Autotrophic biofilms sustained by deeply sourced groundwater host diverse bacteria implicated in sulfur and hydrogen metabolism.

BACKGROUND: Biofilms in sulfide-rich springs present intricate microbial communities that play pivotal roles in biogeochemical cycling. We studied chemoautotrophically based biofilms that host diverse CPR bacteria and grow in sulfide-rich springs to investigate microbial controls on biogeochemical cycling. RESULTS: Sulfide springs biofilms were investigated using bulk geochemical analysis, genome-resolved metagenomics, and scanning transmission X-ray microscopy (STXM) at room temperature and 87 K. Chemolithotrophic sulfur-oxidizing bacteria, including Thiothrix and Beggiatoa, dominate the biofilms, which also contain CPR Gracilibacteria, Absconditabacteria, Saccharibacteria, Peregrinibacteria, Berkelbacteria, Microgenomates, and Parcubacteria. STXM imaging revealed ultra-small cells near the surfaces of filamentous bacteria that may be CPR bacterial episymbionts. STXM and NEXAFS spectroscopy at carbon K and sulfur L2,3 edges show that filamentous bacteria contain protein-encapsulated spherical elemental sulfur granules, indicating that they are sulfur oxidizers, likely Thiothrix. Berkelbacteria and Moranbacteria in the same biofilm sample are predicted to have a novel electron bifurcating group 3b [NiFe]-hydrogenase, putatively a sulfhydrogenase, potentially linked to sulfur metabolism via redox cofactors. This complex could potentially contribute to symbioses, for example, with sulfur-oxidizing bacteria such as Thiothrix that is based on cryptic sulfur cycling. One Doudnabacteria genome encodes adjacent sulfur dioxygenase and rhodanese genes that may convert thiosulfate to sulfite. We find similar conserved genomic architecture associated with CPR bacteria from other sulfur-rich subsurface ecosystems. CONCLUSIONS: Our combined metagenomic, geochemical, spectromicroscopic, and structural bioinformatics analyses of biofilms growing in sulfide-rich springs revealed consortia that contain CPR bacteria and sulfur-oxidizing Proteobacteria, including Thiothrix, and bacteria from a new family within Beggiatoales. We infer roles for CPR bacteria in sulfur and hydrogen cycling. Video Abstract.

Fate of Dissolved Nitrogen in a Horizontal Levee: Seasonal Fluctuations in Nitrate Removal Processes.

Horizontal levees are a nature-based approach for removing nitrogen from municipal wastewater effluent while simultaneously providing additional benefits, such as flood control. To assess nitrogen removal mechanisms and the efficacy of a horizontal levee, we monitored an experimental system receiving nitrified municipal wastewater effluent for 2 years. Based on mass balances and microbial gene abundance data, we determined that much of the applied nitrogen was most likely removed by heterotrophic denitrifiers that consumed labile organic carbon from decaying plants and added wood chips. Fe(III) and sulfate reduction driven by decay of labile organic carbon also produced Fe(II) sulfide minerals. During winter months, when heterotrophic activity was lower, strong correlations between sulfate release and nitrogen removal suggested that autotrophic denitrifiers oxidized Fe(II) sulfides using nitrate as an electron acceptor. These trends were seasonal, with Fe(II) sulfide minerals formed during summer fueling denitrification during the subsequent winter. Overall, around 30% of gaseous nitrogen losses in the winter were attributable to autotrophic denitrifiers. To predict long-term nitrogen removal, we developed an electron-transfer model that accounted for the production and consumption of electron donors. The model indicated that the labile organic carbon released from wood chips may be capable of supporting nitrogen removal from wastewater effluent for several decades with sulfide minerals, decaying vegetation, and root exudates likely sustaining nitrogen removal over a longer timescale.

Nanoparticle cellular internalization is not required for RNA delivery to mature plant leaves.

