The role of microaerophilic Fe‐oxidizing micro‐organisms in producing banded iron formations

Despite the historical and economic significance of banded iron formations (BIFs), we have yet to resolve the formation mechanisms. On modern Earth, neutrophilic microaerophilic Fe‐oxidizing micro‐organisms (FeOM) produce copious amounts of Fe oxyhydroxides, leading us to wonder whether similar organisms played a role in producing BIFs. To evaluate this, we review the current knowledge of modern microaerophilic FeOM in the context of BIF paleoenvironmental studies. In modern environments wherever Fe(II) and O₂ co‐exist, microaerophilic FeOM proliferate. These organisms grow in a variety of environments, including the marine water column redoxcline, which is where BIF precursor minerals likely formed. FeOM can grow across a range of O₂ concentrations, measured as low as 2 μm to date, although lower concentrations have not been tested. While some extant FeOM can tolerate high O₂ concentrations, many FeOM appear to prefer and thrive at low O₂ concentrations (~3–25 μm ). These are similar to the estimated dissolved O₂ concentrations in the few hundred million years prior to the ‘Great Oxidation Event’ (GOE). We compare biotic and abiotic Fe oxidation kinetics in the presence of varying levels of O₂ and show that microaerophilic FeOM contribute substantially to Fe oxidation, at rates fast enough to account for BIF deposition. Based on this synthesis, we propose that microaerophilic FeOM were capable of playing a significant role in depositing the largest, most well‐known BIFs associated with the GOE, as well as afterward when global O₂ levels increased.     

Biogeochemical probing of microbial communities in a basalt‐hosted hot spring at Kverkfjöll volcano,Iceland

We investigated bacterial and archaeal communities along an ice‐fed surficial hot spring at Kverkfjöll volcano—a partially ice‐covered basaltic volcano at Vatnajökull glacier, Iceland, using biomolecular (16S rRNA, apsA, mcrA, amoA, nifH genes) and stable isotope techniques. The hot spring environment is characterized by high temperatures and low dissolved oxygen concentrations at the source (68°C and <1 mg/L (±0.1%)) changing to lower temperatures and higher dissolved oxygen downstream (34.7°C and 5.9 mg/L), with sulfate the dominant anion (225 mg/L at the source). Sediments are comprised of detrital basalt, low‐temperature alteration phases and pyrite, with <0.4 wt. % total organic carbon (TOC). 16S rRNA gene profiles reveal that organisms affiliated with Hydrogenobaculum (54%–87% bacterial population) and Thermoproteales (35%–63% archaeal population) dominate the micro‐oxic hot spring source, while sulfur‐oxidizing archaea (Sulfolobales, 57%–82%), and putative sulfur‐oxidizing and heterotrophic bacterial groups dominate oxic downstream environments. The δ¹³C_org (‰ V‐PDB) values for sediment TOC and microbial biomass range from ?9.4‰ at the spring's source decreasing to ?12.6‰ downstream. A reverse effect isotope fractionation of ~3‰ between sediment sulfide (δ³⁴S ~0‰) and dissolved water sulfate (δ³⁴S +3.2‰), and δ¹⁸O values of ~ ?5.3‰ suggest pyrite forms abiogenically from volcanic sulfide, followed by abiogenic and microbial oxidation. These environments represent an unexplored surficial geothermal environment analogous to transient volcanogenic habitats during putative “snowball Earth” scenarios and volcano–ice geothermal environments on Mars.     

Effect of Reduced Sulfur Species on Chemolithoautotrophic Pyrite Oxidation with Nitrate

We compared the response at neutral pH of some denitrifiers to different electron donors such as reduced sulfur (pyrite, S(0), and marcasite) and reduced Fe. Chemolithoautotrophic oxidation of pyrite with nitrate as electron acceptor was not possible when the pyrite was in a pure crystalline form, whereas oxidation of synthesized FeS₂ of low crystallinity and of S(0) with nitrate as electron acceptor was possible. Neither nitrite nor sulfate was formed when Fe(II)-oxidizing strain Acidovorax sp. BoFeN1 was tested. Microbial reduction of nitrate appears to be induced via S oxidation but not via Fe oxidation.     

