Dissimilatory perchlorate reduction: a review

The horizontal gene transfer of perchlorate reduction genomic island in three bacteria from an ecological niche

Three new strains of dissimilatory perchlorate-reducing bacteria (DPRB), QD19-16, QD1-5, and P3-1, were isolated from an active sludge. Phylogenetic trees based on 16S rRNA genes indicated that QD19-16, QD1-5, and P3-1 belonged to Brucella, Acidovorax, and Citrobacter, respectively, expanding the distribution of DPRB in the Proteobacteria. The three strains were gram-negative and facultative anaerobes with rod-shaped cells without flagella, which were 1.0-1.6 μm long and 0.5-0.6 μm wide. The three DPRB strains utilized similar broad spectrum of electron donors and acceptors and demonstrated a similar capability to reduce perchlorate within 6 days. The enzyme activity of perchlorate reductase in QD19-16 toward chlorate was higher than that toward perchlorate. The high sequence similarity of the perchlorate reductase operon and chlorite dismutase genes in the perchlorate reduction genomic islands (PRI) of the three strains implied that they were monophyletic origin from a common ancestral PRI. Two transposase genes (tnp1 and tnp2) were found in the PRIs of strain QD19-16 and QD1-5, but were absent in the strain P3-1 PRI. The presence of fragments of IR sequences in the P3-1 PRI suggested that P3-1 PRI had previously contained these two tnp genes. Therefore, it is plausible to suggest that a common ancestral PRI transferred across the strains Brucella sp. QD19-16, Acidovorax sp. QD1-5, and Citrobacter sp. P3-1 through horizontal gene transfer, facilitated by transposases. These results provided a direct evidence of horizontal gene transfer of PRI that could jump across phylogenetically unrelated bacteria through transposase. KEY POINTS: • Three new DPRB strains can effectively remove high concentration of perchlorate. • The PRIs of three DPRB strains are acquired from a single ancestral PRI. • PRIs are incorporated into different bacteria genome through HGT by transposase.

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.

The effect of reflux ratio on sulfur disproportionation tendency in anaerobic baffled reactor with the heterotrophic combining sulfur autotrophic processes under high concentration perchlorate stress

In a laboratory scale, an anaerobic baffled reactor (ABR) consisting of eight compartments, the heterotrophic combining sulfur autotrophic processes under different reflux ratios were constructed to achieve effective perchlorate removal and alleviate sulfur disproportionation reaction. Perchlorate was efficiently removed with effluent perchlorate concentration below 0.5 μg/L when the influent perchlorate concentration was 1030 mg/L during stages I ~ V, indicating that heterotrophic combining sulfur autotrophic perchlorate reduction processes can effectively achieve high concentration perchlorate removal. Furthermore, the 100% reflux ratio could reduce the contact time between sulfur particles and water; thus, the sulfur disproportionation reaction was inhibited. However, the inhibition effect of reflux on sulfur disproportionation was attenuated due to dilute perchlorate concentration when a reflux ratio of 150% and 200% was implemented. Meanwhile, the content of extracellular polymeric substances (EPS) in the heterotrophic unit (36.79 ~ 45.71 mg/g VSS) was higher than that in the sulfur autotrophic unit (22.19 ~ 25.77 mg/g VSS), indicating that high concentration perchlorate stress in the heterotrophic unit promoted EPS secretion. Thereinto, the PN content of sulfur autotrophic unit decreased in stage III and stage V due to decreasing perchlorate concentration in the autotrophic unit. Meanwhile, the PS content increased with increasing reflux in the autotrophic unit, which was conducive to the formation of biofilm. Furthermore, the high-throughput sequencing result showed that Proteobacteria, Chloroflexi, Firmicutes, and Bacteroidetes were the dominant phyla and Longilinea, Diaphorobacter, Acinetobacter, and Nitrobacter were the dominant genus in ABR, which were associated with heterotrophic or autotrophic perchlorate reduction and beneficial for effective perchlorate removal. The study indicated that reflux was a reasonable strategy for alleviating sulfur disproportionation in heterotrophic combining sulfur autotrophic perchlorate removal processes.

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.

Effective removal of nitrate in water by continuous-flow electro-dialysis ion exchange membrane bioreactor (CF-EDIMB): Performance optimization and microbial analysis

The use of nitrogen fertilizer has been causing nitrate pollution in groundwater, and there is an urgent need for efficient approach to remove nitrate from groundwater. In our job, a novel continuous-flow electrodialysis ion exchange membrane bioreactor system (CF-EDIMB) was set up to remove nitrate (NO3-) from water for the first time. Nitrate removal was positively dependent on water chamber HRT and voltage; voltage had significant effect on the water chamber effluent pH; acetate utilization efficiency was closely correlated with acetate dosage. The optimal conditions forecasted through response surface method (RSM) were given as follows: water chamber HRT was 20 h, biological chamber HRT was 24 h, voltage was 6.65 V and acetate dosage was 454.99 mg/L, dedicating to nitrate removal of 81.90% (83.70% in prediction), water chamber effluent pH of 7.10 (7.00 in prediction) and acetate utilization efficiency of 92.87% (96.51% in prediction). Meanwhile, microorganisms are crucial for nitrate removal, and the microbial community was not sensitive to the variation of acetate dosage. The microbial analysis results indicated that when CF-EDIMB system was operated for 20 d, the sulfate-reducing bacteria Sediminibacterium appeared in the biological chamber, and the effluent sulfate concentration of biological chamber was decreased. During the whole operation, Thauera was the dominant genus. Denitrifying functional genes nirS presented a better expression than the gene narG, and there was no accumulation of nitrite.

Oxyanion Removal from Impaired Water by Donnan Dialysis Plug Flow Contactors

In the last twenty-five years, extensive work has been done on ion exchange membrane bioreactors (IEMB) combining Donnan dialysis and anaerobic reduction to remove trace oxyanions (e.g., perchlorate, nitrate, chlorate, arsenate) from contaminated water sources. Most studies used Donnan dialysis contactors with high recirculation rates on the feed side, so under continuous operation, the effective concentration on the feed side of the membrane is the same as the exit concentration (CSTR mode). We have built, characterized, and modelled a plug flow Donnan dialysis contactor (PFR) that maximizes concentration on the feed side and operated it on feed solutions spiked with perchlorate and nitrate ion using ACS and PCA-100 anion exchange membranes. At identical feed inlet concentrations with the ACS membrane, membrane area loading rates are three-fold greater, and fluxes are more than double in the PFR contactor than in the CSTR contactor. A model based on the nonlinear adsorption of perchlorate in ACS membrane correctly predicted the trace ion concentration as a function of space-time in experiments with ACS. For PCA membrane, a linear flux dependence on feed concentration correctly described trace ion feed concentration as a function of space-time. Anion permeability for PCA-100 was high enough that the overall mass transfer was affected by the film boundary layer resistance. These results provide a basis for efficiently scaling up Donnan dialysis contactors and incorporating them in full-scale IEMB setups.

