulfate effect. Overall, this study shed light on the underlying mechanism of sulfate inhibition on selenate reduction and laid the foundation for applying the technology to practical wastewaters.

Granular activated carbon assisted nitrate-dependent anaerobic methane oxidation-membrane bioreactor: Strengthening effect and mechanisms

Eutrophication and global warming are two main urgent environmental problems around the world. Nitrate-dependent Anaerobic Methane Oxidation (NdAMO) is a bioprocess coupling nitrate reduction with anaerobic methane oxidation, which could mitigate of these two environmental issues simultaneously. In this study, a newly granular active carbon-NdAMO-membrane bioreactor (GAC-NdAMO-MBR) system was established to evaluate its nitrogen removal efficiency, membrane fouling property and the probable strengthening mechanism was also uncovered. Results indicated that the nitrate removal rate in GAC-NdAMO-MBR reached 31.85 ± 3.19 mgN·L^-1·d^-1 while it was only 10.35 ± 2.02 mgN·L^-1·d^-1 in NdAMO-MBR system (lack of GAC), which was multiplied three-fold. The membrane flux decay rate of GAC- NdAMO -MBR was 0.15 L/m²·h·d while it was 0.49 L/m²·h·d without GAC, and the addition of GAC could extend membrane fouling time for 2.5 times. Notablely, the relative abundance of NdAMO bacteria sharply increased from 27.15% to 56.91% after GAC addition while the NdAMO archaea showed similar variation trend. The physicochemical property of GAC mainly contributed the strengthening effect. The porous structure of GAC absorbed methane and adhered by microorganism, which enhance microorganism amount and metabolic activity. The mechanical strength of GAC scoured membrane surface to mitigate external fouling and pores absorbed EPS to reduce internal fouling. The combined effects could improve NdAMO microorganism growth and metabolism activity and finally improved nitrogen removal performance and controlled membrane fouling. These findings could deep the knowledge of NdAMO process and help extend its application potential in environment science and engineering.

Influence of non-dechlorinating microbes on trichloroethene reduction based on vitamin B12 synthesis in anaerobic cultures

In this study, the YH consortium, an ethene-producing culture, was used to evaluate the effect of vitamin B₁₂ (VB₁₂) on trichloroethene (TCE) dechlorination by transferring the original TCE-reducing culture with or without adding exogenous VB₁₂. Ultra-high performance liquid chromatography - tandem mass spectrometry (UPLC-MS/MS) was applied to detect the concentrations of VB₁₂ and its lower ligand 5,6-dimethylbenzimidazole (DMB) in the cultures. After three successive VB₁₂ starvation cycles, the dechlorination of TCE stopped mostly at cis-dichloroethene (cDCE), and no ethene was found; methane production increased significantly, and no VB₁₂ was detected. Results suggest that the co-cultured microbes may not be able to provide enough VB₁₂ as a cofactor for the growth of Dehalococcoides in the YH culture, possibly due to the competition for corrinoids between Dehalococcoides and methanogens. The relative abundances of 16 S rRNA gene of Dehalococcoides and reductive dehalogenase genes tceA or vcrA were lower in the cultures without VB₁₂ compared with the cultures with VB₁₂. VB₁₂ limitation changed the microbial community structures of the consortia. In the absence of VB₁₂, the microbial community shifted from dominance of Chloroflexi to Proteobacteria after three consecutive VB₁₂ starvation cycles, and the dechlorinating genus Dehalococcoides declined from 42.9% to 13.5%. In addition, Geobacter, Clostridium, and Desulfovibrio were also present in the cultures without VB₁₂. Furthermore, the abundance of archaea increased under VB₁₂ limited conditions. Methanobacterium and Methanosarcina were the predominant archaea in the culture without VB₁₂.

How nitrate affects perchlorate reduction in a methane-based biofilm batch reactor

Nitrate (NO₃^-) affected perchlorate (ClO₄^-) reduction in a membrane batch biofilm reactor (MBBR), even though the electron donor, CH₄, was available well in excess of its demand. For example, the perchlorate-reduction rate was 1.7 mmol/m²-d when perchlorate was the sole electron acceptor, but it dropped to 0.64 mmol/m²-d when nitrate also was present. The perchlorate-reduction rate returned to 1.60 mmol/m²-d after all nitrate was consumed. Denitratisoma and Azospirillum were main genera involved in perchlorate and nitrate reduction, and both could utilize NO₃^- and ClO₄^- as electron acceptors. Results of the reverse transcription-polymerase chain reaction (RT-PCR) showed that transcript abundances of nitrate reductase (narG), nitrite reductase (nirS), and perchlorate reductase (pcrA) increased when the perchlorate and nitrate concentrations were higher. Specifically, pcrA transcripts correlated to the sum of perchlorate and nitrate, rather than perchlorate individually. Analysis based on Density Functional Theory (DFT) suggests that bacteria able to utilize both acceptors, preferred NO₃^- over ClO₄^- due to nitrate reduction having lower energy barriers for proton and electron transfers.

