Description of the novel perchlorate-reducing bacteria Dechlorobacter hydrogenophilus gen. nov., sp. nov.and Propionivibrio militaris, sp. nov

Metabolic versatility of anaerobic sludge towards platform chemical production from waste glycerol

Waste glycerol is produced in excess by several industries, such as during biodiesel production. In this work, the metabolic versatility of anaerobic sludge was explored towards waste glycerol valorization. By applying different environmental (methanogenic and sulfate-reducing) conditions, three distinct microbial cultures were obtained from the same inoculum (anaerobic granular sludge), with high microbial specialization, within three different phyla (Thermodesulfobacteriota, Euryarchaeota and Pseudomonadota). The cultures are capable of glycerol conversion through different pathways: (i) glycerol conversion to methane by a bacterium closely related to Solidesulfovibrio alcoholivorans (99.8% 16S rRNA gene identity), in syntrophic relationship with Methanofollis liminatans (98.8% identity), (ii) fermentation to propionate by Propionivibrio pelophilus strain asp66 (98.6% identity), with a propionate yield of 0.88 mmol mmol-1 (0.71 mg mg-1) and a propionate purity of 80-97% and (iii) acetate production coupled to sulfate reduction by Desulfolutivibrio sulfoxidireducens (98.3% identity). In conclusion, starting from the same inoculum, we could drive the metabolic and functional potential of the microbiota towards the formation of several valuable products that can be used in industrial applications or as energy carriers. KEY POINTS: Versatility of anaerobic cultures was explored for waste glycerol valorization Different environmental conditions lead to metabolic specialization Biocommodities such as propionate, acetate and methane were produced.

In situ incubation of iron(II)-bearing minerals and Fe(0) reveals insights into metabolic flexibility of chemolithotrophic bacteria in a nitrate polluted karst aquifer

Groundwater nitrate pollution is a major reason for deteriorating water quality and threatens human and animal health. Yet, mitigating groundwater contamination naturally is often complicated since most aquifers are limited in bioavailable carbon. Since metabolically flexible microbes might have advantages for survival, this study presents a detailed description and first results on our modification of the BacTrap© method, aiming to determine the prevailing microbial community's potential to utilize chemolithotrophic pathways. Our microbial trapping devices (MTDs) were amended with four different iron sources and incubated in seven groundwater monitoring wells for ∼3 months to promote growth of nitrate-reducing Fe(II)-oxidizing bacteria (NRFeOxB) in a nitrate-contaminated karst aquifer. Phylogenetic analysis based on 16S rRNA gene sequences implies that the identity of the iron source influenced the microbial community's composition. In addition, high throughput amplicon sequencing revealed increased relative 16S rRNA gene abundances of OTUs affiliated to genera such as Thiobacillus, Rhodobacter, Pseudomonas, Albidiferax, and Sideroxydans. MTD-derived enrichments set up with Fe(II)/nitrate/acetate to isolate potential NRFeOxB, were dominated by e.g., Acidovorax spp., Paracoccus spp. and Propionivibrio spp. MTDs are a cost-effective approach for investigating microorganisms in groundwater and our data not only solidifies the MTD's capacity to provide insights into the metabolic flexibility of the aquifer's microbial community, but also substantiates its metabolic potential for anaerobic Fe(II) oxidation.

Exploring the stability of an A-stage-EBPR system for simultaneous biological removal of organic matter and phosphorus

This work evaluates the performance and stability of a continuous anaerobic/aerobic A-stage system with integrated enhanced biological phosphorus removal (A-stage-EBPR) under different operational conditions. Dissolved oxygen (DO) in the aerobic reactor was tested in the 0.2-2 mgDO/L range using real wastewater amended with propionic acid, obtaining almost full simultaneous COD and P removal without nitrification in the range 0.5-1 mgDO/L, but failing at 0.2 mgDO/L. Anaerobic purge was tested to evaluate a possible mainstream P-recovery strategy, generating a P-enriched stream containing 22% of influent P. COD and N mass balances indicated that about 43% of the influent COD could be redirected to the anaerobic digestion for methane production and 66% of influent NH₄⁺-N was discharged in the effluent for the following N-removal B-stage. Finally, when the system was switched to glutamate as sole carbon source, successful EBPR activity and COD removal were maintained for two months, but after this period settleability problems appeared with biomass loss. Microbial community analysis indicated that Propionivibrio, Thiothrix and Lewinella were the most abundant species when propionic acid was the carbon source and Propionivibrio was the most favoured with glutamate. Thiothrix, Hydrogenophaga, Dechloromonas and Desulfobacter appeared as the dominant polyphosphate-accumulating organisms (PAOs) under different operation stages.

Distinct oxygen isotope fractionations driven by different electron donors during microbial nitrate reduction in lake sediments

Microbial nitrate reduction can be driven by organic carbon oxidation, as well as by inorganic electron donors, such as reduced forms of sulfur and iron. An apparent inverse oxygen isotope fractionation effect was observed during nitrate reduction in sediment incubations from five sampling sites of a freshwater lake, Hongze Lake, China. Incubations with organic and inorganic electron donor additions were performed. Especially, the inverse oxygen isotope effect was intensified after glucose addition, whereas the incubations with sulfide and Fe²⁺ showed normal fractionation factors. Nitrate reductase encoding genes, napA and narG, were analysed with metagenomics. Higher napA/narG ratios were associated with higher oxygen fractionation factors. The most abundant clade (59%) of NapA in the incubation with glucose was affiliated with Rhodocyclales. In contrast, it only accounted for 8%-9% of NapA in the incubations with sulfide and Fe²⁺ . Differences in nitrate reductases might explain different oxygen isotope effects. Our findings also suggested that large variance of O-nitrate isotope fractionations might have to be considered in the interpretation of natural isotope records.