A review of perchlorate contamination: Analysis and remediation strategies

Perchlorate has been categorised as a potential contaminant and researched for years, but there are still many unknowns regarding this anion's contamination impacts. In this review, the basic information about perchlorate is summarised and evaluated, including the physical-chemical properties, fate and transportation, toxicity, analysis, and remediation. Especially, recent advances on analysis and remediation are emphasised and evaluated, such as in-situ imaging analysis and on-site bio-remediation respectively. The high solubility and persistence of the perchlorate anion mean its contamination is different from others, particularly in terms of analysis and remediation that might lead to secondary contamination. The knowledge gaps are listed for future research.

Synergistic effect of electrotrophic perchlorate reducing microorganisms and chemically modified electrodes for enhancing bioelectrochemical perchlorate removal

Perchlorate has been described as an emerging pollutant that compromises water sources and human health. In this study, a new electrotrophic perchlorate reducing microorganism (EPRM) isolated from the Atacama Desert, Dechloromonas sp. CS-1, was evaluated for perchlorate removal in water in a bioelectrochemical reactor (BER) with a chemically modified electrode. BERs were operated for 17 days under batch mode conditions with an applied potential of -500 mV vs. Ag/AgCl. Surface analysis (i.e., SEM, XPS, FT-IR, RAMAN spectroscopy) on the modified electrode demonstrated heterogeneous transformation of the carbon fibers with the incorporation of nitrogen functional groups and the oxidation of the carbonaceous material. The BERs with the modified electrode and the presence of the EAM reached high cathodic efficiency (90.79 ± 9.157%) and removal rate (0.34 ± 0.007 mol m-3-day) compared with both control conditions. The observed catalytic enhancement of CS-1 was confirmed by a reduction in the charge transfer resistance obtained by electrochemical impedance spectroscopy (EIS). Finally, an electrochemical kinetic study revealed an eight-electron perchlorate bioreduction reaction at -638.33 ± 24.132 mV vs. Ag/AgCl. Therefore, our results show the synergistic effect of EPRM and chemically modified electrodes on perchlorate removal in a BER.

Anoxic granular activated sludge process for simultaneous removal of hazardous perchlorate and nitrate

An airtight, anoxic bubble-column sequencing batch reactor (SBR) was developed for the rapid cultivation of perchlorate (ClO₄^-) and nitrate (NO₃^-) reducing granular sludge (GS) in this study. Feast/famine conditions and shear force selection pressures in tandem with a short settling time (2-min) as a hydraulic section pressure resulted in the accelerated formation of anoxic granular activated sludge (AxGS). ClO₄^- and NO₃^- were efficiently (>99.9%) reduced over long-term (>500-d) steady-state operation. Specific NO₃^- reduction, ClO₄^- reduction, chloride production, and non-purgeable dissolved organic carbon (DOC) oxidation rates of 5.77 ± 0.54 mg NO₃^--N/g VSS·h, 8.13 ± 0^.74 mg ClO₄^-/g VSS·h, 2.40 ± 0.₄0 mg Cl^-/g VSS·h, and 16.0 ± 0.06 mg DOC/g VSS·h were recorded within the reactor under steady-state conditions, respectively. The AxGS biomass cultivated in this study exhibited faster specific ClO₄^- reduction, NO₃^- reduction, and DOC oxidation rates than flocculated biomass cultivated under similar conditions and AxGS biomass operated in an up-flow anaerobic sludge blank (UASB) bioreactor receiving the same influent loading. EPS peptide identification revealed a suite of extracellular catabolic enzymes. Dechloromonas species were present in high abundance throughout the entirety of this study. This is one of the initial studies on anoxic granulation to simultaneously treat hazardous chemicals and adds to the science of the granular activated sludge process.

Fermentation broth of food waste: A sustainable electron donor for perchlorate biodegradation

Microbial reduction has been considered an effective way to remove perchlorate (ClO₄^-), during which, additional electron donors and carbon sources are required. This work aims to study the potential of fermentation broth of food waste (FBFW) serving as an electron donor for ClO₄^- biodegradation, and further investigates the variance of the microbial community. The results showed that FBFW without anaerobic inoculum at 96 h (F-96) exhibited the highest ClO₄^- removal rate of 127.09 mg/L/d, attributed to higher acetate and lower ammonium contents in the F-96 system. In a 5 L continuous stirred-tank reactor (CSTR), with a 217.39 g/m³·d ClO₄^- loading rate, 100% removal efficiency of ClO₄^- was achieved, indicating that the application of FBFW in the CSTR showed satisfactory performance for ClO₄^- degradation. Moreover, the microbial community analysis revealed that Proteobacteria and Dechloromonas contributed positively to ClO₄^- degradation. Therefore, this study provided a novel approach for the recovery and utilization of food waste, by employing it as a cost-effective electron donor for ClO₄^- biodegradation.

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.

Electrotrophic perchlorate reduction by a psychrotolerant acidophile isolated from an acid rock drainage in Antarctica

A new extremophilic isolate (USS-CCA7) was obtained from an acidic environment (pH ∼ 3.2) in Antarctica phylogenetically related to Acidithiobacillus ferrivorans; its electrotrophic capacities were evaluated in a three-electrode electrochemical cell. Cyclic voltammetry showed cathodic peaks of -428 mV, -536 mV, and -634 mV (vs. Ag/AgCl; pH = 1.7; 3 M KCl) for nitrate, oxygen, and perchlorate, respectively. The catalytic role of this microorganism was also observed by a decrease in the charge transfer resistance registered via electrochemical impedance spectroscopy. Five-day chronoamperometry of culture at pH = 1.7, USS-CCA7 showed a perchlorate removal rate of 19.106 ± 1.689 mgL^-1 day^-1 and a cathodic efficiency of 112 ± 5.2  %. Growth on electrodes was observed by epifluorescence and scanning electron microscopy. Interestingly, the results showed that toward higher pH, the cathodic peak of perchlorate is reduced in the voltammetric profiles. This study highlights the use of this psychrotolerant acidophile for the bioremediation of harsh perchlorate-pressured terrestrial under acidic conditions.

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.

Biological Reduction of Perchlorate from Spent Modified Reed Mediated by an Enriched Microbial Culture

Disposal of untreated perchlorate-laden modified reed (MR) generated in the treatment of contaminated water risks further contamination issues. The primary aim of this study was to assess the enriched microbial culture (EMC)-mediated biodegradation of perchlorate bound to spent quaternary amine-MR adsorbent. The culture was enriched to utilize perchlorate as an electron acceptor and brewer’s yeast as an electron donor. Kinetics experiments were performed to determine biodegradation rates and the total time for complete removal. The secondary aim of this research was the development and testing of a microwave-assisted extraction (MAE)-facilitated accurate perchlorate quantitation method. Extraction efficiency was optimized by adjusting the nitric acid concentration, resulting in 90 ± 5% recovery. Results show that EMC can degrade aqueous perchlorate effectively and consistently. EMC can also effectively degrade adsorbed perchlorate from spent MR. In conclusion, anaerobic biodegradation of perchlorate from spent MR can be used as an efficient, cost-effective, and environmentally friendly alternative or supplement to other treatment options, such as ion exchange and incineration. This approach can also be an effective solution to the waste generated by using MR to remove perchlorate from contaminated water.

