Geothermal springs within Yellowstone National Park (YNP) often contain arsenic (As) at concentrations of 10-40 microM, levels that are considered toxic to many organisms. Arsenite (As(III)) is often the predominant valence state at the point of discharge but is rapidly oxidized to arsenate (As(V)) during transport in shallow surface water. The current study was designed to establish rates and possible mechanisms of As(III) oxidation and to characterize the geochemical environment associated with predominant microbial mats in a representative acid-sulfate-chloride (pH 3.1) thermal (58-62 degrees C) spring in Norris Basin, YNP. At the spring origin, total soluble As was predominantly As(III) at concentrations of 33 microM. No oxidation of As(III) was detected over the first 2.7 m downstream from the spring source, corresponding to an area dominated by a yellow filamentous S0-rich microbial mat However, rapid oxidation of As(III) to As(V) was observed between 2.7 and 5.6 m, corresponding to termination of the S0-rich mats, decreases in dissolved sulfide, and commencement of a brown Fe/As-rich mat. Rates of As(II) oxidation were estimated, yielding an apparent first-order rate constant of 1.2 min(-1) (half-life = 0.58 min). The oxidation of As(III) was shown to require live organisms present just prior to and within the Fe/As-rich mat. Complementary analytical tools used to characterize the brown mat revealed an As:Fe molar ratio of 0.7 and suggested that this filamentous microbial mat contains iron(III) oxyhydroxide coprecipitated with As(V). Results from the current work are the first to provide a comprehensive characterization of microbially mediated As(III) oxidation and the geochemical environments associated with microbial mats in acid-sulfate-chloride springs of YNP.

Electron microscopy (EM), denaturing gradient gel electrophoresis (DGGE) and 16S rDNA sequencing were used to examine the structure and diversity of microbial mats present in an acid-sulphate-chloride (pH 3.1) thermal (58-62 degrees C) spring in Norris Basin, Yellowstone National Park, WY, USA, exhibiting rapid rates of arsenite oxidation. Initial visual assessments, scanning EM and geochemical measurements revealed the presence of three distinct mat types. Analysis of 16S rDNA fragments with DGGE confirmed the presence of different bacterial and archaeal communities within these zones. Changes in the microbial community appeared to coincide with arsenite oxidation activity. Phylogenetic analysis of 1400 bp 16S rDNA sequences revealed that clone libraries prepared from both arsenic redox active and inactive bacterial communities were dominated by sequences phylogenetically related to Hydrogenobacter acidophilus and Desulphurella sp. The appearance of archaeal 16S rDNA sequences coincided with the start of arsenite oxidation, and sequences were obtained showing affiliation with both Crenarchaeota and Euryarchaeota. The majority of archaeal sequences were most similar to sequences obtained from marine hydrothermal vents and other acidic hot springs, although the level of similarity was typically just 90%. Arsenite oxidation in this system may result from the activities of these unknown archaeal taxa and/or the previously unreported arsenic redox activity of H. acidophilus- or Desulphurella-like organisms. If the latter, arsenite oxidation must be inhibited in the initial high-sulphide zone of the spring, where no change in the distribution of arsenite versus arsenate was observed.

