Microbial Perchlorate Reduction Driven by Ethane and Propane

Microbial oxidation of short-chain gaseous alkanes

Short-chain gaseous alkanes (SCGAs), including ethane, propane and butane, are major components of natural gas and their atmospheric emissions impact global air quality and tropospheric chemistry. Many microbial taxa can degrade SCGAs aerobically and anaerobically to CO2, acting as the major biological sink of these compounds and reducing their negative impacts on climate. Environmental metagenomics and cultivation efforts have expanded our understanding of SCGA-oxidizing microorganisms. In this Review, we discuss recent discoveries in the diversity, physiology and metabolism of aerobic and anaerobic SCGA-oxidizing microorganisms, highlight their climate implications and discuss how knowledge of these processes can help develop biotechnologies for environmental remediation and value-added chemical production.

Groundwater chromate removal by autotrophic sulfur disproportionation

Chromate [Cr(VI)] contamination in groundwater is a global environmental challenge. Traditional elemental sulfur-based biotechnologies for Cr(VI) removal depend heavily on the synthesis of dissolved organic carbon to fuel heterotrophic Cr(VI) reduction, a bottleneck in the remediation process. Here we show an alternative approach by leveraging sulfur-disproportionating bacteria (SDB) inherent to groundwater ecosystems, offering a novel and efficient Cr(VI) removal strategy. We implemented SDB within a sulfur-packed bed reactor for treating Cr(VI)-contaminated groundwater, achieving a notable removal rate of 6.19 mg L-1 h-1 under oligotrophic conditions. We identified the chemical reduction of Cr(VI) via sulfide, produced through sulfur disproportionation, as a key mechanism, alongside microbial Cr(VI) reduction within the sulfur-based biosystem. Genome-centric metagenomic analysis revealed a symbiotic relationship among SDB, sulfur-oxidizing, and chromate-reducing bacteria within the reactor, suggesting that Cr(VI) detoxification by these microbial communities enhances the sulfur-disproportionation process. This research highlights the significance of sulfur disproportionation in the cryptic sulfur cycle in Cr(VI)-contaminated groundwater and proposes its practical application in groundwater remediation efforts.

Simultaneous Biogas Upgrading and Valuable Chemical Production Using Homoacetogens in a Membrane Biofilm Reactor

Biogas produced from anaerobic digestion usually contains impurities, particularly with a high content of CO2 (15-60%), thus decreasing its caloric value and limiting its application as an energy source. H2-driven biogas upgrading using homoacetogens is a promising approach for upgrading biogas to biomethane and converting CO2 to acetate simultaneously. Herein, we developed a novel membrane biofilm reactor (MBfR) with H2 and biogas separately supplied via bubbleless hollow fiber membranes. The gas-permeable hollow fibers of the MBfR enabled high H2 and CO2 utilization efficiencies (∼98% and ∼97%, respectively) and achieved concurrent biomethane (∼94%) and acetate (∼450 mg/L/d) production. High-throughput 16S rRNA gene amplicon sequencing suggested that enriched microbial communities were dominated by Acetobacterium (38-48% relative abundance). In addition, reverse transcription quantitative PCR of the functional marker gene formyltetrahydrofolate synthetase showed that its expression level increased with increasing H2 and CO2 utilization efficiencies. These results indicate that Acetobacterium plays a key role in CO2 to acetate conversion. These findings are expected to facilitate energy-positive wastewater treatment and contribute to the development of a new solution to biogas upgrading.

Biogenic Palladium Improved Perchlorate Reduction during Nitrate Co-Reduction by Diverting Electron Flow in a Hydrogenotrophic Biofilm