Impacts of co-contaminants and dilution on perchlorate biodegradation using various carbon sources

This research investigates the biodegradation of perchlorate in the presence of the co-contaminants nitrate and chlorate using soluble and slow-release carbon sources. In addition, the impact of bio-augmentation and dilution, which results in lower total dissolved salts (TDS) and contaminant levels, is examined. Laboratory microcosms were conducted using actual groundwater and soils from a contaminated aquifer. The results revealed that both soluble and slow-release carbon sources support biodegradation of contaminants in the sequence nitrate > chlorate > perchlorate. Degradation rates, including and excluding lag times, revealed that the overall impact of the presence of co-contaminants depends on degradation kinetics and the relative concentrations of the contaminants. When the lag time caused by the presence of the co-contaminants is considered, the degradation rates for chlorate and perchlorate were two to three times slower. The results also show that dilution causes lower initial contaminant concentrations, and consequently, slower degradation rates, which is not desirable. On the other hand, the dilution resulting from the injection of amendments to support remediation promotes desirably lower salinity levels. However, the salinity associated with the presence of sulfate does not inhibit biodegradation. The naturally occurring bacteria were able to support the degradation of all contaminants. Bio-augmentation was effective only in diluted microcosms. Proteobacteria and Firmicutes were the dominant phyla identified in the microcosms.

Environmental occurrence, toxicity and remediation of perchlorate - A review

Perchlorate (ClO4-) comes under the class of contaminants called the emerging contaminants that will impact environment in the near future. A strong oxidizer by nature, perchlorate has received significant observation due to its occurrence, reactive nature, and persistence in varied environments such as surface water, groundwater, soil, and food. Perchlorate finds its use in number of industrial products ranging from missile fuel, fertilizers, and fireworks. Perchlorate exposure occurs when naturally occurring or manmade perchlorate in water or food is ingested. Perchlorate ingestion affects iodide absorption into the thyroid, thereby causing a decrease in the synthesis of thyroid hormone, a very crucial component needed for metabolism, neural development, and a number of other physiological functions in the body. Perchlorate remediation from ground water and drinking water is carried out through a series of physical-chemical techniques like ion (particle) transfer and reverse osmosis. However, the generation of waste through these processes are difficult to manage, so the need for alternative treatment methods occur. This review talks about the hybrid technologies that are currently researched and gaining momentum in the treatment of emerging contaminants, namely perchlorate.

Parapatric speciation of Meiothermus in serpentinite-hosted aquifers in Oman

The factors that control the distribution and evolution of microbial life in subsurface environments remain enigmatic due to challenges associated with sampling fluids from discrete depth intervals via boreholes while avoiding mixing of fluids. Here, using an inflatable packer system, fracture waters were isolated and collected from three discrete depth intervals spanning >130 m in a borehole intersecting an ultramafic rock formation undergoing serpentinization in the Samail Ophiolite, Sultanate of Oman. Near surface aquifer waters were moderately reducing and had alkaline pH while deeper aquifer waters were reduced and had hyperalkaline pH, indicating extensive influence by serpentinization. Metagenomic sequencing and analysis of DNA from filtered biomass collected from discrete depth intervals revealed an abundance of aerobes in near surface waters and a greater proportion of anaerobes at depth. Yet the abundance of the putatively obligate aerobe, Meiothermus, increased with depth, providing an opportunity to evaluate the influence of chemical and spatial variation on its distribution and speciation. Two clades of Meiothermus metagenome assembled genomes (MAGs) were identified that correspond to surface and deep populations termed Types I (S) and II (D), respectively; both clades comprised an apparently Oman-specific lineage indicating a common ancestor. Type II (D) clade MAGs encoded fewer genes and were undergoing slower genome replication as inferred from read mapping. Further, single nucleotide variants (SNVs) and mobile genetic elements identified among MAGs revealed detectable, albeit limited, evidence for gene flow/recombination between spatially segregated Type I (S) and Type II (D) populations. Together, these observations indicate that chemical variation generated by serpentinization, combined with physical barriers that reduce/limit dispersal and gene flow, allowed for the parapatric speciation of Meiothermus in the Samail Ophiolite or a geologic precursor. Further, Meiothermus genomic data suggest that deep and shallow aquifer fluids in the Samail Ophiolite may mix over shorter time scales than has been previously estimated from geochemical data.

Perchlorate removal by a combined heterotrophic and bio-electrochemical hydrogen autotrophic system

Here, a novel combined heterotrophic and bio-electrochemical hydrogen autotrophic (CHBHA) system was developed to remove perchlorate under low chemical dosages and energy consumption. The perchlorate removal performance at various hydraulic retention times (HRTs) and acetate dosages was investigated. For influent containing 10 ± 0.10 mg/L perchlorate, the optimal removal efficiency by the CHBHA system was 98.96 ± 1.62 %, 92.99 ± 2.99 %, 97.85 ± 0.41 %, and 98.24 ± 1.56 % at different operating stages. Perchlorate was mainly removed in the heterotrophic part (H-part) at a sufficient HRT (6 h) and acetate dosage (14.75 mg/L). At other stages, perchlorate was synergistically removed by the H-part and electrochemical hydrogen autotrophic part (E-part). Since the H-part removed some perchlorate, the E-part's applied current decreased, thus reducing energy costs. The maximum current efficiency of CHBHA system was 22.09 %. Compared with the single E-part system, the combined system used 65 % less energy. Perchlorate was converted into active chlorine in the E-part, which improved the effluent quality. The bacterial community structures of the two parts were significantly different. Comamonas, Dechloromonas, Acinetobacter, and Chryseobacterium were enriched in the H-part, and the dominant genera in the E-part were Thauera, Azonexus, Hydrogenophaga, and Tissierella.

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.

Impact of Acetate in Reduction of Perchlorate by Mixed Microbial Culture under the Influence of Nitrate and Sulfate

The biological reduction of slow degradation contaminants such as perchlorate (ClO4−) is considered to be a promising water treatment technology. The process is based on the ability of a specific mixed microbial culture to use perchlorate as an electron acceptor in the absence of oxygen. In this study, batch experiments were conducted to investigate the effect of nitrate on perchlorate reduction, the kinetic parameters of the Monod equation and the optimal ratio of acetate to perchlorate for the perchlorate reducing bacterial consortium. The results of this study suggest that acclimated microbial cultures can be applied to treat wastewater containing high concentrations of perchlorate. Reactor experiments were carried out with different hydraulic retention times (HRTs) to determine the optimal operating conditions. A fixed optimal HRT and the effect of nitrate on perchlorate reduction were investigated with various concentrations of the electron donor. The results showed that perchlorate reduction occurred after nitrate removal. Moreover, the presence of sulfate in wastewater had no effect on the perchlorate reduction. However, it had little effect on biomass concentration in the presence of nitrate during exposure to a mixed microbial culture, considering the nitrate as the inhibitor of perchlorate reduction by reducing the degradation rate. The batch scale experiment results illustrated that for efficient operation of perchlorate reduction, the optimal acetate to perchlorate ratio of 1.4:1.0 would be enough. Moreover, these experiments found the following results: the kinetic parameters equivalent to Y = 0.281 mg biomass/mg perchlorate, Ks = 37.619 mg/L and qmax = 0.042 mg perchlorate/mg biomass/h. In addition, anoxic–aerobic experimental reactor results verify the optimal HRT of 6 h for continuous application. Furthermore, it also illustrated that using 600 mg/L of acetate as a carbon source is responsible for 100% of nitrate reduction with less than 50% of the perchlorate reduction, whereas at 1000 mg/L acetate, approximately 100% reduction was recorded.