Rapidly growing interest in the nanoparticle-mediated delivery of DNA and RNA to plants requires a better understanding of how nanoparticles and their cargoes translocate in plant tissues and into plant cells. However, little is known about how the size and shape of nanoparticles influence transport in plants and the delivery efficiency of their cargoes, limiting the development of nanotechnology in plant systems. In this study we employed non-biolistically delivered DNA-modified gold nanoparticles (AuNPs) of various sizes (5-20 nm) and shapes (spheres and rods) to systematically investigate their transport following infiltration into Nicotiana benthamiana leaves. Generally, smaller AuNPs demonstrated more rapid, higher and longer-lasting levels of association with plant cell walls compared with larger AuNPs. We observed internalization of rod-shaped but not spherical AuNPs into plant cells, yet, surprisingly, 10 nm spherical AuNPs functionalized with small-interfering RNA (siRNA) were the most efficient at siRNA delivery and inducing gene silencing in mature plant leaves. These results indicate the importance of nanoparticle size in efficient biomolecule delivery and, counterintuitively, demonstrate that efficient cargo delivery is possible and potentially optimal in the absence of nanoparticle cellular internalization. Overall, our results highlight nanoparticle features of importance for transport within plant tissues, providing a mechanistic overview of how nanoparticles can be designed to achieve efficacious biocargo delivery for future developments in plant nanobiotechnology.

Ligand-Free Processable Perovskite Semiconductor Ink.

Traditional covalent semiconductors require complex processing methods for device fabrication due to their high cohesive energies. Here, we develop a stable, ligand-free perovskite semiconductor ink that can be used to make patterned semiconductor-based optoelectronics in one step. The perovskite ink is formed via the dissolution of crystals of vacancy-ordered double perovskite Cs2TeX6 (X = Cl-, Br-, I-) in polar aprotic solvents, leading to the stabilization of isolated [TeX6]2- octahedral anions and free Cs+ cations without the presence of ligands. The stabilization of the fundamental perovskite ionic octahedral building blocks in solution creates multifunctional inks with the ability to reversibly transform between the liquid ink and the solid-state perovskite crystalline system in air within minutes. These easily processable inks can be patterned onto various materials via dropcasting, spraying or painting, and stamping, highlighting the crucial role of solvated octahedral complexes toward the rapid formation of phase-pure perovskite structures in ambient conditions.

Origin of enhanced water oxidation activity in an iridium single atom anchored on NiFe oxyhydroxide catalyst.

The efficiency of the synthesis of renewable fuels and feedstocks from electrical sources is limited, at present, by the sluggish water oxidation reaction. Single-atom catalysts (SACs) with a controllable coordination environment and exceptional atom utilization efficiency open new paradigms toward designing high-performance water oxidation catalysts. Here, using operando X-ray absorption spectroscopy measurements with calculations of spectra and electrochemical activity, we demonstrate that the origin of water oxidation activity of IrNiFe SACs is the presence of highly oxidized Ir single atom (Ir5.3+) in the NiFe oxyhydroxide under operating conditions. We show that the optimal water oxidation catalyst could be achieved by systematically increasing the oxidation state and modulating the coordination environment of the Ir active sites anchored atop the NiFe oxyhydroxide layers. Based on the proposed mechanism, we have successfully anchored Ir single-atom sites on NiFe oxyhydroxides (Ir0.1/Ni9Fe SAC) via a unique in situ cryogenic-photochemical reduction method that delivers an overpotential of 183 mV at 10 mA ⋅ cm-2 and retains its performance following 100 h of operation in 1 M KOH electrolyte, outperforming the reported catalysts and the commercial IrO2 catalysts. These findings open the avenue toward an atomic-level understanding of the oxygen evolution of catalytic centers under in operando conditions.

Endophyte-Promoted Phosphorus Solubilization in Populus.

