A Microbial Arsenic Cycle in a Salt-Saturated, Extreme Environment

Coupled arsenotrophy in a hot spring photosynthetic biofilm at Mono Lake, California

Red-pigmented biofilms grow on rock and cobble surfaces present in anoxic hot springs located on Paoha Island in Mono Lake. The bacterial community was dominated ( approximately 85% of 16S rRNA gene clones) by sequences from the photosynthetic Ectothiorhodospira genus. Scraped biofilm materials incubated under anoxic conditions rapidly oxidized As(III) to As(V) in the light via anoxygenic photosynthesis but could also readily reduce As(V) to As(III) in the dark at comparable rates. Back-labeling experiments with (73)As(V) demonstrated that reduction to (73)As(III) also occurred in the light, thereby illustrating the cooccurrence of these two anaerobic processes as an example of closely coupled arsenotrophy. Oxic biofilms also oxidized As(III) to As(V). Biofilms incubated with [(14)C]acetate oxidized the radiolabel to (14)CO(2) in the light but not the dark, indicating a capacity for photoheterotrophy but not chemoheterotrophy. Anoxic, dark-incubated samples demonstrated As(V) reduction linked to additions of hydrogen or sulfide but not acetate. Chemoautotrophy linked to As(V) as measured by dark fixation of [(14)C]bicarbonate into cell material was stimulated by either H(2) or HS(-). Functional genes for the arsenate respiratory reductase (arrA) and arsenic resistance (arsB) were detected in sequenced amplicons of extracted DNA, with about half of the arrA sequences closely related ( approximately 98% translated amino acid identity) to those from the family Ectothiorhodospiraceae. Surprisingly, no authentic PCR products for arsenite oxidase (aoxB) were obtained, despite observing aerobic arsenite oxidation activity. Collectively, these results demonstrate close linkages of these arsenic redox processes occurring within these biofilms.

Anoxic oxidation of arsenite linked to chemolithotrophic denitrification in continuous bioreactors

In this study, the anoxic oxidation of arsenite (As(III)) linked to chemolithotrophic denitrification was shown to be feasible in continuous bioreactors. Biological oxidation of As(III) was stable over prolonged periods of operation ranging up to 3 years in continuous denitrifying bioreactors with granular biofilms. As(III) was removed with a high conversion efficiency (>92%) to arsenate (As(V)) in periods with high volumetric loadings (e.g., 3.5-5.1 mmol As L(reactor) (-1) day(-1)). The maximum specific activity of sampled granular sludge from the bioreactors was 0.98 +/- 0.04 mmol As(V) formed g(-1) VSS day(-1) when determined at an initial concentration of 0.5 mM As(III). The microbial population adapted to high influent concentrations of As(III) up to 5.2 mM. However, the As(III) oxidation process was severely inhibited when 7.6-8.1 mM As(III) was fed. Activity was restored upon lowering the As(III) concentration to 3.8 mM. Several experimental strategies were utilized to demonstrate a dependence of the nitrate removal on As(III) oxidation as well as a dependence of the As(III) removal on nitrate reduction. The molar stoichiometric ratio of As(V) formed to nitrate removed (corrected for endogenous denitrification) in the bioreactors approximated 2.5, indicating complete denitrification was occurring. As(III) oxidation was also shown to be linked to the complete denitrification of NO(3) (-) to N(2) gas by demonstrating a significantly enhanced production of N(2) beyond the background endogenous production in a batch bioassay spiked with 3.5 mM As(III). The N(2) production also corresponded closely to the expected stoichiometry of 2.5 mol As(III) mol(-1) N(2)-N for complete denitrification.

Arsenic detoxification potential of aox genes in arsenite-oxidizing bacteria isolated from natural and constructed wetlands in the Republic of Korea

Arsenic is subject to microbial interactions, which support a wide range of biogeochemical transformations of elements in natural environments such as wetlands. The arsenic detoxification potential of the bacterial strains was investigated with the arsenite oxidation gene, aox genotype, which were isolated from the natural and constructed wetlands. The isolates were able to grow in the presence of 10 mM of sodium arsenite (As(III) as NaAsO(2)) and 1 mM of D: +glucose. Phylogenetic analysis based on 16S rRNA gene sequencing indicated that these isolated strains resembled members of the genus that have arsenic-resistant systems (Acinetobacter sp., Aeromonas sp., Agrobacterium sp., Comamonas sp., Enterobacter sp., Pantoea sp., and Pseudomonas sp.) with sequence similarities of 81-98%. One bacterial isolate identified as Pseudomonas stutzeri strain GIST-BDan2 (EF429003) showed the activity of arsenite oxidation and existence of aoxB and aoxR gene, which could play an important role in arsenite oxidation to arsenate. This reaction may be considered as arsenic detoxification process. The results of a batch test showed that P. stutzeri GIST-BDan2 (EF429003) completely oxidized in 1 mM of As(III) to As(V) within 25-30 h. In this study, microbial activity was evaluated to provide a better understanding of arsenic biogeochemical cycle in both natural and constructed wetlands, where ecological niches for microorganisms could be different, with a specific focus on arsenic oxidation/reduction and detoxification.