Perchlorate-reducing bacteria from Antarctic marine sediments

Perchlorate is a contaminant that can persist in groundwater and soil, and is frequently detected in different ecosystems at concentrations relevant to human health. This study isolated and characterised halotolerant bacteria that can potentially perform perchlorate reduction. Bacterial microorganisms were isolated from marine sediments on Deception, Horseshoe and Half Moon Islands of Antarctica. The results of the 16S ribosomal RNA (rRNA) gene sequence analysis indicated that the isolates were phylogenetically related to Psychrobacter cryohalolentis, Psychrobacter urativorans, Idiomarina loihiensis, Psychrobacter nivimaris, Sporosarcina aquimarina and Pseudomonas lactis. The isolates grew at a sodium chloride concentration of up to 30% and a perchlorate concentration of up to 10,000 mg/L, which showed their ability to survive in saline conditions and high perchlorate concentrations. Between 21.6 and 40% of perchlorate was degraded by the isolated bacteria. P. cryohalolentis and P. urativorans degraded 30.3% and 32.6% of perchlorate, respectively. I. loihiensis degraded 40% of perchlorate, and P. nivimaris, S. aquimarina and P. lactis degraded 22%, 21.8% and 21.6% of perchlorate, respectively. I. loihiensis had the highest reduction in perchlorate, whereas P. lactis had the lowest reduction. This study is significant as it is the first finding of P. cryohalolentis and. P. lactis on the Antarctic continent. In conclusion, these bacteria isolated from marine sediments on Antarctica offer promising resources for the bioremediation of perchlorate contamination due to their ability to degrade perchlorate, showing their potential use as a biological system to reduce perchlorate in high-salinity ecosystems.

Microbial Communities in Saltpan Sediments Show Tolerance to Mars Analog Conditions, but Susceptibility to Chloride and Perchlorate Toxicity

Brines at or near the surface of present-day Mars are a potential explanation for seasonally recurring dark streaks on the walls of craters, termed recurring slope lineae (RSL). Deliquescence and freezing point depression are possible drivers of brine stability, attributable to the high salinity observed in martian regolith including chlorides and perchlorates. Investigation of life, which may inhabit RSL, and the cellular mechanisms necessary for survival, must consider the tolerance of highly variable hydration, freeze-thaw cycles, and high osmolarity in addition to the anaerobic, oligotrophic, and irradiated environment. We propose the saltpan, an ephemeral, hypersaline wetland as an analogue for putative RSL hydrology. Saltpan sediment archaeal and bacterial communities showed tolerance of the Mars-analogous atmosphere, hydration, minerology, salinity, and temperature. Although active growth and a shift to well-adapted taxa were observed, susceptibility to low-concentration chloride and perchlorate addition suggested that such a composition was insufficient for beneficial water retention relative to added salt stress.

Formation of anaerobic granular sludge (AnGS) to treat high-strength perchlorate wastewater via anaerobic baffled reactor (ABR) system: Electron transfer characteristic, bacterial community and positive feedback mechanism

Anaerobic granular sludge (AnGS) was cultured to treat high-strength perchlorate (reaching to 4800 mg/L) wastewater by an anaerobic baffled reactor (ABR) system with five equal-volume compartments (C1-C5 compartments). Inoculated sludge completely granulated on day 104 with granule size of 0.50-0.75 mm and perchlorate removal efficiency reaching to 97% (influent perchlorate of 2000-4800 mg/L). The Cyclic voltammetry (CV) capacitance increased from 487.5, 465.8 and 407.8 μF to 576.5, 552.4, 549.6 μF in C1, C3 and C5 compartments of ABR system, respectively, suggesting the electron transfer capacity was enhanced under high-strength perchlorate stress. Meanwhile, adenosine triphosphate (ATP) value and electron transport system activity (ETSA) increased to 25.05, 22.87, 20.43 and 6.22, 4.87, 3.95 of C1, C3 and C5 compartments, respectively. The results suggested that high-strength perchlorate stress improved the microbial metabolic activity, which promoted secretion of extracellular polymeric substances (EPS). The more EPS could facilitate the formation and stability of AnGS under high-strength perchlorate stress. In addition, more reasonable metabolic division of labor in functional bacterial (Thauera and Comamonas) was beneficial to AnGS formation, which achieved high-strength perchlorate efficient removal. Finally, a positive feedback mechanism between AnGS formation and high-strength perchlorate removal was established through EPS, microbial metabolic activity and electron transfer characteristic in ABR system. However, excessive perchlorate (5800 mg/L) would exceed the treatment capacity of AnGS, which resulted in the deterioration of removal performance. This work provided an effective information for AnGS application to treat high-strength perchlorate wastewater.

Chlorate addition enhances perchlorate reduction in denitrifying membrane-biofilm reactors

Perchlorate is a widespread drinking water contaminant with regulatory standards ranging from 2 to 18 μg/L. The hydrogen-based membrane-biofilm reactor (MBfR) can effectively reduce perchlorate, but it is challenging to achieve low-µg/L levels. We explored chlorate addition to increase the abundance of perchlorate-reducing bacteria (PRB) and improve removals. MBfR reactors were operated with and without chlorate addition. Results show that chlorate doubled the abundance of putative PRB (e.g., Rhodocyclales) and improved perchlorate reduction to 23 ± 17 µg/L, compared to 53 ± 37 µg/L in the control. Sulfate reduction was substantially inhibited during chlorate addition, but quickly recovered once suspended. Our results suggest that chlorate addition can enhance perchlorate reduction by providing a selective pressure for PRB. It also decreases net sulfate reduction. KEY POINTS: • Chlorate increased the abundance of perchlorate-reducing bacteria • Chlorate addition improved perchlorate removal • Chlorate appeared to suppress sulfate reduction.