Dark aerobic sulfide oxidation by anoxygenic phototrophs in anoxic waters

Anoxygenic phototrophic sulfide oxidation by green and purple sulfur bacteria (PSB) plays a key role in sulfide removal from anoxic shallow sediments and stratified waters. Although some PSB can also oxidize sulfide with nitrate and oxygen, little is known about the prevalence of this chemolithotrophic lifestyle in the environment. In this study, we investigated the role of these phototrophs in light-independent sulfide removal in the chemocline of Lake Cadagno. Our temporally resolved, high-resolution chemical profiles indicated that dark sulfide oxidation was coupled to high oxygen consumption rates of ~9 μM O₂·h⁻¹. Single-cell analyses of lake water incubated with ¹³CO₂ in the dark revealed that Chromatium okenii was to a large extent responsible for aerobic sulfide oxidation and it accounted for up to 40% of total dark carbon fixation. The genome of Chr. okenii reconstructed from the Lake Cadagno metagenome confirms its capacity for microaerophilic growth and provides further insights into its metabolic capabilities. Moreover, our genomic and single-cell data indicated that other PSB grow microaerobically in these apparently anoxic waters. Altogether, our observations suggest that aerobic respiration may not only play an underappreciated role in anoxic environments but also that organisms typically considered strict anaerobes may be involved.     

Iron oxidation stimulates organic matter decomposition in humid tropical forest soils

Humid tropical forests have the fastest rates of organic matter decomposition globally, which often coincide with fluctuating oxygen (O₂) availability in surface soils. Microbial iron (Fe) reduction generates reduced iron [Fe(II)] under anaerobic conditions, which oxidizes to Fe(III) under subsequent aerobic conditions. We demonstrate that Fe (II) oxidation stimulates organic matter decomposition via two mechanisms: (i) organic matter oxidation, likely driven by reactive oxygen species; and (ii) increased dissolved organic carbon (DOC) availability, likely driven by acidification. Phenol oxidative activity increased linearly with Fe(II) concentrations (P < 0.0001, pseudo R² = 0.79) in soils sampled within and among five tropical forest sites. A similar pattern occurred in the absence of soil, suggesting an abiotic driver of this reaction. No phenol oxidative activity occurred in soils under anaerobic conditions, implying the importance of oxidants such as O₂ or hydrogen peroxide (H₂O₂) in addition to Fe(II). Reactions between Fe(II) and H₂O₂ generate hydroxyl radical, a strong nonselective oxidant of organic compounds. We found increasing consumption of H₂O₂ as soil Fe(II) concentrations increased, suggesting that reactive oxygen species produced by Fe(II) oxidation explained variation in phenol oxidative activity among samples. Amending soils with Fe(II) at field concentrations stimulated short‐term C mineralization by up to 270%, likely via a second mechanism. Oxidation of Fe(II) drove a decrease in pH and a monotonic increase in DOC; a decline of two pH units doubled DOC, likely stimulating microbial respiration. We obtained similar results by manipulating soil acidity independently of Fe(II), implying that Fe(II) oxidation affected C substrate availability via pH fluctuations, in addition to producing reactive oxygen species. Iron oxidation coupled to organic matter decomposition contributes to rapid rates of C cycling across humid tropical forests in spite of periodic O₂ limitation, and may help explain the rapid turnover of complex C molecules in these soils.     

Fe(III) reduction during pyruvate fermentation by Desulfotomaculum reducens strain MI‐1

Desulfotomaculum reducens MI‐1 is a Gram‐positive, sulfate‐reducing bacterium also capable of reducing several metals, among which is Fe(III). Very limited knowledge is available on the potential mechanism(s) of metal reduction among Gram‐positive bacteria, despite their preponderance in the microbial communities that inhabit some inhospitable environments (e.g., thermal or hyperthermal ecosystems, extreme pH or salinity environments, heavy metal or radionuclide contaminated sediments). Here, we show that in the presence of pyruvate, this micro‐organism is capable of reducing both soluble Fe(III)‐citrate and solid‐phase hydrous ferric oxide, although growth is sustained by pyruvate fermentation rather than Fe(III) respiration. Despite the fact that Fe(III) reduction does not support direct energy conservation, D. reducens uses it as a complementary means of discarding excess reducing equivalent after H₂ accumulation in the culture headspace renders proton reduction unfavorable. Thus, Fe(III) reduction permits the oxidation of greater amounts of pyruvate than fermentation alone. Fe(III) reduction by D. reducens is mediated by a soluble electron carrier, most likely riboflavin. Additionally, an intracellular electron storage molecule acts as a capacitor and accumulates electrons during pyruvate oxidation for slow release to Fe(III). The reductase responsible for the transfer of electrons from the capacitor to the soluble carrier has not been identified, but data presented here argue against the involvement of c‐type cytochromes.     