Arsenic-resistant bacteria associated with roots of the wild Cirsium arvense (L.) plant from an arsenic polluted soil, and screening of potential plant growth-promoting characteristics

A rhizobacterial community, associated with the roots of wild thistle Cirsium arvense (L.) growing in an arsenic polluted soil, was studied by fluorescence in situ hybridization (FISH) analysis in conjunction with cultivation-based methods. In the bulk, rhizosphere, and rhizoplane fractions of the soil, the qualitative picture obtained by FISH analysis of the main phylogenetic bacterial groups was similar and was predominantly comprised of Alphaproteobacteria, Betaproteobacteria, and Gammaproteobacteria. The arsenic-resistant isolates belonged to 13 genera, the most abundant being those of Bacillus, Achromobacter, Brevundimonas, Microbacterium, and Ochrobactrum. Most bacteria grew in the presence of high arsenic concentrations (over 100mM arsenate and 10mM arsenite). Most strains possessed the ArsC, ArsB and ACR3 genes homologous to arsenate reductase and to the two classes of arsenite efflux pumps, respectively, peculiar to the ars operon of the arsenic detoxification system. ArsB and ACR3 were present simultaneously in highly resistant strains. An inconsistency between 16S rRNA phylogenetic affiliations and the arsenate reductase sequences of the strains was observed, indicating possible horizontal transfer of arsenic resistance genes in the soil bacterial community. Several isolates were able to reduce arsenate and to oxidise arsenite. In particular, Ancylobacter dichloromethanicum strain As3-1b possessed both characteristics, and arsenite oxidation occurred in the strain also under chemoautotrophic conditions. Some rhizobacteria produced siderophores, indole acetic acid and 1-amino-cyclopropane-1-carboxylic acid deaminase, thus possessing potential plant growth-promoting traits.

Arsenite and ferrous iron oxidation linked to chemolithotrophic denitrification for the immobilization of arsenic in anoxic environments

The objective of this study was to explore a bioremediation strategy based on injecting NO3- to support the anoxic oxidation of ferrous iron (Fe(II)) and arsenite (As(II)) in the subsurface as a means to immobilize As in the form of arsenate (As(V)) adsorbed onto biogenic ferric (Fe(III)) (hydr)oxides. Continuous flow sand filled columns were used to simulate a natural anaerobic groundwater and sediment system with co-occurring As(III) and Fe(II) in the presence (column SF1) or absence (column SF2) of nitrate, respectively. During operation for 250 days, the average influent arsenic concentration of 567 microg L(-1) was reduced to 10.6 (+/-9.6) microg L(-1) in the effluent of column SF1. The cumulative removal of Fe(II) and As(II) in SF1 was 6.5 to 10-fold higher than that in SF2 Extraction and measurement of the mass of iron and arsenic immobilized on the sand packing of the columns were close to the iron and arsenic removed from the aqueous phase during column operation. The dominant speciation of the immobilized iron and arsenic was Fe(III) and As(V) in SF1, compared with Fe(II) and As(III) in SF2. The speciation was confirmed by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The results indicate that microbial oxidation of As(III) and Fe(II) linked to denitrification resulted in the enhanced immobilization of aqueous arsenic in anaerobic environments by forming Fe(III) (hydr)oxide coated sands with adsorbed As(V).

Biotransformation of arsenic by a Yellowstone thermoacidophilic eukaryotic alga

Arsenic is the most common toxic substance in the environment, ranking first on the Superfund list of hazardous substances. It is introduced primarily from geochemical sources and is acted on biologically, creating an arsenic biogeocycle. Geothermal environments are known for their elevated arsenic content and thus provide an excellent setting in which to study microbial redox transformations of arsenic. To date, most studies of microbial communities in geothermal environments have focused on Bacteria and Archaea, with little attention to eukaryotic microorganisms. Here, we show the potential of an extremophilic eukaryotic alga of the order Cyanidiales to influence arsenic cycling at elevated temperatures. Cyanidioschyzon sp. isolate 5508 oxidized arsenite [As(III)] to arsenate [As(V)], reduced As(V) to As(III), and methylated As(III) to form trimethylarsine oxide (TMAO) and dimethylarsenate [DMAs(V)]. Two arsenic methyltransferase genes, CmarsM7 and CmarsM8, were cloned from this organism and demonstrated to confer resistance to As(III) in an arsenite hypersensitive strain of Escherichia coli. The 2 recombinant CmArsMs were purified and shown to transform As(III) into monomethylarsenite, DMAs(V), TMAO, and trimethylarsine gas, with a T(opt) of 60-70 degrees C. These studies illustrate the importance of eukaryotic microorganisms to the biogeochemical cycling of arsenic in geothermal systems, offer a molecular explanation for how these algae tolerate arsenic in their environment, and provide the characterization of algal methyltransferases.