Constructed wetlands for rural domestic wastewater treatment: A coupling of tidal strategy, in-situ bio-regeneration of zeolite and Fe(Ⅱ)-oxygen denitrification

Constructed wetlands (CWs) offer a promising method to treat domestic wastewater in rural areas, but CWs usually limiting in nitrogen removal and large area. In this study, zeolite and pyrite were used to construct tidal wetlands to address the problems of insufficient oxygen supply and carbon source. The results show that the fully drained wetland achieved the highest ammonia removal load of 34.67 ± 1.72 g/(m²·d) with 8.57 ± 1.13 mg/L of effluent. Pyrite was found to compensate for the lack of carbon source in the denitrification process in half-drained wetland, which achieved a 78.36 ± 5.3% TN removal rate with 7.09 ± 1.85 mg/L effluent concentration. Pyrite released Fe(II) to promote nitrate reduction for denitrification in the subsequent flooded period. Microbial community analysis indicates that the tidal flow constructed wetlands simultaneously achieved nitrification and denitrification by the coupling of in-situ zeolite regeneration and Fe(II) oxidation denitrification.

Bioelectricity generation using long-term operated biocathode: RFLP based microbial diversity analysis

In the present work, power generation and substrate removal efficiencies of long-term operated microbial fuel cells, containing abiotic cathodes and biocathodes, were evaluated for 220 days. Among the two microbial fuel cell (MFC) types, the one containing biocathode showed higher power density (54 mW/m2), current density (122 mA/m2) coulombic efficiency (33%), and substrate removal efficiency (94%) than the abiotic cathode containing MFC. Voltammetric analysis also witnessed higher and sustainable electron discharge for the MFC with biocathode, when compared with the abiotic cathode MFC. Over the tested period, both MFC have shown a cell voltage drop, after 150 and 165, days, for the MFC with biocathode and abiotic cathodes, respectively. Polymerase chain reaction (PCR) based restriction fragment length polymorphism (RFLP) analysis identified 281 clones. Bacteria belonging to Acinetobacter, Acidovorax, Pseudomonas and Burkholderia were observed in the abiotic cathode MFC. Bacteria belonging to Geobacter, Cupriavidus and Acidobacteria were observed in the biocathode MFC. Almost similar types of archaea (Methanosarcinales, Methanolinea, Nitrososphaera and Methanomicrobiales) were observed in both MFCs.

Niveibacterium microcysteis sp. nov., isolated from a cyanobacterial bloom sample

A novel bacterial strain, named HC41^T, was isolated from a cyanobacterial bloom sample and was characterized as Gram-stain-negative, rod-shaped and non-motile. According to 16S rRNA phylogenetic analyses, this strain HC41^T belongs to the family Rhodocyclaceae and is most closely related to Niveibacterium umoris KACC 17062^T (=MIC 2059^T; 98.63 %) and Uliginosibacterium gangwonense 5YN10-9 ^T (=KACC 11603^T; 93.64 %). The genome size and DNA G+C content of strain HC41^T were 4.8 Mbp and 64.17 mol%, respectively. Moreover, the average nucleotide identity, digital DNA-DNA hybridization and amino acid identity values between strain HC41^T and N. umoris KACC 17062^T were 81.8, 43.1 and 90.89 %, respectively. Additionally, strain HC41^T exhibited weak catalase and oxidase activities and had no motility (swimming and swarming motilities). The cells grew at 11-40 °C (optimum, 30 °C), pH 5.5-8.0 (optimum, pH 7) and with 0-1.0 % (w/v) NaCl (optimum, 0 % NaCl) in Reasoner's 2A medium. Its major respiratory quinone was ubiquinone-8 and its major polar lipids were diphosphatidylglycerol, phosphatidylglycerol and phosphatidylethanolamine. Furthermore, C_16 : 0 and summed feature 3 (C_16 : 1 ω7c and/or C_16 : 1 ω6c; C_16 : 1 ω6c and/or C_16 : 1 ω7c) were the predominant cellular fatty acids in strain HC41^T according to fatty acid methyl ester analysis. Based on its genotypic and phenotypic characteristics, strain HC41^T was identified as representing a novel Niveibacterium species, for which the name Niveibacterium microcysteis sp. nov. is proposed (=KACC 22091^T=DSM 111425^T).

Effects of sludge age on anaerobic acidification of waste activated sludge: Volatile fatty acids production and phosphorus release

Effects of sludge age on volatile fatty acids (VFAs) production and Phosphorus (P) release during anaerobic acidification of waste activated sludge (WAS) were investigated. Sequencing batch reactors (SBR) fed with simulating domestic sewage were applied to produce WAS of different sludge ages, and batch tests were used for anaerobic acidification. The maximum dissolved total organic carbon, release of  PO₄³⁺-P, and accumulation of acetate (C2), propionate (C3), butyrate (C4), and valerate (C5) decreased by 56.2%, 55.8%, 52.6%, 43.7%, 82.4% and 84.8%, respectively, as the sludge age of WAS increased from 5 to 40 days. Limited degradation of protein played a dominating role in decreasing DTOC and VFAs production. Moreover, the increase in molecular weight of organics and organic nitrogen content in the supernatant after acidification suggested that the refractory protein in WAS increased as sludge age extended. Although the production of C2, C3, C4, and C5 from WAS decreased as the sludge age increased, the proportions of C2 and C3 in VFAs increased, which might be due to the declined production of C5 from protein and the faded genus Dechlorobacter. Keeping sludge age of WAS at a relatively low level (<10 days) is more appropriate for anaerobic acidification of WAS as internal carbon sources and P resource.

Characteristics of microbial denitrification under different aeration intensities: Performance, mechanism, and co-occurrence network

This study aimed to explore how dissolved oxygen (DO) affected the characteristics and mechanisms of denitrification in mixed bacterial consortia. We analyzed denitrification efficiency, intracellular nicotinamide adenine dinucleotide (NADH), relative expression of functional genes, and potential co-occurrence network of microorganisms. Results showed that the total nitrogen (TN) removal rates at different aeration intensities (0.00, 0.25, 0.63, and 1.25 L/(L·min)) were 0.93, 1.45, 0.86, and 0.53 mg/(L·min), respectively, which were higher than previously reported values for pure culture. The optimal aeration intensity was 0.25 L/(L·min), at which the maximum NADH accumulation rate and highest relative abundance of napA, nirK, and nosZ were achieved. With increased aeration intensity, the amount of electron flux to nitrate decreased and nitrate assimilation increased. On one hand, nitrate reduction was primarily inhibited by oxygen through competition for electron donors of a certain single strain. On the other hand, oxygen was consumed rapidly by bacteria by stimulating carbon metabolism to create an optimal denitrification niche for denitrifying microorganisms. Denitrification was performed via inter-genus cooperation (competitive interactions and symbiotic relationships) between keystone taxa (Azoarcus, Paracoccus, Thauera, Stappia, and Pseudomonas) and other heterotrophic bacteria (OHB) in aeration reactors. However, in the non-aeration case, which was primarily carried out based on intra-genus syntrophy within genus Propionivibrio, the co-occurrence network constructed the optimal niche contributing to the high TN removal efficiency. Overall, this study enhanced our knowledge about the molecular ecological mechanisms of aerobic denitrification in mixed bacterial consortia and has theoretical guiding significance for further practical application.