On the energetics of chemolithotrophy in nonequilibrium systems: case studies of geothermal springs in Yellowstone National Park

Chemolithotrophic micro‐organisms are important primary producers in high‐temperature geothermal environments and may catalyse a number of different energetically favourable redox reactions as a primary energy source. Analysis of geochemical constituents followed by chemical speciation and subsequent calculation of reaction free energies (ΔG_rxn) is a useful tool for evaluating the thermodynamic favourability and potential energy available for microbial metabolism. The primary goal of this study was to examine relationships among geochemical gradients and microbial population distribution, and to evaluate the utility of energetic approaches for predicting microbial metabolism from free‐energy calculations, utilizing as examples, several geothermal habitats in Yellowstone National Park where thorough geochemical and phylogenetic analyses have been performed. Acidic (pH ～ 3) and near‐neutral (pH ～ 6–7) geothermal springs were chosen for their range in geochemical properties. Aqueous and solid phase samples obtained from the source pools and the outflow channels of each spring were characterized for all major chemical constituents using laboratory and field methods to accurately measure the concentrations of predominant oxidized and reduced species. Reaction free energies (ΔG_rxn) for 33 oxidation–reduction reactions potentially important to chemolithotrophic micro‐organisms were calculated at relevant spring temperatures after calculating ion activities using an aqueous equilibrium model. Free‐energy values exhibit significant variation among sites for reactions with pH dependence. For example, free‐energy values for reactions involving Fe³⁺ are especially variable across sites due in large part to the pH dependence of Fe³⁺ activity, and exhibit changes of up to 40 kJ mol^?1 electron from acidic to near neutral geothermal springs. Many of the detected 16S rRNA gene sequences represent organisms whose metabolisms are consistent with exergonic processes. However, sensitivity analyses demonstrated that reaction free energies do not generally represent the steep gradients in local geochemical conditions resulting from air–water gas exchange and solid phase deposition that are important in defining microbial habitats and 16S rRNA gene sequence distribution within geothermal outflow channels.     

Magnetite formation from ferrihydrite by hyperthermophilic archaea from Endeavour Segment,Juan de Fuca Ridge hydrothermal vent chimneys

Hyperthermophilic iron reducers are common in hydrothermal chimneys found along the Endeavour Segment in the northeastern Pacific Ocean based on culture‐dependent estimates. However, information on the availability of Fe(III) (oxyhydr) oxides within these chimneys, the types of Fe(III) (oxyhydr) oxides utilized by the organisms, rates and environmental constraints of hyperthermophilic iron reduction, and mineral end products is needed to determine their biogeochemical significance and are addressed in this study. Thin‐section petrography on the interior of a hydrothermal chimney from the Dante edifice at Endeavour showed a thin coat of Fe(III) (oxyhydr) oxide associated with amorphous silica on the exposed outer surfaces of pyrrhotite, sphalerite, and chalcopyrite in pore spaces, along with anhydrite precipitation in the pores that is indicative of seawater ingress. The iron sulfide minerals were likely oxidized to Fe(III) (oxyhydr) oxide with increasing pH and E_h due to cooling and seawater exposure, providing reactants for bioreduction. Culture‐dependent estimates of hyperthermophilic iron reducer abundances in this sample were 1740 and 10 cells per gram (dry weight) of material from the outer surface and the marcasite‐sphalerite‐rich interior, respectively. Two hyperthermophilic iron reducers, Hyperthermus sp. Ro04 and Pyrodictium sp. Su06, were isolated from other active hydrothermal chimneys on the Endeavour Segment. Strain Ro04 is a neutrophilic (pH_opt 7–8) heterotroph, while strain Su06 is a mildly acidophilic (pH_opt 5), hydrogenotrophic autotroph, both with optimal growth temperatures of 90–92 °C. Mössbauer spectroscopy of the iron oxides before and after growth demonstrated that both organisms form nanophase (<12 nm) magnetite [Fe₃O₄] from laboratory‐synthesized ferrihydrite [Fe₁₀O₁₄(OH)₂] with no detectable mineral intermediates. They produced up to 40 mm Fe²⁺ in a growth‐dependent manner, while all abiotic and biotic controls produced <3 mm Fe²⁺. Hyperthermophilic iron reducers may have a growth advantage over other hyperthermophiles in hydrothermal systems that are mildly acidic where mineral weathering at increased temperatures occurs.     