Arsenic-resistant bacteria isolated from agricultural soils of Bangladesh and characterization of arsenate-reducing strains

AIMS

To analyse the arsenic-resistant bacterial communities of two agricultural soils of Bangladesh, to isolate arsenic-resistant bacteria, to study their potential role in arsenic transformation and to investigate the genetic determinants for arsenic resistance among the isolates.

METHODS AND RESULTS

Enrichment cultures were performed in a minimal medium in the presence of As(III) and As(V) to isolate resistant bacteria. Twenty-one arsenic-resistant bacteria belonging to different genera of Gram-positive and Gram-negative bacteria were isolated. The isolates, with the exception of Oceanimonas doudoroffii Dhal Rw, reduced 2 mmol l(-1) As(V) completely to As(III) in aerobic conditions. Putative gene fragments for arsenite efflux pumps were amplified in isolates from Dhal soil and a putative arsenate reductase gene fragment was amplified from a Bacillus sp. from Rice soil.

CONCLUSIONS

Phylogenetically diverse arsenic-resistant bacteria present in agricultural soils of Bangladesh are capable of reducing arsenate to arsenite under aerobic conditions apparently for detoxification purpose.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study provides results on identification, levels of arsenic resistance and reduction of arsenate by the bacterial isolates which could play an important role in arsenic cycling in the two arsenic-contaminated soils in Bangladesh.

Ecophysiology of "Halarsenatibacter silvermanii" strain SLAS-1T, gen. nov., sp. nov., a facultative chemoautotrophic arsenate respirer from salt-saturated Searles Lake, California

Searles Lake occupies a closed basin harboring salt-saturated, alkaline brines that have exceptionally high concentrations of arsenic oxyanions. Strain SLAS-1(T) was previously isolated from Searles Lake (R. S. Oremland, T. R. Kulp, J. Switzer Blum, S. E. Hoeft, S. Baesman, L. G. Miller, and J. F. Stolz, Science 308:1305-1308, 2005). We now describe this extremophile with regard to its substrate affinities, its unusual mode of motility, sequenced arrABD gene cluster, cell envelope lipids, and its phylogenetic alignment within the order Halanaerobacteriales, assigning it the name "Halarsenatibacter silvermanii" strain SLAS-1(T). We also report on the substrate dynamics of an anaerobic enrichment culture obtained from Searles Lake that grows under conditions of salt saturation and whose members include a novel sulfate reducer of the order Desulfovibriales, the archaeon Halorhabdus utahensis, as well as a close homolog of strain SLAS-1(T).

Bacterial succession within an ephemeral hypereutrophic Mojave Desert playa Lake

Ephemerally wet playas are conspicuous features of arid landscapes worldwide; however, they have not been well studied as habitats for microorganisms. We tracked the geochemistry and microbial community in Silver Lake playa, California, over one flooding/desiccation cycle following the unusually wet winter of 2004-2005. Over the course of the study, total dissolved solids increased by approximately 10-fold and pH increased by nearly one unit. As the lake contracted and temperatures increased over the summer, a moderately dense planktonic population of approximately 1x10(6) cells ml(-1) of culturable heterotrophs was replaced by a dense population of more than 1x10(9) cells ml(-1), which appears to be the highest concentration of culturable planktonic heterotrophs reported in any natural aquatic ecosystem. This correlated with a dramatic depletion of nitrate as well as changes in the microbial community, as assessed by small subunit ribosomal RNA gene sequencing of bacterial isolates and uncultivated clones. Isolates from the early-phase flooded playa were primarily Actinobacteria, Firmicutes, and Bacteroidetes, yet clone libraries were dominated by Betaproteobacteria and yet uncultivated Actinobacteria. Isolates from the late-flooded phase ecosystem were predominantly Proteobacteria, particularly alkalitolerant isolates of Rhodobaca, Porphyrobacter, Hydrogenophaga, Alishwenella, and relatives of Thauera; however, clone libraries were composed almost entirely of Synechococcus (Cyanobacteria). A sample taken after the playa surface was completely desiccated contained diverse culturable Actinobacteria typically isolated from soils. In total, 205 isolates and 166 clones represented 82 and 44 species-level groups, respectively, including a wide diversity of Proteobacteria, Bacteroidetes, Actinobacteria, Firmicutes, Gemmatimonadetes, Acidobacteria, and Cyanobacteria.

