Regina Lohmayer

Disproportionation of elemental sulfur by haloalkaliphilic bacteria from soda lakes

Microbial disproportionation of elemental sulfur to sulfide and sulfate is a poorly characterized part of the anoxic sulfur cycle. So far, only a few bacterial strains have been described that can couple this reaction to cell growth. Continuous removal of the produced sulfide, for instance by oxidation and/or precipitation with metal ions such as iron, is essential to keep the reaction exergonic. Hitherto, the process has exclusively been reported for neutrophilic anaerobic bacteria. Here, we report for the first time disproportionation of elemental sulfur by three pure cultures of haloalkaliphilic bacteria isolated from soda lakes: the Deltaproteobacteria Desulfurivibrio alkaliphilus and Desulfurivibrio sp. AMeS2, and a member of the Clostridia, Dethiobacter alkaliphilus. All cultures grew in saline media at pH 10 by sulfur disproportionation in the absence of metals as sulfide scavengers. Our data indicate that polysulfides are the dominant sulfur species under highly alkaline conditions and that they might be disproportionated. Furthermore, we report the first organism (Dt. alkaliphilus) from the class Clostridia that is able to grow by sulfur disproportionation.

Occurrence of Surface Polysulfides during the Interaction between Ferric (Hydr)Oxides and Aqueous Sulfide

Polysulfides are often referred to as key reactants in the sulfur cycle, especially during the interaction of ferric (hydr)oxides and sulfide, forming ferrous-sulphide minerals. Despite their potential relevance, the extent of polysulfide formation and its relevance for product formation pathways remains enigmatic. We applied cryogenic X-ray Photoelectron Spectroscopy and wet chemical analysis to study sulfur oxidation products during the reaction of goethite and lepidocrocite with aqueous sulfide at different initial Fe/S molar ratios under anoxic conditions at neutral pH. The higher reactivity of lepidocrocite leads to faster and higher electron turnover compared to goethite. We were able to demonstrate for the first time the occurrence of surface-associated polysulfides being the main oxidation products in the presence of both minerals, with a predominance of disulfide (S2(2-)(surf)), and elemental sulfur. Concentrations of aqueous polysulfide species were negligible (<1%). With prior sulfide fixation by zinc acetate, the surface-associated polysulfides could be precipitated as zerovalent sulfur (S°), which was extracted by methanol thereafter. Of the generated S°, 20-34% were associated with S2(2-)(surf). Varying the Fe/S ratio revealed that surface polysulfide formation only becomes dominant when the remaining aqueous sulfide concentration is low (<0.03 mmol L(-1)). We hypothesize these novel surface sulfur species, particularly surface disulfide, to act as pyrite precursors. We further propose that these species play an overlooked role in the sulfur cycle.

Anaerobic Chemolithotrophic Growth of the Haloalkaliphilic Bacterium Strain MLMS‑1 by Disproportionation of Monothioarsenate

A novel chemolithotrophic metabolism based on a mixed arsenic-sulfur species has been discovered for the anaerobic deltaproteobacterium, strain MLMS-1, a haloalkaliphile isolated from Mono Lake, California, U.S. Strain MLMS-1 is the first reported obligate arsenate-respiring chemoautotroph which grows by coupling arsenate reduction to arsenite with the oxidation of sulfide to sulfate. In that pathway the formation of a mixed arsenic-sulfur species was reported. That species was assumed to be monothioarsenite ([H2As(III)S(-II)O2](-)), formed as an intermediate by abiotic reaction of arsenite with sulfide. We now report that this species is monothioarsenate ([HAs(V)S(-II)O3](2-)) as revealed by X-ray absorption spectroscopy. Monothioarsenate forms by abiotic reaction of arsenite with zerovalent sulfur. Monothioarsenate is kinetically stable under a wide range of pH and redox conditions. However, it was metabolized rapidly by strain MLMS-1 when incubated with arsenate. Incubations using monothioarsenate confirmed that strain MLMS-1 was able to grow (μ = 0.017 h(-1)) on this substrate via a disproportionation reaction by oxidizing the thio-group-sulfur (S(-II)) to zerovalent sulfur or sulfate while concurrently reducing the central arsenic atom (As(V)) to arsenite. Monothioarsenate disproportionation could be widespread in nature beyond the already studied arsenic and sulfide rich hot springs and soda lakes where it was discovered.