Presence of perchlorate in marine sediments from Antarctica during 2017-2020

Perchlorate of natural origin is a persistent pollutant that affects thyroid function by inhibiting iodine uptake, and this pollutant is frequently detected in different ecosystems at concentrations that can harm human health. In this study, we measured the perchlorate concentrations in 3,000 marine sediment samples from January to March in 2017, 2018, 2019, and 2020 during the 3^rd, 4^th, 5^th, and 6^th Colombian Scientific Expeditions to Antarctica. The sampling zones were located at 15 different points on the South Shetland Islands and Antarctic Peninsula, and they were measured using a selective perchlorate electrode. The concentration data indicate that perchlorate reached a minimum concentration of 90 ppm on Horseshoe Island and a maximum concentration of 465 ppm on Deception Island, suggesting a spatial variation in perchlorate concentrations that can be attributed to the natural formation of this pollutant due to volcanic eruptions. Additionally, homogeneous distribution of perchlorate was not observed in Antarctica.

Simultaneous reduction of perchlorate and nitrate using fast-settling anoxic sludge

Fast-settling, anoxic sludge (FAS) was cultivated and utilized in this study to simultaneously reduce elevated levels of perchlorate and nitrate in an anaerobic sequencing batch reactor (AnSBR). Average perchlorate and nitrate removal efficiencies of 96.5 ± 8.44 % and 99.8 ± 0.32 %, respectively, were achieved from an average perchlorate and nitrate loading rate of 159 ± 101 g ClO₄^-/m³·d and 10.8 ± 7.25 g NO₃^--N/m³·d, respectively, throughout long-term operation (>500-d). Batch activity tests revealed a preferential utilization of nitrate over perchlorate, where significant perchlorate reduction inhibition occurred when nitrate was present as a competing electron acceptor under carbon-limiting conditions. Specific perchlorate and nitrate reduction rates were shown to increase as the hydraulic retention time (HRT) of the AnSBR was step-wise decreased and subsequently the perchlorate and nitrate loading rates were step-wise increased. Functional, mRNA-based expression of the nitrite reductase (nirS and nirK), nitrous oxide reductase (nosZ), perchlorate reductase subunit A (pcrA), and the chlorite dismutase (cld) genes illustrated the simultaneous activity of heterotrophic denitrification and perchlorate reduction occurring throughout a complete standard reactor operational cycle, and allowed for expression trends to be documented as the HRT of the AnSBR was reduced from 5-d to 1.25-d. Nitrous oxide (N₂O) production was detected as a result of incomplete denitrification, where the largest N₂O production occurred at the highest nitrate loading rates investigated in this study. Thauera species were heavily enriched at a longer HRT of 5-d, but were out-competed by Dechloromonas species as the HRT of the AnSBR was step-wise reduced to 1.25-d.

Catalytic enhancement of hydrogenation reduction and oxygen transfer reaction for perchlorate removal: A review

Perchlorate is the main contaminant in surface water and groundwater, and it is of current urgency to remove due to its high water solubility, mobility, and endocrine-disrupting properties. The conversion of perchlorate into harmless chloride ions by using appropriate catalysts is the most promising and effective route to overcome its high activation energy and kinetic stability. Perchlorate is usually reduced in two ways: (1) indirect reduction via oxygen atom transfer (OAT) reaction or (2) hydrodeoxygenation through highly active reducing H atoms. This paper discusses the mechanisms underlying both the OAT reaction catalyzed by homogenous rhenium-oxo complexes or biological Mo-based enzymes and the heterogeneous hydrogenation for perchlorate reduction. Particular emphasis is placed on the factors affecting the catalytic process and the synergy between the (1) and (2) reactions. For completeness, the applicability of different electrolysis devices, electrodes, and bioreactors is also illustrated. Finally, this article gives prospects for the synthesis and application of catalysts in different pathways.

Using a sulfur autotrophic fluidized bed reactor for simultaneous perchlorate and nitrate removal from water: S disproportionation prediction and system optimization

The sulfur autotrophic reduction (SAR) process is promising in co-reduction of perchlorate and nitrate from aqueous solution. To further understand the reaction process, we developed a sulfur autotrophic fluidized bed reactor where the proceeding extent of sulfur (S) disproportionation was predicted by Response surface methodology (RSM) for the first time. Three fundamental reaction parameters including the hydraulic retention time (HRT), co-existing nitrate concentration ([Formula: see text]) and recirculation ratio (R) were considered for reactor optimization. The results demonstrated that S disproportionation was promoted by long HRT and high R, whereas was inhibited by high [Formula: see text]. Also, the optimal HRT, [Formula: see text] and R were 0.50 h, 10.00 mg/L and 14, respectively, the bioreactor can achieve high reduction efficiency of perchlorate and nitrate (> 98.45%), and generate less sulfate (236.07 mg/L). High-throughput sequencing showed that Chlorobaculum was related to S disproportionation, and Sulfurovum was associated with nitrate/perchlorate reducing. All results indicate that the sulfur autotrophic fluidized bed reactor is a promising candidate for the treatment of perchlorate and nitrate wastewater in future practical applications.

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.

Perchlorate - properties, toxicity and human health effects: an updated review

Interest in perchlorate as environmental pollutant has increased since 1997, when high concentrations have been found in the waters of the Colorado River, USA. Perchlorate is very persistent in nature and it is slowly degraded. Although harmful effects of large doses of perchlorate on thyroid function have been proven, the environmental effects are still unclear. The primary objective of the present review is to collect prevailing data of perchlorate exposure and to discuss its impact on human health. The results show that more than 50% of reviewed works found significant associations of perchlorate exposure and human health. This review consists of the following sections: general information of perchlorate sources, its properties and determination methods, role and sources in human body including food and water intake, overview of the scientific literature on the research on the effect of perchlorate on human health from 2010 to 2020. Finally, conclusions and recommendations on future perchlorate studies concerning human exposure are presented.

Transcriptional response to prolonged perchlorate exposure in the methanogen Methanosarcina barkeri and implications for Martian habitability

Observations of trace methane (CH₄) in the Martian atmosphere are significant to the astrobiology community given the overwhelming contribution of biological methanogenesis to atmospheric CH₄ on Earth. Previous studies have shown that methanogenic Archaea can generate CH₄ when incubated with perchlorates, highly oxidizing chaotropic salts which have been found across the Martian surface. However, the regulatory mechanisms behind this remain completely unexplored. In this study we performed comparative transcriptomics on the methanogen Methanosarcina barkeri, which was incubated at 30˚C and 0˚C with 10-20 mM calcium-, magnesium-, or sodium perchlorate. Consistent with prior studies, we observed decreased CH₄ production and apparent perchlorate reduction, with the latter process proceeding by heretofore essentially unknown mechanisms. Transcriptomic responses of M. barkeri to perchlorates include up-regulation of osmoprotectant transporters and selection against redox-sensitive amino acids. Increased expression of methylamine methanogenesis genes suggest competition for H₂ with perchlorate reduction, which we propose is catalyzed by up-regulated molybdenum-containing enzymes and maintained by siphoning diffused H₂ from energy-conserving hydrogenases. Methanogenesis regulatory patterns suggest Mars' freezing temperatures alone pose greater constraints to CH₄ production than perchlorates. These findings increase our understanding of methanogen survival in extreme environments and confers continued consideration of a potential biological contribution to Martian CH₄.