Spatial Changes in Microbial Communities along Different Functional Zones of a Free-Water Surface Wetland

Constructed wetlands (CWs) are complicated ecosystems that include vegetation, sediments, and the associated microbiome mediating numerous processes in wastewater treatment. CWs have various functional zones where contrasting biochemical processes occur. Since these zones are characterized by different particle-size composition, physicochemical conditions, and vegetation, one can expect the presence of distinct microbiomes across different CW zones. Here, we investigated spatial changes in microbiomes along different functional zones of a free-water surface wetland located in Moscow, Russia. The microbiome structure was analyzed using Illumina MiSeq amplicon sequencing. We also determined particle diameter and surface area of sediments, as well as chemical composition of organic pollutants in different CW zones. Specific organic particle aggregates similar to activated sludge flocs were identified in the sediments. The highest accumulation of hydrocarbons was found in the zones with predominant sedimentation of fine fractions. Phytofilters had the highest rate of organic pollutants decomposition and predominance of Smithella, Ignavibacterium, and Methanothrix. The sedimentation tank had lower microbial diversity, and higher relative abundances of Parcubacteria, Proteiniclasticum, and Macellibacteroides, as well as higher predicted abundances of genes related to methanogenesis and methanotrophy. Thus, spatial changes in microbiomes of constructed wetlands can be associated with different types of wastewater treatment processes.

Isolation and Characterization of Facultative-Anaerobic Antimonate-Reducing Bacteria

Microbial antimonate (Sb(V)) reduction is a promising approach to remove Sb(V) from wastewater. However, current knowledge regarding microbial Sb(V) reduction is limited to strictly anaerobic conditions. This study was the first to isolate three facultative-anaerobic Sb(V)-reducing bacterial strains from the sludge collected from a wastewater treatment facility in an antimony products plant. Two of the isolated strains, designated Dechloromonas sp. AR-2 and Propionivibrio sp. AR-3, were characterized based on their Sb(V)-reducing abilities. When cultivated under anaerobic conditions with Sb(V) and acetate as the electron acceptor and donor, respectively, both strains could efficiently reduce 5.0 mM Sb(V), removing most of it from the water phase within 7 d. Along with Sb(V) reduction by the strains, white precipitates, which were likely amorphous Sb(OH)₃ solids, were formed with a minor generation of soluble antimonite. Additionally, respiratory Sb(V) reduction by both strains occurred not only under anaerobic but also microaerobic conditions. It was suggested that Sb(V) reduction and the growth abilities of the strains under microaerobic conditions presented a substantial advantage of the use of strains AR-2 and AR-3 for practical applications to Sb(V)-containing wastewater treatment.

Corroding copper and steel exposed to intermittently flowing tap water promote biofilm formation and growth of Legionella pneumophila

The information about the impact of copper pipes on the growth of Legionella pneumophila in premise plumbing is controversial. For this reason, pipe segments of copper, stainless steel (SS), mild steel (MS), polyethylene, chlorinated polyvinylchloride (CPVC) and glass (controls) were exposed to intermittently flowing (20 min stagnation time) nonchlorinated tap water of 37 °C or 16 °C (ambient temperature) during six months to study the impact of metals on biofilm formation and growth of L. pneumophila. Biofilm concentrations (BfC, measured as ATP) on copper were 3 (at 37 °C) to 6 (at 16 °C) times higher than on SS. The maximum colony counts of L. pneumophila on the materials tested at 37 °C showed a quadratic relationship with the associated BfCs, with highest values on copper and MS. The average Cu concentration on the glass control of copper (glass-copper) was more than two log units lower than the Fe concentration on glass-MS, suggesting that copper released less corrosion by-products than MS. The release of corrosion by-products with attached biomass from MS most likely enhanced biofilm formation on glass-MS. Cloning and 16S RNA gene sequence analysis of the predominating biofilm bacteria revealed that an uncultured Xanthobacteraceae bacterium and Reyranella accounted for 75% of the bacterial community on copper at 37 °C. The nitrite-oxidizing Nitrospira moscoviensis, which can also utilize hydrogen (H₂) and formate, accounted for >50% of the bacterial abundance in the biofilms on MS and glass-MS at 37 °C. The predominating presence of the strictly anaerobic non-fermentative Fe(III)-reducing Geobacter and the Fe(II)-oxidizing Gallionella on MS exposed to tap water of 16 °C indicated anoxic niches and the availability of H₂, low molecular weight carboxylic acids (LMWCAs) and Fe(II) at the MS surface. LMWCAs likely also promoted bacterial growth on copper, but the release mechanisms from natural organic matter at the surface of corroding metals are unclear. The effects of water stagnation time and flow dynamics on biofilm formation on copper requires further investigation.

Identification of a parasitic symbiosis between respiratory metabolisms in the biogeochemical chlorine cycle

A key step in the chlorine cycle is the reduction of perchlorate (ClO4-) and chlorate (ClO3-) to chloride by microbial respiratory pathways. Perchlorate-reducing bacteria and chlorate-reducing bacteria differ in that the latter cannot use perchlorate, the most oxidized chlorine compound. However, a recent study identified a bacterium with the chlorate reduction pathway dominating a community provided only perchlorate. Here we confirm a metabolic interaction between perchlorate- and chlorate-reducing bacteria and define its mechanism. Perchlorate-reducing bacteria supported the growth of chlorate-reducing bacteria to up to 90% of total cells in communities and co-cultures. Chlorate-reducing bacteria required the gene for chlorate reductase to grow in co-culture with perchlorate-reducing bacteria, demonstrating that chlorate is responsible for the interaction, not the subsequent intermediates chlorite and oxygen. Modeling of the interaction suggested that cells specialized for chlorate reduction have a competitive advantage for consuming chlorate produced from perchlorate, especially at high concentrations of perchlorate, because perchlorate and chlorate compete for a single enzyme in perchlorate-reducing cells. We conclude that perchlorate-reducing bacteria inadvertently support large populations of chlorate-reducing bacteria in a parasitic relationship through the release of the intermediate chlorate. An implication of these findings is that undetected chlorate-reducing bacteria have likely negatively impacted efforts to bioremediate perchlorate pollution for decades.

