Arsenic Reduces Methane Emissions from Paddy Soils: Insights from Continental Investigation and Laboratory Incubations.

Arsenic (As) contamination and methane (CH4) emissions co-occur in rice paddies. However, how As impacts CH4 production, oxidation, and emission dynamics is unknown. Here, we investigated the abundances and activities of CH4-cycling microbes from 132 paddy soils with different As concentrations across continental China using metagenomics and the reverse transcription polymerase chain reaction. Our results revealed that As was a crucial factor affecting the abundance and distribution patterns of the mcrA gene, which is responsible for CH4 production and anaerobic CH4 oxidation. Laboratory incubation experiments showed that adding 30 mg kg-1 arsenate increased 13CO2 production by 10-fold, ultimately decreasing CH4 emissions by 68.5%. The inhibition of CH4 emissions by As was induced through three aspects: (1) the toxicity of As decreased the abundance and activity of the methanogens; (2) the adaptability and response of methanotrophs to As is beneficial for CH4 oxidation under As stress; and (3) the more robust arsenate reduction would anaerobically consume more CH4 in paddies. Additionally, significant positive correlations were observed between arsC and pmoA gene abundance in both the observational study and incubation experiment. These findings enhance our understanding of the mechanisms underlying the interactions between As and CH4 cycling in soils.

Sulfate-mediated Fe(III) mineral reduction accelerates arsenic mobilization by a Desulfovibrio strain isolated from paddy soil.

The biogeochemical cycling of arsenic (As) is often intertwined with iron (Fe) and sulfur (S) cycles, wherein Fe(III)- and sulfate-reducing bacteria (SRB) play a crucial role. Here, we isolated strain DS-1, a strictly anaerobic Fe(III)- and sulfate-reducing bacterium, from As-contaminated paddy soil. Using 16S rRNA gene sequence analysis, strain DS-1 was identified as a member of the genus Desulfovibrio. Strain DS-1 utilized energy derived from ferrihydrite reduction to support its cellular growth. Under anoxic sulfate-reducing conditions, the presence of strain DS-1 significantly increased As mobilization compared to sulfate-free conditions. Mechanistically, SRB-produced sulfide reacts with Fe(III) to form FeS, which disrupts Fe(III) minerals, thereby enhancing As release. These findings highlight the critical role of redox disequilibrium in As mobilization and suggest that SRB-produced sulfide may permeate to the rice rhizosphere, increasing As mobilization through Fe(III) reduction.

Arsenic immobilization and greenhouse gas emission depend on quantity and frequency of nitrogen fertilization in paddy soil.

Nitrogen (N) fertilization in paddy soils decreases arsenic mobility and methane emissions. However, it is unknown how quantity and frequency of N fertilization affects the interlinked redox reactions of iron(II)-driven denitrification, iron mineral (trans-)formation with subsequent arsenic (im-)mobilization, methane and nitrous oxide emissions, and how this links to microbiome composition. Thus, we incubated paddy soil from Vercelli, Italy, over 129 days and applied nitrate fertilizer at different concentrations (control: 0, low: ∼35, medium: ∼100, high: ∼200 mg N kg-1 soil-1) once at the beginning and after 49 days. In the high N treatment, nitrate reduction was coupled to oxidation of dissolved and solid-phase iron(II), while naturally occurring arsenic was retained on iron minerals due to suppression of reductive iron(III) mineral dissolution. In the low N treatment, 40 µg L-1 of arsenic was mobilized into solution after nitrate depletion, with 69 % being immobilized after a second nitrate application. In the non-fertilized control, concentrations of dissolved arsenic were as high as 76 µg L-1, driven by mobilization of 36 % of the initial mineral-bound arsenic. Generally, N fertilization led to 1.5-fold higher total GHG emissions (sum of CO2, CH4 and N2O as CO2 equivalents), 158-fold higher N2O, and 7.5-fold lower CH4 emissions compared to non-fertilization. On day 37, Gallionellaceae, Comamonadaceae and Rhodospirillales were more abundant in the high N treatment compared to the non-fertilized control, indicating their potential role as key players in nitrate reduction coupled to iron(II) oxidation. The findings underscore the dual effect of N fertilization, immobilizing arsenic in the short-term (low/medium N) or long-term (high N), while simultaneously increasing N2O and lowering CH4 emissions. This highlights the significance of both the quantity and frequency of N fertilizer application in paddy soils.