The effect of step-feeding distribution ratio on high concentration perchlorate removal performance in ABR system with heterotrophic combined sulfur autotrophic process

In a lab-scale anaerobic baffled reactor (ABR) with eight compartments, the heterotrophic and sulfur autotrophic processes were combined to remove perchlorate. And then, the step-feeding distribution ratio of the heterotrophic perchlorate reduction unit (HPR unit) was optimized to achieve efficient removal of high concentration perchlorate. Under the optimized step-feeding distribution ratio, the perchlorate removal efficiency reached to 99.8% with the influent concentration of 1300 mg/L, indicating that the removal performance of step-feeding was better than that of normal-feeding. A mass balance results showed that the perchlorate removal capacity of the C1-C5 compartments were 11.8 ± 0.6, 13.2 ± 0.2, 11.7 ± 1.0, 8.8 ± 0.2 and 9.8 ± 1.0 g/d during the stage VIII, indicating that the step-feeding can effectively relieve pollutant loading of C1 compartment and improve the perchlorate removal capacity of the C2-C5 compartments. Moreover, the high-throughput sequencing analysis showed that bacterial community was significant difference between the HPR and sulfur autotrophic perchlorate removal (SAPR) units. Principal component analysis (PCA) showed that perchlorate removal was more positive correlation with the forward compartments than the posterior compartments of HPR unit. The study confirms that the optimized step-feeding ratio is beneficial to remove the high concentration perchlorate via combining heterotrophic and sulfur autotrophic processes.

Inhibition of perchlorate biodegradation by ferric and ferrous iron

Previous observations from in-situ biological treatments in the subsurface of a perchlorate-contaminated site revealed multiple reduction processes occurring parallel to perchlorate degradation. Iron reduction was accelerated and correlated with a decline in the efficiency of the in-situ perchlorate reduction. In the current study, we examined the influence of iron forms on perchlorate reduction. A series of kinetic laboratory experiments were conducted, using an indigenous mixed perchlorate-reducing culture, enriched from the polluted soil that was undergoing bioremediation. The results show that ferrous iron was a non-competitive inhibitor with a 41% decrease in µ_max for perchlorate reduction. Moreover, chlorate was accumulated in all samples treated with ferrous iron, indicating a disruption to the chlorate reduction step. Ferric iron, however, had less impact on perchlorate degradation with non-competitive inhibition reaching a 23% decrease in µ_max. Scanning electron microscopy (SEM) revealed that the presence of ferrous iron in the perchlorate degradation enrichment culture initiated cell encrustation. We propose that during perchlorate reduction and the emission of oxygen from chlorite dismutation, the chemical oxidation of ferrous iron occurred near the bacteria's surface where the enzyme is located, forming an oxidized iron crust layer that can directly affect the perchlorate reduction enzymatic system.

Effect of Chlorination on Microbiological Quality of Effluent of a Full-Scale Wastewater Treatment Plant

The evaluation of effluent wastewater quality mainly relies on the assessment of conventional bacterial indicators, such as fecal coliforms and enterococci; however, little is known about opportunistic pathogens, which can resist chlorination and may be transmitted in aquatic environments. In contrast to conventional microbiological methods, high-throughput molecular techniques can provide an accurate evaluation of effluent quality, although a limited number of studies have been performed in this direction. In this work, high-throughput amplicon sequencing was employed to assess the effectiveness of chlorination as a disinfection method for secondary effluents. Common inhabitants of the intestinal tract, such as Bacteroides, Arcobacter and Clostridium, and activated sludge denitrifiers capable of forming biofilms, such as Acidovorax, Pseudomonas and Thauera, were identified in the chlorinated effluent. Chloroflexi with dechlorination capability and the bacteria involved in enhanced biological phosphorus removal, i.e., Candidatus Accumulibacter and Candidatus Competibacter, were also found to resist chlorination. No detection of Escherichia indicates the lack of fecal coliform contamination. Mycobacterium spp. were absent in the chlorinated effluent, whereas toxin-producing cyanobacteria of the genera Anabaena and Microcystis were identified in low abundances. Chlorination significantly affected the filamentous bacteria Nocardioides and Gordonia, whereas Zoogloea proliferated in the disinfected effluent. Moreover, perchlorate/chlorate- and organochlorine-reducing bacteria resisted chlorination.

A Multiplex Immunosensor for Detecting Perchlorate-Reducing Bacteria for Environmental Monitoring and Planetary Exploration

Perchlorate anions are produced by chemical industries and are important contaminants in certain natural ecosystems. Perchlorate also occurs in some natural and uncontaminated environments such as the Atacama Desert, the high Arctic or the Antarctic Dry Valleys, and is especially abundant on the surface of Mars. As some bacterial strains are capable of using perchlorate as an electron acceptor under anaerobic conditions, their detection is relevant for environmental monitoring on Earth as well as for the search for life on Mars. We have developed an antibody microarray with 20 polyclonal antibodies to detect perchlorate-reducing bacteria (PRB) strains and two crucial and highly conserved enzymes involved in perchlorate respiration: perchlorate reductase and chlorite dismutase. We determined the cross-reactivity, the working concentration, and the limit of detection of each antibody individually and in a multiplex format by Fluorescent Sandwich Microarray Immunoassay. Although most of them exhibited relatively high sensitivity and specificity, we applied a deconvolution method based on graph theory to discriminate between specific signals and cross-reactions from related microorganisms. We validated the system by analyzing multiple bacterial isolates, crude extracts from contaminated reactors and salt-rich natural samples from the high Arctic. The PRB detecting chip (PRBCHIP) allowed us to detect and classify environmental isolates as well as to detect similar strains by using crude extracts obtained from 0.5 g even from soils with low organic-matter levels (<10³ cells/g of soil). Our results demonstrated that PRBCHIP is a valuable tool for sensitive and reliable detection of perchlorate-reducing bacteria for research purposes, environmental monitoring and planetary exploration.

Perchlorate Contamination: Sources, Effects, and Technologies for Remediation

Perchlorate is a persistent pollutant, generated via natural and anthropogenic processes, that possesses a high potential for endocrine disruption in humans and biota. It inhibits iodine fixation, a major reason for eliminating this pollutant from ecosystems. Remediation of perchlorate can be achieved with various physicochemical treatments, especially at low concentrations. However, microbiological approaches using microorganisms, such as those from the genera Dechloromonas, Serratia, Propionivibrio, Wolinella, and Azospirillum, are promising when perchlorate pollution is extensive. Perchlorate-reducing bacteria, isolated from harsh environments, for example saline soils, mine sediments, thermal waters, wastewater treatment plants, underground gas storage facilities, and remote areas, including the Antarctica, can provide removal yields from 20 to 100%. Perchlorate reduction, carried out by a series of enzymes, such as perchlorate reductase and superoxide chlorite, depends on pH, temperature, salt concentration, metabolic inhibitors, nutritional conditions, time of contact, and cellular concentration. Microbial degradation is cost-effective, simple to implement, and environmentally friendly, rendering it a viable method for alleviating perchlorate pollution in the environment.