Role of soil-derived dissolved substances in arsenic transport and transformation in laboratory experiments

Dissolved substances derived from soil may interact with both soil surfaces and with arsenic and subsequently influence arsenic mobility and species transformation. The purpose of this study was to investigate arsenic transport and transformation in porous media with a specific focus on the impact of soil-derived dissolved substances, mainly consisting of inorganic colloids and dissolved organic matter (DOM), on these processes. Arsenic transport and transformation through columns, which were packed with uncoated sand (UC) or naturally coated sand (NC) and fed with arsenate (AsV) or monomethylarsonic acid (MMA) spiked influents, were investigated in the presence or absence of soil-derived dissolved substances. The presence of soil-derived inorganic colloids and/or DOM clearly enhanced As transport through the column, with the fraction of As leached out of column (referring to the total amount added) being increased from 23 to 46% (UC) and 21 to 50% (NC) in AsV experiments while 46 to 64% (UC) and 28 to 63% (NC) in MMA experiments. The association of arsenic with DOM and the competitive adsorption between arsenic and DOM could account for, at least partly, the enhanced As movement. Distinct species transformation of As during transport through soil columns was observed. When AsV was the initial species spiked in the influent solutions, only arsenite (AsIII) was detected in the effluents for UC columns; while both AsIII (dominant) and AsV were present for NC columns, with AsIII being the dominant species. When MMA was initially spiked in the influent solutions, all method detectable As species, AsIII, AsV, MMA, and dimethylarsenic acid (DMA) were present in the effluents for both soil columns. These results indicate that risk assessment associated with As contamination, particularly due to previous organoarsenical pesticide applications, should take into account the role of soil-derived dissolved substances in promoting As transport and As species transformation.

Isolation and diversity analysis of arsenite-resistant bacteria in communities enriched from deep-sea sediments of the Southwest Indian Ocean Ridge

Microorganisms play an important role in the geobiocycling of arsenic element. However, little is known about the bacteria involved in this process in oceanic environments. In this report, arsenite-resistant bacteria were detected in deep-sea sediments on the Southwest Indian Ridge. From arsenite enriched cultures, 54 isolates were obtained, which showed varied tolerance to arsenite of 2-80 mM. Phylogenetic analysis based on 16S rRNA showed that they mainly belonged to Proteobacteria and Actinobacteria. Denaturing gradient gel electrophoresis revealed that Microbacterium esteraromaticum was the dominant member in the arsenite enriched communities, and this was reconfirmed by 16S rRNA gene library analyses. Thus, M. esteraromaticum showed highest resistant to arsenite among the detected bacteria. These results indicate that there are quite diverse bacteria of arsenite resistance inhabiting the deep sea sediment, which may play a role in the geobiocycling of arsenic element in marine environments.

Volatile arsenic species released from Escherichia coli expressing the AsIII S-adenosylmethionine methyltransferase gene

Biological systems, ranging from bacteria and fungi to humans, can methylate arsenic. Recent studies have suggested that the AsIII S-adenosylmethionine methyltransferase (arsM) gene in bacteria was responsible for the removal of arsenic as the volatile arsines from the bacteria. However, there has been no direct measure of the arsines released from bacteria cultures. We describe here an integrated system incorporating the bacterial incubation and volatile arsenic species analysis, and we demonstrate its application to the identification of the volatile arsines produced in bacterial cultures. The headspace of the bacterial cultures was purged with helium, and the volatile arsenic species were trapped in a chromatographic column immersed in liquid nitrogen. The cryogenically trapped arsines [AsH3, (CH3)AsH2, (CH3)2AsH, and (CH3)3As] were separated by gas chromatography and were detected by inductively coupled plasma mass spectrometry. A hydride generation system was coupled to the bacterial culture system, allowing for spiking standards and for generating calibration arsines necessary for quantitative analysis. Both bacteria containing the arsM gene or its variant arsMC2 gene were able to produce 400-500 ng of trimethylarsine. No trimethylarsine was detectable in bacteria lacking the arsM gene (containing the vector plasmid as negative control). These results confirm that arsM is responsible for releasing arsenic as volatile species from the arsenic-resistant bacteria. Our results also show traces of AsH3, CH3AsH2, and (CH3)2AsH in cultures of bacteria expressing arsM. The method detection limits for AsH3, CH3AsH2, (CH3)2AsH, and (CH3)3As were 0.5, 0.5, 0.7, and 0.6 pg, respectively. The ability to quantify trace levels of these volatile arsenic species makes it possible to study the biotransformation and biochemical roles of the evolution of these volatile arsenic species by biological systems.