Extent of As(III) versus As(V) adsorption on iron (oxyhydr) oxides depends on the presence of vacancy cluster-like micropore sites: Insights into a seesaw effect.

Iron (oxyhydr)oxides are ubiquitous in terrestrial environments and play a crucial role in controling the fate of arsenic in sediments and groundwater. Although there is evidence that different iron (oxyhydr)oxides have different affinities towards As(III) and As(V), it is still unclear why As(V) adsorption on some iron (oxyhydr)oxides is larger than As(III) adsorption, while it is opposite for other ones. In this study, six typical iron (oxyhydr)oxides are selected to evaluate their adsorption capacities for As(III) and As(V). The characteristics of these iron minerals such as morphology, arsenic adsorption species, and pore size distribution are carefully examined using transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDS), positron annihilation lifetime (PAL) spectroscopy, and X-ray absorption spectroscopy (XAS). We confirm a seesaw effect occurred in different iron minerals for As(III) and As(V) immobilization, i.e., at pH 6.0, adsorption of As(V) on hematite (0.73 µmol m-2) and magnetite (0.33 µmol m-2) is higher than for As(III) (0.61 µmol m-2 and 0.27 µmol m-2, respectively), for goethite and lepidocrocite it is almost equal, while As(III) sorption on ferrihydrite (5.77 µmol m-2) and schwertmannite (28.41 µmol m-2) showed higher sorption than As(V) (1.53 µmol m-2 and 12.99 µmol m-2, respectively). PAL analysis demonstrates that ferrihydrite and schwertmannite have a large concentration of vacancy cluster-like micropores, significantly more than goethite and lepidocrocite, followed by hematite and magnetite. The difference of adsorption of As(III) and As(V) to different iron (oxyhydr)oxides is due to differences in the abundance of vacancy cluster-like micropore sites, which are conducive for smaller size As(III) immobilization but not for larger size of As(V). The findings of this study provide novel insights into a seesaw effect for As(III) and As(V) immobilization on naturally occurring iron mineral.

Directional long-distance electron transfer from reduced to oxidized zones in the subsurface.

Electron transfer (ET) is the fundamental redox process of life and element cycling. The ET distance is normally as short as nanometers or micrometers in the subsurface. However, the redox gradient in the subsurface is as long as centimeters or even meters. This gap triggers an intriguing question whether directional long-distance ET from reduced to oxidized zones exists along the redox gradient. By using electron-donating capacity variation as a proxy of ET, we show that ET can last over 10 cm along the redox gradient in sediment columns, through a directional long-distance ET chain from reduced to oxidized zones constituted by a series of short-distance electron hopping reactions. Microbial and chemical processes synergistically mediate the long-distance ET chain, with an estimated flux of 6.73 µmol e-/cm2 per day. This directional long-distance ET represents an overlooked but important "remote" source of electrons for local biogeochemical and environmental processes.

Emerging investigator series: preferential adsorption and coprecipitation of permafrost organic matter with poorly crystalline iron minerals.

Future permafrost thaw will likely lead to substantial release of greenhouse gases due to thawing of previously unavailable organic carbon (OC). Accurate predictions of this release are limited by poor knowledge of the bioavailability of mobilized OC during thaw. Organic carbon bioavailability decreases due to adsorption to, or coprecipitation with, poorly crystalline ferric iron (Fe(III)) (oxyhydr)oxide minerals but the maximum binding extent and binding selectivity of permafrost OC to these minerals is unknown. We therefore utilized water-extractable organic matter (WEOM) from soils across a permafrost thaw gradient to quantify adsorption and coprecipitation processes with poorly crystalline Fe(III) (oxyhydr)oxides. We found that the maximum adsorption capacity of WEOM from intact and partly thawed permafrost soils was similar (204 and 226 mg C g-1 ferrihydrite, respectively) but decreased to 81 mg C g-1 ferrihydrite for WEOM from the fully thawed site. In comparison, coprecipitation of WEOM from intact and partly thawed soils with Fe immobilized up to 925 and 1532 mg C g-1 Fe respectively due to formation of precipitated Fe(III)-OC phases. Analysis of the OC composition before and after adsorption/coprecipitation revealed that high molecular weight, oxygen-rich, carboxylic- and aromatic-rich OC was preferentially bound to Fe(III) minerals relative to low molecular weight, aliphatic-rich compounds which may be more bioavailable. This selective binding effect was stronger after adsorption than coprecipitation. Our results suggest that OC binding by Fe(III) (oxyhydr)oxides sharply decreases under fully thawed conditions and that small, aliphatic OC molecules that may be readily bioavailable are less protected across all thaw stages.