Microbial reduction of perchlorate (ClO4-) is emerging as a cost-effective strategy for groundwater remediation. However, the effectiveness of perchlorate reduction can be suppressed by the common co-contamination of nitrate (NO3-). We propose a means to overcome the limitation of ClO4- reduction: depositing palladium nanoparticles (Pd0NPs) within the matrix of a hydrogenotrophic biofilm. Two H2-based membrane biofilm reactors (MBfRs) were operated in parallel in long-term continuous and batch modes: one system had only a biofilm (bio-MBfR), while the other incorporated biogenic Pd0NPs in the biofilm matrix (bioPd-MBfR). For long-term co-reduction, bioPd-MBfR had a distinct advantage of oxyanion reduction fluxes, and it particularly alleviated the competitive advantage of NO3- reduction over ClO4- reduction. Batch tests also demonstrated that bioPd-MBfR gave more rapid reduction rates for ClO4- and ClO3- compared to those of bio-MBfR. Both biofilm communities were dominated by bacteria known to be perchlorate and nitrate reducers. Functional-gene abundances reflecting the intracellular electron flow from H2 to NADH to the reductases were supplanted by extracellular electron flow with the addition of Pd0NPs.

Impact of Nitrate on the Removal of Pollutants from Water in Reducing Gas-Based Membrane Biofilm Reactors: A Review

The membrane biofilm reactor (MBfR) is a novel wastewater treatment technology, garnering attention due to its high gas utilization rate and effective pollutant removal capability. This paper outlines the working mechanism, advantages, and disadvantages of MBfR, and the denitrification pathways, assessing the efficacy of MBfR in removing oxidized pollutants (sulfate (SO4-), perchlorate (ClO4-)), heavy metal ions (chromates (Cr(VI)), selenates (Se(VI))), and organic pollutants (tetracycline (TC), p-chloronitrobenzene (p-CNB)), and delves into the role of related microorganisms. Specifically, through the addition of nitrates (NO3-), this paper analyzes its impact on the removal efficiency of other pollutants and explores the changes in microbial communities. The results of the study show that NO3- inhibits the removal of other pollutants (oxidizing pollutants, heavy metal ions and organic pollutants), etc., in the simultaneous removal of multiple pollutants by MBfR.

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.

Thermodynamic Inhibition of Microbial Sulfur Disproportionation in a Multisubunit Designed Sulfur-Siderite Packed Bioreactor

Sulfur disproportionation (S0DP) poses a challenge to the robust application of sulfur autotrophic denitrification due to unpredictable sulfide production, which risks the safety of downstream ecosystems. This study explored the S0DP occurrence boundaries with nitrate loading and temperature effects. The boundary values increased with the increase in temperature, exhibiting below 0.15 and 0.53 kg-N/m3/d of nitrate loading at 20 and 30 °C, respectively. A pilot-scale sulfur-siderite packed bioreactor (150 m3/d treatment capacity) was optimally designed with multiple subunits to dynamically distribute the loading of sulfur-heterologous electron acceptors. Operating two active and one standby subunit achieved an effective denitrification rate of 0.31 kg-N/m3/d at 20 °C. For the standby subunit, involving oxygen by aeration effectively transformed the facultative S0DP functional community from S0DP metabolism to aerobic respiration, but with enormous sulfur consumption resulting in ongoing sulfate production of over 3000 mg/L. Meanwhile, acidification by the sulfur oxidation process could reduce the pH to as low as 2.5, which evaluated the Gibbs free energy (ΔG) of the S0DP reaction to +2.56 kJ, thermodynamically suppressing the S0DP occurrence. Therefore, a multisubunit design along with S0DP inhibition strategies of short-term aeration and long-term acidification is suggested for managing S0DP in various practical sulfur-packed bioreactors.

Co-metabolic biodegradation of chlorinated ethene in an oxygen- and ethane-based membrane biofilm reactor

Groundwater contamination by chlorinated ethenes is an urgent concern worldwide. One approach for detoxifying chlorinated ethenes is aerobic co-metabilims using ethane (C2H6) as the primary substrate. This study evaluated long-term continuous biodegradation of three chlorinated alkenes in a membrane biofilm reactor (MBfR) that delivered C2H6 and O2 via gas-transfer membranes. During 133 days of continuous operation, removals of dichloroethane (DCE), trichloroethene (TCE), and tetrachloroethene (PCE) were as high as 94 % and with effluent concentrations below 5 μM. In situ batch tests showed that the co-metabolic kinetics were faster with more chlorination. C2H6-oxidizing Comamonadaceae and "others," such as Methylococcaceae, oxidized C2H6 via monooxyenation reactions. The abundant non-ethane monooxygenases, particularly propane monooxygenase, appears to have been responsible for C2H6 aerobic metabolism and co-metabolism of chlorinated ethenes. This work proves that the C2H6 + O2 MBfR is a platform for ex-situ bioremediation of chlorinated ethenes, and the generalized action of the monooxygenases may make it applicable for other chlorinated organic contaminants.

