Nutritional Aspects of Dissimilatory Sulfate Reduction in the Human Large Intestine

In contrast to other anaerobic ecosystems, such as marine and estuarine sediments, there is a lack of information on the nutritional requirements of human gut sulfate-reducing bacteria (SRB). Various substrates stimulated sulfate reduction in mixed culture, including short-chain fatty acids and other organic acids, alcohols, and amino acids (but not sugars or aromatic compounds). However, the use of sodium molybdate as a specific inhibitor of sulfate reduction caused an accumulation of ethanol and malonate only, and reduced the rate of utilization of lactate. This indicates the importance of these electron donors for sulfate reduction. Since ethanol and lactate are primarily utilized by members of the Desulfovibrio genus, the results suggest a physiologically important role for this group.  Experiments with two strains of Desulfovibrio desulfuricans isolated from human feces demonstrated that both were able to reduce sulfite, thiosulfate or nitrate in the absence of sulfate. In addition, one strain (DsvUC1) was able to grow by fermentative metabolism, although the second strain (DsvFD1) showed more restricted fermentative growth. The data indicate that desulfovibrios are ecologically the most significant group of SRB in the human colon, and that colonic isolates belonging to this genus are versatile, in terms of both the electron acceptors and donors that they are able to utilize. Received: 24 March 1997 / Accepted: 10 June 1997     

Dissimilatory Selenate Reduction Potentials in a Diversity of Sediment Types

We measured potential rates of bacterial dissimilatory reduction of ⁷⁵SeO₄²⁻ to ⁷⁵Se⁰ in a diversity of sediment types, with salinities ranging from freshwater (salinity = 1 g/liter) to hypersaline (salinity = 320 g/liter and with pH values ranging from 7.1 to 9.8. Significant biological selenate reduction occurred in all samples with salinities from 1 to 250 g/liter but not in samples with a salinity of 320 g/liter. Potential selenate reduction rates (25 nmol of SeO₄²⁻ per ml of sediment added with isotope) ranged from 0.07 to 22 μmol of SeO₄²⁻ reduced liter⁻¹ h⁻¹. Activity followed Michaelis-Menten kinetics in relation to SeO₄²⁻ concentration (K_m of selenate = 7.9 to 720 μM). There was no linear correlation between potential rates of SeO₄²⁻ reduction and salinity, pH, concentrations of total Se, porosity, or organic carbon in the sediments. However, potential selenate reduction was correlated with apparent K_m for selenate and with potential rates of denitrification (r = 0.92 and 0.81, respectively). NO₃⁻, NO₂⁻, MoO₄²⁻, and WO₄²⁻ inhibited selenate reduction activity to different extents in sediments from both Hunter Drain and Massie Slough, Nev. Sulfate partially inhibited activity in sediment from freshwater (salinity = 1 g/liter) Massie Slough samples but not from the saline (salinity = 60 g/liter) Hunter Drain samples. We conclude that dissimilatory selenate reduction in sediments is widespread in nature. In addition, in situ selenate reduction is a first-order reaction, because the ambient concentrations of selenium oxyanions in the sediments were orders of magnitude less than their K_ms.     

Archaeal and Bacterial Glycerol Dialkyl Glycerol Tetraether Lipids in Hot Springs of Yellowstone National Park

Glycerol dialkyl glycerol tetraethers (GDGTs) are core membrane lipids originally thought to be produced mainly by (hyper)thermophilic archaea. Environmental screening of low-temperature environments showed, however, the abundant presence of structurally diverse GDGTs from both bacterial and archaeal sources. In this study, we examined the occurrences and distribution of GDGTs in hot spring environments in Yellowstone National Park with high temperatures (47 to 83°C) and mostly neutral to alkaline pHs. GDGTs with 0 to 4 cyclopentane moieties were dominant in all samples and are likely derived from both (hyper)thermophilic Crenarchaeota and Euryarchaeota. GDGTs with 4 to 8 cyclopentane moieties, likely derived from the crenarchaeotal order Sulfolobales and the euryarchaeotal order Thermoplasmatales, are usually present in much lower abundance, consistent with the relatively high pH values of the hot springs. The relative abundances of cyclopentane-containing GDGTs did not correlate with in situ temperature and pH, suggesting that other environmental and possibly genetic factors play a role as well. Crenarchaeol, a biomarker thought to be specific for nonthermophilic group I Crenarchaeota, was also found in most hot springs, though in relatively low concentrations, i.e., <5% of total GDGTs. Its abundance did not correlate with temperature, as has been reported previously. Instead, the cooccurrence of relatively abundant nonisoprenoid GDGTs thought to be derived from soil bacteria suggests a predominantly allochthonous source for crenarchaeol in these hot spring environments. Finally, the distribution of bacterial branched GDGTs suggests that they may be derived from the geothermally heated soils surrounding the hot springs.     