Interactions between iron and carbon in permafrost thaw ponds.

Thawing permafrost forms "thaw ponds" that accumulate and transport organic carbon (OC), redox-active iron (Fe), and other elements. Although Fe has been shown to act as a control on the microbial degradation of OC in permafrost soils, the role of iron in carbon cycling in thaw ponds remains poorly understood. Here, we investigated Fe-OC interactions in thaw ponds in partially and fully thawed soils ("bog" and "fen" thaw ponds, respectively) in a permafrost peatland complex in Abisko, Sweden, using size separation (large particulate fraction (LPF), small particulate fraction (SPF), and dissolved fraction (DF)), acid extractions, scanning electron microscopy (SEM), Fe K-edge X-ray absorption spectroscopy (XAS), and Fourier Transform Infrared (FTIR) spectroscopy. The bulk total Fe (total suspended Fe) in the bogs ranged from 135 mg/L (mean = 13 mg/L) whereas the fens exhibited higher total Fe (1.5 to 212 mg/L, mean = 30 mg/L). The concentration of bulk total OC (TOC) in the bog thaw ponds ranged from 50 to 352 mg/L (mean = 170 mg/L), higher than the TOC concentration in the fen thaw ponds (8.5 to 268 mg/L, mean = 17 mg/L). The concentration of 1 M HCl-extractable Fe in the bog ponds was slightly lower than that in the fens (93 ± 1.2 and 137 ± 3.5 mg/L Fe, respectively) with Fe predominantly (>75 %) in the DF in both thaw stages. Fe K-edge XAS analysis showed that while Fe(II) was the predominant species in LPF, Fe(III) was more abundant in the DF, indicating that the stage of thawing and particle size may control Fe redox state. Furthermore, Fe(II) and Fe(III) were partially complexed with natural organic matter (NOM, 8 to 80 %) in both thaw ponds. Results of our work suggest that Fe and OC released during permafrost thaw into thaw ponds (re-)associate, potentially protecting OC from microbial decomposition while also stabilizing the redox state of Fe.

Morphological, Microstructural, and In Situ Chemical Characteristics of Siderite Produced by Iron-Reducing Bacteria.

Dissimilatory iron-reducing bacteria (DIRB) oxidize organic matter or hydrogen and reduce ferric iron to form Fe(II)-bearing minerals, such as magnetite and siderite. However, compared with magnetite, which was extensively studied, the mineralization process and mechanisms of siderite remain unclear. Here, with the combination of advanced electron microscopy and synchrotron-based scanning transmission X-ray microscopy (STXM) approaches, we studied in detail the morphological, structural, and chemical features of biogenic siderite via a growth experiment with Shewanella oneidensis MR-4. Results showed that along with the growth of cells, Fe(II) ions were increasingly released into solution and reacted with CO32- to form micrometer-sized siderite minerals with spindle, rod, peanut, dumbbell, and sphere shapes. They are composed of many single-crystal siderite plates that are fanned out from the center of the particles. Additionally, STXM revealed Fh and organic molecules inside siderite. This suggests that the siderite crystals might assemble around a Fh-organic molecule core and then continue to grow radially. This study illustrates the biomineralization and assembly of siderite by a successive multistep growth process induced by DIRB, also provides evidences that the distinctive shapes and the presence of organic molecules inside might be morphological and chemical features for biogenic siderite.

Simultaneously decreasing arsenic and cadmium in rice by soil sulfate and limestone amendment under intermittent flooding.