Carbon Source Competition in Biological Selenate Reduction under Other Oxyanions Contamination

Selenate removal in drinking water is being vigorously debated due to the various health issues concerned. As a viable treatment option, this study investigated a fixed-bed biofilm reactor (FBBR) with internal recycling. The experimental design tested how hydraulic loading rate and electron donor affect selenate reduction together with other oxyanions. The tested accompanying oxyanions were nitrate and perchlorate and experiments were designed to test how an FBBR responded to the limited electron donor condition. The results showed that the reactor achieved almost complete selenate reduction with the initial hydraulic loading rate of 12 m3/m2/day (influent concentration of 1416 µg SeO42−/L). Increasing the hydraulic loading rates to 16.24 and 48 m3/m2/day led to a gradual decline in selenate removal efficiency. A sufficient external carbon source (C:N of 3.3:1) achieved an almost complete reduction of nitrate as well as selenate. The FBBR acclimated to selenate instantaneously and reduced nitrate via synergistic denitrification. An experiment with another oxyanion addition, perchlorate (459 µg ClO4−/L), revealed that perchlorate-reducing bacteria were more strongly associated with carbon limitation than selenate-reducing bacteria, which can help us to understand parallel reactions in FBBRs. This research provides a framework to further study the use of electron donor-controlled FBBRs for simultaneous reduction of selenate and other oxyanions threatening the drinking water-related environment and public health.

Synergistic Zerovalent Iron (Fe0) and Microbiological Trichloroethene and Perchlorate Reductions Are Determined by the Concentration and Speciation of Fe

Trichloroethene (TCE) and perchlorate (ClO₄^-) are cocontaminants at multiple Superfund sites. Fe⁰ is often used during TCE bioremediation with Dehalococcoides mccartyi to establish anoxic conditions in the aquifer. However, the synergy between Fe⁰ abiotic reactions and microbiological TCE and ClO₄^- reductions is poorly understood and seldom addressed in the literature. Here, we investigated the effects of Fe⁰ and its oxidation product, Fe²⁺, at field-relevant concentrations in promoting microbial TCE and ClO₄^- reductions. Using semibatch microcosms with a Superfund site soil and groundwater, we showed that the high Fe⁰ concentration (16.5 g L^-1) expected during Fe⁰in situ injection mostly yielded TCE abiotic reduction to ethene/ethane. However, such concentrations obscured dechlorination by D. mccartyi, impeded ClO₄^- reduction, and enhanced SO₄^2- reduction and methanogenesis. Fe²⁺ at 0.25 g L^-1 substantially delayed conversion of TCE to ethene when compared to no-Fe controls. A low concentration of aged-Fe⁰ synergistically promoted microbiological TCE dechlorination to ethene while achieving complete ClO₄^- reduction. Collectively, these results illustrate scenarios relevant at or downstream of Fe⁰ injection zones when Fe⁰ is used to facilitate microbial dechlorination. Results also underscore the potential detrimental effects of Fe⁰ and bioaugmentation cultures coinjection for in situ treatment of chlorinated ethenes and ClO₄^-.

Perchlorate bioreduction by anaerobic granular sludge immobilised with Fe-HA complex: Performance, extracellular polymeric substances and microbial community structure

An iron-humic acid (Fe-HA) complex was used as a redox mediator in perchlorate (ClO₄^-) bioreduction. Bioreduction performance, extracellular polymeric substances (EPS), and microbial community structure were comprehensively explored using different types of anaerobic granular sludge (AnGS) immobilised without the Fe-HA complex (AnGS_CON) and with the Fe-HA complex (AnGS_FH). The ClO₄^- was completely removed by AnGS_CON by day 20, while the ClO₄^- was completely removed by AnGS_FH by day 6. The AnGS immobilised with the Fe-HA complex significantly increased the ClO₄^- bioreduction. The acceleration of ClO₄^- bioreduction was also explained by the mixed liquor volatile suspended solids (MLVSS), MLVSS/mixed liquor suspended solids (MLSS), EPS composition, and microbial community structure. Compared with AnGS_CON, the MLVSS and MLVSS/MLSS of the AnGS_FH increased 1.4- and 1.2-fold, respectively. Humic substances in the EPS were stimulated by the Fe-HA complex. The microbial community structure analysis indicated that perchlorate and quinone reducing bacteria were enriched by the Fe-HA complex. Based on the analysis, the ClO₄^- bioreduction mechanism of the AnGS_FH was revealed because the Fe-HA complex in the AnGS increased the biomass concentration, biological activity, and redox-active mediator and shifted the microbial community structure.

Aerobic microbial communities in the sediments of a marine oxygen minimum zone

The ecology of aerobic microorganisms is never explored in marine oxygen minimum zone (OMZ) sediments. Here we reveal aerobic bacterial communities along ∼3 m sediment-horizons of the eastern Arabian Sea OMZ. Sulfide-containing sediment-cores retrieved from 530 mbsl (meters beneath the sea-level) and 580 mbsl were explored at 15–30 cm intervals, using metagenomics, pure-culture-isolation, genomics and metatranscriptomics. Genes for aerobic respiration, and oxidation of methane/ammonia/alcohols/thiosulfate/sulfite/organosulfur-compounds, were detected in the metagenomes from all 25 sediment-samples explored. Most probable numbers for aerobic chemolithoautotrophs and chemoorganoheterotrophs at individual sample-sites were up to 1.1 × 10⁷ (g sediment)^-1. The sediment-sample collected from 275 cmbsf (centimeters beneath the seafloor) of the 530-mbsl-core yielded many such obligately aerobic isolates belonging to Cereibacter, Guyparkeria, Halomonas, Methylophaga, Pseudomonas and Sulfitobacter which died upon anaerobic incubation, despite being provided with all possible electron acceptors and fermentative substrates. High percentages of metatranscriptomic reads from the 275 cmbsf sediment-sample, and metagenomic reads from all 25 sediment-samples, matched the isolates’ genomic sequences including those for aerobic metabolisms, genetic/environmental information processing and cell division, thereby illustrating the bacteria's in-situ activity, and ubiquity across the sediment-horizons, respectively. The findings hold critical implications for organic carbon sequestration/remineralization, and inorganic compounds oxidation, within the sediment realm of global marine OMZs.

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.

Ultrafine Re/Pd nanoparticles on polydopamine modified carbon nanotubes for efficient perchlorate reduction and reusability

This study synthesized nanocomposite catalysts via a modification of Re/Pd codoped carbon nanotubes (CNTs) with different concentrations of polydopamine (PDA), which were used for perchlorate (ClO₄^-) reduction. The loads, dispersion and reducibility of Re/Pd nanoparticles increased yet their particle sizes significantly decreased with the increase of PDA concentrations. The average diameter of Re/Pd codoped D₂CNT (CNT modified by 2 mg/mL PDA) with a narrow size distribution was measured to be 2 nm. The ultrafine Re/Pd codoped D₂CNT catalysts represented outstanding catalytic reduction activity for the conversion of ClO₄^- to Cl^- with TOF of 17.34 h^-1 under the room H₂ atmospheric pressure, which was about 8 times than that of the unmodified catalysts. Furthermore, PDA modification minimized the dissociation of Re by chemical bonding between Re and CNTs carrier and maintained good stability of nanocomposite. This study inspires us to apply green bionic methods to enhance the catalytic reduction of perchlorate by changing the physical properties of Re/Pd nanoparticles.