Chemical and steady-state kinetic analyses of a heterologously expressed heme dependent chlorite dismutase

Chlorite dismutase carries out the heme-catalyzed decomposition of ClO2- to Cl- and O2, an unusual transformation with biotechnological and bioremediative applications. The enzyme has been successfully overexpressed for the first time in highly functional form in Escherichia coli and its steady state kinetics studied. The purified enzyme is abundant (55 mg/L cell culture), highly active (approximately 4.7 x 10(3) micromol of ClO2- min(-1) mg(-1) subunit) and nearly stoichiometric in heme; further, it shares spectroscopic and physicochemical features with chlorite dismutases previously isolated from three organisms. A careful study of the enzyme's steady state kinetics has been carried out. ClO2- consumption and O2 release rates were measured, yielding comparable values of kcat (4.5 x 10(5) min(-1)), K(m) (approximately 215 microM), and kcat/Km (3.5 x 10(7) M(-1) s(-1) via either method (4 degrees C, pH 6.8; all values referenced per heme-containing subunit). ClO2-:O2 stoichiometry exhibited a 1:1 relationship under all conditions measured. Though the value of kcat/Km indicates near diffusion control of the reaction, viscosogens had no effect on k(cat)/K(m) or V(max). The product O2 did not inhibit the reaction at saturating [O2], but Cl- is a mixed inhibitor with relatively high values of KI (225 mM for enzyme and 95.6 mM for the enzyme-substrate complex), indicating a relatively low affinity of the heme iron for halogen ions. Chlorite irreversibly inactivates the enzyme after approximately 1.7 x 10(4) turnovers (per heme) and with a half-life of 0.39 min, resulting in bleaching of the heme chromophore. The inactivation K(I) (K(inact)) of 166 microM is similar in magnitude to Km, consistent with a common Michaelis complex on the pathway to both reaction and inactivation. The one-electron peroxidase substrate guaiacol offers incomplete protection of the enzyme from inactivation. Mechanisms in keeping with the available data and the properties of other well-described heme enzymes are proposed.

Analysis of underivatized amino acids in geological samples using ion-pairing liquid chromatography and electrospray tandem mass spectrometry

The capability of detecting biomarkers, such as amino acids, in chemically complex field samples is essential to establishing the knowledge required to search for chemical signatures of life in future planetary explorations. However, due to the complexities of in situ investigations, it is important to establish a new analytical scheme that utilizes a minimal amount of sample preparation. This paper reports the feasibility of a novel and sensitive technique, which has been established to quantitate amino acids in terrestrial crust samples directly without derivatization using volatile ion-pairing liquid chromatography and tandem mass spectrometry equipped with an electrospray ionization source. Adequate separation of 20 underivatized amino acids was achieved on a C(18) capillary column within 26 min with nonafluoropentanoic acid (NFPA) as ion-pairing reagent. Each amino acid was identified from its retention time as well as from its characteristic parent-to-daughter ion transition. Using tandem mass spectrometry as a detection technique allows co-elution of some amino acids, as it is more specific than traditional spectrophotometric methods. In the present study, terrestrial samples collected from 3 different locations were analyzed for their water-extractable free amino acid contents, following the removal of metal and organic interferences via ion exchange procedures. This is the first time that amino acids in geological samples were directly determined quantitatively without complicated derivatization steps. Depending on the amino acid, the detection limits varied from 0.02 to 5.7 pmol with the use of a 1 microl sample injection loop.

A new role for sulfur in arsenic cycling

Sulfur and arsenic often coexist in the environment and share similar microbial redox transformations. We examined the effects of sulfide on aerobic arsenite oxidation in alkaline lake water samples and in laboratory enrichment cultures. Significant arsenite oxidation occurred only in treatments with bacteria present, and production of arsenate was greatly enhanced by the addition of sulfide or thiosulfate. IC-ICP-MS analysis of samples showed that mono- and dithioarsenate formed in arsenite + sulfide amended lake water. Our data indicate that these two thioarsenic compounds are fairly stable in sterile alkaline solutions, but are transformed predominantly to arsenate when bacteria are present. Enrichment culture experiments suggest that sulfur-oxidizing bacteria use free or arsenic-bound sulfur as a growth substrate and directly or indirectly transform arsenite and thioarsenates to arsenate during growth. Increases in cell density resulted in more rapid conversion of arsenite and thioarsenates. The rate and extent of these processes appearto be controlled bythe concentration of bacteria and the ratio of reduced sulfur to arsenite present. Sulfur-driven arsenite oxidation and microbial thioarsenate transformation may be important biogeochemical processes in the arsenic cycle of our study site (Mono Lake, CA, USA) and other alkaline environments as well.