Sulfur Species as Redox Partners and Electron Shuttles for Ferrihydrite Reduction by Sulfurospirillum deleyianum

ABSTRACT Iron(III) (oxyhydr)oxides can represent the dominant microbial electron acceptors under anoxic conditions in many aquatic environments, which makes understanding the mechanisms and processes regulating their dissolution and transformation particularly important. In a previous laboratory-based study, it has been shown that 0.05 mM thiosulfate can reduce 6 mM ferrihydrite indirectly via enzymatic reduction of thiosulfate to sulfide by the sulfur-reducing bacterium Sulfurospirillum deleyianum, followed by abiotic reduction of ferrihydrite coupled to reoxidation of sulfide. Thiosulfate, elemental sulfur, and polysulfides were proposed as reoxidized sulfur species functioning as electron shuttles. However, the exact electron transfer pathway remained unknown. Here, we present a detailed analysis of the sulfur species involved. Apart from thiosulfate, substoichiometric amounts of sulfite, tetrathionate, sulfide, or polysulfides also initiated ferrihydrite reduction. The portion of thiosulfate produced during abiotic ferrihydrite-dependent reoxidation of sulfide was about 10% of the total sulfur at maximum. The main abiotic oxidation product was elemental sulfur attached to the iron mineral surface, which indicates that direct contact between microorganisms and ferrihydrite is necessary to maintain the iron reduction process. Polysulfides were not detected in the liquid phase. Minor amounts were found associated either with microorganisms or the mineral phase. The abiotic oxidation of sulfide in the reaction with ferrihydrite was identified as rate determining. Cysteine, added as a sulfur source and a reducing agent, also led to abiotic ferrihydrite reduction and therefore should be eliminated when sulfur redox reactions are investigated. Overall, we could demonstrate the large impact of intermediate sulfur species on biogeochemical iron transformations.

Sulfidization of Organic Freshwater Flocs from a Minerotrophic Peatland: Speciation Changes of Iron, Sulfur, and Arsenic

Iron-rich organic flocs are frequently observed in surface waters of wetlands and show a high affinity for trace metal(loid)s. Under low-flow stream conditions, flocs may settle, become buried, and eventually be subjected to reducing conditions facilitating trace metal(loid) release. In this study, we reacted freshwater flocs (704-1280 mg As/kg) from a minerotrophic peatland (Gola di Lago, Switzerland) with sulfide (5.2 mM, S(-II)spike/Fe = 0.75-1.62 mol/mol) at neutral pH and studied the speciation changes of Fe, S, and As at 25 ± 1 °C over 1 week through a combination of synchrotron X-ray techniques and wet-chemical analyses. Sulfidization of floc ferrihydrite and nanocrystalline lepidocrocite caused the rapid formation of mackinawite (52-81% of Fesolid at day 7) as well as solid-phase associated S(0) and polysulfides. Ferrihydrite was preferentially reduced over lepidocrocite, although neoformation of lepidocrocite from ferrihydrite could not be excluded. Sulfide-reacted flocs contained primarily arsenate (47-72%) which preferentially adsorbed to Fe(III)-(oxyhydr)oxides, despite abundant mackinawite precipitation. At higher S(-II)spike/Fe molar ratios (≥1.0), the formation of an orpiment-like phase accounted for up to 35% of solid-phase As. Despite Fe and As sulfide precipitation and the presence of residual Fe(III)-(oxyhydr)oxides, mobilization of As was recorded in all samples (Asaq = 0.45-7.0 μM at 7 days). Aqueous As speciation analyses documented the formation of thioarsenates contributing up to 33% of Asaq. Our findings show that freshwater flocs from the Gola di Lago peatland may become a source of As under sulfate-reducing conditions and emphasize the pivotal role Fe-rich organic freshwater flocs play in trace metal(loid) cycling in S-rich wetlands characterized by oscillating redox conditions.