Pyrite formation in marshes during early diagenesis

Pyrite was removed from peat cores by draining the sediments and allowing the pyrite to oxidize. Then the peat cores were placed back into intertidal salt marsh sediments to incubate. Pyrite accumulated rapidly in peat incubated in situ. A greater accumulation of pyrite was observed in peat that contained living grass than peat in which the grass had been killed.

Resin‐imbedded samples of peat from nearby sediments showed that small single crystals of pyrite were abundant, supporting the idea that pyrite in marshes forms rapidly through direct precipitation. Pyrite was also observed filling vascular channels in roots. It had been proposed that pyrite fills root channels in freshwater environments where the primary sulfur source used by sulfate‐reducing bacteria is organic sulfur rather than sulfate. The widespread occurrence of pyrite filling vascular channels in salt marsh peat makes it unlikely that pyrite morphology can be used to infer the salinity of the overlying water.

Marsh sediments are characterized by higher carbon/sulfur ratios and pyritization (Fe‐pyritel(Fe‐pyrite + Fe‐HCl)) indices than marine subtidal sediments. Within wide ranges these indices do not seem to be very sensitive to salinity of flooding water or carbon concentrations in sediments. Oxidation and iron availability appear to be the major controls on pyrite accumulation in marshes. While pyrite concentrations in submerged sediments can be used as indicators of relative rates of sulfate reduction, sulfur storage in intertidal marsh sediments is not as tightly linked to this microbial process.     

In situ analysis of oxygen consumption and diffusive transport in high‐temperature acidic iron‐oxide microbial mats

The role of dissolved oxygen as a principal electron acceptor for microbial metabolism was investigated within Fe(III)‐oxide microbial mats that form in acidic geothermal springs of Yellowstone National Park (USA). Specific goals of the study were to measure and model dissolved oxygen profiles within high‐temperature (65–75°C) acidic (pH = 2.7–3.8) Fe(III)‐oxide microbial mats, and correlate the abundance of aerobic, iron‐oxidizing Metallosphaera yellowstonensis organisms and mRNA gene expression levels to Fe(II)‐oxidizing habitats shown to consume oxygen. In situ oxygen microprofiles were obtained perpendicular to the direction of convective flow across the aqueous phase/Fe(III)‐oxide microbial mat interface using oxygen microsensors. Dissolved oxygen concentrations dropped from ～ 50–60 μM in the bulk‐fluid/mat surface to below detection (< 0.3 μM) at a depth of ～ 700 μm (～ 10% of the total mat depth). Net areal oxygen fluxes into the microbial mats were estimated to range from 1.4–1.6 × 10^?4 μmol cm^?2 s^?1. Dimensionless parameters were used to model dissolved oxygen profiles and establish that mass transfer rates limit the oxygen consumption. A zone of higher dissolved oxygen at the mat surface promotes Fe(III)‐oxide biomineralization, which was supported using molecular analysis of Metallosphaera yellowstonensis 16S rRNA gene copy numbers and mRNA expression of haem Cu oxidases (FoxA) associated with Fe(II)‐oxidation.     

Chemical and microbiological studies of sulfide‐mediated manganese reduction 1

Laboratory studies of manganese reduction by naturally occurring reduced inorganic compounds were undertaken, both to study further possible in situ mechanisms of manganese reduction and to examine how manganese redox reactions might be coupled to other biogeochemical processes. Chemical manganese reduction by sulfide (in the presence of excess manganese oxide) was found to be rapid and complete, with all sulfide being oxidized within 5–10 min. The reduction of δMnO₂ by sulfide involves a two‐electron transfer, with S° the predominant oxidized sulfur product. Using a marine sulfate‐reducing bacterium (Desulfovibrio sp.), the kinetics of sulfide‐dependent, bacterially mediated manganese reduction were studied; the rate‐limiting step was bacterial sulfide production. These findings suggest that in stratified marine environments (such as the Black Sea, Saanich Inlet, or certain coastal sediments) manganese reduction should occur just below the oxic‐anoxic (O₂/H₂S) interface or redox boundary as a result of the chemical reaction between manganese oxides and sulfide produced by sulfate‐reducing bacteria.     

