Dissimilatory perchlorate reduction: a review

Heterotrophic-autotrophic sequential system for reductive nitrate and perchlorate removal

Nitrate and perchlorate were identified as significant water contaminants all over the world. This study aims at evaluating the performances of the heterotrophic-autotrophic sequential denitrification process for reductive nitrate and perchlorate removal from drinking water. The reduced nitrate concentration in the heterotrophic reactor increased with increasing methanol concentrations and the remaining nitrate/nitrite was further removed in the following autotrophic denitrifying process. The performances of the sequential process were studied under varying nitrate loads of [Formula: see text] at a fixed hydraulic retention time of 2 h. The C/N ratio in the heterotrophic reactor varied between 1.24 and 2.77 throughout the study. Nitrate and perchlorate reduced completely with maximum initial concentrations of [Formula: see text] and 1000 µg/L, respectively. The maximum denitrification rate for the heterotrophic reactor was [Formula: see text] when the bioreactor was fed with [Formula: see text] and 277 mg/L methanol. For the autotrophic reactor, the highest denitrification rate was [Formula: see text] in the first period when the heterotrophic reactor performance was low. Perchlorate reduction was initiated in the heterotrophic reactor, but completed in the following autotrophic process. Effluent sulphate concentration was below the drinking water standard level of 250 mg/L and pH was in the neutral level.

Perchlorate in Lake Water from an Operating Diamond Mine

Mining-related perchlorate [ClO4(-)] in the receiving environment was investigated at the operating open-pit and underground Diavik diamond mine, Northwest Territories, Canada. Samples were collected over four years and ClO4(-) was measured in various mine waters, the 560 km(2) ultraoligotrophic receiving lake, background lake water and snow distal from the mine. Groundwaters from the underground mine had variable ClO4(-) concentrations, up to 157 μg L(-1), and were typically an order of magnitude higher than concentrations in combined mine waters prior to treatment and discharge to the lake. Snow core samples had a mean ClO4(-) concentration of 0.021 μg L(-1) (n=16). Snow and lake water Cl(-)/ClO4(-) ratios suggest evapoconcentration was not an important process affecting lake ClO4(-) concentrations. The multiyear mean ClO4(-) concentrations in the lake were 0.30 μg L(-1) (n = 114) in open water and 0.24 μg L(-1) (n = 107) under ice, much below the Canadian drinking water guideline of 6 μg L(-1). Receiving lake concentrations of ClO4(-) generally decreased year over year and ClO4(-) was not likely [biogeo]chemically attenuated within the receiving lake. The discharge of treated mine water was shown to contribute mining-related ClO4(-) to the lake and the low concentrations after 12 years of mining were attributed to the large volume of the receiving lake.

Electron donors and co-contaminants affect microbial community composition and activity in perchlorate degradation

Although microbial reduction of perchlorate (ClO4(-)) is a promising and effective method, our knowledge on the changes in microbial communities during ClO4(-) degradation is limited, especially when different electron donors are supplied and/or other contaminants are present. Here, we examined the effects of acetate and hydrogen as electron donors and nitrate and ammonium as co-contaminants on ClO4(-) degradation by anaerobic microcosms using six treatments. The process of degradation was divided into the lag stage (SI) and the accelerated stage (SII). Quantitative PCR was used to quantify four genes: pcrA (encoding perchlorate reductase), cld (encoding chlorite dismutase), nirS (encoding copper and cytochrome cd1 nitrite reductase), and 16S rRNA. While the degradation of ClO4(-) with acetate, nitrate, and ammonia system (PNA) was the fastest with the highest abundance of the four genes, it was the slowest in the autotrophic system (HYP). The pcrA gene accumulated in SI and played a key role in initiating the accelerated degradation of ClO4(-) when its abundance reached a peak. Degradation in SII was primarily maintained by the cld gene. Acetate inhibited the growth of perchlorate-reducing bacteria (PRB), but its effect was weakened by nitrate (NO3(-)), which promoted the growth of PRB in SI, and therefore, accelerated the ClO4(-) degradation rate. In addition, ammonia (NH4(+)), as nitrogen sources, accelerated the growth of PRB. The bacterial communities' structure and diversity were significantly affected by electron donors and co-contaminants. Under heterotrophic conditions, both ammonia and nitrate promoted Azospira as the most dominant genera, a fact that might significantly influence the rate of ClO4(-) natural attenuation by degradation.