Phosphorus is one of the essential nutrients for plant growth, but it may be relatively unavailable to plants because of its chemistry. In soil, the majority of phosphorus is present in the form of a phosphate, usually as metal complexes making it bound to minerals or organic matter. Therefore, inorganic phosphate solubilization is an important process of plant growth promotion by plant associated bacteria and fungi. Non-nodulating plant species have been shown to thrive in low-nutrient environments, in some instances by relying on plant associated microorganisms called endophytes. These microorganisms live within the plant and help supply nutrients for the plant. Despite their potential enormous environmental importance, there are a limited number of studies looking at the direct molecular impact of phosphate solubilizing endophytic bacteria on the host plant. In this work, we studied the impact of two endophyte strains of wild poplar (Populus trichocarpa) that solubilize phosphate. Using a combination of x-ray imaging, spectroscopy methods, and proteomics, we report direct evidence of endophyte-promoted phosphorus uptake in poplar. We found that the solubilized phosphate may react and become insoluble once inside plant tissue, suggesting that endophytes may aid in the re-release of phosphate. Using synchrotron x-ray fluorescence spectromicroscopy, we visualized the nutrient phosphorus inside poplar roots inoculated by the selected endophytes and found the phosphorus in both forms of organic and inorganic phosphates inside the root. Tomography-based root imaging revealed a markedly different root biomass and root architecture for poplar samples inoculated with the phosphate solubilizing bacteria strains. Proteomics characterization on poplar roots coupled with protein network analysis revealed novel proteins and metabolic pathways with possible involvement in endophyte enriched phosphorus uptake. These findings suggest an important role of endophytes for phosphorus acquisition and provide a deeper understanding of the critical symbiotic associations between poplar and the endophytic bacteria.

Electrochemically scrambled nanocrystals are catalytically active for CO2-to-multicarbons.

Promotion of C-C bonds is one of the key fundamental questions in the field of CO2 electroreduction. Much progress has occurred in developing bulk-derived Cu-based electrodes for CO2-to-multicarbons (CO2-to-C2+), especially in the widely studied class of high-surface-area "oxide-derived" copper. However, fundamental understanding into the structural characteristics responsible for efficient C-C formation is restricted by the intrinsic activity of these catalysts often being comparable to polycrystalline copper foil. By closely probing a Cu nanoparticle (NP) ensemble catalyst active for CO2-to-C2+, we show that bias-induced rapid fusion or "electrochemical scrambling" of Cu NPs creates disordered structures intrinsically active for low overpotential C2+ formation, exhibiting around sevenfold enhancement in C2+ turnover over crystalline Cu. Integrating ex situ, passivated ex situ, and in situ analyses reveals that the scrambled state exhibits several structural signatures: a distinct transition to single-crystal Cu2O cubes upon air exposure, low crystallinity upon passivation, and high mobility under bias. These findings suggest that disordered copper structures facilitate C-C bond formation from CO2 and that electrochemical nanocrystal scrambling is an avenue toward creating such catalysts.

Selenium Metabolism in Hemp (Cannabis sativa L.)-Potential for Phytoremediation and Biofortification.

Selenium (Se) deficiency and toxicity affect over a billion people worldwide. Plants can mitigate both problems, via Se biofortification and phytoremediation. Here we explore the potential of hemp (Cannabis sativa L.) for these phytotechnologies. Field surveys in naturally seleniferous agricultural areas in Colorado, United States, found 15-25 µg of Se/g in seed and 5-10 µg of Se/g dry weight (DW) in flowers and leaves. Thus, 4 g of this hemp seed provides the U.S. recommended daily allowance of 55-75 µg of Se. In controlled greenhouse experiments, hemp seedlings grown in Turface supplied with 40-320 µM selenate showed complete tolerance up to 160 µM and accumulated up to 1300 mg of Se/kg shoot dry weight. Mature hemp grown in Turface supplied with 5-80 µM selenate was completely tolerant up to 40 µM selenate and accumulated up to 200 mg of Se/kg DW in leaves, flowers, and seeds. Synchrotron X-ray fluorescence and X-ray absorption spectroscopies of selenate-supplied hemp showed Se to accumulate mainly in the leaf vasculature and in the seed embryos, with predominant Se speciation in C-Se-C forms (57-75% in leaf and more than 86% in seeds). Aqueous seed extracts were found by liquid chromatography mass spectrometry to contain selenomethionine and methyl-selenocysteine (1:1-3 ratio), both excellent dietary Se sources. Floral concentrations of medicinal cannabidiol (CBD) and terpenoids were not affected by Se. We conclude that hemp has good potential for Se phytoremediation while producing Se-biofortified dietary products.

Identification and physiological comparison of plant species that show positive or negative co-occurrence with selenium hyperaccumulators.

