Pelobacter seleniigenes sp. nov., a selenate-respiring bacterium

Genomic Insights into Selenate Reduction by Anaerobacillus Species

Selenium (Se), a potentially toxic trace element, undergoes complex biogeochemical cycling in the environment, largely driven by microbial activity. The reduction in selenate or selenite to elemental selenium is an environmentally beneficial process, as it decreases both Se toxicity and mobility. This reduction is catalyzed by enzymes encoded by various related genes. The link between Se reduction gene clusters and specific taxonomic groups is significant for elucidating the ecological roles and processes of Se reduction in diverse environments. In this study, a new species of Se-reducing microorganism belonging to the genus Anaerobacillus was isolated from a mining site. A comparative analysis of the growth characteristics reveals that Anaerobacillus species exhibit notable metabolic versatility, particularly in their fermentation abilities and utilization of diverse electron donors and acceptors. Genome analysis identified a diverse array of gene clusters associated with selenate uptake (sul, pst), selenate reduction (ser), and selenite reduction (hig, frd, trx, and bsh). Since selenate reduction is the first crucial step in Se reduction, genes linked to selenate reductase are the focus. The serA gene clusters analysis suggests that the serA gene is highly conserved across Anaerobacillus species. The surrounding genes of serA show significant variability in both presence and gene size. This evolutionary difference in coenzyme utilization and serA regulation suggests distinct survival strategies among Anaerobacillus species. This study offers insights into Se bio-transformations and the adaptive strategies of Se-reducing microorganisms.

Sulfidogenic Microbial Communities of the Uzen High-Temperature Oil Field in Kazakhstan

Application of seawater for secondary oil recovery stimulates the development of sulfidogenic bacteria in the oil field leading to microbially influenced corrosion of steel equipment, oil souring, and environmental issues. The aim of this work was to investigate potential sulfide producers in the high-temperature Uzen oil field (Republic of Kazakhstan) exploited with seawater flooding and the possibility of suppressing growth of sulfidogens in both planktonic and biofilm forms. Approaches used in the study included 16S rRNA and dsrAB gene sequencing, scanning electron microscopy, and culture-based techniques. Thermophilic hydrogenotrophic methanogens of the genus Methanothermococcus (phylum Euryarchaeota) predominated in water from the zone not affected by seawater flooding. Methanogens were accompanied by fermentative bacteria of the genera Thermovirga, Defliviitoga, Geotoga, and Thermosipho (phylum Thermotogae), which are potential thiosulfate- or/and sulfur-reducers. In the sulfate- and sulfide-rich formation water, the share of Desulfonauticus sulfate-reducing bacteria (SRB) increased. Thermodesulforhabdus, Thermodesulfobacterium, Desulfotomaculum, Desulfovibrio, and Desulfoglaeba were also detected. Mesophilic denitrifying bacteria of the genera Marinobacter, Halomonas, and Pelobacter inhabited the near-bottom zone of injection wells. Nitrate did not suppress sulfidogenesis in mesophilic enrichments because denitrifiers reduced nitrate to dinitrogen; however, thermophilic denitrifiers produced nitrite, an inhibitor of SRB. Enrichments and a pure culture Desulfovibrio alaskensis Kaz19 formed biofilms highly resistant to biocides. Our results suggest that seawater injection and temperature of the environment determine the composition and functional activity of prokaryotes in the Uzen oil field.

Selenium respiration in anaerobic bacteria: Does energy generation pay off?

Selenium (Se) respiration in bacteria was revealed for the first time at the end of 1980s. Although thermodynamically-favorable, energy-dense and documented in phylogenetically-diverse bacteria, this metabolic process appears to be accompanied by a number of challenges and numerous unanswered questions. Selenium oxyanions, SeO₄^2- and SeO₃^2-, are reduced to elemental Se (Se⁰) through anaerobic respiration, the end product being solid and displaying a considerable size (up to 500 nm) at the bacterial scale. Compared to other electron acceptors used in anaerobic respiration (e.g. N, S, Fe, Mn, and As), Se is one of the few elements whose end product is solid. Furthermore, unlike other known bacterial intracellular accumulations such as volutin (inorganic polyphosphate), S⁰, glycogen or magnetite, Se⁰ has not been shown to play a nutritional or ecological role for its host. In the context of anaerobic respiration of Se oxyanions, biogenic Se⁰ appears to be a by-product, a waste that needs proper handling, and this raises the question of the evolutionary implications of this process. Why would bacteria use a respiratory substrate that is useful, in the first place, and then highly detrimental? Interestingly, in certain artificial ecosystems (e.g. upflow bioreactors) Se⁰ might help bacterial cells to increase their density and buoyancy and thus avoid biomass wash-out, ensuring survival. This review article provides an in-depth analysis of selenium respiration (model selenium respiring bacteria, thermodynamics, respiratory enzymes, and genetic determinants), complemented by an extensive discussion about the evolutionary implications and the properties of biogenic Se⁰ using published and original/unpublished results.