Study of microbial perchlorate reduction: considering of multiple pH, electron acceptors and donors

Bioremediation of perchlorate-cotaminated water by a heterotrophic perchlorate reducing bacterium creates a multiple electron acceptor-donor system. We experimentally determined the perchlorate reduction by Azospira sp. KJ at multiple pH, electron acceptors and donors systems; this was the aim of this study. Perchlorate reduction was drastically inhibited at the pH 6.0, and the maximum reduction of perchlorate by Azospira sp. KJ was observed at pH value of 8.0. Perchlorate reduction was retarded in ClO4(-)-ClO3(-), ClO4(-)-ClO3(-)-NO3(-),and ClO4(-)-NO3(-) acceptor systems, while being completely inhibited by the additional O2 in the ClO4(-)-O2 acceptor system. The reduction proceeded as an order of ClO3(-), ClO4(-), and NO3(-) in the ClO4(-)-ClO3(-)-NO3(-) system. K(S), v(max), and q(max) obtained at different e(-) acceptor and donor conditions are calculated as 140.5-190.6 mg/L, 8.7-13.2 mg-perchlorate/L-h, and 0.094-0.16 mg-perchlorate/mg-DW-h, respectively.

Mechanism of chlorite degradation to chloride and dioxygen by the enzyme chlorite dismutase

Heme b containing chlorite dismutase (Cld) catalyses the conversion of chlorite to chloride and dioxygen which includes an unusual OO bond formation. This review summarizes our knowledge about the interaction of chlorite with heme enzymes and introduces the biological role, phylogeny and structure of functional chlorite dismutases with differences in overall structure and subunit architecture. The paper sums up the available experimental and computational studies on chlorite degradation by water soluble porphyrin complexes as well as a model based on the active site of Cld. Finally, it reports the available biochemical and biophysical data of Clds from different organisms which allow the presentation of a general reaction mechanism. It includes binding of chlorite to ferric Cld followed by subsequent heterolytic OCl bond cleavage leading to the formation of Compound I and hypochlorite, which finally recombine for production of chloride and O2. The role of the Cld-typical distal arginine in catalysis is discussed together with the pH dependence of the reaction and the role of transiently produced hypochlorite in irreversible inactivation of the enzyme.

Chlorate reductase is cotranscribed with cytochrome c and other downstream genes in the gene cluster for chlorate respiration of Ideonella dechloratans

The chlorate-respiring bacterium Ideonella dechloratans is a facultative anaerobe that can use both oxygen and chlorate as terminal electron acceptors. The genes for the enzymes chlorate reductase (clrABDC) and chlorite dismutase, necessary for chlorate metabolism and probably acquired by lateral gene transfer, are located in a gene cluster that also includes other genes potentially important for chlorate metabolism. Among those are a gene for cytochrome c (cyc) whose gene product may serve as an electron carrier during chlorate reduction, a cofactor biosynthesis gene (mobB) and a predicted transcriptional regulator (arsR). Only chlorate reductase and chlorite dismutase have been shown to be expressed in vivo. Here, we report the in vivo production of a single polycistronic transcript covering eight open reading frames including clrABDC, cyc, mobB and arsR. Transcription levels of the cyc and clrA genes were compared to each other by the use of qRT-PCR in RNA preparations from cells grown under aerobic or chlorate reducing anaerobic conditions. The two genes showed the same mRNA levels under both growth regimes, indicating that no transcription termination occurs between them. Higher transcription levels were observed at growth without external oxygen supply. Implications for electron pathway integration following lateral gene transfer are discussed.

Phenotypic and genotypic description of Sedimenticola selenatireducens strain CUZ, a marine (per)chlorate-respiring gammaproteobacterium, and its close relative the chlorate-respiring Sedimenticola strain NSS