In these studies we identified and compared the properties of plant species that showed positive or negative co-occurrence with selenium (Se) hyperaccumulators in their natural habitat. The main questions addressed were: which species are most abundant directly adjacent to hyperaccumulators, and which are absent? How do Se accumulation and tolerance compare in species found to positively or negatively co-occur with hyperaccumulators? Approaches included field surveys, X-ray microprobe analysis of field samples, and a lab Se tolerance and accumulation study. When 54 hyperaccumulators across two naturally seleniferous sites were surveyed for their five nearest neighboring species, and the relative abundance of these species around hyperaccumulators compared to that in the overall vegetation, some species were identified to positively or negatively co-occur with hyperaccumulators. Several positively co-occurring species showed high Se accumulation capability (up to 900 mg Se per kg dry weight), which may reflect Se tolerance. Leaf X-ray microprobe analysis found relatively more organic forms of Se in two positively co-occurring species than in a negatively co-occurring one. There were elevated soil Se levels around Se hyperaccumulators, and neighbors of Se hyperaccumulators had a higher tissue Se concentration as compared to when the same species grew elsewhere in the area. The elevated soil Se levels around Se hyperaccumulators - likely resulting from litter deposition- may significantly affect the local plant community, facilitating Se-tolerant plant community members but lowering the fitness of Se-sensitive members.

Selenium tolerance, accumulation, localization and speciation in a Cardamine hyperaccumulator and a non-hyperaccumulator.

Cardamine violifolia (family Brassicaceae) is the first discovered selenium hyperaccumulator from the genus Cardamine with unique properties in terms of selenium accumulation, i.e., high abundance of selenolanthionine. In our study, a fully comprehensive experiment was conducted with the comparison of a non-hyperaccumulator Cardamine species, Cardamine pratensis, covering growth characteristics, chlorophyll fluorescence, spatial selenium/sulfur distribution patterns through elemental analyses (synchrotron-based X-Ray Fluorescence and ICP-OES) and speciation data through selenium K-edge micro X-ray absorption near-edge structure analysis (µXANES) and strong cation exchange (SCX)-ICP-MS. The results revealed remarkable differences in contrast to other selenium hyperaccumulators as neither Cardamine species showed evidence of growth stimulation by selenium. Also, selenite uptake was not inhibited by phosphate for either of the Cardamine species. Sulfate inhibited selenate uptake, but the two Cardamine species did not show any difference in this respect. However, µXRF derived speciation maps and selenium/sulfur uptake characteristics provided results that are similar to other formerly reported hyperaccumulator and non-hyperaccumulator Brassicaceae species. µXANES showed organic selenium, "C-Se-C", in seedlings of both species and also in mature C. violifolia plants. In contrast, selenate-supplied mature C. pratensis contained approximately half "C-Se-C" and half selenate. SCX-ICP-MS data showed evidence of the lack of selenocystine in any of the Cardamine plant extracts. Thus, C. violifolia shows clear selenium-related physiological and biochemical differences compared to C. pratensis and other selenium hyperaccumulators.

Selenium Accumulation, Speciation and Localization in Brazil Nuts (Bertholletia excelsa H.B.K.).

More than a billion people worldwide may be selenium (Se) deficient, and supplementation with Se-rich Brazil nuts may be a good strategy to prevent deficiency. Since different forms of Se have different nutritional value, and Se is toxic at elevated levels, careful seed characterization is important. Variation in Se concentration and correlations of this element with other nutrients were found in two batches of commercially available nuts. Selenium tissue localization and speciation were further determined. Mean Se levels were between 28 and 49 mg kg-1, with up to 8-fold seed-to-seed variation (n = 13) within batches. Brazil nut Se was mainly in organic form. While present throughout the seed, Se was most concentrated in a ring 1 to 2 mm below the surface. While healthy, Brazil nuts should be consumed in moderation. Consumption of one seed (5 g) from a high-Se area meets its recommended daily allowance; the recommended serving size of 30 g may exceed the allowable daily intake (400 µg) or even its toxicity threshold (1200 µg). Based on these findings, the recommended serving size may be re-evaluated, consumers should be warned not to exceed the serving size and the seed may be sold as part of mixed nuts, to avoid excess Se intake.

Bioinspired Metal-Organic Framework Catalysts for Selective Methane Oxidation to Methanol.