High Rates of Sulfate Reduction in a Low-Sulfate Hot Spring Microbial Mat Are Driven by a Low Level of Diversity of Sulfate-Respiring Microorganisms

The importance of sulfate respiration in the microbial mat found in the low-sulfate thermal outflow of Mushroom Spring in Yellowstone National Park was evaluated using a combination of molecular, microelectrode, and radiotracer studies. Despite very low sulfate concentrations, this mat community was shown to sustain a highly active sulfur cycle. The highest rates of sulfate respiration were measured close to the surface of the mat late in the day when photosynthetic oxygen production ceased and were associated with a Thermodesulfovibrio-like population. Reduced activity at greater depths was correlated with novel populations of sulfate-reducing microorganisms, unrelated to characterized species, and most likely due to both sulfate and carbon limitation.     

PCR Detection and Analysis of the Free-Living Amoeba Naegleria in Hot Springs in Yellowstone and Grand Teton National Parks

Free-living thermotolerant amoebae pose a significant health risk to people who soak and swim in habitats suitable for their growth, such as hot springs. In this survey of 23 different hot springs in Yellowstone and Grand Teton National Parks, we used PCR with primer sets specific for Naegleria to detect three sequence types that represent species not previously described, as well as a fourth sequence type identified as the pathogen Naegleria fowleri.     

Bar-Coded Pyrosequencing Reveals Shared Bacterial Community Properties along the Temperature Gradients of Two Alkaline Hot Springs in Yellowstone National Park

An understanding of how communities are organized is a fundamental goal of ecology but one which has historically been elusive for microbial systems. We used a bar-coded pyrosequencing approach targeting the V3 region of the bacterial small-subunit rRNA gene to address the factors that structure communities along the thermal gradients of two alkaline hot springs in the Lower Geyser Basin of Yellowstone National Park. The filtered data set included a total of nearly 34,000 sequences from 39 environmental samples. Each was assigned to one of 391 operational taxonomic units (OTUs) identified by their unique V3 sequence signatures. Although the two hot springs differed in their OTU compositions, community resemblance and diversity changed with strikingly similar dynamics along the two outflow channels. Two lines of evidence suggest that these community properties are controlled primarily by environmental temperature. First, community resemblance decayed exponentially with increasing differences in temperature between samples but was only weakly correlated with physical distance. Second, diversity decreased with increasing temperature at the same rate along both gradients but was uncorrelated with other measured environmental variables. This study also provides novel insights into the nature of the ecological interactions among important taxa in these communities. A strong negative association was observed between cyanobacteria and the Chloroflexi, which together accounted for ∼70% of the sequences sampled. This pattern contradicts the longstanding hypothesis that coadapted lineages of these bacteria maintain tightly cooccurring distributions along these gradients as a result of a producer-consumer relationship. We propose that they instead compete for some limiting resource(s).Elucidating how biodiversity is distributed and the mechanisms underlying those patterns is a central goal of ecology. Although microorganisms make critical contributions to ecosystem function through their participation in biogeochemical cycles, we still have only a limited understanding of the factors that control the spatial structure and diversity of microbial communities (37). For example, although it is clear that microbial community composition is influenced by environmental variation (11, 17, 18, 28), the question of how diversity changes along environmental gradients remains generally unresolved. Of particular interest is the relationship between microbial diversity and temperature, as this environmental variable is strongly correlated with diversity over a broad range of spatial scales for many plant and animal taxa in terrestrial, freshwater, and marine ecosystems (1, 12, 36, 39, 43). In addition, because spatially resolved abundance data for individual microbial taxa are scarce, we have only limited information regarding the patterns of association among microorganisms. Consequently, microbial ecology has developed with little clarity regarding the potential roles of either negative or positive biotic interactions for structuring communities or whether microbial community organization along environmental gradients conforms to either individualistic (e.g., see references 13 and 20) or organismal (e.g., see reference 9) community ecology paradigms developed for macroscopic organisms.Hot spring microbial ecosystems present an excellent opportunity to investigate fundamental questions regarding community organization. Steep temperature gradients enable the explicit investigation of the importance of environmental variation for structuring diversity while controlling for other factors that typically vary across sampling locations on larger geographic scales, such as solar energy availability and geological history. Decades of biological research on the alkaline-silica hot springs of Yellowstone National Park (reviewed in reference 49) further inform predictions regarding the nature of the ecological interactions among abundant community members.These systems are particularly notable for the presence of ecologically diverse groups of cyanobacteria and photosynthetic green nonsulfur bacteria (i.e., the Chloroflexi). The former group includes lineages of Synechococcus, the most thermotolerant of which delimits the thermal maximum for photosynthesis, whereas the latter includes divergent “green” (the genus Chloroflexus and relatives) and “red” (the genus Roseiflexus) clades. The conventional view is that interactions among cyanobacteria and the Chloroflexi are generally positive. Specifically, it has been proposed that coadapted lineages of Synechococcus and Chloroflexi maintain tightly cooccurring distributions due to a producer-consumer relationship in which the Chloroflexi grow as photoheterotrophs on low-molecular-weight organic compounds excreted by the cyanobacteria (48, 49). In turn, the filamentous Chloroflexi were previously suggested to provide a matrix within which Synechococcus cells become stably embedded (5). According to this model, we would expect to observe coincident peaks in abundance between coadapted lineages of cyanobacteria and Chloroflexi along alkaline hot spring gradients as well as a general positive correlation between the abundances of both groups. However, other evidence raises the possibility that these groups may not share a strict cross-feeding relationship. The genus Chloroflexus is metabolically flexible in laboratory culture, and certain strains have been grown as photoautotrophs with hydrogen sulfide or hydrogen as an electron donor (16, 26, 30). Although Roseiflexus is yet to be grown autotrophically in the laboratory, comparative genomics of laboratory strains and metagenomic data from microbial mat communities have also revealed the presence of Roseiflexus genes involved in the autotrophic hydroxypropionate pathway (24). In addition, stable carbon isotope data (46, 47) suggest that certain members of the Chloroflexi may have the capacity to grow autotrophically in situ. The issue of the nature of the ecological interactions between cyanobacteria and the Chloroflexi therefore requires clarification.In the present study, we investigated the patterns of distribution of bacteria along the temperature gradients of two alkaline-silica hot springs in Yellowstone National Park to determine how community properties changed in response to temperature and whether the realized distributions of cyanobacteria and the Chloroflexi meet the prediction of the coadaptation hypothesis. To do so, we used bar-coded mass parallel pyrosequencing of the V3 variable region of the bacterial small-subunit (SSU) rRNA gene to simultaneously interrogate the sequence diversity of environmental samples from multiple locations along both hot springs. This strategy is distinct from recent applications of next-generation DNA sequencing technology for investigating microbial diversity, as previous efforts focused principally on the deep sampling of one or a few discrete habitats for the purpose of quantifying the magnitude of diversity (e.g., see references 21, 38, and 41). We report that community properties changed with similar dynamics in response to temperature despite differences between hot springs in water chemistries and taxon composition, and we reject the coadaptation hypothesis for these communities based on a strong negative association in abundances of cyanobacteria and Chloroflexi.     