Biogeochemical cyclic activity of bacterial arsB in arsenic-contaminated mines

Biogeochemical cyclic activity of the ars (arsenic resistance system) operon is arsB influx/efflux encoded by the ecological of Pseudomonas putida. This suggests that studying arsenite-oxidizing bacteria may lead to a better understanding of molecular geomicrobiology, which can be applied to the bioremediation of arsenic-contaminated mines. This is the first report in which multiple arsB-binding mechanisms have been used on indigenous bacteria. In ArsB (strains OS-5; ABB83931; OS-19; ABB04282 and RW-28; ABB88574), there are ten putative enzyme, Histidine (His) 131, His 133, His 137, Arginine (Arg) 135, Arg 137, Arg 161, Trptohan (Trp) 142, Trp 164, Trp 166, and Trp 171, which are each located in different regions of the partial sequence. The adenosine triphosphate (ATP)-binding cassette transports, binding affinities and associating ratable constants show that As-binding is comparatively insensitive to the location of the residues within the moderately stable alpha-helical structure. The alpha-helical structures in ArsB-permease and anion permease arsB have been shown to import/export arsenic in P. putida. We proposed that arsB residues, His 131, His 133, His 137, Arg 135, Arg 137, Arg 161, Trp 142, Trp 164, Trp 166, and Trp 171 are required for arsenic binding and activation of arsA/arsB or arsAB. This arsB influx/efflux pum-ping is important, and the effect in arsenic species change and mobility in mine soil has got a significantly ecological role because it allows arsenic oxidizing/reducing bacteria to control biogeochemical cycle of abandoned mines.

Effects of imposed salinity gradients on dissimilatory arsenate reduction, sulfate reduction, and other microbial processes in sediments from two California soda lakes

Salinity effects on microbial community structure and on potential rates of arsenate reduction, arsenite oxidation, sulfate reduction, denitrification, and methanogenesis were examined in sediment slurries from two California soda lakes. We conducted experiments with Mono Lake and Searles Lake sediments over a wide range of salt concentrations (25 to 346 g liter(-1)). With the exception of sulfate reduction, rates of all processes demonstrated an inverse relationship to total salinity. However, each of these processes persisted at low but detectable rates at salt saturation. Denaturing gradient gel electrophoresis analysis of partial 16S rRNA genes amplified from As(V) reduction slurries revealed that distinct microbial populations grew at low (25 to 50 g liter(-1)), intermediate (100 to 200 g liter(-1)), and high (>300 g liter(-1)) salinity. At intermediate and high salinities, a close relative of a cultivated As-respiring halophile was present. These results suggest that organisms adapted to more dilute conditions can remain viable at high salinity and rapidly repopulate the lake during periods of rising lake level. In contrast to As reduction, sulfate reduction in Mono Lake slurries was undetectable at salt saturation. Furthermore, sulfate reduction was excluded from Searles Lake sediments at any salinity despite the presence of abundant sulfate. Sulfate reduction occurred in Searles Lake sediment slurries only following inoculation with Mono Lake sediment, indicating the absence of sulfate-reducing flora. Experiments with borate-amended Mono Lake slurries suggest that the notably high (0.46 molal) concentration of borate in the Searles Lake brine was responsible for the exclusion of sulfate reducers from that ecosystem.

A tale of two oxidation states: bacterial colonization of arsenic-rich environments

Microbial biotransformations have a major impact on contamination by toxic elements, which threatens public health in developing and industrial countries. Finding a means of preserving natural environments-including ground and surface waters-from arsenic constitutes a major challenge facing modern society. Although this metalloid is ubiquitous on Earth, thus far no bacterium thriving in arsenic-contaminated environments has been fully characterized. In-depth exploration of the genome of the beta-proteobacterium Herminiimonas arsenicoxydans with regard to physiology, genetics, and proteomics, revealed that it possesses heretofore unsuspected mechanisms for coping with arsenic. Aside from multiple biochemical processes such as arsenic oxidation, reduction, and efflux, H. arsenicoxydans also exhibits positive chemotaxis and motility towards arsenic and metalloid scavenging by exopolysaccharides. These observations demonstrate the existence of a novel strategy to efficiently colonize arsenic-rich environments, which extends beyond oxidoreduction reactions. Such a microbial mechanism of detoxification, which is possibly exploitable for bioremediation applications of contaminated sites, may have played a crucial role in the occupation of ancient ecological niches on earth.