Microbial nitrate reduction in propane- or butane-based membrane biofilm reactors under oxygen-limiting conditions

Nitrate contamination has been commonly detected in water environments and poses serious hazards to human health. Previously methane was proposed as a promising electron donor to remove nitrate from contaminated water. Compared with pure methane, natural gas, which not only contains methane but also other short chain gaseous alkanes (SCGAs), is less expensive and more widely available, representing a more attractive electron source for removing oxidized contaminants. However, it remains unknown if these SCGAs can be utilized as electron donors for nitrate reduction. Here, two lab-scale membrane biofilm reactors (MBfRs) separately supplied with propane and butane were operated under oxygen-limiting conditions to test its feasibility of microbial nitrate reduction. Long-term performance suggested nitrate could be continuously removed at a rate of ∼40-50 mg N/L/d using propane/butane as electron donors. In the absence of propane/butane, nitrate removal rates significantly decreased both in the long-term operation (∼2-10 and ∼4-9 mg N/L/d for propane- and butane-based MBfRs, respectively) and batch tests, indicating nitrate bio-reduction was driven by propane/butane. The consumption rates of nitrate and propane/butane dramatically decreased under anaerobic conditions, but recovered after resupplying limited oxygen, suggesting oxygen was an essential triggering factor for propane/butane-based nitrate reduction. High-throughput sequencing targeting 16S rRNA, bmoX and narG genes indicated Mycobacterium/Rhodococcus/Thauera were the potential microorganisms oxidizing propane/butane, while various denitrifiers (e.g. Dechloromonas, Denitratisoma, Zoogloea, Acidovorax, Variovorax, Pseudogulbenkiania and Rhodanobacter) might perform nitrate reduction in the biofilms. Our findings provide evidence to link SCGA oxidation with nitrate reduction under oxygen-limiting conditions and may ultimately facilitate the design of cost-effective techniques for ex-situ groundwater remediation using natural gas.

Generation, toxicity, and reduction of chlorinated byproducts: Overcome bottlenecks of electrochemical advanced oxidation technology to treat high chloride wastewater

Electrochemical advanced oxidation process (EAOP) is recommended for high-strength refractory organics wastewater treatment, but the accompanying chlorinated byproduct generation becomes a bottleneck that limits the application of this technology to actual wastewater. In this study, we applied EAOP (0.4-40 mA cm^-2) to treat ultrafiltration effluent of an actual landfill leachate, and quantitatively assessed the toxicities of the dominant chlorinated byproducts in EAOP-treated effluent. Considering both toxic effect and dose, it followed the order: active chlorine > chlorate > perchlorate > organochlorines. The toxic active chlorine could spontaneously decompose by settling. And secondary bioreactor originally serving for denitrification could be used to reduce perchlorate and chlorate. The effects of residual active chlorine and extra carbon addition on simultaneous denitrification, perchlorate, and chlorate reduction were investigated. It seemed that 20 mg of active chlorine was an acceptable level to bioactivity, and sufficient electron donors favored the removal of chlorate and perchlorate. Pseudomonas was identified as an active chlorine tolerant chlorate-reducing bacteria. And Thauera was responsible for perchlorate reduction under the conditions of sufficient carbon source supply. Our results confirmed that the perchlorate and chlorate concentrations in the effluent below their health advisory levels were achievable, solving the issue of toxic chlorinated byproduct generation during EAOP. This study provided a solution to realistic application of EAOP to treat high chloride wastewater.