Evaluation of Rules to Distinguish Unique Female Grizzly Bears With Cubs in Yellowstone

ABSTRACT The United States Fish and Wildlife Service uses counts of unduplicated female grizzly bears (Ursus arctos) with cubs-of-the-year to establish limits of sustainable mortality in the Greater Yellowstone Ecosystem, USA. Sightings are clustered into observations of unique bears based on an empirically derived rule set. The method has never been tested or verified. To evaluate the rule set, we used data from radiocollared females obtained during 1975–2004 to simulate populations under varying densities, distributions, and sighting frequencies. We tested individual rules and rule-set performance, using custom software to apply the rule-set and cluster sightings. Results indicated most rules were violated to some degree, and rule-based clustering consistently underestimated the minimum number of females and total population size derived from a nonparametric estimator (Chao2). We conclude that the current rule set returns conservative estimates, but with minor improvements, counts of unduplicated females-with-cubs can serve as a reasonable index of population size useful for establishing annual mortality limits. For the Yellowstone population, the index is more practical and cost-effective than capture-mark-recapture using either DNA hair snagging or aerial surveys with radiomarked bears. The method has useful application in other ecosystems, but we recommend rules used to distinguish unique females be adapted to local conditions and tested.     

Draft Genome Sequence of Ureibacillus thermosphaericus Strain Thermo-BF, Isolated from Ramsar Hot Springs in Iran

Ureibacillus thermosphaericus strain Thermo-BF is an aerobic, thermophilic bacillus which has been characterized to biosynthesize gold nanoparticles. Here we present the draft genome sequence of Ureibacillus thermosphaericus strain Thermo-BF which consists of a 2,864,162-bp chromosome. This is the first report of a shotgun sequenced draft genome of a species in the Ureibacillus genus.     

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.     