Chemolithotrophic growth of the aerobic hyperthermophilic bacterium Thermocrinis ruber OC 14/7/2 on monothioarsenate and arsenite.

Novel insights are provided regarding aerobic chemolithotrophic growth of Thermocrinis ruber OC14/7/2 on the electron donors arsenite and monothioarsenate. Thermocrinis ruber is a hyperthermophilic bacterium that thrives in pH-neutral to alkaline hot springs and grows on hydrogen, elemental sulfur, and thiosulfate. Our study showed that T. ruber can also utilize arsenite as sole electron donor producing arsenate. Growth rates of 0.024 h(-1) were lower than for oxidation of thiosulfate to sulfate (μ = 0.247 h(-1)). Fast growth was observed on monothioarsenate (μ = 0.359 h(-1)), comprising different abiotic and biotic redox interactions. The initial dominant process was abiotic transformation of monothioarsenate to arsenate and elemental sulfur, followed by microbial oxidation of sulfur to sulfate. Elevated microbial activity during stationary growth of T. ruber might be explained by microbial oxidation of thiosulfate and arsenite, both also products of abiotic monothioarsenate transformation. However, the observed rapid decrease of monothioarsenate, exceeding concentrations in equilibrium with its products, also indicates direct microbial oxidation of arsenic-bond S(-II) to sulfate. Free sulfide was oxidized abiotically too fast to play a role as electron donor for T. ruber. Our present laboratory and previous field studies suggest that thioarsenates can either indirectly or directly be used by (hyper)thermophiles in arsenic-sulfidic environments.

Pyrobaculum yellowstonensis strain WP30 respires on elemental sulfur and/or arsenate in circumneutral sulfidic geothermal sediments of Yellowstone National Park

ABSTRACT Thermoproteales (phylum Crenarchaeota) populations are abundant in high-temperature (>70°C) environments of Yellowstone National Park (YNP) and are important in mediating the biogeochemical cycles of sulfur, arsenic, and carbon. The objectives of this study were to determine the specific physiological attributes of the isolate Pyrobaculum yellowstonensis strain WP30, which was obtained from an elemental sulfur sediment (Josephs Coat Hot Spring [JCHS], 80°C, pH 6.1, 135 μM As) and relate this organism to geochemical processes occurring in situ. Strain WP30 is a chemoorganoheterotroph and requires elemental sulfur and/or arsenate as an electron acceptor. Growth in the presence of elemental sulfur and arsenate resulted in the formation of thioarsenates and polysulfides. The complete genome of this organism was sequenced (1.99 Mb, 58% G+C content), revealing numerous metabolic pathways for the degradation of carbohydrates, amino acids, and lipids. Multiple dimethyl sulfoxide-molybdopterin (DMSO-MPT) oxidoreductase genes, which are implicated in the reduction of sulfur and arsenic, were identified. Pathways for the de novo synthesis of nearly all required cofactors and metabolites were identified. The comparative genomics of P. yellowstonensis and the assembled metagenome sequence from JCHS showed that this organism is highly related (∼95% average nucleotide sequence identity) to in situ populations. The physiological attributes and metabolic capabilities of P. yellowstonensis provide an important foundation for developing an understanding of the distribution and function of these populations in YNP.

Ion-pair chromatography coupled to inductively coupled plasma-mass spectrometry (IPC-ICP-MS) as a method for thiomolybdate speciation in natural waters.