Microbial vanadate and nitrate reductions coupled with anaerobic methane oxidation in groundwater

Vanadate contaminant in groundwater receives increasing attentions, but little is known on its biogeochemical transformation with gaseous electron donors. This study investigated bio-reduction of vanadate coupled with anaerobic methane oxidation and its relationship with nitrate reduction. Results showed 95.8 ± 3.1% of 1 mM vanadate was removed within 7 days using methane as the sole electron donor. Tetravalent vanadium compounds were the main reduction products, which precipitated naturally in groundwater environment. The introduction of nitrate inhibited vanadate reduction, though both were reduced in parallel. Accumulations of volatile fatty acids (VFAs) were observed from methane oxidation. Preliminary microbial community structure and metabolite analyses indicated that vanadate was likely reduced via Methylomonas coupled with methane oxidation or through synergistic relationships between methane oxidizing bacteria and heterotrophic vanadate reducers with VFAs served as the intermediates.

Nitrogen Removal Performance and Metabolic Pathways Analysis of a Novel Aerobic Denitrifying Halotolerant Pseudomonas balearica strain RAD-17

An aerobic denitrification strain, Pseudomonas balearica RAD-17, was identified and showed efficient inorganic nitrogen removal ability. The average NO₃⁻-N, NO₂⁻-N, and total ammonium nitrogen (TAN) removal rate (>95% removal efficiency) in a batch test was 6.22 mg/(L∙h), 6.30 mg/(L∙h), and 1.56 mg/(L∙h), respectively. Meanwhile, optimal incubate conditions were obtained through single factor experiments. For nitrogen removal pathways, the transcriptional results proved that respiratory nitrate reductases encoded by napA, which was primarily performed in aerobic denitrification and cell assimilation, were conducted by gluS and gluD genes for ammonium metabolism. In addition, adding the strain RAD-17 into actual wastewater showed obvious higher denitrification performance than in the no inoculum group (84.22% vs. 22.54%), and the maximum cell abundance achieved 28.5 ± 4.5% in a ratio of total cell numbers. Overall, the efficient nitrogen removal performance plus strong environmental fitness makes the strain RAD-17 a potential alternative for RAS (recirculating aquaculture system) effluent treatment.

Model-based assessment of chromate reduction and nitrate effect in a methane-based membrane biofilm reactor

Chromate contamination can pose a high risk to both the environment and public health. Previous studies have shown that CH₄-based membrane biofilm reactor (MBfR) is a promising method for chromate removal. In this study, we developed a multispecies biofilm model to study chromate reduction and its interaction with nitrate reduction in a CH₄-based MBfR. The model-simulated results were consistent with the experimental data reported in the literature. The model showed that the presence of nitrate in the influent promoted the growth of heterotrophs, while suppressing methanotrophs and chromate reducers. Moreover, it indicated that a biofilm thickness of 150 μm and an influent dissolved oxygen concentration of 0.5 mg O₂/L could improve the reactor performance by increasing the chromate removal efficiency under the simulated conditions.

Multi-omics analysis reveals niche and fitness differences in typical denitrification microbial aggregations

Suspended floc and fixed biofilm are two commonly applied strategies for heterotrophic denitrification in wastewater treatment. These two strategies use different carbon sources and reside within different ecological niches for microbial aggregation, which were hypothesized to show distinct microbial structures and metabolic fitness. We surveyed three floc reactors and three biofilm reactors for denitrification and determined if there were distinct microbial aggregations. Multiple molecular omics approaches were used to determine the microbial community composition, co-occurrence network and metabolic pathways. Proteobacteria was the dominating and most active phylum among all samples. Carbon source played an important role in shaping the microbial community composition while the distribution of functional protein was largely influenced by salinity. We found that the topological network features had different ecological patterns and that the microorganisms in the biofilm reactors had more nodes but less interactions than those in floc reactors. The large niche differences in the biofilm reactors explained the observed high microbial diversity, functional redundancy and resulting high system stability. We also observed a lower proportion of denitrifiers and higher resistance to oxygen and salinity perturbation in the biofilm reactors than the floc reactors. Our findings support our hypothesis that niche differences caused a distinct microbial structure and increased microbial ecology distribution, which has the potential to improve system efficiency and stability.