The source waters of acid‐sulphate‐chloride (ASC) geothermal springs located in Norris Geyser Basin, Yellowstone National Park contain several reduced chemical species, including H2, H2S, As(III), and Fe(II), which may serve as electron donors driving chemolithotrophic metabolism. Microorganisms thriving in these environments must also cope with high temperatures, low pH (∼3), and high concentrations of sulphide, As(III), and boron. The goal of the current study was to correlate the temporal and spatial distribution of bacterial and archaeal populations with changes in temperature and geochemical energy gradients occurring throughout a newly formed (redirected) outflow channel of an ASC spring. A suite of complimentary analyses including aqueous geochemistry, microscopy, solid phase identification, and 16S rDNA sequence distribution were used to correlate the appearance of specific microbial populations with biogeochemical processes mediating S, Fe, and As cycling and subsequent biomineralization of As(V)‐rich hydrous ferric oxide (HFO) mats. Rapid As(III) oxidation (maximum first order rate constants ranged from 4 to 5 min−1, t1/2 = 0.17 − 0.14 min) was correlated with the appearance of Hydrogenobaculum and Thiomonas–like populations, whereas the biogenesis of As(V)‐rich HFO microbial mats (mole ratios of As:Fe ∼0.7) was correlated with the appearance of Metallosphaera, Acidimicrobium, and Thiomonas–like populations. Several 16S sequences detected near the source were closely related to sequences of chemolithotrophic hyperthermophilic populations including Stygiolobus and Hydrogenobaculum organisms that are known H2 oxidizers. The use of H2, reduced S(–II,0), Fe(II) and perhaps As(III) by different organisms represented throughout the outflow channel was supported by thermodynamic calculations, confirming highly exergonic redox couples with these electron donors. Results from this work demonstrated that chemical energy gradients play an important role in establishing distinct community structure as a function of distance from geothermal spring discharge.

Increased mobilization of As under anaerobic conditions is of great concern in As contaminated soils and sediments. The identification of important release mechanisms may assist in designing safe and effective remediation strategies. In this study we investigated the effect of microbial reduction of aqueous arsenate (As(V)) on the solubilization of As(V) sorbed to ferrihydrite, in the absence of reductive dissolution of the Fe(III)-oxide solid phase. The addition of 0.1, 1.0, and 5.0 mM As(V) to serum bottles containing 10 mmol L -1 Fe(III) as ferrihydrite resulted in the sorption of 98, 75, and 20% of the applied As(V), respectively. Inoculation with an As(V) reducing, glucose fermenting microorganism (CN8) was followed by complete reduction of aqueous As(V) to As(III) at nontoxic As concentrations (up to 1.0 mM), but no reduction or dissolution of the Fe(III) solid phase was observed. Despite rapid reduction of aqueous As(V) to As(III), sorbed phase As remained primarily as As(V), and desorption of As(V) was too slow to cause a significant increase in aqueous As concentration over the 24-day experiment. Our study suggests that the reduction of aqueous As(V) may play a relatively minor role in the solubilization of As(V) sorbed to Fe (hydr)oxide. Arsenic release from contaminated soils and sediments may proceed considerably faster under conditions favoring dissimilatory Fe(III) reduction leading to the dissolution of sorbing phases.

Reduction of arsenate [As(V)] to arsenite[As(III)] influences the mobility and toxicity of arsenic (As), yet the mechanisms controlling the rate of reduction in soils and natural waters are poorly understood. The goal of this study was to determine processes affecting reduction rates of both aqueous and sorbed phase As(V). Reduction experiments were conducted anaerobically in serum bottles with a range of glucose and As(V) concentrations. Serum bottles were inoculated with microorganisms extracted directly from an agricultural soil having naturally elevated concentrations of As (unenriched population), or with a pure culture isolate obtained from the same soil after enrichment for As(V) reduction. At As(V) concentrations ranging from 6 to 600 μM, the rate of As(V) reduction by the soil isolate was first order with respect to both As(V) concentration and microbial biomass. Reduction rates of As(V) with the soil isolate were 2 to10 fold greater than in the unenriched population, suggesting As(V) reducers represented only a subset of the unenriched population. Compiled data indicated that the pure culture isolate was fermenting glucose, and potentially reducing As(V) as a detoxification mechanism. In a parallel study, reduction rates of As(V) with the unenriched population were evaluated in the presence of goethite or ferrihydrite. When redox potential decreased from 500 to near 0 mV, aqueous As concentrations decreased by approximately 30% in a goethite suspension with a high As surface coverage, yet increased by seven fold in a goethite suspension with a low As surface coverage. In a ferrihydrite suspension, aqueous As concentrations during reduction increased approximately 100 fold faster than in a goethite suspension at similar initial aqueous As(V) concentrations, corresponding to differences in Fe oxide surface areas and reductive dissolution rates. The results indicate that rates of As mobilization during reduction in soils are highly dependent on oxide surface area and As surface coverage.