High-rate biological selenate reduction in a sequencing batch reactor for recovery of hexagonal selenium

Recovery of selenium (Se) from wastewater provides a solution for both securing Se supply and preventing Se pollution. Here, we developed a high-rate process for biological selenate reduction to elemental selenium. Distinctive from other studies, we aimed for a process with selenate as the main biological electron sink, with minimal formation of methane or sulfide. A sequencing batch reactor, fed with an influent containing 120 mgSe L^-1 selenate and ethanol as electron donor and carbon source, was operated for 495 days. The high rates (419 ± 17 mgSe L^-1 day^-1) were recorded between day 446 and day 495 for a hydraulic retention time of 6 h. The maximum conversion efficiency of selenate amounted to 96% with a volumetric conversion rate of 444 mgSe L^-1 day^-1, which is 6 times higher than the rates reported in the literature thus far. At the end of the experiment, a highly enriched selenate reducing biomass had developed, with a specific activity of 856 ± 26 mgSe^-1day^-1g_biomass^-1, which was nearly 1000-fold higher than that of the inoculum. No evidence was found for the formation of methane, sulfide, or volatile reduced selenium compounds like dimethyl-selenide or H₂Se, revealing a high selectivity. Ethanol was incompletely oxidized to acetate. The produced elemental selenium partially accumulated in the reactor as pure (≥80% Se of the total mixture of biomass sludge flocs and flaky aggregates, and ~100% of the specific flaky aggregates) selenium black hexagonal needles, with cluster sizes between 20 and 200 µm. The new process may serve as the basis for a high-rate technology to remove and recover pure selenium from wastewater or process streams with high selectivity.

16s rRNA metagenomic analysis reveals predominance of Crtl and CruF genes in Arabian Sea coast of India

Microbial communities perform crucial biogeochemical cycles in distinct ecosystems. Halophilic microbial communities are enriched in the saline areas. Hence, haloarchaea have been primarily studied in salterns and marine biosystems with the aim to harness haloarcheal carotenoids biosynthesis. In this study, sediment from several distinct biosystems (mangrove, seashore, estuary, river, lake, salt pan and island) across the Arabian coastal region of India were collected and analyzed though 16s rRNA metagenomic and whole genome approach to elucidated the dominant representative genre, haloarcheal diversity, and the prevalence of Crtl and CruF genes. We found that the microbial diversity in mangrove sediment (794 OTUs) was highest and lowest in lake and river (558-560 OTUs). Moreover, the bacterial domain dominated in all biosystems (96.00-99.45%). Top 10 abundant genera were involved in biochemical cycles such as sulfur, methane, ammonia, hydrocarbon degradation, and antibiotics production. The Archaea was mainly composed of Haloarchaea, Methanobacteria, Methanococci, Methanomicrobia and Crenarchaeota. Carotenoid gene, Crtl, was observed in a major portion (abundance 60%; diversity 45%) of microbial community. Interestingly, we found that all species under haloarcheal class that were represented in fresh as well as marine biosystems encodes CruF gene (bacterioruberin carotenoid). Our study demonstrates the high microbial diversity in various ecosystems, enrichment of Crtl gene, and also shows that Crtl and CruF genes are highly abundant in haloarcheal genera. The finding of ecosystems specific Crtl and CruF encoding genera opens up a promising area in bioprospecting the carotenoid derivatives from the wide range of natural biosystems.