Application of slow-release carbon sources embedded in polymer for stable and extended power generation in microbial fuel cells

Stable and long-term power output is a prerequisite for the application of the energy recovered from microbial fuel cells (MFCs). In this study, a novel fuel supplying strategy based on slow-release carbon embedded in polymer gels was attempted in MFCs aimed to achieve a sustainable power generation. Polymer gels containing starch acetate as the carbon source (40% (w/w)) were prepared, and the effects of its loading dosage on power generation and microbial community structure were investigated. Results showed that the MFCs once fed with 20.0 g/L, 37.5 g/L and 55.0 g/L polymer gels attained a long-term power generation periods of 110, 140 and 170 days, respectively, with a maximum power density of 386-427 mW/m². The MFC with a medium loading dosage (37.5 g/L polymer gels) performed best. MFCs fed with the slow-release carbon enriched a distinct microbial community comparing to the control MFC with acetate as the carbon source, with the genera Geobacter, Sphaerochaeta, Christensenellaceae, Aminiphilus and Proteiniphilum significantly enriched on the anode electrode, and Sphaerochaeta, Proteiniphilum and Bacteroidetes in the anolyte. This carbon source providing method will promote the application of MFCs as a sustainable and stable power source for environmental monitoring and remediation.

DMSO Reductase Family: Phylogenetics and Applications of Extremophiles

Dimethyl sulfoxide reductases (DMSO) are molybdoenzymes widespread in all domains of life. They catalyse not only redox reactions, but also hydroxylation/hydration and oxygen transfer processes. Although literature on DMSO is abundant, the biological significance of these enzymes in anaerobic respiration and the molecular mechanisms beyond the expression of genes coding for them are still scarce. In this review, a deep revision of the literature reported on DMSO as well as the use of bioinformatics tools and free software has been developed in order to highlight the relevance of DMSO reductases on anaerobic processes connected to different biogeochemical cycles. Special emphasis has been addressed to DMSO from extremophilic organisms and their role in nitrogen cycle. Besides, an updated overview of phylogeny of DMSOs as well as potential applications of some DMSO reductases on bioremediation approaches are also described.

Improving biocathode community multifunctionality by polarity inversion for simultaneous bioelectroreduction processes in domestic wastewater

Bioelectrochemical systems (BESs) have been tentatively applied for wastewater treatment processes, but the complex composition of wastewater could lead to difficulties in establishing functional biofilm or result in performance instability. Few studies have investigated the enrichment of biocathode with domestic wastewater (DW) and the function. A biocathode with multi-pollutant removal capabilities was enriched based on polarity inverted bioanode, which was established with DW. The biocathode function was examined using model pollutants (nitrate, nitrobenzene and Acid Orange 7) supplemented as sole or mixed electron acceptors. When compared to the anaerobic control treatment, the biofilm demonstrated significantly enhanced reduction abilities in the open circuit. For the closed circuit, their removal efficiencies were further enhanced for both the sole and mixed substrates conditions. The bioanodes community structure and diversity markedly changed after operating for 50 d as biocathodes. The biocathode multifunctionality and stability could be related to the maintenance of organic matters fermentative bacteria (mainly belonging to Bacteroidetes, Firmicutes and Synergistetes) and the enrichment of versatile pollutant-reducing bacteria (e.g. Pseudomonas, Thauera and Comamonas from Proteobacteria). Other pollutants, such as perchlorate, sulfate, heavy metals, and halogenated organics, may also work as potential electron acceptors. This study provides a new strategy to improve the biocathode community multifunctionality for simultaneous bioelectroreduction, which can be combined with other wastewater treatment processes in actual application.

Characterization of diversified Sb(V)-reducing bacterial communities by various organic or inorganic electron donors

This study aims to enrich Sb(V)-reducing bacterial communities from Sb-contaminated soils using various electron donors for bioremediation of Sb-contaminated sites and recovery of Sb from wastewater. When the organic electron donors were used, Sb(V) reduction rates were 2-24 times faster but electron recoveries were 24-59% lower compared to the culture using inorganic electron donor. The morphological crystallizations of the antimony-reduced precipitates were completely different depending on the electron donor. Different microbial populations were enriched with various electron donors but most commonly, only Proteobacteria and Firmicutes phyla were enriched from a diversified soil microbial community. Geobacter sp. seemed to be an important bacterium in organic electron donors-fed cultures whereas an unclassified Rhodocyclaceae was dominant in inorganic electron donor-fed cultures. The results indicated that organic electron donors especially sugar groups were preferable options to obtain rapid Sb(V)-reduction whereas inorganic electron donor like H₂ was better option to achieve high electron recovery.

One-step production of C6-C8 carboxylates by mixed culture solely grown on CO

BACKGROUND

This study aimed at producing C6-C8 medium-chain carboxylates (MCCAs) directly from gaseous CO using mixed culture. The yield and C2-C8 product composition were investigated when CO was continuously fed with gradually increasing partial pressure.

RESULTS

The maximal concentrations of n-caproate, n-heptylate, and n-caprylate were 1.892, 1.635, and 1.033 mmol L^-1, which were achieved at the maximal production rates of 0.276, 0.442, and 0.112 mmol L^-1 day^-1, respectively. Microbial analysis revealed that long-term acclimation and high CO partial pressure were important to establish a CO-tolerant and CO-utilizing chain-elongating microbiome, rich in Acinetobacter, Alcaligenes, and Rhodobacteraceae and capable of forming MCCAs solely from CO.

CONCLUSIONS

These results demonstrated that carboxylate and syngas platform could be integrated in a shared growth vessel, and could be a promising one-step technique to convert gaseous syngas to preferable liquid biochemicals, thereby avoiding the necessity to coordinate syngas fermentation to short-chain carboxylates and short-to-medium-chain elongation. Thus, this method could provide an alternative solution for the utilization of waste-derived syngas and expand the resource of promising biofuels.