真菌异化硝酸盐还原机理的研究进展

真菌异化硝酸盐还原途径的发现打破了反硝化仅存在于原核细胞这一传统观念。真菌异化硝酸盐还原途径是在环境中氧供给受限的情况下发生的, 包括反硝化和氨的发酵。硝酸盐能诱导产生反硝化作用的酶, 其中, 硝酸盐还原酶与亚硝酸还原酶位于线粒体中, 它们所催化的酶促反应能偶联呼吸链ATP合成酶合成ATP, 同时产生NO。与参与反硝化作用前两个酶不同, 真菌NO还原酶能以NADH为直接电子供体将NO还原为N2O, 在NAD+的再生和自由基NO的脱毒中起着重要作用。氨发酵则将硝酸盐还原成NH4+, 同时偶联乙酸的生成和底物水平磷酸化。此文从参与该过程的关键酶、关键酶的表达调节、真菌与细菌异化硝酸盐还原的比较等角度综述了真菌异化硝酸盐还原的最新研究进展。     

Metabolic Engineering of an Aerobic Sulfate Reduction Pathway and Its Application to Precipitation of Cadmium on the Cell Surface

The conversion of sulfate to an excess of free sulfide requires stringent reductive conditions. Dissimilatory sulfate reduction is used in nature by sulfate-reducing bacteria for respiration and results in the conversion of sulfate to sulfide. However, this dissimilatory sulfate reduction pathway is inhibited by oxygen and is thus limited to anaerobic environments. As an alternative, we have metabolically engineered a novel aerobic sulfate reduction pathway for the secretion of sulfides. The assimilatory sulfate reduction pathway was redirected to overproduce cysteine, and excess cysteine was converted to sulfide by cysteine desulfhydrase. As a potential application for this pathway, a bacterium was engineered with this pathway and was used to aerobically precipitate cadmium as cadmium sulfide, which was deposited on the cell surface. To maximize sulfide production and cadmium precipitation, the production of cysteine desulfhydrase was modulated to achieve an optimal balance between the production and degradation of cysteine.     

The Role of Extracellular Polymeric Substances in the Silicification of Calothrix: Evidence from Microbial Mat Communities in Hot Springs at Yellowstone National Park,USA

We provide nanoscale evidence of the role of sheath exopolymers in the silicification of the sheathed cyanobacteria Calothrix. Electron microscope observations of silicified Calothrix cells revealed that silica accretes directly onto EPS sheath fibrils to produce an open web of silica particles that could remain permeable to nutrients and waste products. We also found that silicified Calothrix cells from different microhabitats contained morphologically distinct silica particles. Differences in silicification texture suggest that environmental variables may influence silicification at the nanoscale. We develop a framework based on aggregation kinetics to address silicification processes in Calothrix and other sheathed cyanobacteria.     

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 (~90 g/liter) and Searles Lake (~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 ×10⁴ to 3.2 × 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.     

Reduction of Respiration by High Ambient CO2 and the Resulting Error in Measurements of Respiration Made with O2 Electrodes

Respiration rates of Lemna gibba fronds and Orobanche aegyptiacaand Lactuca sativa seedlings, were measured with a Clark typeoxygen electrode in the presence or absence of a carbon-dioxideabsorber (KOH) in the gas phase. Measured respiration ratesin the presence of KOH were 17-34% higher than in its absence.The suppression of respiration by high CO₂ concentrations, [CO₂],was confirmed by parallel studies of CO₂ efflux, made by infraredgas spectrometry. These results are consistent with other reportsof reduced rates of respiration at high [CO₂]. Measurements of respiration quotients of Lemna and Lactuca weremade at 0 and 100 Pa [CO₂]. Results did not support the possibilityof induced dark fixation of CO₂ at the ambient atmospheric [CO₂]predicted for the next century (35-100 Pa). It is concluded that the numerous reports of respiration measurementsmade with O₂ electrodes, in the absence of a CO₂ absorber, maycontain a significant errorCopyright 1993, 1999 Academic Press Lemna gibba, Lactuca sativa, Orobanche aegyptiaca, CO₂ accumulation, O₂ electrode, respiration, dark CO₂ fixation, respiration quotient, atmospheric CO₂     

Abiotic Reduction of 4-Chloronitrobenzene to 4-Chloroaniline in a Dissimilatory Iron-Reducing Enrichment Culture

4-chloronitrobenzene (4-Cl-NB) was rapidly reduced to 4-chloroaniline with half-lives of minutes in a dissimilatory Fe(III)-reducing enrichment culture. The initial pseudo-first-order rate constants at 25°C ranged from 0.11 to 0.19 per minute. The linear Arrhenius correlation in a temperature range of 6 to 85°C and the unchanged reactivity after pasteurization indicated that the nitroreduction occurred abiotically. A fine-grained black solid which was identified as poorly crystalline magnetite (Fe₃O₄) by X-ray diffraction accumulated in the enrichments. Magnetite produced by the Fe(III)-reducing bacterium Geobacter metallireducens GS-15 and synthetic magnetite also reduced 4-Cl-NB. These results suggest that the reduction of 4-Cl-NB by the enrichment material was a surface-mediated reaction by dissimilatory formed Fe(II) associated with magnetite.     