Dissolved oxygen has no inhibition on methane oxidation coupled to selenate reduction in a membrane biofilm reactor

Methane oxidation coupled to selenate reduction has been suggested as a promising technology to bio-remediate selenium contaminated environments. However, the effect of dissolved oxygen (DO) on this process remained unclear. Here, we investigate the feasibility of selenate removal at two distinct DO concentrations. A membrane biofilm reactor (MBfR) was initially fed with ∼5 mg Se/L and then lowered to ∼1 mg Se/L of selenate, under anoxic condition containing ∼0.2 mg/L of influent DO. Selenate removal reached approximately 90% without selenite accumulation after one-month operation. Then 6-7 mg/L of DO was introduced and showed no apparent effect on selenate reduction in the subsequent operation. Electron microscopy suggested elevated oxygen exposure did not affect microbial shapes. 16S rDNA sequencing showed the aerobic methanotroph Methylocystis increased, while possible selenate reducers, Ignavibacterium and Bradyrhizobium, maintained stable after oxygen boost. Gene analysis indicated that nitrate/nitrite reductases positively correlated with selenate removal flux and were not remarkably affected by oxygen addition. Reversely, enzymes related with aerobic methane oxidation were obviously improved. This study provides a potential technology for selenate removal from oxygenated environments in a methane-based MBfR.

Anaerobic methane oxidation coupled to chromate reduction in a methane-based membrane biofilm batch reactor

Chromate can be reduced by methanotrophs in a membrane biofilm reactor (MBfR). In this study, we cultivated a Cr(VI)-reducing biofilm in a methane (CH₄)-based membrane biofilm batch reactor (MBBR) under anaerobic conditions. The Cr(VI) reduction rate increased to 0.28 mg/L day when the chromate concentration was ≤ 2.2 mg/L but declined sharply to 0.01 mg/L day when the Cr(VI) concentration increased to 6 mg/L. Isotope tracing experiments showed that part of the ¹³C-labeled CH₄ was transformed to ¹³CO₂, suggesting that the biofilm may reduce Cr(VI) by anaerobic methane oxidation (AnMO). Microbial community analysis showed that a methanogen, i.e., Methanobacterium, dominated in the biofilm, suggesting that this genus is probably capable of carrying out AnMO. The abundance of Methylomonas, an aerobic methanotroph, decreased significantly, while Meiothermus, a potential chromate-reducing bacterium, was enriched in the biofilm. Overall, the results showed that the anaerobic environment inhibited the activity of aerobic methanotrophs while promoting AnMO bacterial enrichment, and high Cr(VI) loading reduced Cr(VI) flux by inhibiting the methane oxidation process.

Methane oxidation coupled to vanadate reduction in a membrane biofilm batch reactor under hypoxic condition

This study shows vanadate (V(V)) reduction in a methane (CH₄) based membrane biofilm batch reactor when the concentration of dissolved oxygen (O₂) was extremely low. V(IV) was the dominant products formed from V(V) bio-reduction, and majority of produced V(IV) transformed into precipitates with green color. Quantitative polymerase chain reaction and Illumina sequencing analysis showed that archaea methanosarcina were significantly enriched. Metagenomic predictive analysis further showed the enrichment of genes associated with reverse methanogenesis pathway, the key CH₄-activating mechanism for anaerobic methane oxidation (AnMO), as well as the enrichment of genes related to acetate synthesis, in archaea. The enrichment of aerobic methanotrophs Methylococcus and Methylomonas implied their role in CH₄ activation using trace level of O₂, or their participation in V(V) reduction.

Microbial reduction of bromate: current status and prospects

Bromate is a disinfection byproduct (DBP) that forms during the ozonation of bromide-containing natural water, which may cause health risks to humans. In this review, we provide an overview of the mechanism of bromate formation, microbial communities and bioreactors that are responsible for bromate reduction. Bromate can be formed through two pathways of bromide oxidation by ozone or by ^·OH, and it can be removed by biological approaches. Members belonging to phyla of Spirochaetes, Proteobacteria, Firmicutes, Actinobacteria, Clostridium, Deinococcus-Thermus and Bacteroidetes have been identified as capable of reducing bromate to bromide. Multiple configurations of biofilm bioreactors have been employed to cultivate microbial communities to perform bromate removal. The rapid development of multiomics has and will continue to accelerate the elucidation of the mechanisms involved in bromate and other DBP conversions, as well as the interaction patterns among different bacterial subdivisions in the bioremoval of DBPs.