Proposal to reclassify the proteobacterial classes Deltaproteobacteria and Oligoflexia, and the phylum Thermodesulfobacteria into four phyla reflecting major functional capabilities

The class Deltaproteobacteria comprises an ecologically and metabolically diverse group of bacteria best known for dissimilatory sulphate reduction and predatory behaviour. Although this lineage is the fourth described class of the phylum Proteobacteria , it rarely affiliates with other proteobacterial classes and is frequently not recovered as a monophyletic unit in phylogenetic analyses. Indeed, one branch of the class Deltaproteobacteria encompassing Bdellovibrio-like predators was recently reclassified into a separate proteobacterial class, the Oligoflexia . Here we systematically explore the phylogeny of taxa currently assigned to these classes using 120 conserved single-copy marker genes as well as rRNA genes. The overwhelming majority of markers reject the inclusion of the classes Deltaproteobacteria and Oligoflexia in the phylum Proteobacteria . Instead, the great majority of currently recognized members of the class Deltaproteobacteria are better classified into four novel phylum-level lineages. We propose the names Desulfobacterota phyl. nov. and Myxococcota phyl. nov. for two of these phyla, based on the oldest validly published names in each lineage, and retain the placeholder name SAR324 for the third phylum pending formal description of type material. Members of the class Oligoflexia represent a separate phylum for which we propose the name Bdellovibrionota phyl. nov. based on priority in the literature and general recognition of the genus Bdellovibrio. Desulfobacterota phyl. nov. includes the taxa previously classified in the phylum Thermodesulfobacteria , and these reclassifications imply that the ability of sulphate reduction was vertically inherited in the Thermodesulfobacteria rather than laterally acquired as previously inferred. Our analysis also indicates the independent acquisition of predatory behaviour in the phyla Myxococcota and Bdellovibrionota, which is consistent with their distinct modes of action. This work represents a stable reclassification of one of the most taxonomically challenging areas of the bacterial tree and provides a robust framework for future ecological and systematic studies.

Microbial selenium metabolism: a brief history, biogeochemistry and ecophysiology

Selenium is an essential trace element for organisms from all three domains of life. Microorganisms, in particular, mediate reductive transformations of selenium that govern the element's mobility and bioavailability in terrestrial and aquatic environments. Selenium metabolism is not just ubiquitous but an ancient feature of life likely extending back to the universal common ancestor of all cellular lineages. As with the sulfur biogeochemical cycle, reductive transformations of selenium serve two metabolic functions: assimilation into macromolecules and dissimilatory reduction during anaerobic respiration. This review begins with a historical overview of how research in both aspects of selenium metabolism has developed. We then provide an overview of the global selenium biogeochemical cycle, emphasizing the central role of microorganisms in the cycle. This serves as a basis for a robust discussion of current models for the evolution of the selenium biogeochemical cycle over geologic time, and how knowledge of the evolution and ecophysiology of selenium metabolism can enrich and refine these models. We conclude with a discussion of the ecophysiological function of selenium-respiring prokaryotes within the cycle, and the tantalizing possibility of oxidative selenium transformations during chemolithoautotrophic growth.

Respiratory Selenite Reductase from Bacillus selenitireducens Strain MLS10

The putative respiratory selenite [Se(IV)] reductase (Srr) from Bacillus selenitireducens MLS10 has been identified through a polyphasic approach involving genomics, proteomics, and enzymology. Nondenaturing gel assays were used to identify Srr in cell fractions, and the active band was shown to contain a single protein of 80 kDa. The protein was identified through liquid chromatography-tandem mass spectrometry (LC-MS/MS) as a homolog of the catalytic subunit of polysulfide reductase (PsrA). It was found to be encoded as part of an operon that contains six genes that we designated srrE, srrA, srrB, srrC, srrD, and srrF SrrA is the catalytic subunit (80 kDa), with a twin-arginine translocation (TAT) leader sequence indicative of a periplasmic protein and one putative 4Fe-4S binding site. SrrB is a small subunit (17 kDa) with four putative 4Fe-4S binding sites, SrrC (43 kDa) is an anchoring subunit, and SrrD (24 kDa) is a chaperon protein. Both SrrE (38 kDa) and SrrF (45 kDa) were annotated as rhodanese domain-containing proteins. Phylogenetic analysis revealed that SrrA belonged to the PsrA/PhsA clade but that it did not define a distinct subgroup, based on the putative homologs that were subsequently identified from other known selenite-respiring bacteria (e.g., Desulfurispirillum indicum and Pyrobaculum aerophilum). The enzyme appeared to be specific for Se(IV), showing no activity with selenate, arsenate, or thiosulfate, with a K_m of 145 ± 53 μM, a V _max of 23 ± 2.5 μM min^-1, and a k _cat of 23 ± 2.68 s^-1 These results further our understanding of the mechanisms of selenium biotransformation and its biogeochemical cycle.IMPORTANCE Selenium is an essential element for life, with Se(IV) reduction a key step in its biogeochemical cycle. This report identifies for the first time a dissimilatory Se(IV) reductase, Srr, from a known selenite-respiring bacterium, the haloalkalophilic Bacillus selenitireducens strain MLS10. The work extends the versatility of the complex iron-sulfur molybdoenzyme (CISM) superfamily in electron transfer involving chalcogen substrates with different redox potentials. Further, it underscores the importance of biochemical and enzymological approaches in establishing the functionality of these enzymes.