Hydrogen thresholds and steady-state concentrations associated with microbial arsenate respiration

H2 thresholds for microbial respiration of arsenate (As(V)) were investigated in a pure culture of Sulfurospirillum arsenophilum. H2 was consumed to threshold concentrations of 0.03-0.09 nmol/L with As(V) as terminal electron acceptor, allowing for a Gibbs free-energy yield of 36-41 kJ per mol of reaction. These thresholds are among the lowest measured for anaerobic respirers and fall into the range of denitrifiers or Fe(III)-reducers. In sediments from an arsenic-contaminated aquifer in the Red River flood plain, Vietnam, H2 levels decreased to 0.4-2 nmol/L when As(V) was added under anoxic conditions. When As-(V) was depleted, H2 concentrations rebounded by a factor of 10, a level similar to that observed in arsenic-free controls. The sediment-associated microbial population completely reduced millimolar levels of As(V) to arsenite (As-(III)) within a few days. The rate of As(V)-reduction was essentially the same in sediments amended with a pure culture of S. arsenophilum. These findings together with a review of observed H2 threshold and steady-state values suggest that microbial As(V)-respirers have a competitive advantage over several other anaerobic respirers through their ability to thrive at low H2 levels.

Alkalilimnicola ehrlichii sp. nov., a novel, arsenite-oxidizing haloalkaliphilic gammaproteobacterium capable of chemoautotrophic or heterotrophic growth with nitrate or oxygen as the electron acceptor

A facultative chemoautotrophic bacterium, strain MLHE-1T, was isolated from Mono Lake, an alkaline hypersaline soda lake in California, USA. Cells of strain MLHE-1T were Gram-negative, short motile rods that grew with inorganic electron donors (arsenite, hydrogen, sulfide or thiosulfate) coupled with the reduction of nitrate to nitrite. No aerobic growth was attained with arsenite or sulfide, but hydrogen sustained both aerobic and anaerobic growth. No growth occurred when nitrite or nitrous oxide was substituted for nitrate. Heterotrophic growth was observed under aerobic and anaerobic (nitrate) conditions. Cells of strain MLHE-1T could oxidize but not grow on CO, while CH4 neither supported growth nor was it oxidized. When grown chemoautotrophically, strain MLHE-1T assimilated inorganic carbon via the Calvin–Benson–Bassham reductive pentose phosphate pathway, with the activity of ribulose 1,5-bisphosphate carboxylase (RuBisCO) functioning optimally at 0.1 M NaCl and at pH 7.3. Strain MLHE-1T grew over broad ranges of pH (7.3–10.0; optimum, 9.3), salinity (15–190 g l−1; optimum 30 g l−1) and temperature (13–40 °C; optimum, 30 °C). Phylogenetic analysis of 16S rRNA gene sequences placed strain MLHE-1T in the class Gammaproteobacteria (family Ectothiorhodospiraceae) and most closely related to Alkalispirillum mobile (98.5 %) and Alkalilimnicola halodurans (98.6 %), although none of these three haloalkaliphilic micro-organisms were capable of photoautotrophic growth and only strain MLHE-1T was able to oxidize As(III). On the basis of physiological characteristics and DNA–DNA hybridization data, it is suggested that strain MLHE-1T represents a novel species within the genus Alkalilimnicola for which the name Alkalilimnicola ehrlichii is proposed. The type strain is MLHE-1T (=DSM 17681T=ATCC BAA-1101T). Aspects of the annotated full genome of Alkalilimnicola ehrlichii are discussed in the light of its physiology.

Biotransformation of 3-nitro-4-hydroxybenzene arsonic acid (roxarsone) and release of inorganic arsenic by Clostridium species

The extensive use of 3-nitro-4-hydroxybenzene arsonic acid (roxarsone) in the production of broiler chickens can lead to increased soil arsenic concentration and arsenic contaminated dust. While roxarsone is the dominant arsenic species in fresh litter, inorganic As (V) predominates in composted litter. Microbial activity has been implicated as the cause, but neither the specific processes nor the organisms have been identified. Here we demonstrate the rapid biotransformation of roxarsone under anaerobic conditions by Clostridium species in chicken litter enrichments and a pure culture of a fresh water arsenate respiring species (Clostridium sp. strain OhILAs). The main products were 3-amino-4-hydroxybenzene arsonic acid and inorganic arsenic. Growth experiments and genomic analysis indicate strain OhILAs may use roxarsone as a terminal electron acceptor for anaerobic respiration. Electronic structure analysis suggests that the reducing equivalents should go to the nitro group, while liberation of inorganic arsenic from the intact benzene ring by cleaving the C-As bond is unlikely. Clostridium and Lactobacillus species are common in the chicken cecum and litter. Thus, the organic-rich manure and anaerobic conditions typically associated with composting provide the conditions necessary for the native microbial populations to transform the roxarsone in the litter releasing the more toxic inorganic arsenic.