Molybdenum precipitates preferentially under reducing conditions; therefore, its occurrence in sediment records is used as an indicator of paleoredox conditions. Although thiomolybdates (MoO4-xSx(2-) with x = 1-4) supposedly are necessary intermediates in the process of molybdenum precipitation under anoxic conditions, there is no information about their abundance in natural environments, because of a lack of element-specific methods with sufficiently low detection limits. Here, we optimized ion-pair chromatographic separation for coupling to an inductively coupled plasma-mass spectrometry detector (IPC-ICP-MS). 2-Propanol (10%-25% gradient) replaced the previously used acetonitrile (25%-75%) as the solvent, to reduce the carbon load into the plasma. In synthetic solutions, formation of thiomolybdates was found to occur spontaneously in the presence of excess sulfide and the degree of thiolation was highest at pH 7. Excess hydroxyl led to a transformation of thiomolybdates to molybdate. Under acidic to neutral conditions, precipitation of molybdenum and hydrolysis of tetrathiomolybdate were observed. Flash-freezing was found to be suitable to stabilize tetrathiomolybdate, with <4% transformation over more than two months. High ionic strengths matrices (>2 mM) negatively affected the detection of molybdate, which eluted mainly in the dead volume, but had no negative effect on higher thiolated molybdates. Detection limits were ∼10 nM. With the newly developed IPC-ICP-MS method, thiomolybdates were found to form spontaneously in euxinic marine waters after adding a molybdate spike and occur naturally in sulfidic geothermal waters.

Experimental Confirmation of Isotope Fractionation in Thiomolybdates Using Ion Chromatographic Separation and Detection by Multicollector ICPMS

Molybdenum 98Mo/95Mo isotope ratios are a sediment paleo proxy for the redox state of the ancient ocean. Under sulfidic conditions, no fractionation between seawater and sediment should be observed if molybdate (MoO42-) is quantitatively transformed to tetrathiomolybdate (MoS42-) and precipitated. However, quantum mechanical calculations previously suggested that incomplete sulfidation could be associated with substantial fractionation. To experimentally confirm isotope fractionation in thiomolybdates, a new approach for determination of isotope ratios of individual thiomolybdate species was developed that uses chromatography (HPLC-UV) to separate individual thiomolybdates, collecting each peak and analyzing isotope ratios with multicollector inductively coupled plasma mass spectrometry (MC-ICPMS). Using commercially available MoO42- and MoS42- standards, the method was evaluated and excellent reproducibility and accuracy were obtained. For species with longer retention times, complete chromatographic peaks had to be collected to avoid isotope fractionation within peaks. Isotope fractionation during formation of thiomolybdates could be experimentally proven for the first time in the reaction of MoO42- with 20-fold or 50-fold excess of sulfide. The previously calculated isotope fractionation for MoS42- was confirmed, and the result for MoO2S22- was in the predicted range. Isotopic fractionation during MoS42- transformation with pressurized air was dominated by kinetic fractionation. Further optimization and online-coupling of the HPLC-MC-ICPMS approach for determination of low concentrations in natural samples will greatly help to obtain more accurate species-selective isotope information.

Thioarsenate toxicity and tolerance in the model system Arabidopsis thaliana

Thioarsenates form from arsenite under sulfate-reducing conditions, e.g., in rice paddy soils, and are structural analogues of arsenate. Even though rice is one of the most important sources of human arsenic intake, nothing is published about uptake, toxicity, or tolerance of thioarsenates in plants. Experiments using the model system Arabidopsis thaliana showed that monothioarsenate is less toxic than arsenite, but more toxic than arsenate at concentrations ≥25 μM As, reflected in stronger seedling growth inhibition on agar plates. Despite higher toxicity, total As accumulation in roots was lower upon exposure to monothioarsenate compared to arsenate, and a higher root efflux was confirmed. Root-shoot translocation was higher for monothioarsenate than for arsenate. Compared to the wild type (Col-0), both arsenate and monothioarsenate induced higher toxicity in phytochelatin (PC)-deficient mutants (cad1-3) as well as in glutathione biosynthesis (cad2) and PC transport (abcc12) mutants, demonstrating the important role of the PC pathway, not only for arsenate, but also for monothioarsenate detoxification. In Col-0, monothioarsenate induced relatively higher accumulation of PCs than arsenate. The observed differences in plant uptake, toxicity, and tolerance of thioarsenate vs oxyarsenate show that studying the effects of As on plants should include experiments with thiolated As species.