Water management in paddy soils can effectively reduce the soil-to-rice grain transfer of either As or Cd, but not of both elements simultaneously due to the higher mobility of As under reducing and Cd under oxidizing soil conditions. Limestone amendment, the common form of liming, is well known for decreasing Cd accumulation in rice grown on acidic soils. Sulfate amendment was suggested to effectively decrease As accumulation in rice, especially under intermittent soil flooding. To study the unknown effects of combined sulfate and limestone amendment under intermittent flooding for simultaneously decreasing As and Cd in rice, we performed a pot experiment using an acidic sandy loam paddy soil. We also included a clay loam paddy soil to study the role of soil texture in low-As rice production under intermittent flooding. We found that liming not only decreased rice Cd concentrations but also greatly decreased dimethylarsenate (DMA) accumulation in rice. We hypothesize that this is due to suppressed sulfate reduction, As methylation, and As thiolation by liming in the sulfate-amended soil and a higher share of deprotonated DMA at higher pH which is taken up less readily than protonated DMA. Decreased gene abundance of potential soil sulfate-reducers by liming further supported our hypothesis. Combined sulfate and limestone amendment to the acidic sandy loam soil produced rice with 43% lower inorganic As, 72% lower DMA, and 68% lower Cd compared to the control soil without amendment. A tradeoff between soil aeration and water availability was observed for the clay loam soil, suggesting difficulties to decrease As in rice while avoiding plant water stress under intermittent flooding in fine-textured soils. Our results suggest that combining sulfate amendment, liming, and intermittent flooding can help to secure rice safety when the presence of both As and Cd in coarse-textured soils is of concern.

Phototrophic Fe(II) oxidation by Rhodopseudomonas palustris TIE-1 in organic and Fe(II)-rich conditions.

Rhodopseudomonas palustris TIE-1 grows photoautotrophically with Fe(II) as an electron donor and photoheterotrophically with a variety of organic substrates. However, it is unclear whether R. palustris TIE-1 conducts Fe(II) oxidation in conditions where organic substrates and Fe(II) are available simultaneously. In addition, the effect of organic co-substrates on Fe(II) oxidation rates or the identity of Fe(III) minerals formed is unknown. We incubated R. palustris TIE-1 with 2 mM Fe(II), amended with 0.6 mM organic co-substrate, and in the presence/absence of CO2 . We found that in the absence of CO2 , only the organic co-substrates acetate, lactate and pyruvate, but not Fe(II), were consumed. When CO2 was present, Fe(II) and all organic substrates were consumed. Acetate, butyrate and pyruvate were consumed before Fe(II) oxidation commenced, whereas lactate and glucose were consumed at the same time as Fe(II) oxidation proceeded. Lactate, pyruvate and glucose increased the Fe(II) oxidation rate significantly (by up to threefold in the case of lactate). 57 Fe Mössbauer spectroscopy revealed that short-range ordered Fe(III) oxyhydroxides were formed under all conditions. This study demonstrates phototrophic Fe(II) oxidation proceeds even in the presence of organic compounds, and that the simultaneous oxidation of organic substrates can stimulate Fe(II) oxidation.

Phototrophic Fe(II) oxidation benefits from light/dark cycles.

Phototrophic Fe(II)-oxidizers use Fe(II) as electron donor for CO2 fixation thus linking Fe(II) oxidation, ATP formation, and growth directly to the availability of sunlight. We compared the effect of short (10 h light/14 h dark) and long (2-3 days light/2-3 days dark) light/dark cycles to constant light conditions for the phototrophic Fe(II)-oxidizer Chlorobium ferrooxidans KoFox. Fe(II) oxidation was completed first in the setup with constant light (9 mM Fe(II) oxidised within 8.9 days) compared to the light/dark cycles but both short and long light/dark cycles showed faster maximum Fe(II) oxidation rates. In the short and long cycle, Fe(II) oxidation rates reached 3.5 ± 1.0 and 2.6 ± 0.3 mM/d, respectively, compared to 2.1 ± 0.3 mM/d in the constant light setup. Maximum Fe(II) oxidation was significantly faster in the short cycle compared to the constant light setup. Cell growth reached roughly equivalent cell numbers across all three light conditions (from 0.2-2.0 × 106 cells/mL to 1.1-1.4 × 108 cells/mL) and took place in both the light and dark phases of incubation. SEM images showed different mineral structures independent of the light setup and 57 Fe Mössbauer spectroscopy confirmed the formation of poorly crystalline Fe(III) oxyhydroxides (such as ferrihydrite) in all three setups. Our results suggest that periods of darkness have a significant impact on phototrophic Fe(II)-oxidizers and significantly influence rates of Fe(II) oxidation.