Fermentative Bacteria Influence the Competition between Denitrifiers and DNRA Bacteria

Denitrification and dissimilatory reduction to ammonium (DNRA) are competing nitrate-reduction processes that entail important biogeochemical consequences for nitrogen retention/removal in natural and man-made ecosystems. The nature of the available carbon source and electron donor have been suggested to play an important role on the outcome of this microbial competition. In this study, the influence of lactate as fermentable carbon source on the competition for nitrate was investigated for varying ratios of lactate and nitrate in the influent (Lac/N ratio). The study was conducted in an open chemostat culture, enriched from activated sludge, under strict anoxia. The mechanistic explanation of the conversions observed was based on integration of results from specific batch tests with biomass from the chemostat, molecular analysis of the biomass enriched, and a computational model. At high Lac/N ratio (2.97 mol/mol) both fermentative and respiratory nitrate reduction to ammonium occurred, coupled to partial oxidation of lactate to acetate, and to acetate oxidation respectively. Remaining lactate was fermented to propionate and acetate. At a decreased Lac/N ratio (1.15 mol/mol), the molar percentage of nitrate reduced to ammonium decreased to 58%, even though lactate was supplied in adequate amounts for full ammonification and nitrate remained the growth limiting compound. Data evaluation at this Lac/N ratio suggested conversions were comparable to the higher Lac/N ratio, except for lactate oxidation to acetate that was coupled to denitrification instead of ammonification. Respiratory DNRA on acetate was likely catalyzed by two Geobacter species related to G. luticola and G. lovleyi. Two Clostridiales members were likely responsible for lactate fermentation and partial lactate fermentation to acetate coupled to fermentative DNRA. An organism related to Propionivibrio militaris was identified as the organism likely responsible for denitrification. The results of this study clearly show that not only the ratio of available substrates, but also the nature of the electron donor influences the outcome of competition between DNRA and denitrification. Apparently, fermentative bacteria are competitive for the electron donor and thereby alter the ratio of available substrates for nitrate reduction.

Characterization of an anaerobic marine microbial community exposed to combined fluxes of perchlorate and salinity

The recent recognition of the environmental prevalence of perchlorate and its discovery on Mars, Earth's moon, and in meteorites, in addition to its novel application to controlling oil reservoir sulfidogenesis, has resulted in a renewed interest in this exotic ion and its associated microbiology. However, while plentiful data exists on freshwater perchlorate respiring organisms, information on their halophilic counterparts and microbial communities is scarce. Here, we investigated the temporal evolving structure of perchlorate respiring communities under a range of NaCl concentrations (1, 3, 5, 7, and 10 % wt/vol) using marine sediment amended with acetate and perchlorate. In general, perchlorate consumption rates were inversely proportional to NaCl concentration with the most rapid rate observed at 1 % NaCl. At 10 % NaCl, no perchlorate removal was observed. Transcriptional analysis of the 16S rRNA gene indicated that salinity impacted microbial community structure and the most active members were in families Rhodocyclaceae (1 and 3 % NaCl), Pseudomonadaceae (1 NaCl), Campylobacteraceae (1, 5, and 7 % NaCl), Sedimenticolaceae (3 % NaCl), Desulfuromonadaceae (5 and 7 % NaCl), Pelobacteraceae (5 % NaCl), Helicobacteraceae (5 and 7 % NaCl), and V1B07b93 (7 %). Novel isolates of genera Sedimenticola, Marinobacter, Denitromonas, Azoarcus, and Pseudomonas were obtained and their perchlorate respiring capacity confirmed. Although the obligate anaerobic, sulfur-reducing Desulfuromonadaceae species were dominant at 5 and 7 % NaCl, their enrichment may result from biological sulfur cycling, ensuing from the innate ability of DPRB to oxidize sulfide. Additionally, our results demonstrated enrichment of an archaeon of phylum Parvarchaeota at 5 % NaCl. To date, this phylum has only been described in metagenomic experiments of acid mine drainage and is unexpected in a marine community. These studies identify the intrinsic capacity of marine systems to respire perchlorate and significantly expand the known diversity of organisms capable of this novel metabolism.

PCE dechlorination by non-Dehalococcoides in a microbial electrochemical system

The bioremediation of tetrachloroethene (perchloroethene; PCE) contaminated sites generally requires a supply of some fermentable organic substrates as an electron donor. On the other hand, organic substrates can induce the massive growth of microorganisms around the injection wells, which can foul the contaminated subsurface environment. In this study, PCE dechlorination to ethene was performed in a microbial electrochemical system (MES) using the electrode (a cathode polarized at −500 mV vs. standard hydrogen electrode) as the electron donor. Denaturing gel gradient electrophoresis and pyrosequencing revealed a variety of non-Dehalococcoides bacteria dominant in MES, such as Acinetobacter sp. (25.7 % for AS1 in suspension of M3), Rhodopseudomonas sp. (10.5 % for AE1 and 10.1 % for AE2 in anodic biofilm of M3), Pseudomonas aeruginosa (22.4 % for BS1 in suspension of M4), and Enterobacter sp. (21.7 % for BE1 in anodic biofilm of M4) which are capable of electron transfer, hydrogen production and dechlorination. The Dehalococcoides group, however, was not detected in this system. Therefore, these results suggest that a range of bacterial species outside the Dehalococcoides can play an important role in the microbial electrochemical dechlorination process, which may lead to innovative bioremediation technology.

(Per)chlorate in Biology on Earth and Beyond

Respiration of perchlorate and chlorate [collectively, (per)chlorate] was only recognized in the last 20 years, yet substantial advances have been made in our understanding of the underlying metabolisms. Although it was once considered solely anthropogenic, pervasive natural sources, both terrestrial and extraterrestrial, indicate an ancient (per)chlorate presence across our solar system. These discoveries stimulated interest in (per)chlorate microbiology, and the application of advanced approaches highlights exciting new facets. Forward and reverse genetics revealed new information regarding underlying molecular biology and associated regulatory mechanisms. Structural and functional analysis characterized core enzymes and identified novel reaction sequences. Comparative genomics elucidated evolutionary aspects, and stress analysis identified novel response mechanisms to reactive chlorine species. Finally, systems biology identified unique metabolic versatility and novel mechanisms of (per)chlorate respiration, including symbiosis and a hybrid enzymatic-abiotic metabolism. While many published studies focus on (per)chlorate and their basic metabolism, this review highlights seminal advances made over the last decade and identifies new directions and potential novel applications.