Two (per)chlorate-reducing bacteria, strains CUZ and NSS, were isolated from marine sediments in Berkeley and San Diego, CA, respectively. Strain CUZ respired both perchlorate and chlorate [collectively designated (per)chlorate], while strain NSS respired only chlorate. Phylogenetic analysis classified both strains as close relatives of the gammaproteobacterium Sedimenticola selenatireducens. Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) preparations showed the presence of rod-shaped, motile cells containing one polar flagellum. Optimum growth for strain CUZ was observed at 25 to 30 °C, pH 7, and 4% NaCl, while strain NSS grew optimally at 37 to 42 °C, pH 7.5 to 8, and 1.5 to 2.5% NaCl. Both strains oxidized hydrogen, sulfide, various organic acids, and aromatics, such as benzoate and phenylacetate, as electron donors coupled to oxygen, nitrate, and (per)chlorate or chlorate as electron acceptors. The draft genome of strain CUZ carried the requisite (per)chlorate reduction island (PRI) for (per)chlorate respiration, while that of strain NSS carried the composite chlorate reduction transposon responsible for chlorate metabolism. The PRI of strain CUZ encoded a perchlorate reductase (Pcr), which reduced both perchlorate and chlorate, while the genome of strain NSS included a gene for a distinct chlorate reductase (Clr) that reduced only chlorate. When both (per)chlorate and nitrate were present, (per)chlorate was preferentially utilized if the inoculum was pregrown on (per)chlorate. Historically, (per)chlorate-reducing bacteria (PRB) and chlorate-reducing bacteria (CRB) have been isolated primarily from freshwater, mesophilic environments. This study describes the isolation and characterization of two highly related marine halophiles, one a PRB and the other a CRB, and thus broadens the known phylogenetic and physiological diversity of these unusual metabolisms.

Characterisation of chlorate reduction in the haloarchaeon Haloferax mediterranei

BACKGROUND

Haloferax mediterranei is a denitrifying haloarchaeon using nitrate as a respiratory electron acceptor under anaerobic conditions in a reaction catalysed by pNarGH. Other ions such as bromate, perchlorate and chlorate can also be reduced.

METHODS

Hfx. mediterranei cells were grown anaerobically with nitrate as electron acceptor and chlorate reductase activity measured in whole cells and purified nitrate reductase.

RESULTS

No genes encoding (per)chlorate reductases have been detected either in the Hfx. mediterranei genome or in other haloarchaea. However, a gene encoding a chlorite dismutase that is predicted to be exported across the cytoplasmic membrane has been identified in Hfx. mediterranei genome. Cells did not grow anaerobically in presence of chlorate as the unique electron acceptor. However, cells anaerobically grown with nitrate and then transferred to chlorate-containing growth medium can grow a few generations. Chlorate reduction by the whole cells, as well as by pure pNarGH, has been characterised. No clear chlorite dismutase activity could be detected.

CONCLUSIONS

Hfx. mediterranei pNarGH has its active site on the outer-face of the cytoplasmic membrane and reacts with chlorate and perchlorate. Biochemical characterisation of this enzymatic activity suggests that Hfx. mediterranei or its pure pNarGH could be of great interest for waste water treatments or to better understand biological chlorate reduction in early Earth or Martian environments.

GENERAL SIGNIFICANCE

Some archaea species reduce (per)chlorate. However, results here presented as well as those recently reported by Liebensteiner and co-workers [1] suggest that complete perchlorate reduction in archaea follows different rules in terms of biological reactions.

Removal of perchlorate from aqueous solution by cross-linked Fe(III)-chitosan complex

Cross-linked Fe(III)-chitosan composite (Fe-CB) was used as the adsorbent for removing perchlorate from the aqueous solution. The adsorption experiments were carried out by varying contact time, initial concentrations, temperatures, pH, and the presence of co-existing anions. The morphology of the adsorbent was discussed using FT-IR and SEM with X-EDS analysis. The pH ranging from 3.0-10.2 exhibited very little effect on the adsorption capability. The perchlorate uptake onto Fe-CB obeyed Langmuir isotherm model. The adsorption process was rapid and the kinetics data obeyed the pseudo second-order model well. The eluent of 2.5% (W/V) NaCl could regenerate the exhausted adsorbent efficiently. The adsorption mechanism was also discussed.

Chlorite dismutases - a heme enzyme family for use in bioremediation and generation of molecular oxygen

Chlorite is a serious environmental concern, as rising concentrations of this harmful anthropogenic compound have been detected in groundwater, drinking water, and soil. Chlorite dismutases (Clds) are therefore important molecules in bioremediation as Clds catalyze the degradation of chlorite to chloride and molecular oxygen. Clds are heme b-containing oxidoreductases present in numerous bacterial and archaeal phyla. This review presents the phylogeny of functional Clds and Cld-like proteins, and demonstrates the close relationship of this novel enzyme family to the recently discovered dye-decolorizing peroxidases. The available X-ray structures, biophysical and enzymatic properties, as well as a proposed reaction mechanism, are presented and critically discussed. Open questions about structure-function relationships are addressed, including the nature of the catalytically relevant redox and reaction intermediates and the mechanism of inactivation of Clds during turnover. Based on analysis of currently available data, chlorite dismutase from "Candidatus Nitrospira defluvii" is suggested as a model Cld for future application in biotechnology and bioremediation. Additionally, Clds can be used in various applications as local generators of molecular oxygen, a reactivity already exploited by microbes that must perform aerobic metabolic pathways in the absence of molecular oxygen. For biotechnologists in the field of chemical engineering and bioremediation, this review provides the biochemical and biophysical background of the Cld enzyme family as well as critically assesses Cld's technological potential.