In situ incubation of iron(II)-bearing minerals and Fe(0) reveals insights into metabolic flexibility of chemolithotrophic bacteria in a nitrate polluted karst aquifer.

Groundwater nitrate pollution is a major reason for deteriorating water quality and threatens human and animal health. Yet, mitigating groundwater contamination naturally is often complicated since most aquifers are limited in bioavailable carbon. Since metabolically flexible microbes might have advantages for survival, this study presents a detailed description and first results on our modification of the BacTrap© method, aiming to determine the prevailing microbial community's potential to utilize chemolithotrophic pathways. Our microbial trapping devices (MTDs) were amended with four different iron sources and incubated in seven groundwater monitoring wells for ∼3 months to promote growth of nitrate-reducing Fe(II)-oxidizing bacteria (NRFeOxB) in a nitrate-contaminated karst aquifer. Phylogenetic analysis based on 16S rRNA gene sequences implies that the identity of the iron source influenced the microbial community's composition. In addition, high throughput amplicon sequencing revealed increased relative 16S rRNA gene abundances of OTUs affiliated to genera such as Thiobacillus, Rhodobacter, Pseudomonas, Albidiferax, and Sideroxydans. MTD-derived enrichments set up with Fe(II)/nitrate/acetate to isolate potential NRFeOxB, were dominated by e.g., Acidovorax spp., Paracoccus spp. and Propionivibrio spp. MTDs are a cost-effective approach for investigating microorganisms in groundwater and our data not only solidifies the MTD's capacity to provide insights into the metabolic flexibility of the aquifer's microbial community, but also substantiates its metabolic potential for anaerobic Fe(II) oxidation.

The interplay between microbial communities and soil properties.

In recent years, there has been considerable progress in determining the soil properties that influence the structure of the soil microbiome. By contrast, the effects of microorganisms on their soil habitat have received less attention with most previous studies focusing on microbial contributions to soil carbon and nitrogen dynamics. However, soil microorganisms are not only involved in nutrient cycling and organic matter transformations but also alter the soil habitat through various biochemical and biophysical mechanisms. Such microbially mediated modifications of soil properties can have local impacts on microbiome assembly with pronounced ecological ramifications. In this Review, we describe the processes by which microorganisms modify the soil environment, considering soil physics, hydrology and chemistry. We explore how microorganism-soil interactions can generate feedback loops and discuss how microbially mediated modifications of soil properties can serve as an alternative avenue for the management and manipulation of microbiomes to combat soil threats and global change.

Cu(II) and Cd(II) Removal Efficiency of Microbially Redox-Activated Magnetite Nanoparticles.

Heavy metal pollutants in the environment are of global concern due to their risk of contaminating drinking water and food supplies. Removal of these metals can be achieved by adsorption to mixed-valent magnetite nanoparticles (MNPs) due to their high surface area, reactivity, and ability for magnetic recovery. The adsorption capacity and overall efficiency of MNPs are influenced by redox state as well as surface charge, the latter of which is directly related to solution pH. However, the influence of microbial redox cycling of iron (Fe) in magnetite alongside the change of pH on the metal adsorption process by MNPs remains an open question. Here we investigated adsorption of Cd2+ and Cu2+ by MNPs at different pH values that were modified by microbial Fe(II) oxidation or Fe(III) reduction. We found that the maximum adsorption capacity increased with pH for Cd2+ from 256 µmol/g Fe at pH 5.0 to 478 µmol/g Fe at pH 7.3 and for Cu2+ from 229 µmol/g Fe at pH 5.0 to 274 µmol/g Fe at pH 5.5. Microbially reduced MNPs exhibited the greatest adsorption for both Cu2+ and Cd2+ (632 µmol/g Fe at pH 7.3 for Cd2+ and 530 µmol/g Fe at pH 5.5 for Cu2+). Magnetite oxidation also enhanced adsorption of Cu2+ but inhibited Cd2+. Our results show that microbial modification of MNPs has an important impact on the (im-)mobilization of aqueous contaminations like Cu2+ and Cd2+ and that a change in stoichiometry of the MNPs can have a greater influence than a change of pH.