"Candidatus Propionivibrio aalborgensis": A Novel Glycogen Accumulating Organism Abundant in Full-Scale Enhanced Biological Phosphorus Removal Plants

Enhanced biological phosphorus removal (EBPR) is widely used to remove phosphorus from wastewater. The process relies on polyphosphate accumulating organisms (PAOs) that are able to take up phosphorus in excess of what is needed for growth, whereby phosphorus can be removed from the wastewater by wasting the biomass. However, glycogen accumulating organisms (GAOs) may reduce the EBPR efficiency as they compete for substrates with PAOs, but do not store excessive amounts of polyphosphate. PAOs and GAOs are thought to be phylogenetically unrelated, with the model PAO being the betaproteobacterial “Candidatus Accumulibacter phosphatis” (Accumulibacter) and the model GAO being the gammaproteobacterial “Candidatus Competibacter phosphatis”. Here, we report the discovery of a GAO from the genus Propionivibrio, which is closely related to Accumulibacter. Propionivibrio sp. are targeted by the canonical fluorescence in situ hybridization probes used to target Accumulibacter (PAOmix), but do not store excessive amounts of polyphosphate in situ. A laboratory scale reactor, operated to enrich for PAOs, surprisingly contained co-dominant populations of Propionivibrio and Accumulibacter. Metagenomic sequencing of multiple time-points enabled recovery of near complete population genomes from both genera. Annotation of the Propionivibrio genome confirmed their potential for the GAO phenotype and a basic metabolic model is proposed for their metabolism in the EBPR environment. Using newly designed fluorescence in situ hybridization (FISH) probes, analyses of full-scale EBPR plants revealed that Propionivibrio is a common member of the community, constituting up to 3% of the biovolume. To avoid overestimation of Accumulibacter abundance in situ, we recommend the use of the FISH probe PAO651 instead of the commonly applied PAOmix probe set.

Influence of Oxygen and Nitrate on Fe (Hydr)oxide Mineral Transformation and Soil Microbial Communities during Redox Cycling

Oscillations between reducing and oxidizing conditions are observed at the interface of anaerobic/oxic and anaerobic/anoxic environments, and are often stimulated by an alternating flux of electron donors (e.g., organic carbon) and electron acceptors (e.g., O2 and NO3(-)). In iron (Fe) rich soils and sediments, these oscillations may stimulate the growth of both Fe-reducing bacteria (FeRB) and Fe-oxidizing bacteria (FeOB), and their metabolism may induce cycling between Fe(II) and Fe(III), promoting the transformation of Fe (hydr)oxide minerals. Here, we examine the mineralogical evolution of lepidocrocite and ferrihydrite, and the adaptation of a natural microbial community to alternating Fe-reducing (anaerobic with addition of glucose) and Fe-oxidizing (with addition of nitrate or air) conditions. The growth of FeRB (e.g., Geobacter) is stimulated under anaerobic conditions in the presence of glucose. However, the abundance of these organisms depends on the availability of Fe(III) (hydr)oxides. Redox cycling with nitrate results in decreased Fe(II) oxidation thereby decreasing the availability of Fe(III) for FeRB. Additionally, magnetite is detected as the main product of both lepidocrocite and ferrihydrite reduction. In contrast, introduction of air results in increased Fe(II) oxidation, increasing the availability of Fe(III) and the abundance of Geobacter. In the lepidocrocite reactors, Fe(II) oxidation by dissolved O2 promotes the formation of ferrihydrite and lepidocrocite, whereas in the ferrihydrite reactors we observe a decrease in magnetite stoichiometry (e.g., oxidation). Understanding Fe (hydr)oxide transformation under environmentally relevant redox cycling conditions provides insight into nutrient availability and transport, contaminant mobility, and microbial metabolism in soils and sediments.

Perchlorate Reductase Is Distinguished by Active Site Aromatic Gate Residues

Perchlorate is an important ion on both Earth and Mars. Perchlorate reductase (PcrAB), a specialized member of the dimethylsulfoxide reductase superfamily, catalyzes the first step of microbial perchlorate respiration, but little is known about the biochemistry, specificity, structure, and mechanism of PcrAB. Here we characterize the biophysics and phylogeny of this enzyme and report the 1.86-Å resolution PcrAB complex crystal structure. Biochemical analysis revealed a relatively high perchlorate affinity (Km = 6 μm) and a characteristic substrate inhibition compared with the highly similar respiratory nitrate reductase NarGHI, which has a relatively much lower affinity for perchlorate (Km = 1.1 mm) and no substrate inhibition. Structural analysis of oxidized and reduced PcrAB with and without the substrate analog SeO3 (2-) bound to the active site identified key residues in the positively charged and funnel-shaped substrate access tunnel that gated substrate entrance and product release while trapping transiently produced chlorate. The structures suggest gating was associated with shifts of a Phe residue between open and closed conformations plus an Asp residue carboxylate shift between monodentate and bidentate coordination to the active site molybdenum atom. Taken together, structural and mutational analyses of gate residues suggest key roles of these gate residues for substrate entrance and product release. Our combined results provide the first detailed structural insight into the mechanism of biological perchlorate reduction, a critical component of the chlorine redox cycle on Earth.

The Perchlorate Reduction Genomic Island: Mechanisms and Pathways of Evolution by Horizontal Gene Transfer

Background

Perchlorate is a widely distributed anion that is toxic to humans, but serves as a valuable electron acceptor for several lineages of bacteria. The ability to utilize perchlorate is conferred by a horizontally transferred piece of DNA called the perchlorate reduction genomic island (PRI).

Methods

We compared genomes of perchlorate reducers using phylogenomics, SNP mapping, and differences in genomic architecture to interrogate the evolutionary history of perchlorate respiration.