Atmospheric hydrogen peroxide and Eoarchean iron formations

It is widely accepted that photosynthetic bacteria played a crucial role in Fe(II) oxidation and the precipitation of iron formations (IF) during the Late Archean–Early Paleoproterozoic (2.7–2.4 Ga). It is less clear whether microbes similarly caused the deposition of the oldest IF at ca. 3.8 Ga, which would imply photosynthesis having already evolved by that time. Abiological alternatives, such as the direct oxidation of dissolved Fe(II) by ultraviolet radiation may have occurred, but its importance has been discounted in environments where the injection of high concentrations of dissolved iron directly into the photic zone led to chemical precipitation reactions that overwhelmed photooxidation rates. However, an outstanding possibility remains with respect to photochemical reactions occurring in the atmosphere that might generate hydrogen peroxide (H₂O₂), a recognized strong oxidant for ferrous iron. Here, we modeled the amount of H₂O₂ that could be produced in an Eoarchean atmosphere using updated solar fluxes and plausible CO₂, O₂, and CH₄ mixing ratios. Irrespective of the atmospheric simulations, the upper limit of H₂O₂ rainout was calculated to be <10⁶ molecules cm^?2 s^?1. Using conservative Fe(III) sedimentation rates predicted for submarine hydrothermal settings in the Eoarchean, we demonstrate that the flux of H₂O₂ was insufficient by several orders of magnitude to account for IF deposition (requiring ~10¹¹ H₂O₂ molecules cm^?2 s^?1). This finding further constrains the plausible Fe(II) oxidation mechanisms in Eoarchean seawater, leaving, in our opinion, anoxygenic phototrophic Fe(II)‐oxidizing micro‐organisms the most likely mechanism responsible for Earth's oldest IF.     

Metagenomes from High-Temperature Chemotrophic Systems Reveal Geochemical Controls on Microbial Community Structure and Function

The Yellowstone caldera contains the most numerous and diverse geothermal systems on Earth, yielding an extensive array of unique high-temperature environments that host a variety of deeply-rooted and understudied Archaea, Bacteria and Eukarya. The combination of extreme temperature and chemical conditions encountered in geothermal environments often results in considerably less microbial diversity than other terrestrial habitats and offers a tremendous opportunity for studying the structure and function of indigenous microbial communities and for establishing linkages between putative metabolisms and element cycling. Metagenome sequence (14–15,000 Sanger reads per site) was obtained for five high-temperature (>65°C) chemotrophic microbial communities sampled from geothermal springs (or pools) in Yellowstone National Park (YNP) that exhibit a wide range in geochemistry including pH, dissolved sulfide, dissolved oxygen and ferrous iron. Metagenome data revealed significant differences in the predominant phyla associated with each of these geochemical environments. Novel members of the Sulfolobales are dominant in low pH environments, while other Crenarchaeota including distantly-related Thermoproteales and Desulfurococcales populations dominate in suboxic sulfidic sediments. Several novel archaeal groups are well represented in an acidic (pH 3) Fe-oxyhydroxide mat, where a higher O₂ influx is accompanied with an increase in archaeal diversity. The presence or absence of genes and pathways important in S oxidation-reduction, H₂-oxidation, and aerobic respiration (terminal oxidation) provide insight regarding the metabolic strategies of indigenous organisms present in geothermal systems. Multiple-pathway and protein-specific functional analysis of metagenome sequence data corroborated results from phylogenetic analyses and clearly demonstrate major differences in metabolic potential across sites. The distribution of functional genes involved in electron transport is consistent with the hypothesis that geochemical parameters (e.g., pH, sulfide, Fe, O₂) control microbial community structure and function in YNP geothermal springs.     

Linking geochemical processes with microbial community analysis: successional dynamics in an arsenic-rich, acid-sulphate-chloride geothermal spring

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, t_1/2 = 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.     