Sustainable Immobilization of Arsenic by Man-Made Aerenchymatous Tissues in Paddy Soil.

Arsenic (As) is a major environmental pollutant and poses a significant health risk to humans through rice consumption. Elevating the soil redox potential (Eh) has been shown to reduce As bioavailability and decrease As accumulation in rice grains. However, sustainable methods for managing the Eh of rice paddies are lacking. To address this issue, we propose a new approach that uses man-made aerenchymatous tissues (MAT) to increase soil Eh by mimicking O2 release from wet plant roots. Our study demonstrated that the MAT method sustainably increased the soil Eh levels from -119 to -80.7 mV (∼30%), over approximately 100 days and within a radius of around 5 cm from the surface of the MAT. Moreover, it resulted in a significant reduction (-28.5% to -63.3%) in dissolved organic carbon, Fe, Mn, and As concentrations. MAT-induced Fe(III) (oxyhydr)oxide minerals served as additional adsorption sites for dissolved As in soil porewater. Furthermore, MAT promoted the oxidation of arsenite to the less mobile arsenate by significantly enhancing the relative abundance of the aioA gene (130% increase in the 0-5 cm soil zone around MAT). The decrease in As bioavailability significantly reduced As accumulation in rice grains (-30.0%). This work offers a low-cost and sustainable method for mitigating As release in rice paddies by addressing the issue of soil Eh management.

Novel Insight into Microbially Mediated Nitrate-Reducing Fe(II) Oxidation by Acidovorax sp. Strain BoFeN1 Using Dual N-O Isotope Fractionation.

Microbially mediated nitrate reduction coupled with Fe(II) oxidation (NRFO) plays an important role in the Fe/N interactions in pH-neutral anoxic environments. However, the relative contributions of the chemical and microbial processes to NRFO are still unclear. In this study, N-O isotope fractionation during NRFO was investigated. The ratios of O and N isotope enrichment factors (18ε:15ε)-NO3- indicated that the main nitrate reductase functioning in Acidovorax sp. strain BoFeN1 was membrane-bound dissimilatory nitrate reductase (Nar). N-O isotope fractionation during chemodenitrification [Fe(II) + NO2-], microbial nitrite reduction (cells + NO2-), and the coupled process [cells + NO2- + Fe(II)] was explored. The ratios of (18ε:15ε)-NO2- were 0.58 ± 0.05 during chemodenitrification and -0.41 ± 0.11 during microbial nitrite reduction, indicating that N-O isotopes can be used to distinguish chemical from biological reactions. The (18ε:15ε)-NO2- of 0.70 ± 0.05 during the coupled process was close to that obtained for chemodenitrification, indicating that chemodenitrification played a more important role than biological reactions during the coupled process. The results of kinetic modeling showed that the relative contribution of chemodenitrification was 99.3% during the coupled process, which was consistent with that of isotope fractionation. This study provides a better understanding of chemical and biological mechanisms of NRFO using N-O isotopes and kinetic modeling.

Quantitative Contribution of Oxygen Vacancy Defects to Arsenate Immobilization on Hematite.

Hematite is a common iron oxide in natural environments, which has been observed to influence the transport and fate of arsenate by its association with hematite. Although oxygen vacancies were demonstrated to exist in hematite, their contributions to the arsenate immobilization have not been quantified. In this study, hematite samples with tunable oxygen vacancy defect (OVD) concentrations were synthesized by treating defect-free hematite using different NaBH4 solutions. The vacancy defects were characterized by positron annihilation lifetime spectroscopy, Doppler broadening of annihilation radiation, extended X-ray absorption fine structure (EXAFS), thermogravimetric mass spectrometry (TG-MS), electron paramagnetic resonance (EPR), and X-ray photoelectron spectroscopy (XPS). The results revealed that oxygen vacancy was the primary defect type existing on the hematite surface. TG-MS combined with EPR analysis allowed quantification of OVD concentrations in hematite. Batch experiments revealed that OVDs had a positive effect on arsenate adsorption, which could be quantitatively described by a linear relationship between the OVD concentration (Cdef, mmol m-2) and the enhanced arsenate adsorption amount caused by defects (ΔQm, µmol m-2) (ΔQm = 20.94 Cdef, R2 = 0.9813). NH3-diffuse reflectance infrared Fourier transform (NH3-DRIFT) analysis and density functional theory (DFT) calculations demonstrated that OVDs in hematite were beneficial to the improvement in adsorption strength of surface-active sites, thus considerably promoting the immobilization of arsenate.