Microbial Community Changes in a Chlorinated Solvents Polluted Aquifer Over the Field Scale Treatment With Poly-3-Hydroxybutyrate as Amendment

This study investigated the organohalide-respiring bacteria (OHRB) and the supporting microbial populations operating in a pilot scale plant employing poly-3-hydroxybutyrate (PHB), a biodegradable polymer produced by bacteria from waste streams, for the in situ bioremediation of groundwater contaminated by chlorinated solvents. The bioremediation was performed in ground treatment units, including PHB reactors as slow release source of electron donors, where groundwater extracted from the wells flows through before the re-infiltration to the low permeability zones of the aquifer. The coupling of the biological treatment with groundwater recirculation allowed to drastically reducing the contamination level and the remediation time by efficiently stimulating the growth of autochthonous OHRB and enhancing the mobilization of the pollutants. Quantitative PCR performed along the external treatment unit showed that the PHB reactor may efficiently act as an external incubator to growing Dehalococcoides mccartyi, known to be capable of fully converting chlorinated ethenes to innocuous end-products. The slow release source of electron donors for the bioremediation process allowed the establishment of a stable population of D. mccartyi, mainly carrying bvcA and vcrA genes which are implicated in the metabolic conversion of vinyl chloride to harmless ethene. Next generation sequencing was performed to analyze the phylogenetic diversity of the groundwater microbiome before and after the bioremediation treatment and allowed the identification of the microorganisms working closely with organohalide-respiring bacteria.

Control of Sulfide Production in High Salinity Bakken Shale Oil Reservoirs by Halophilic Bacteria Reducing Nitrate to Nitrite

Microbial communities in shale oil fields are still poorly known. We obtained samples of injection, produced and facility waters from a Bakken shale oil field in Saskatchewan, Canada with a resident temperature of 60°C. The injection water had a lower salinity (0.7 Meq of NaCl) than produced or facility waters (0.6-3.6 Meq of NaCl). Salinities of the latter decreased with time, likely due to injection of low salinity water, which had 15-30 mM sulfate. Batch cultures of field samples showed sulfate-reducing and nitrate-reducing bacteria activities at different salinities (0, 0.5, 0.75, 1.0, 1.5, and 2.5 M NaCl). Notably, at high salinity nitrite accumulated, which was not observed at low salinity, indicating potential for nitrate-mediated souring control at high salinity. Continuous culture chemostats were established in media with volatile fatty acids (a mixture of acetate, propionate and butyrate) or lactate as electron donor and nitrate or sulfate as electron acceptor at 0.5 to 2.5 M NaCl. Microbial community analyses of these cultures indicated high proportions of Halanaerobium, Desulfovermiculus, Halomonas, and Marinobacter in cultures at 2.5 M NaCl, whereas Desulfovibrio, Geoalkalibacter, and Dethiosulfatibacter were dominant at 0.5 M NaCl. Use of bioreactors to study the effect of nitrate injection on sulfate reduction showed that accumulation of nitrite inhibited SRB activity at 2.5 M but not at 0.5 M NaCl. High proportions of Halanaerobium and Desulfovermiculus were found at 2.5 M NaCl in the absence of nitrate, whereas high proportions of Halomonas and no SRB were found in the presence of nitrate. A diverse microbial community dominated by the SRB Desulfovibrio was observed at 0.5 M NaCl both in the presence and absence of nitrate. Our results suggest that nitrate injection can prevent souring provided that the salinity is maintained at a high level. Thus, reinjection of high salinity produced water amended with nitrate maybe be a cost effective method for souring control.