Seasonal cycling of Fe in saltmarsh sediments

This study combines an analysis of porewater chemistry with new, solid phase wet chemical extractions to examine the seasonal cycling of Fe in vegetated and unvegetated (cyanobacterial mat) saltmarsh sediments. Saltmarsh sediments are shown to contain more solid phase reactive Fe than other marine sediments studied so far. From the partitioning and speciation of solid Fe, and solid/soluble reduced S analysis in 10 sediment cores, we have observed that a majority of solid Fe in these sediments is cycled rapidly and completely between oxidized reactive Fe and reduced Fe as pyrite. Vegetated porewaters showed a lower pH and much higher Fe(II) concentrations on average than unvegetated porewaters in the top 10 cm, whereas sulfate, alkalinity, and sulfide concentrations were similar in the two environments. The amorphous Fe(III) oxide fraction showed a high negative correlation to solid and soluble reduced S (r ² = –0.86 and –0.71, respectively) in surface vegetated sediments whereas the crystalline Fe(III) oxide fraction showed a high negative correlation (r ² = –0.96) to sulfide only at depth. Though reactive Fe was observed in unvegetated sediments, no seasonal trend was apparent and the speciation of solid Fe revealed that most of it was reduced. Solid phase and porewater chemistry support the dominant role of the biota (Spartina alterniflora and bacteria) in controlling the reactivity of Fe and suggest that the current definition of solid phase, reactive Fe should be expanded to include crystalline Fe(III) minerals which are available for pyrite formation in saltmarsh sediments. In support of previous saltmarsh studies, we present evidence that the redox cycle of solid Fe is controlled by sulfate reduction and sediment oxidation which respond to both annual cycles (light, temperature) and to short-term, episodic effects such as weather and tides.     

Linking microbial oxidation of arsenic with detection and phylogenetic analysis of arsenite oxidase genes in diverse geothermal environments

The identification and characterization of genes involved in the microbial oxidation of arsenite will contribute to our understanding of factors controlling As cycling in natural systems. Towards this goal, we recently characterized the widespread occurrence of aerobic arsenite oxidase genes (aroA‐like) from pure‐culture bacterial isolates, soils, sediments and geothermal mats, but were unable to detect these genes in all geothermal systems where we have observed microbial arsenite oxidation. Consequently, the objectives of the current study were to measure arsenite‐oxidation rates in geochemically diverse thermal habitats in Yellowstone National Park (YNP) ranging in pH from 2.6 to 8, and to identify corresponding 16S rRNA and aroA genotypes associated with these arsenite‐oxidizing environments. Geochemical analyses, including measurement of arsenite‐oxidation rates within geothermal outflow channels, were combined with 16S rRNA gene and aroA functional gene analysis using newly designed primers to capture previously undescribed aroA‐like arsenite oxidase gene diversity. The majority of bacterial 16S rRNA gene sequences found in acidic (pH 2.6–3.6) Fe‐oxyhydroxide microbial mats were closely related to Hydrogenobaculum spp. (members of the bacterial order Aquificales), while the predominant sequences from near‐neutral (pH 6.2–8) springs were affiliated with other Aquificales including Sulfurihydrogenibium spp., Thermocrinis spp. and Hydrogenobacter spp., as well as members of the Deinococci, Thermodesulfobacteria and β‐Proteobacteria. Modified primers designed around previously characterized and newly identified aroA‐like genes successfully amplified new lineages of aroA‐like genes associated with members of the Aquificales across all geothermal systems examined. The expression of Aquificales aroA‐like genes was also confirmed in situ, and the resultant cDNA sequences were consistent with aroA genotypes identified in the same environments. The aroA sequences identified in the current study expand the phylogenetic distribution of known Mo‐pterin arsenite oxidase genes, and suggest the importance of three prominent genera of the order Aquificales in arsenite oxidation across geochemically distinct geothermal habitats ranging in pH from 2.6 to 8.     

The selectivity of a sequential extraction procedure for the determination of iron oxyhydroxides andiron sulfides in lake sediments