Goethite Formed in the Periplasmic Space of Pseudomonas sp. JM-7 during Fe Cycling Enhances Its Denitrification in Water.

Denitrification-driven Fe(II) oxidation is an important microbial metabolism that connects iron and nitrogen cycling in the environment. The formation of Fe(III) minerals in the periplasmic space has a significant effect on microbial metabolism and electron transfer, but direct evidence of iron ions entering the periplasm and resulting in periplasmic mineral precipitation and electron conduction properties has yet to be conclusively determined. Here, we investigated the pathways and amounts of iron, with different valence states and morphologies, entering the periplasmic space of the denitrifier Pseudomonas sp. JM-7 (P. JM-7), and the possible effects on the electron transfer and the denitrifying ability. When consistently provided with Fe(II) ions (from siderite (FeCO3)), the dissolved Fe(II) ions entered the periplasmic space and were oxidized to Fe(III), leading to the formation of a 25 nm thick crystalline goethite crust, which functioned as a semiconductor, accelerating the transfer of electrons from the intracellular to the extracellular matrix. This consequently doubled the denitrification rate and increased the electron transport capacity by 4-30 times (0.015-0.04 µA). However, as the Fe(II) concentration further increased to above 4 mM, the Fe(II) ions tended to preferentially nucleate, oxidize, and crystallize on the outer surface of P. JM-7, leading to the formation of a densely crystallized goethite layer, which significantly slowed down the metabolism of P. JM-7. In contrast to the Fe(II) conditions, regardless of the initial concentration of Fe(III), it was challenging for Fe(III) ions to form goethite in the periplasmic space. This work has shed light on the likely effects of iron on environmental microorganisms, improved our understanding of globally significant iron and nitrogen geochemical cycles in water, and expanded our ability to study and control these important processes.

Critical nanomaterial attributes of iron-carbohydrate nanoparticles: Leveraging orthogonal methods to resolve the 3-dimensional structure.

Intravenous iron-carbohydrate nanomedicines are widely used to treat iron deficiency and iron deficiency anemia across a wide breadth of patient populations. These colloidal solutions of nanoparticles are complex drugs which inherently makes physicochemical characterization more challenging than small molecule drugs. There have been advancements in physicochemical characterization techniques such as dynamic light scattering and zeta potential measurement, that have provided a better understanding of the physical structure of these drug products in vitro. However, establishment and validation of complementary and orthogonal approaches are necessary to better understand the 3-dimensional physical structure of the iron-carbohydrate complexes, particularly with regard to their physical state in the context of the nanoparticle interaction with biological components such as whole blood (i.e. the nano-bio interface).

Microbially influenced corrosion and rust tubercle formation on sheet piles in freshwater systems.

The extent of how complex natural microbial communities contribute to metal corrosion is still not fully resolved, especially not for freshwater environments. In order to elucidate the key processes, we investigated rust tubercles forming massively on sheet piles along the river Havel (Germany) applying a complementary set of techniques. In-situ microsensor profiling revealed steep gradients of O2 , redox potential and pH within the tubercle. Micro-computed tomography and scanning electron microscopy showed a multi-layered inner structure with chambers and channels and various organisms embedded in the mineral matrix. Using Mössbauer spectroscopy we identified typical corrosion products including electrically conductive iron (Fe) minerals. Determination of bacterial gene copy numbers and sequencing of 16S rRNA and 18S rRNA amplicons supported a densely populated tubercle matrix with a phylogenetically and metabolically diverse microbial community. Based on our results and previous models of physic(electro)chemical reactions, we propose here a comprehensive concept of tubercle formation highlighting the crucial reactions and microorganisms involved (such as phototrophs, fermenting bacteria, dissimilatory sulphate and Fe(III) reducers) in metal corrosion in freshwaters.