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.

A Diverse Community of Metal(loid) Oxide Respiring Bacteria Is Associated with Tube Worms in the Vicinity of the Juan de Fuca Ridge Black Smoker Field

Epibiotic bacteria associated with tube worms living in the vicinity of deep sea hydrothermal vents of the Juan de Fuca Ridge in the Pacific Ocean were investigated for the ability to respire anaerobically on tellurite, tellurate, selenite, selenate, metavanadate and orthovanadate as terminal electron acceptors. Out of 107 isolates tested, 106 were capable of respiration on one or more of these oxides, indicating that metal(loid) oxide based respiration is not only much more prevalent in nature than is generally believed, but also is an important mode of energy generation in the habitat. Partial 16S rRNA gene sequencing revealed the bacterial community to be rich and highly diverse, containing many potentially new species. Furthermore, it appears that the worms not only possess a close symbiotic relationship with chemolithotrophic sulfide-oxidizing bacteria, but also with the metal(loid) oxide transformers. Possibly they protect the worms through reduction of the toxic compounds that would otherwise be harmful to the host.

Nitrate shaped the selenate-reducing microbial community in a hydrogen-based biofilm reactor

To study the effect of nitrate (NO3(-)) on selenate (SeO4(2-)) reduction, we tested a H2-based biofilm with a range of influent NO3(-) loadings. When SeO4(2-) was the only electron acceptor (stage 1), 40% of the influent SeO4(2-) was reduced to insoluble elemental selenium (Se(0)). SeO4(2-) reduction was dramatically inhibited when NO3(-) was added at a surface loading larger than 1.14 g of N m(-2) day(-1), when H2 delivery became limiting and only 80% of the input NO3(-) was reduced (stage 2). In stage 3, when NO3(-) was again removed from the influent, SeO4(2-) reduction was re-established and increased to 60% conversion to Se(0). SeO4(2-) reduction remained stable at 60% in stages 4 and 5, when the NO3(-) surface loading was re-introduced at ≤ 0.53 g of N m(-2) day(-1), allowing for complete NO3(-) reduction. The selenate-reducing microbial community was significantly reshaped by the high NO3(-) surface loading in stage 2, and it remained stable through stages 3-5. In particular, the abundance of α-Proteobacteria decreased from 30% in stage 1 to less than 10% of total bacteria in stage 2. β-Proteobacteria, which represented about 55% of total bacteria in the biofilm in stage 1, increased to more than 90% of phylotypes in stage 2. Hydrogenophaga, an autotrophic denitrifier, was positively correlated with NO3(-) flux. Thus, introducing a NO3(-) loading high enough to cause H2 limitation and suppress SeO4(2-) reduction had a long-lasting effect on the microbial community structure, which was confirmed by principal coordinate analysis, although SeO4(2-) reduction remained intact.

Prokaryotic diversity of an active mud volcano in the Usu City of Xinjiang, China

The Usu mud volcanoes are the largest group of terrestrial mud volcanoes in China. The volcanoes are located in a typical arid and semi-arid region, and the group consists of 36 erupting active mud volcanoes. In this study, the prokaryotic diversity and community structure in the sediment of an active mud volcano were investigated by constructing bacterial and archaeal clone libraries of the 16S rRNA gene. A total of 100 bacterial and 100 archaeal clones were analysed and found to comprise 11 and 7 distinct phylotypes, respectively. The bacterial phylotypes were classified into three phyla (Proteobacteria, Actinobacteria, and Fusobacteria). Of these, Proteobacteria were the most abundant bacterial group, with Deltaproteobacteria dominating the sediment community, and these were affiliated with the order Desulfuromonadales. The archaeal phylotypes were all closely related to uncultivated species, and the majority of the members were related to the orders Methanosarcinales and Halobacteriales of the Euryarchaeota originating from methane hydrate bearing or alkaline sediments. The rest of the archaeal phylotypes belonged to the phylum Crenarchaeota, with representatives from similar habitats. These results suggested that a large number of novel microbial groups and potential methanogenesis may exist in this unique ecosystem.