Physiological and genetic description of dissimilatory perchlorate reduction by the novel marine bacterium Arcobacter sp. strain CAB

A novel dissimilatory perchlorate-reducing bacterium (DPRB), Arcobacter sp. strain CAB, was isolated from a marina in Berkeley, CA. Phylogenetically, this halophile was most closely related to Arcobacter defluvii strain SW30-2 and Arcobacter ellisii. With acetate as the electron donor, strain CAB completely reduced perchlorate (ClO₄⁻) or chlorate (ClO₃⁻) [collectively designated (per)chlorate] to innocuous chloride (Cl⁻), likely using the perchlorate reductase (Pcr) and chlorite dismutase (Cld) enzymes. When grown with perchlorate, optimum growth was observed at 25 to 30°C, pH 7, and 3% NaCl. Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) preparations were dominated by free-swimming straight rods with 1 to 2 polar flagella per cell. Strain CAB utilized a variety of organic acids, fructose, and hydrogen as electron donors coupled to (per)chlorate reduction. Further, under anoxic growth conditions strain CAB utilized the biogenic oxygen produced as a result of chlorite dismutation to oxidize catechol via the meta-cleavage pathway of aerobic catechol degradation and the catechol 2,3-dioxygenase enzyme. In addition to (per)chlorate, oxygen and nitrate were alternatively used as electron acceptors. The 3.48-Mb draft genome encoded a distinct perchlorate reduction island (PRI) containing several transposases. The genome lacks the pcrC gene, which was previously thought to be essential for (per)chlorate reduction, and appears to use an unrelated Arcobacter c-type cytochrome to perform the same function.

The study of dissimilatory perchlorate-reducing bacteria (DPRB) has largely focused on freshwater, mesophilic, neutral-pH environments. This study identifies a novel marine DPRB in the genus Arcobacter that represents the first description of a DPRB associated with the Campylobacteraceae. Strain CAB is currently the only epsilonproteobacterial DPRB in pure culture. The genome of strain CAB lacks the pcrC gene found in all other DPRB tested, demonstrating a new variation on the (per)chlorate reduction pathway. The ability of strain CAB to oxidize catechol via the oxygenase-dependent meta-cleavage pathway in the absence of external oxygen by using the biogenic oxygen produced from the dismutation of chlorite provides a valuable model for understanding the anaerobic degradation of a broad diversity of xenobiotics which are recalcitrant to anaerobic metabolism but labile to oxygenase-dependent mechanisms.

Perchlorate reduction by an isolated Serratia marcescens strain under high salt and extreme pH

An isolated Serratia marcescens strain exhibited growth-coupled perchlorate (ClO4 -) reduction under anoxic conditions. Perchlorate was reduced completely with stoichiometric chloride buildup and equimolar acetate consumption. Polymerase chain reaction confirmed the presence of pcrA and cld genes coding for key enzymes involved in the ClO4 - degradation pathway. The isolate degraded ClO4 - under high salt (up to 15% NaCl) and a wide range of pH (4.0-9.0), as well as simultaneously reduced nitrate and ClO4 -.

Metal-induced decomposition of perchlorate in pressurized hot water

Decomposition of perchlorate (ClO(4)(-)) in pressurized hot water (PHW) was investigated. Although ClO(4)(-) demonstrated little reactivity in pure PHW up to 300°C, addition of zerovalent metals to the reaction system enhanced the decomposition of ClO(4)(-) to Cl(-) with an increasing order of activity of (no metal)≈Al < Cu < Zn < Ni << Fe: the addition of iron powder led to the most efficient decomposition of ClO(4)(-). When the iron powder was added to an aqueous ClO(4)(-) solution (104 μM) and the mixture was heated at 150°C, ClO(4)(-) concentration fell below 0.58 μM (58 μg L(-1), detection limit of ion chromatography) in 1 h, and Cl(-) was formed with the yield of 85% after 6 h. The decomposition was accompanied by transformation of the zerovalent iron to Fe(3)O(4). This method was successfully used in the decomposition of ClO(4)(-) in a water sample contaminated with this compound, following fireworks display at Albany, New York, USA.