Perchlorate reduction kinetics and genome-resolved metagenomics identify metabolic interactions in acclimated saline lake perchlorate-reducing consortia

Perchlorate is a widely detected environmental contaminant in surface and underground water, that seriously impacts human health by inhibiting the uptake of thyroidal radioiodine. Perchlorate reduction due to saline lake microorganisms is not as well understood as that in marine environments. In this study, we enriched a perchlorate-reducing microbial consortium collected from saline lake sediments and found that the perchlorate reduction kinetics of the enriched consortium fit the Michaelis-Menten kinetics well, with a maximum specific substrate reduction rate (q_max) of 0.596 ± 0.001 mg ClO₄^-/mg DW/h and half-saturation constant (K_s) of 16.549 ± 0.488 mg ClO₄^-/L. Furthermore, we used improved metagenome binning to reconstruct high-quality metagenome-assembled genomes from the metagenomes of the microbial consortia, including the perchlorate-reducing bacteria (PRB) Dechloromonas agitata and Wolinella succinogenes, with the genome of W. succinogenes harboring complete functional genes for perchlorate reduction being the first recovered. Given that the electrons were directly transferred to the electronic carrier cytochrome c-553 from the quinone pool, the electron transfer pathway of W. succinogenes was shorter and more efficient than the canonical pattern. This finding provides a theoretical basis for microbial remediation of sites contaminated by high concentrations of perchlorate. Metagenomic binning and metatranscriptomic analyses revealed the gene transcription variation of perchlorate reductase pcr and chlorite dismutase cld by PRB and the synergistic metabolic mechanism.

Polyhydroxyalkanoate-driven current generation via acetate by an anaerobic methanotrophic consortium

Anaerobic oxidation of methane (AOM) is an important microbial process mitigating methane (CH₄) emission from natural sediments. Anaerobic methanotrophic archaea (ANME) have been shown to mediate AOM coupled to the reduction of several compounds, either directly (i.e. nitrate, metal oxides) or in consortia with syntrophic bacterial partners (i.e. sulfate). However, the mechanisms underlying extracellular electron transfer (EET) between ANME and their bacterial partners or external electron acceptors are poorly understood. In this study, we investigated electron and carbon flow for an anaerobic methanotrophic consortium dominated by 'Candidatus Methanoperedens nitroreducens' in a CH₄-fed microbial electrolysis cell (MEC). Acetate was identified as a likely intermediate for the methanotrophic consortium, which stimulated the growth of the known electroactive genus Geobacter. Electrochemical characterization, stoichiometric calculations of the system, along with stable isotope-based assays, revealed that acetate was not produced from CH₄ directly. In the absence of CH₄, current was still generated and the microbial community remained largely unchanged. A substantial portion of the generated current in the absence of CH₄ was linked to the oxidation of the intracellular polyhydroxybutyrate (PHB) and the breakdown of extracellular polymeric substances (EPSs). The ability of 'Ca. M. nitroreducens' to use stored PHB as a carbon and energy source, and its ability to donate acetate as a diffusible electron carrier expands the known metabolic diversity of this lineage that likely underpins its success in natural systems.

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.

Catalytic Membrane Reactors: The Industrial Applications Perspective

Catalytic membrane reactors have been widely used in different production industries around the world. Applying a catalytic membrane reactor (CMR) reduces waste generation from a cleaner process perspective and reduces energy consumption in line with the process intensification strategy. A CMR combines a chemical or biochemical reaction with a membrane separation process in a single unit by improving the performance of the process in terms of conversion and selectivity. The core of the CMR is the membrane which can be polymeric or inorganic depending on the operating conditions of the catalytic process. Besides, the membrane can be inert or catalytically active. The number of studies devoted to applying CMR with higher membrane area per unit volume in multi-phase reactions remains very limited for both catalytic polymeric and inorganic membranes. The various bio-based catalytic membrane system is also used in a different commercial application. The opportunities and advantages offered by applying catalytic membrane reactors to multi-phase systems need to be further explored. In this review, the preparation and the application of inorganic membrane reactors in the different catalytic processes as water gas shift (WGS), Fisher Tropsch synthesis (FTS), selective CO oxidation (CO SeLox), and so on, have been discussed.