Desulfurispirillum indicum sp. nov., a selenate- and selenite-respiring bacterium isolated from an estuarine canal

Strain S5T, a novel bacterium that was isolated for its capability to respire selenate to elemental selenium, is described. In addition to selenate respiration, it was also capable of dissimilatory selenite, arsenate and nitrate reduction with short-chain organic acids such as pyruvate, lactate and acetate as the carbon sources and electron donors. The isolate was unable to grow fermentatively. Strain S5T was isolated from sediment of an estuarine canal in Chennai, India. Phylogenetic analysis of the 16S rRNA gene of this novel isolate revealed that it belonged to the family Chrysiogenaceae with sequence similarities of 92 and 98 %, respectively, with the type strains of Chrysiogenes arsenatis and Desulfurispirillum alkaliphilum, its closest known relatives. Strain S5T and D. alkaliphilum were closely related in terms of their 16S rRNA gene phylogeny; however, they varied greatly in their genomic DNA G+C content (56 mol% versus 45 mol%) and cellular fatty acid compositions, as well as in many metabolic capabilities. Strain S5T represents a novel species for which the name Desulfurispirillum indicum sp. nov. is proposed; the type strain is S5T (=DSM 22839T =ATCC BAA-1389T).

Development of a genetic transfer system in selenate-respiring bacterium Citrobacter sp. strain JSA which was isolated from natural freshwater sediment

Conjugative mating between the selenate-reducing bacterium Citrobacter sp. strain JSA and Escherichia coli S17-1 harboring the broad-host-range plasmid pKT230 or pKT240 (IncQ) allowed genetic transfer to strain JSA at a maximum frequency of 2.5×10(-5) (pKT230) and 5.1×10(-6) (pKT240) per recipient JSA cell. Kanamycin-resistant (selection marker of pKT230 and pKT240) transconjugants were routinely obtained with this method, and we confirmed that both vectors were also successfully transferred and replicated in strain JSA without alteration of the replicon. Furthermore, an electroporation method has also allowed transformation of JSA at a frequency of 10(-7) to 10(-6) transformants per μg vector DNA (per recipient cell), and PCR and hybridization analyses revealed that pKT230 and pKT240 are stably maintained in transformed JSA cells. These results indicated that both InQ plasmids can be used as vectors for gene transfer to selenate-reducing strain JSA. This is the first study to demonstrate an effective method for genetic transfer in a selenate-reducing Citrobacter bacterium and will aid in the elucidation of the selenium oxyanion reduction mechanism in this genus of environmental selenate-respiring isolates.

Geobacter uraniireducens sp. nov., isolated from subsurface sediment undergoing uranium bioremediation

A Gram-negative, rod-shaped, motile bacterium, strain Rf4T, which conserves energy from dissimilatory Fe(III) reduction concomitant with acetate oxidation, was isolated from subsurface sediment undergoing uranium bioremediation. The 16S rRNA gene sequence of strain Rf4T matched sequences recovered in 16S rRNA gene clone libraries constructed from DNA extracted from groundwater sampled at the same time as the source sediment. Cells of strain Rf4T were regular, motile rods, 1.2-2.0 microm long and 0.5-0.6 microm in diameter, with rounded ends. Cells had one lateral flagellum. Growth was optimal at pH 6.5-7.0 and 32 degrees C. With acetate as the electron donor, strain Rf4T used Fe(III), Mn(IV), anthraquinone-2,6-disulfonate, malate and fumarate as electron acceptors and reduced U(VI) in cell suspensions. With poorly crystalline Fe(III) oxide as the electron acceptor, strain Rf4T oxidized the following electron donors: acetate, lactate, pyruvate and ethanol. Phylogenetic analysis of the 16S rRNA gene sequence of strain Rf4T placed it in the genus Geobacter. Strain Rf4T was most closely related to 'Geobacter humireducens' JW3 (95.9 % sequence similarity), Geobacter bremensis Dfr1T (95.4 %) and Geobacter bemidjiensis BemT (95.1 %). Based on phylogenetic analysis and phenotypic differences between strain Rf4T and closely related Geobacter species, this strain is described as a representative of a novel species, Geobacter uraniireducens sp. nov. The type strain is Rf4T (=ATCC BAA-1134T =JCM 13001T).