Results

Here we report on the PRI of 13 genomes of perchlorate-reducing bacteria from four different classes of Phylum Proteobacteria (the Alpha-, Beta-, Gamma- and Epsilonproteobacteria). Among the different phylogenetic classes, the island varies considerably in genetic content as well as in its putative mechanism and location of integration. However, the islands of the densely sampled genera Azospira and Magnetospirillum have striking nucleotide identity despite divergent genomes, implying horizontal transfer and positive selection within narrow phylogenetic taxa. We also assess the phylogenetic origin of accessory genes in the various incarnations of the island, which can be traced to chromosomal paralogs from phylogenetically similar organisms.

Conclusion

These observations suggest a complex phylogenetic history where the island is rarely transferred at the class level but undergoes frequent and continuous transfer within narrow phylogenetic groups. This restricted transfer is seen directly by the independent integration of near-identical islands within a genus and indirectly due to the acquisition of lineage-specific accessory genes. The genomic reversibility of perchlorate reduction may present a unique equilibrium for a metabolism that confers a competitive advantage only in the presence of an electron acceptor, which although widely distributed, is generally present at low concentrations in nature.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-2011-5) contains supplementary material, which is available to authorized users.

Propane biostimulation in biologically activated carbon (BAC) selects for bacterial clades adept at degrading persistent water pollutants

Biologically activated carbon (BAC) can be used in both municipal water and hazardous waste remediation applications to enhance contaminant attenuation in water; however, questions remain about how selective pressures can be applied to increase the capabilities of microbial communities to attenuate recalcitrant contaminants. Here we utilized flow-through laboratory columns seeded with municipally derived BAC and exposed to water from a local drinking water facility to query how propane biostimulation impacts resident microorganisms. Ecological analyses using high throughput phylogenetic sequencing revealed that while propane did not increase the total number of microbiological species, it did select for bacterial communities that were distinct from those without propane. Temporal extractions demonstrated that microbial succession was rapid and established in approximately 2 months. A higher density of propane monooxygenase genes and bacterial clades including the Pelosinus and Dechloromonas genera suggest an enhanced potential for the degradation of persistent water pollutants in propane-stimulated systems. However, the ecological selective pressure was exhausted in less than 15 cm of transit in this flow-through scenario (25 hour retention) indicating a pronounced zonation that could limit the size of a biostimulated zone and require physical mixing, hydraulic manipulation, or other strategies to increase the spatial impact of biostimulation in flow-through scenarios.

Production of dioxygen in the dark: dismutases of oxyanions

O₂-generating reactions are exceedingly rare in biology and difficult to mimic synthetically. Perchlorate-respiring bacteria enzymatically detoxify chlorite (ClO₂(-) ), the end product of the perchlorate (ClO(4)(-) ) respiratory pathway, by rapidly converting it to dioxygen (O₂) and chloride (Cl(-)). This reaction is catalyzed by a heme-containing protein, called chlorite dismutase (Cld), which bears no structural or sequence relationships with known peroxidases or other heme proteins and is part of a large family of proteins with more than one biochemical function. The original assumptions from the 1990s that perchlorate is not a natural product and that perchlorate respiration might be confined to a taxonomically narrow group of species have been called into question, as have the roles of perchlorate respiration and Cld-mediated reactions in the global biogeochemical cycle of chlorine. In this chapter, the chemistry and biochemistry of Cld-mediated O₂generation, as well as the biological and geochemical context of this extraordinary reaction, are described.

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.

Transposon and deletion mutagenesis of genes involved in perchlorate reduction in Azospira suillum PS

Although much work on the biochemistry of the key enzymes of bacterial perchlorate reduction, chlorite dismutase, and perchlorate reductase has been published, understanding of the molecular mechanisms of this metabolism has been somewhat hampered by the lack of a clear model system amenable to genetic manipulation. Using transposon mutagenesis and clean deletions, genes important for perchlorate reduction in Azospira suillum PS have been identified both inside and outside the previously described perchlorate reduction genomic island (PRI). Transposon mutagenesis identified 18 insertions in 11 genes that completely abrogate growth via reduction of perchlorate but have no phenotype during denitrification. Of the mutants deficient in perchlorate reduction, 14 had insertions that were mapped to eight different genes within the PRI, highlighting its importance in this metabolism. To further explore the role of these genes, we also developed systems for constructing unmarked deletions and for complementing these deletions. Using these tools, every core gene in the PRI was systematically deleted; 8 of the 17 genes conserved in the PRI are essential for perchlorate respiration, including 3 genes that comprise a unique histidine kinase system. Interestingly, the other 9 genes in the PRI are not essential for perchlorate reduction and may thus have unknown functions during this metabolism. We present a model detailing our current understanding of perchlorate reduction that incorporates new concepts about this metabolism.

IMPORTANCE

Although perchlorate is generated naturally in the environment, groundwater contamination is largely a result of industrial activity. Bacteria capable of respiring perchlorate and remediating contaminated water have been isolated, but relatively little is known about the biochemistry and genetics of this process. Here we used two complementary approaches to identify genes involved in perchlorate reduction. Most of these genes are located on a genomic island, which is potentially capable of moving between organisms. Some of the genes identified are known to be directly involved in the metabolism of perchlorate, but other new genes likely regulate the metabolism in response to environmental signals. This work has uncovered new questions about the regulation, energetics, and evolution of perchlorate reduction but also presents the tools to address them.

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.

Using a two-stage hydrogen-based membrane biofilm reactor (MBfR) to achieve complete perchlorate reduction in the presence of nitrate and sulfate

We evaluated a strategy for achieving complete reduction of perchlorate (ClO(4)(-)) in the presence of much higher concentrations of sulfate (SO(4)(2-)) and nitrate (NO(3)(-)) in a hydrogen-based membrane biofilm reactor (MBfR). Full ClO(4)(-) reduction was achieved by using a two-stage MBfR with controlled NO(3)(-) surface loadings to each stage. With an equivalent NO(3)(-) surface loading larger than 0.65 ± 0.04 g N/m(2)-day, the lead MBfR removed about 87 ± 4% of NO(3)(-) and 30 ± 8% of ClO(4)(-). This decreased the equivalent surface loading of NO(3)(-) to 0.34 ± 0.04-0.53 ± 0.03 g N/m(2)-day for the lag MBfR, in which ClO(4)(-) was reduced to nondetectable. SO(4)(2-) reduction was eliminated without compromising full ClO(4)(-) reduction using a higher flow rate that gave an equivalent NO(3)(-) surface loading of 0.94 ± 0.05 g N/m(2)-day in the lead MBfR and 0.53 ± 0.03 g N/m(2)-day in the lag MBfR. Results from qPCR and pyrosequencing showed that the lead and lag MBfRs had distinctly different microbial communities when SO(4)(2-) reduction took place. Denitrifying bacteria (DB), quantified using the nirS and nirK genes, dominated the biofilm in the lead MBfR, but perchlorate-reducing bacteria (PRB), quantified using the pcrA gene, became more important in the lag MBfR. The facultative anaerobic bacteria Dechloromonas, Rubrivivax, and Enterobacter were dominant genera in the lead MBfR, where their main function was to reduce NO(3)(-). With a small NO(3)(-) surface loading and full ClO(4)(-) reduction, the dominant genera shifted to ClO(4)(-)-reducing bacteria Sphaerotilus, Rhodocyclaceae, and Rhodobacter in the lag MBfR.