Dissimilatory arsenate and sulfate reduction in sediments of two hypersaline, arsenic-rich soda lakes: Mono and Searles Lakes, California

A radioisotope method was devised to study bacterial respiratory reduction of arsenate in sediments. The following two arsenic-rich soda lakes in California were chosen for comparison on the basis of their different salinities: Mono Lake (approximately 90 g/liter) and Searles Lake (approximately 340 g/liter). Profiles of arsenate reduction and sulfate reduction were constructed for both lakes. Reduction of [73As]arsenate occurred at all depth intervals in the cores from Mono Lake (rate constant [k] = 0.103 to 0.04 h(-1)) and Searles Lake (k = 0.012 to 0.002 h(-1)), and the highest activities occurred in the top sections of each core. In contrast, [35S]sulfate reduction was measurable in Mono Lake (k = 7.6 x10(4) to 3.2 x 10(-6) h(-1)) but not in Searles Lake. Sediment DNA was extracted, PCR amplified, and separated by denaturing gradient gel electrophoresis (DGGE) to obtain phylogenetic markers (i.e., 16S rRNA genes) and a partial functional gene for dissimilatory arsenate reduction (arrA). The amplified arrA gene product showed a similar trend in both lakes; the signal was strongest in surface sediments and decreased to undetectable levels deeper in the sediments. More arrA gene signal was observed in Mono Lake and was detectable at a greater depth, despite the higher arsenate reduction activity observed in Searles Lake. A partial sequence (about 900 bp) was obtained for a clone (SLAS-3) that matched the dominant DGGE band found in deeper parts of the Searles Lake sample (below 3 cm), and this clone was found to be closely related to SLAS-1, a novel extremophilic arsenate respirer previously cultivated from Searles Lake.

Arsenic and Selenium in Microbial Metabolism

Arsenic and selenium are readily metabolized by prokaryotes, participating in a full range of metabolic functions including assimilation, methylation, detoxification, and anaerobic respiration. Arsenic speciation and mobility is affected by microbes through oxidation/reduction reactions as part of resistance and respiratory processes. A robust arsenic cycle has been demonstrated in diverse environments. Respiratory arsenate reductases, arsenic methyltransferases, and new components in arsenic resistance have been recently described. The requirement for selenium stems primarily from its incorporation into selenocysteine and its function in selenoenzymes. Selenium oxyanions can serve as an electron acceptor in anaerobic respiration, forming distinct nanoparticles of elemental selenium that may be enriched in (76)Se. The biogenesis of selenoproteins has been elucidated, and selenium methyltransferases and a respiratory selenate reductase have also been described. This review highlights recent advances in ecology, biochemistry, and molecular biology and provides a prelude to the impact of genomics studies.

Sulfide oxidation coupled to arsenate reduction by a diverse microbial community in a soda lake

We characterized the arsenate-reducing, sulfide-oxidizing population of Mono Lake, California, by analyzing the distribution and diversity of rrnA, cbbL, and dissimilatory arsenate reductase (arrA) genes in environmental DNA, arsenate-plus sulfide-amended lake water, mixed cultures, and isolates. The arsenate-reducing community was diverse. An organism represented by an rrnA sequence previously retrieved from Mono Lake and affiliated with the Desulfobulbaceae (Deltaproteobacteria) appears to be an important member of the arsenate-reducing, sulfide-oxidizing community. Sulfide oxidation coupled with arsenate reduction appears to proceed via a two-electron transfer, resulting in the production of arsenite and an intermediate S compound that is subsequently disproportionated. A realgar-like As/S mineral was formed in some experiments.

Environmental microbes can speciate and cycle arsenic

Naturally occurring arsenic is found predominantly as arsenate [As(V)] or arsenite [As(III)], and can be readily oxidized or reduced by microorganisms. Given the health risks associated with arsenic in groundwater and the interest in arsenic-active microorganisms, we hypothesized that environmental microorganisms could mediate a redox cycling of arsenic that is linked to their metabolism. This hypothesis was tested using an As(V) respiring reducer (strain Y5) and an aerobic chemoautotrophic As(II) oxidizer (strain OL1 ) both isolated from a Superfund site, Onondaga Lake, in Syracuse, NY. Strains were grown separately and together in sealed serum bottles, and the oxic/anoxic condition was the only parameter changed. Initially, under anoxic conditions when both isolates were grown together, 2 mM As(V) was stoichiometrically reduced to As(III) within 14 days. Following complete reduction, sterile ambient air was added and within 24 h As(III) was completely oxidized to As(V). The anoxic-oxic cycle was repeated, and sterile controls showed no abiotic transformation within the 28-day incubation period. These results demonstrate that microorganisms can cycle arsenic in response to dynamic environmental conditions, thereby affecting the speciation, and hence mobility and toxicity of arsenic in the environment.