Particulate methane monooxygenase (pMMO) is an enzyme that oxidizes methane to methanol with high activity and selectivity. Limited success has been achieved in incorporating biologically relevant ligands for the formation of such active site in a synthetic system. Here, we report the design and synthesis of metal-organic framework (MOF) catalysts inspired by pMMO for selective methane oxidation to methanol. By judicious selection of a framework with appropriate topology and chemical functionality, MOF-808 was used to postsynthetically install ligands bearing imidazole units for subsequent metalation with Cu(I) in the presence of dioxygen. The catalysts show high selectivity for methane oxidation to methanol under isothermal conditions at 150 °C. Combined spectroscopies and density functional theory calculations suggest bis(µ-oxo) dicopper species as probable active site of the catalysts.

Characterization of Selenium Accumulation, Localization and Speciation in Buckwheat-Implications for Biofortification.

Buckwheat is an important crop species in areas of selenium (Se) deficiency. To obtain better insight into their Se metabolic properties, common buckwheat (Fagopyrum esculentum) and tartary buckwheat (F. tataricum) were supplied with different concentrations of Se, supplied as selenate, selenite, or Astragalus bisulcatus plant extract (methyl-selenocysteine). Se was supplied at different developmental stages, with different durations, and in the presence or absence of potentially competing ions, sulfate, and phosphate. The plants were analyzed for growth, Se uptake, translocation, accumulation, as well as for Se localization and chemical speciation in the seed. Plants of both buckwheat species were supplied with 20 µM of either of the three forms of Se twice over their growth period. Both species accumulated 15-40 mg Se kg-1 DW in seeds, leaves and stems, from all three selenocompounds. X-ray microprobe analysis showed that the Se in seeds was localized in the embryo, in organic C-Se-C form(s) resembling selenomethionine, methyl-selenocysteine, and γ-glutamyl-methylselenocysteine standards. In short-term (2 and 24 h) Se uptake studies, both buckwheat species showed higher Se uptake rate and shoot Se accumulation when supplied with plant extract (methyl-selenocysteine), compared to selenite or selenate. In long-term (7 days) uptake studies, both species were resistant to selenite up to 50 µM. Tartary buckwheat was also resistant to selenate up to 75 µM Se, but >30 µM selenate inhibited common buckwheat growth. Selenium accumulation was similar in both species. When selenite was supplied, Se levels were 10-20-fold higher in root (up to 900 mg Se kg-1 DW) than shoot, but 4-fold higher in shoot (up to 1,200 mg Se kg-1 DW) than root for selenate-supplied plants. Additionally, sulfate and phosphate supply affected Se uptake, and conversely selenate enhanced S and P accumulation in both species. These findings have relevance for crop Se biofortification applications.

Fungal Endophyte Alternaria tenuissima Can Affect Growth and Selenium Accumulation in Its Hyperaccumulator Host Astragalus bisulcatus.

Endophytes can enhance plant stress tolerance by promoting growth and affecting elemental accumulation, which may be useful in phytoremediation. In earlier studies, up to 35% elemental selenium (Se0) was found in Se hyperaccumulator Astragalus bisulcatus. Since Se0 can be produced by microbes, the plant Se0 was hypothesized to be microbe-derived. Here we characterize a fungal endophyte of A. bisulcatus named A2. It is common in seeds from natural seleniferous habitat containing 1,000-10,000 mg kg-1 Se. We identified A2 as Alternaria tenuissima via 18S rRNA sequence analysis and morphological characterization. X-ray microprobe analysis of A. bisulcatus seeds that did or did not harbor Alternaria, showed that both contained >90% organic seleno-compounds with C-Se-C configuration, likely methylselenocysteine and glutamyl-methylselenocysteine. The seed Se was concentrated in the embryo, not the seed coat. X-ray microprobe analysis of A2 in pure culture showed the fungus produced Se0 when supplied with selenite, but accumulated mainly organic C-Se-C compounds when supplied with selenate. A2 was completely resistant to selenate up to 300 mg L-1, moderately resistant to selenite (50% inhibition at ∼50 mg Se L-1), but relatively sensitive to methylselenocysteine and to Se extracted from A. bisulcatus (50% inhibition at 25 mg Se L-1). Four-week old A. bisulcatus seedlings derived from surface-sterilized seeds containing endophytic Alternaria were up to threefold larger than seeds obtained from seeds not showing evidence of fungal colonization. When supplied with Se, the Alternaria-colonized seedlings had lower shoot Se and sulfur levels than seedlings from uncolonized seeds. In conclusion, A. tenuissima may contribute to the Se0 observed earlier in A. bisulcatus, and affect host growth and Se accumulation. A2 is sensitive to the Se levels found in its host's tissues, but may avoid Se toxicity by occupying low-Se areas (seed coat, apoplast) and converting plant Se to non-toxic Se0. These findings illustrate the potential for hyperaccumulator endophytes to affect plant properties relevant for phytoremediation. Facultative endophytes may also be applicable in bioremediation and biofortification, owing to their capacity to turn toxic inorganic forms of Se into non-toxic or even beneficial, organic forms with anticarcinogenic properties.