Kinetics of nitrate and perchlorate removal and biofilm stratification in an ion exchange membrane bioreactor

The biological degradation of nitrate and perchlorate was investigated in an ion exchange membrane bioreactor (IEMB) using a mixed anoxic microbial culture and ethanol as the carbon source. In this process, a membrane-supported biofilm reduces nitrate and perchlorate delivered through an anion exchange membrane from a polluted water stream, containing 60 mg/L of NO₃⁻ and 100 μg/L of ClO₄⁻. Under ammonia limiting conditions, the perchlorate reduction rate decreased by 10%, whereas the nitrate reduction rate was unaffected. Though nitrate and perchlorate accumulated in the bioreactor, their concentrations in the treated water (2.8 ± 0.5 mg/L of NO₃⁻ and 7.0 ± 0.8 μg/L of ClO₄⁻, respectively) were always below the drinking water regulatory levels, due to Donnan dialysis control of the ionic transport in the system. Kinetic parameters determined for the mixed microbial culture in suspension showed that the nitrate reduction rate was 35 times higher than the maximum perchlorate reduction rate. It was found that perchlorate reduction was inhibited by nitrate, since after nitrate depletion perchlorate reduction rate increased by 77%. The biofilm developed in the IEMB was cryosectioned and the microbial population was analyzed by fluorescence in situ hybridization (FISH). The results obtained seem to indicate that the kinetic advantage of nitrate reduction favored accumulation of denitrifiers near the membrane, whereas per(chlorate) reducing bacteria were mainly positioned at the biofilm outer surface, contacting the biomedium. As a consequence of the biofilm stratification, the reduction of perchlorate and nitrate occur sequentially in space allowing for the removal of both ions in the IEMB.

Microbial metabolism of oxochlorates: a bioenergetic perspective

The microbial metabolism of oxochlorates is part of the biogeochemical cycle of chlorine. Organisms capable of growth using perchlorate or chlorate as respiratory electron acceptors are also interesting for applications in biotreatment of oxochlorate-containing effluents or bioremediation of contaminated areas. In this review, we discuss the reactions of oxochlorate respiration, the corresponding enzymes, and the relation to respiratory electron transport that can contribute to a proton gradient across the cell membrane. Enzymes specific for oxochlorate respiration are oxochlorate reductases and chlorite dismutase. The former belong to DMSO reductase family of molybdenum-containing enzymes. The heme protein chlorite dismutase, which decomposes chlorite into chloride and molecular oxygen, is only distantly related to other proteins with known functions. Pathways for electron transport may be different in perchlorate and chlorate reducers, but appear in both cases to be similar to pathways found in other respiratory systems. This article is part of a Special Issue entitled: Evolutionary aspects bioenergetic systems.

Chlorate reduction capacity and characterisation of chlorate reducing bacteria communities in sediments of the rio Cruces wetland in southern Chile

This study investigated chlorate reduction kinetics in multiple samples of sediments from a longitudinal profile of a wetland located downstream of the effluent discharge of a cellulose plant, including characterisation of the bacterial communities involved. The sediments were exposed to different initial chlorate concentrations in microcosm tests, with and without the addition of acetate as an external electron donor, and in a matrix of natural water or a defined medium. At a high initial chlorate concentration of 100 mg/L, in the absence of an external electron source, the degradation curves presented first-order kinetics, influenced by electron donor availability. The first-order kinetic constant varied between 0.05 and 0.17 day(-1). Subsequently, when the initial chlorate concentration was reduced to 7 mg/L, a zero-order kinetic was obtained, with the kinetic constant presenting values between 1.1 and 1.3 mg/L-day. No correlation was observed between chlorate degradation kinetics and the location of the sampling points or the previous history of exposure to chlorate. Other factors evaluated, such as the availability of organic matter or the chlorate reducing bacteria count, also proved not to have any incidence on the results. The richness of chlorate reducing bacteria species in the different samples analysed were also similar, with the greatest similarity being found between cld genes in the samples from the upstream or downstream sampling points. Additionally, cld genes most similar to those present in PCRB like Dechlorospirillum sp., Alicycliphilus denitrificans, Dechloromonas agitata, Dechloromonas sp. LT1 and Ideonella dechloratans were detected. This study showed that the anaerobic sediments of the Cruces river wetland present a high potential for chlorate natural attenuation, regardless of the previous history of exposure to chlorate. This capacity is associated with the presence of a diverse community of chlorate reducing bacteria.