Effect of Temperature on Rates of Alternative and Cytochrome Pathway Respiration and Their Relationship with the Redox Poise of the Quinone Pool

We investigated the effect of short-term changes in temperature on alternative (Alt) and cytochrome (Cyt) pathway respiration, both in intact tissues and isolated mitochondria of 14-d-old cotyledons of soybean (Glycine max L. cv Stevens). We also established the extent to which temperature alters the interaction between the oxidizing pathways and the level of ubiquinone (UQ) reduction (UQ(r)/UQ(t)). No difference was found between the temperature coefficient of respiration (Q(10); proportional change per 10 degrees C) of Alt and Cyt pathway respiration in cotyledon slices (Q(10) = 1.92 and 1.86, respectively). In isolated mitochondria, the Q(10) of the fully activated Alt pathway (Q(10) = 2.24-2.61) was always equal to, or higher than, that of Cyt c oxidase (COX) alone (Q(10) = 2.08) and the complete Cyt pathway (Q(10) = 2.40-2.55). This was true regardless of substrate or whether ADP was present. There was little difference in the Q(10) of the Cyt pathway with or without ADP; however, the Q(10) of COX was substantially lower in the presence of an uncoupler (Q(10) = 1.61) than its absence (Q(10) = 2.08). The kinetics of Alt and Cyt pathway activity in relation to UQ(r)/UQ(t) were not affected by temperature. For a given UQ(r)/UQ(t) value, the proportion of maximum flux taking place was similar at all temperatures for both pathways (+/-ADP). However, the Q(10) of the Alt and the Cyt pathways (+ADP) increased with increasing UQ(r)/UQ(t). We conclude that the Alt pathway is not less temperature sensitive than the Cyt pathway or COX per se and that changes in the degree of control exerted by individual steps in the respiratory apparatus could result in changes in the Q(10) of mitochondrial O(2) uptake.     

Bacterial Dissimilatory Reduction of Arsenic(V) to Arsenic(III) in Anoxic Sediments

Incubation of anoxic salt marsh sediment slurries with 10 mM As(V) resulted in the disappearance over time of the As(V) in conjunction with its recovery as As(III). No As(V) reduction to As(III) occurred in heat-sterilized or formalin-killed controls or in live sediments incubated in air. The rate of As(V) reduction in slurries was enhanced by addition of the electron donor lactate, H(inf2), or glucose, whereas the respiratory inhibitor/uncoupler dinitrophenol, rotenone, or 2-heptyl-4-hydroxyquinoline N-oxide blocked As(V) reduction. As(V) reduction was also inhibited by tungstate but not by molybdate, sulfate, or phosphate. Nitrate inhibited As(V) reduction by its action as a preferred respiratory electron acceptor rather than as a structural analog of As(V). Nitrate-respiring sediments could reduce As(V) to As(III) once all the nitrate was removed. Chloramphenicol blocked the reduction of As(V) to As(III) in nitrate-respiring sediments, suggesting that nitrate and arsenate were reduced by separate enzyme systems. Oxidation of [2-(sup14)C]acetate to (sup14)CO(inf2) by salt marsh and freshwater sediments was coupled to As(V). Collectively, these results show that reduction of As(V) in sediments proceeds by a dissimilatory process. Bacterial sulfate reduction was completely inhibited by As(V) as well as by As(III).     

Linkage of high rates of sulfate reduction in Yellowstone hot springs to unique sequence types in the dissimilatory sulfate respiration pathway

Diversity, habitat range, and activities of sulfate-reducing prokaryotes within hot springs in Yellowstone National Park were characterized using endogenous activity measurements, molecular characterization, and enrichment. Five major phylogenetic groups were identified using PCR amplification of the dissimilatory sulfite reductase genes (dsrAB) from springs demonstrating significant sulfate reduction rates, including a warm, acidic (pH 2.5) stream and several nearly neutral hot springs with temperatures reaching 89 degrees C. Three of these sequence groups were unrelated to named lineages, suggesting that the diversity and habitat range of sulfate-reducing prokaryotes exceeds that now represented in culture.