Mapping Catalytically Relevant Edge Electronic States of MoS2.

Molybdenum disulfide (MoS2) is a semiconducting transition metal dichalcogenide that is known to be a catalyst for both the hydrogen evolution reaction (HER) as well as for hydro-desulfurization (HDS) of sulfur-rich hydrocarbon fuels. Specifically, the edges of MoS2 nanostructures are known to be far more catalytically active as compared to unmodified basal planes. However, in the absence of the precise details of the geometric and electronic structure of the active catalytic sites, a rational means of modulating edge reactivity remain to be developed. Here we demonstrate using first-principles calculations, X-ray absorption spectroscopy, as well as scanning transmission X-ray microscopy (STXM) imaging that edge corrugations yield distinctive spectroscopic signatures corresponding to increased localization of hybrid Mo 4d states. Independent spectroscopic signatures of such edge states are identified at both the S L2,3 and S K-edges with distinctive spatial localization of such states observed in S L2,3-edge STXM imaging. The presence of such low-energy hybrid states at the edge of the conduction band is seen to correlate with substantially enhanced electrocatalytic activity in terms of a lower Tafel slope and higher exchange current density. These results elucidate the nature of the edge electronic structure and provide a clear framework for its rational manipulation to enhance catalytic activity.

Supported Au Nanoparticles with N-Heterocyclic Carbene Ligands as Active and Stable Heterogeneous Catalysts for Lactonization.

Attachment of N-heterocyclic carbenes (NHCs) on the surface of metal nanoparticle (NP) catalysts permits fine-tuning of catalytic activity and product selectivity. Yet, NHC-coated Au NPs have been seldom used in catalysis beyond hydrogenation chemistry. One challenge in this field has been to develop a platform that permits arbitrary ligand modification without having to compromise NP stability toward aggregation or leaching. Herein, we exploit the strategy of supported dendrimer-encapsulated metal clusters (DEMCs) to achieve aggregation-stable yet active heterogeneous Au NP catalysts with NHC ligands. Dendrimers function as aggregation-inhibitors during the NP synthesis, and NHCs, well-known for their strong attachment to the gold surface, provide a handle to modify the stereochemistry, stereoelectronics, and chemical functionality of the NP surface. Indeed, compared to "ligandless" Au NPs which are virtually inactive below 80 °C, the NHC-ligated Au NP catalysts enable a model lactonization reaction to proceed at 20 °C on the same time scale (hours). Based on Eyring analysis, proto-deauration is the turnover-limiting step accelerated by the NHC ligands. Furthermore, the use of chiral NHCs led to asymmetric induction (up to 16% enantiomeric excess) in the lactonization transformations, which demonstrates the potential of supported DEMCs with ancillary ligands in enantioselective catalysis.

Stable iridium dinuclear heterogeneous catalysts supported on metal-oxide substrate for solar water oxidation.