Preferential Use of the Perchlorate over the Nitrate in the Respiratory Processes Mediated by the Bacterium Azospira sp. OGA 24

Here we report the results obtained for a strain isolated from a polluted site and classified as Azospira sp. OGA 24. The capability of OGA 24 to utilize perchlorate and nitrate and the regulation of pathways were investigated by growth kinetic studies and analysis of messenger RNA (mRNA) expression of the genes of perchlorate reductase alpha subunit (pcrA), chlorite dismutase (cld), and periplasmic nitrate reductase large subunit (napA). In aerobic conditions and in a minimal medium containing 10 mM acetate as carbon source, 5.6 ± 0.34 mmol L−1 perchlorate or 9.7 ± 0.22 mmol L−1 nitrate were efficiently reduced during the growth with 10 mM of either perchlorate or nitrate. In anaerobiosis, napA was completely inhibited in the presence of perchlorate as the only electron acceptor, pcrA was barely detectable in nitrate-reducing conditions. The cell growth kinetics were in accordance with expression data, indicating a separation of nitrate and perchlorate respiration pathways. In the presence of both compounds, anaerobic nitrate consumption was reduced to 50% (4.9 ± 0.4 vs. 9.8 ± 0.15 mmol L−1 without perchlorate), while that of perchlorate was not affected (7.2 ± 0.5 vs. 6.9 ± 0.6 mmol L−1 without nitrate). Expression analysis confirmed the negative effect of perchlorate on nitrate respiration. Based on sequence analysis of the considered genes and 16S ribosomal gene (rDNA), the taxonomic position of Azospira sp. OGA 24 in the perchlorate respiring bacteria (PRB) group was further defined by classifying it in the oryzae species. The respiratory characteristics of OGA 24 strain make it very attractive in terms of potential applications in the bioremediation of environments exposed to perchlorate salts.

Physicochemical Salt Solution Parameters Limit the Survival of Planococcus halocryophilus in Martian Cryobrines

Microorganisms living in sub-zero environments can benefit from the presence of dissolved salts, as they significantly increase the temperature range of liquid water by lowering the freezing point. However, high concentrations of salts can reduce microbial growth and survival, and can evoke a physiological stress response. It remains poorly understood how the physicochemical parameters of brines (e.g. water activity, ionic strength, solubility and hydration shell strength between the ions and the surrounding water molecules) influence the survival of microorganisms. We used the cryo- and halotolerant bacterial strain Planococcus halocryophilus as a model organism to evaluate the degree of stress different salts assert. Cells were incubated in liquid media at -15°C containing single salts at eutectic concentrations (CaCl₂, LiCl, LiI, MgBr₂, MgCl₂, NaBr, NaCl, NaClO₄ and NaI). Four of these salts (LiCl, LiI, MgBr₂ and NaClO₄) were also investigated at concentrations with a low water activity (0.635) and, separately, with a high ionic strength (8 mol/L). Water activity of all solutions was measured at -15°C. This is the first time that water activity has been measured for such a large number of liquid salt solutions at constant sub-zero temperatures (-15°C). Colony-Forming Unit (CFU) counts show that the survival of P. halocryophilus has a negative correlation with the salt concentration, molecular weight of the anion and anion radius; and a positive correlation with the water activity and anions' hydration shell strength. The survival of P. halocryophilus did not show a significant correlation with the ionic strength, the molecular weight of the cation, the hydrated and unhydrated cation and hydrated anion radius, and the cations' hydration bond length. Thus, the water activity, salt concentration and anion parameters play the largest role in the survival of P. halocryophilus in concentrated brines. These findings improve our understanding of the limitations of microbial life in saline environments, which provides a basis for better evaluation of the habitability of extraterrestrial environments such as Martian cryobrines.

A New Record for Microbial Perchlorate Tolerance: Fungal Growth in NaClO4 Brines and its Implications for Putative Life on Mars

The habitability of Mars is strongly dependent on the availability of liquid water, which is essential for life as we know it. One of the few places where liquid water might be found on Mars is in liquid perchlorate brines that could form via deliquescence. As these concentrated perchlorate salt solutions do not occur on Earth as natural environments, it is necessary to investigate in lab experiments the potential of these brines to serve as a microbial habitat. Here, we report on the sodium perchlorate (NaClO₄) tolerances for the halotolerant yeast Debaryomyces hansenii and the filamentous fungus Purpureocillium lilacinum. Microbial growth was determined visually, microscopically and via counting colony forming units (CFU). With the observed growth of D. hansenii in liquid growth medium containing 2.4 M NaClO₄, we found by far the highest microbial perchlorate tolerance reported to date, more than twice as high as the record reported prior (for the bacterium Planococcus halocryophilus). It is plausible to assume that putative Martian microbes could adapt to even higher perchlorate concentrations due to their long exposure to these environments occurring naturally on Mars, which also increases the likelihood of microbial life thriving in the Martian brines.

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.

Effect of alkaline treatment on pathogens, bacterial community and antibiotic resistance genes in different sewage sludges for potential agriculture use

Alkaline treatment is widely used to reduce pathogens in sewage sludge in developing countries and guarantee that it is safe for use in agriculture. The aim of this study was to investigate the effect of alkaline treatment applied to waste-activated (WAS) and Upflow Anaerobic Sludge Blanket (UASB)-sludge on the bacterial community, pathogens (viable helminths eggs and Salmonella spp), and antibiotic resistance genes (ARG). The bacterial community structure was examined through denaturing gel gradient electrophoresis (DGGE), targeting 16S rRNA genes. Polymerase chain reaction (PCR) was applied to evaluate the presence of several ARGs. The conducted alkaline experiment consisted of adding hydrated lime (Ca(OH)₂) to sewage sludges. Samples were taken before and after 2, 24, 48, and 72 hours of treatment. Alkaline treatment changed considerably the bacterial community structure and after 24 hours, shifts in bacterial profiles were more pronounced in the UASB sludge sample than in WAS. Some bacteria remained under extreme pH conditions (pH > 12), such as Azospira oryzae and Dechloromonas denitrificans in the WAS samples, and Geothrix and Geobacter in the UASB sludge samples. The values of pathogens and indicators in the sludge after 24 hours of alkaline treatment meet sanitary law regulations and thus the sludges could have the potential to agricultural distribution. It is important to highlight that ARG, which are not currently present in sanitary regulations, were detected in the sludge samples after the alkaline treatment, which could be a concern for human health.