Interactions between perchlorate and nitrate reductions in the biofilm of a hydrogen-based membrane biofilm reactor

We studied the microbial functional and structural interactions between nitrate (NO(3)(-)) and perchlorate (ClO(4)(-)) reductions in the hydrogen (H(2))-based membrane biofilm reactor (MBfR). When H(2) was not limiting, ClO(4)(-) and NO(3)(-) reductions were complete, and the MBfR's biofilm was composed mainly of bacteria from the ε- and β-proteobacteria classes, with autotrophic genera Sulfuricurvum, Hydrogenophaga, and Dechloromonas dominating the biofilm. Based on functional-gene and pyrosequencing assays, Dechloromonas played the most important role in ClO(4)(-) reduction, while Sulfuricurvum and Hydrogenophaga were responsible for NO(3)(-) reduction. When H(2) delivery was insufficient to completely reduce both electron acceptors, NO(3)(-) reduction out-competed ClO(4)(-) reduction for electrons from H(2), and mixotrophs become important in the MBfR biofilm. β-Proteobacteria became the dominant class, and Azonexus replaced Sulfuricurvum as a main genus. The changes suggest that facultative, NO(3)(-)-reducing bacteria had advantages over strict autotrophs when H(2) was limiting, because organic microbial products became important electron donors when H(2) was severely limiting.

Dissimilatory perchlorate reduction: a review

In the United States anthropogenic activities are mainly responsible for the wide spread perchlorate contamination of drinking water, surface water, groundwater, and soil. Even at microgram levels, perchlorate causes toxicity to flora and fauna and affects growth, metabolism and reproduction in humans and animals. Reports of antithyroid effects of perchlorate and its detection in common food items have raised serious public health concerns, leading to extensive decontamination efforts in recent years. Several physico-chemical removal and biological decontamination processes are being developed. Although promising, ion exchange is a non-selective and incomplete process as it merely transfers perchlorate from water to the resin. The perchlorate-laden spent resins (perchlorate 200-500 mg L(-1)) require regeneration resulting in production of concentrated brine (6-12% NaCl) or caustic waste streams. On the contrary, biological reduction completely degrades perchlorate into O(2) and innocuous Cl(-). High reduction potential of ClO(4)(-)/Cl(-) (E° =∼ 1.28 V) and ClO(3)(-)/Cl(-) pairs (E° =1.03 V) makes these contaminants thermodynamically ideal e(-) acceptors for microbial reduction. In recent years unique dissimilatory perchlorate reducing bacteria have been isolated and detailed studies pertaining to their microbiological, biochemical, genetics and phylogenetic aspects have been undertaken which is the subject of this review article while the various physico-chemical removal and biological reduction processes have been reviewed by others.

(Per)chlorate reduction by an acetogenic bacterium, Sporomusa sp., isolated from an underground gas storage

A mesophilic bacterium, strain An4, was isolated from an underground gas storage reservoir with methanol as substrate and perchlorate as electron acceptor. Cells were Gram-negative, spore-forming, straight to curved rods, 0.5–0.8 μm in diameter, and 2–8 μm in length, growing as single cells or in pairs. The cells grew optimally at 37°C, and the pH optimum was around 7. Strain An4 converted various alcohols, organic acids, fructose, acetoin, and H₂/CO₂ to acetate, usually as the only product. Succinate was decarboxylated to propionate. The isolate was able to respire with (per)chlorate, nitrate, and CO₂. The G+C content of the DNA was 42.6 mol%. Based on the 16S rRNA gene sequence analysis, strain An4 was most closely related to Sporomusa ovata (98% similarity). The bacterium reduced perchlorate and chlorate completely to chloride. Key enzymes, perchlorate reductase and chlorite dismutase, were detected in cell-free extracts.

Magnetospirillum bellicus sp. nov., a novel dissimilatory perchlorate-reducing alphaproteobacterium isolated from a bioelectrical reactor

Previously isolated dissimilatory perchlorate-reducing bacteria (DPRB) have been primarily affiliated with the Betaproteobacteria. Enrichments from the cathodic chamber of a bioelectrical reactor (BER) inoculated from creek water in Berkeley, CA, yielded a novel organism most closely related to a previously described strain, WD (99% 16S rRNA gene identity). Strain VDY(T) has 96% 16S rRNA gene identity to both Magnetospirillum gryphiswaldense and Magnetospirillum magnetotacticum, and along with strain WD, distinguishes a clade of perchlorate-reducing Magnetospirillum species in the Alphaproteobacteria. In spite of the phylogenetic location of VDY(T), attempted PCR for the key magnetosome formation genes mamI and mamL was negative. Strain VDY(T) was motile, non-spore forming, and, in addition to perchlorate, could use oxygen, chlorate, nitrate, nitrite, and nitrous oxide as alternative electron acceptors with acetate as the electron donor. Transient chlorate accumulation occurred during respiration of perchlorate. The organism made use of fermentation end products, such as acetate and ethanol, as carbon sources and electron donors for heterotrophic growth, and in addition, strain VDY(T) could grow chemolithotrophically with hydrogen serving as the electron donor. VDY(T) contains a copy of the RuBisCo cbbM gene, which was expressed under autotrophic but not heterotrophic conditions. DNA-DNA hybridization with strain WD confirmed VDY(T) as a separate species (46.2% identity), and the name Magnetospirillum bellicus sp. nov. (DSM 21662, ATCC BAA-1730) is proposed.