A popular sequential extraction procedure (Tessier et al. 1979) designed t o extract metals partitioned in various sediment phases, was evaluated for its selectivity. Amorphous FeOOH, FeS, and FeS₂ were added separately to natural lake sediments and sequentially extracted. The selectivity of the sequential procedure for the added solid phases was evaluated by determining the difference in the mass of Fe extracted from treated and control sediments. In the experiments where sulfide minerals were added, total S was measured in the residual solids in order to confirm selectivity of the method. Concentrations of total carbon remaining in the solid phase after each extraction step were also measured to determine the selectivity of the sequential procedure for carbon. The procedure was moderately selective for Fe added as FeOOH; a mean of 77 ± 12% (p < 0.05) of the Fe added was extracted in the step designed to reduce Fe-Mn oxyhydroxides. In experiments where FeS was added, a mean of 69 ± 11% (p < 0.05) of the Fe added as FeS was extracted in the fraction designed to oxidize sulfides and organic matter. Approximately 25% of the Fe added as FeS may have been extracted prematurely. Although less precise, total S analyses confirmed that much of the FeS was extracted in the oxidation step, yielding 104 ± 87% (p < 0.05) of the S added as FeS. The procedure was highly selective for FeS₂; 92 ± 14% (p < 0.05) of the Fe added as pyrite was extracted in the sulfide extraction step. Extraction of 80 ± 54% (p < 0.05) of S added as pyrite confirmed that FeS₂ were selectively extracted in the sulfide extraction step. Carbon in the sediments was also selectively extracted in the oxidation step (77 ± 2.4% of total C; p < 0.05). The applications and limitations of sequential extraction procedures as limnological research tools are discussed in light of our results. Request for offprints     

Bacterial formation of phosphatic laminites off Peru

Authigenic phosphatic laminites enclosed in phosphorite crusts from the shelf off Peru (10°01′ S and 10°24′ S) consist of carbonate fluorapatite layers, which contain abundant sulfide minerals including pyrite (FeS₂) and sphalerite (ZnS). Low δ³⁴S_pyrite values (average ?28.8‰) agree with bacterial sulfate reduction and subsequent pyrite formation. Stable sulfur isotopic compositions of sulfate bound in carbonate fluorapatite are lower than that of sulfate from ambient sea water, suggesting bacterial reoxidation of sulfide by sulfide‐oxidizing bacteria. The release of phosphorus and subsequent formation of the autochthonous phosphatic laminites are apparently caused by the activity of sulfate‐reducing bacteria and associated sulfide‐oxidizing bacteria. Following an extraction–phosphorite dissolution–extraction procedure, molecular fossils of sulfate‐reducing bacteria (mono‐O‐alkyl glycerol ethers, di‐O‐alkyl glycerol ethers, as well as the short‐chain branched fatty acids i/ai‐C_15:0, i/ai‐C_17:0 and 10MeC_16:0) are found to be among the most abundant compounds. The fact that these molecular fossils of sulfate‐reducing bacteria are distinctly more abundant after dissolution of the phosphatic laminite reveals that the lipids are tightly bound to the mineral lattice of carbonate fluorapatite. Moreover, compared with the autochthonous laminite, molecular fossils of sulfate‐reducing bacteria are: (1) significantly less abundant and (2) not as tightly bound to the mineral lattice in the other, allochthonous facies of the Peruvian crusts consisting of phosphatic coated grains. These observations confirm the importance of sulfate‐reducing bacteria in the formation of the phosphatic laminite. Model calculations highlight that organic matter degradation by sulfate‐reducing bacteria has the potential to liberate sufficient phosphorus for phosphogenesis.     

Transformation of galena to pyromorphite produces bioavailable sulfur for neutrophilic chemoautotrophy

The aqueous concentration of lead [Pb(II)] in geochemical environments is controlled by the solubility of Pb‐bearing minerals and their weathering products. In contaminated soils, a common method for in situ stabilization of Pb(II) is the addition of phosphate to convert more redox sensitive sulfide minerals into sparingly soluble pyromorphite [Pb₅(PO₄)₃X]. In this study, we conducted experimental studies to investigate the fate of reduced sulfur during the conversion of galena [PbS] to chloropyromorphite [Pb₅(PO₄)₃Cl]. Powder X‐ray diffraction analysis indicated that the reaction of phosphate with galena under oxic conditions resulted in the oxidation of sulfide and formation of elemental sulfur [S₈]. Under oxic abiotic conditions, the S₈ was retained in the solid phase, and negligible concentrations of sulfur as sulfide and thiosulfate were detected in the aqueous phase and only a small amount of sulfate. When PbS reacted in the presence of the chemoautotrophic organism Bosea sp. WAO, the S₈ in the secondary mineral was oxidized to sulfate. Strain WAO produced significantly more sulfate from the secondary S₈ than from the primary galena. Microscopic analysis of mineral–microbe aggregates on mineral‐embedded slide cultures showed that the organism was colocalized and increased in biomass over time on the secondary mineral surface supporting a microbial role. The results of this study indicate that stimulation of sulfur‐oxidizing activity may be a direct consequence of phosphate amendments to Pb(II)‐contaminated soils.