Atomically dispersed catalysts refer to substrate-supported heterogeneous catalysts featuring one or a few active metal atoms that are separated from one another. They represent an important class of materials ranging from single-atom catalysts (SACs) and nanoparticles (NPs). While SACs and NPs have been extensively reported, catalysts featuring a few atoms with well-defined structures are poorly studied. The difficulty in synthesizing such structures has been a critical challenge. Here we report a facile photochemical method that produces catalytic centers consisting of two Ir metal cations, bridged by O and stably bound to a support. Direct evidence unambiguously supporting the dinuclear nature of the catalysts anchored on α-Fe2O3 is obtained by aberration-corrected scanning transmission electron microscopy (AC-STEM). Experimental and computational results further reveal that the threefold hollow binding sites on the OH-terminated surface of α-Fe2O3 anchor the catalysts to provide outstanding stability against detachment or aggregation. The resulting catalysts exhibit high activities toward H2O photooxidation.

Correlative Cryogenic Spectromicroscopy to Investigate Selenium Bioreduction Products.

Accurate mapping of the composition and structure of minerals and associated biological materials is critical in geomicrobiology and environmental research. Here, we have developed an apparatus that allows the correlation of cryogenic transmission electron microscopy (cryo-TEM) and synchrotron hard X-ray microprobe (SHXM) data sets to precisely determine the distribution, valence state, and structure of selenium in biofilms sampled from a contaminated aquifer near Rifle, CO. Results were replicated in the laboratory via anaerobic selenate-reducing enrichment cultures. 16S rRNA analyses of field-derived biofilm indicated the dominance of Betaproteobacteria from the Comamonadaceae family and uncultivated members of the Simplicispira genus. The major product in field and culture-derived biofilms is ∼25-300 nm red amorphous Se0 aggregates of colloidal nanoparticles. Correlative analyses of the cultures provided direct evidence for the microbial dissimilatory reduction of Se(VI) to Se(IV) to Se0. Extended X-ray absorption fine-structure spectroscopy showed red amorphous Se0 with a first shell Se-Se interatomic distance of 2.339 ± 0.003 Å. Complementary scanning transmission X-ray microscopy revealed that these aggregates are strongly associated with a protein-rich biofilm matrix. These findings have important implications for predicting the stability and mobility of Se bioremediation products and understanding of Se biogeochemical cycling. The approach, involving the correlation of cryo-SHXM and cryo-TEM data sets from the same specimen area, is broadly applicable to biological and environmental samples.

Sponge-associated bacteria mineralize arsenic and barium on intracellular vesicles.

Arsenic and barium are ubiquitous environmental toxins that accumulate in higher trophic-level organisms. Whereas metazoans have detoxifying organs to cope with toxic metals, sponges lack organs but harbour a symbiotic microbiome performing various functions. Here we examine the potential roles of microorganisms in arsenic and barium cycles in the sponge Theonella swinhoei, known to accumulate high levels of these metals. We show that a single sponge symbiotic bacterium, Entotheonella sp., constitutes the arsenic- and barium-accumulating entity within the host. These bacteria mineralize both arsenic and barium on intracellular vesicles. Our results indicate that Entotheonella sp. may act as a detoxifying organ for its host.

Do selenium hyperaccumulators affect selenium speciation in neighboring plants and soil? An X-Ray Microprobe Analysis.

Neighbors of Se hyperaccumulators Stanleya pinnata and Astragalus bisulcatus were found earlier to have elevated Se levels. Here we investigate whether Se hyperaccumulators affect Se localization and speciation in surrounding soil and neighboring plants. X-ray fluorescence mapping and X-ray absorption near-edge structure spectroscopy were used to analyze Se localization and speciation in leaves of Artemisia ludoviciana, Symphyotrichum ericoides and Chenopodium album growing next to Se hyperaccumulators or non-accumulators at a seleniferous site. Regardless of neighbors, A. ludoviciana, S. ericoides and C. album accumulated predominantly (73-92%) reduced selenocompounds with XANES spectra similar to the C-Se-C compounds selenomethionine and methyl-selenocysteine. Preliminary data indicate that the largest Se fraction (65-75%), both in soil next to hyperaccumulator S. pinnata and next to nonaccumulator species was reduced Se with spectra similar to C-Se-C standards. These same C-Se-C forms are found in hyperaccumulators. Thus, hyperaccumulator litter may be a source of organic soil Se, but soil microorganisms may also contribute. These findings are relevant for phytoremediation and biofortification since organic Se is more readily accumulated by plants, and more effective for dietary Se supplementation.