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At least 415 records · Page 23Linked to original sources

USGS2015

Detailed description of oil shale organic and mineralogical heterogeneity via fourier transform infrared mircoscopy

Mineralogical and geochemical information on reservoir and source rocks is necessary to assess and produce from petroleum systems. The standard methods in the petroleum industry for obtaining these properties are bulk measurements on homogenized, generally crushed, and pulverized rock samples and can take from hours to days to perform. New methods using Fourier transform infrared (FTIR) spectroscopy have been developed to more rapidly obtain information on mineralogy and geochemistry. However, these methods are also typically performed on bulk, homogenized samples. We present a new approach to rock sample characterization incorporating multivariate analysis and FTIR microscopy to provide non-destructive, spatially resolved mineralogy and geochemistry on whole rock samples. We are able to predict bulk mineralogy and organic carbon content within the same margin of error as standard characterization techniques, including X-ray diffraction (XRD) and total organic carbon (TOC) analysis. Validation of the method was performed using two oil shale samples from the Green River Formation in the Piceance Basin with differing sedimentary structures. One sample represents laminated Green River oil shales, and the other is representative of oil shale breccia. The FTIR microscopy results on the oil shales agree with XRD and LECO TOC data from the homogenized samples but also give additional detail regarding sample heterogeneity by providing information on the distribution of mineral phases and organic content. While measurements for this study were performed on oil shales, the method could also be applied to other geological samples, such as other mudrocks, complex carbonates, and soils.

Energy & Fuels↗

USGS2026

Oil in the Alaska North Slope gas hydrate reservoir: Micro-CT and flow simulation insights into permeability

Gas hydrate-bearing sands on the Alaska North Slope (ANS) host minor volumes of crude oil whose impact on formation permeability has never been quantified relative to a gas hydrate reservoir system. Here, we combine in situ pressure-core microcomputed-tomography (μ-CT), thermogravimetric analysis (TGA), gas-chromatography–mass-spectrometry (GC–MS), scanning-electron microscopy coupled with energy-dispersive spectroscopy (SEM-EDS), and Stokes-flow simulation to (i) quantify oil saturation, (ii) infer its source, and (iii) evaluate its influence on permeability after gas-hydrate dissociation. μ-CT and SEM-EDS imaging identify the host sediment as silty, with some grain-coating clay present. Eight representative 300 3 -voxel subvolumes extracted from a preserved pressure core (Core 15P-3c, from the B1 sand (unit B), HYDRATE 02 Geo Data Well (GDW), 886.96–887.29 m measured depth, MD) exhibit porosities of 0.34–0.37 and an average oil saturation of 0.09 ± 0.03, in comparison to an independent TGA-based estimate of 0.16. GC-MS fingerprints obtained from an adjacent depressurized core (Core 17P-1, 891.37–891.44 m MD), together with oil pore habit revealed by μ-CT, suggest that the oil is partly native but also includes contributions from mineral oil-based drilling fluid contamination. Further analysis of oil–porewater interactions and flushing tests confirms that native oil saturation in the analyzed samples ranges from 0.04 to 0.08 with an average of approximately 0.06, and the oil is highly immobile. Flow simulations demonstrate that the native oil saturation of ≈0.06 reduces permeability by approximately 50% due to both pore blockage and increased flow-path tortuosity. These findings reveal the potential presence of native oil within the ANS gas hydrate reservoir and suggest that future simulation models may need to account for its impact on permeability to improve long-term performance predictions of gas and water production.

Alaska↗

USGS2016

Sulfur species in source rock bitumen before and after hydrous pyrolysis determined by X-ray absorption near-edge structure

The sulfur speciation of source rock bitumen (chloroform-extractable organic matter in sedimentary rocks) was examined using sulfur K-edge X-ray absorption near-edge structure (XANES) spectroscopy for a suite of 11 source rocks from around the world. Sulfur speciation was determined for both the native bitumen in thermally immature rocks and the bitumen produced by thermal maturation of kerogen via hydrous pyrolysis (360 °C for 72 h) and retained within the rock matrix. In this study, the immature bitumens had higher sulfur concentrations than those extracted from samples after hydrous pyrolysis. In addition, dramatic and systematic evolution of the bitumen sulfur moiety distributions following artificial thermal maturation was observed consistently for all samples. Specifically, sulfoxide sulfur (sulfur double bonded to oxygen) is abundant in all immature bitumen samples but decreases substantially following hydrous pyrolysis. The loss in sulfoxide sulfur is associated with a relative increase in the fraction of thiophene sulfur (sulfur bonded to aromatic carbon) to the extent that thiophene is the dominant sulfur form in all post-pyrolysis bitumen samples. This suggests that sulfur moiety distributions might be used for estimating thermal maturity in source rocks based on the character of the extractable organic matter.

Energy & Fuels↗

USGS2019

Experimental study on the impact of thermal maturity on shale microstructures using hydrous pyrolysis

Hydrous pyrolysis was applied to four low-maturity aliquots from the Utica, Excello, Monterey, and Niobrara Shale Formations in North America to create artificial maturation sequences, which could be used to study the impact of maturation on geochemical and microstructural properties. Modified Rock-Eval pyrolysis, reflectance, organic petrology, and Fourier transform infrared spectroscopy (FTIR) were employed to analyze their geochemical properties, while gas adsorption (CO 2 and N 2 ) was used to characterize their pore structures (pores < 200 nm). Organic petrography using white and blue light (fluorescence) before and after hydrous pyrolysis showed that amorphous organic matter cracked into solid bitumen, oil, and gas during hydrous pyrolysis. A reduction of the CH 2 /CH 3 ratio in hydrous pyrolysis residues was observed from FTIR analysis. Rock-Eval pyrolysis showed that kerogens in the four samples were dissimilar, and hydrous pyrolysis residues showed smaller hydrogen index and Sh2 values than starting materials. Results from CO 2 and N 2 gas adsorption analysis showed that pore structures (micropore volume, micropore surface area, meso-macropore volume, and meso-macropore surface area) changed significantly during hydrous pyrolysis. However, changes in pore structure were dissimilar among the four samples, which was attributed to different activation energies of organic matter. A thermodynamic fractal model showed a decrease in fractal dimensions of Utica, Monterey, and Excello after hydrous pyrolysis, indicating a decrease in surface roughness. The pore size heterogeneity in the Utica sample increased as hydrous pyrolysis temperature increased, whereas the pore size heterogeneity distributions in the Monterey and Excello decreased based on the N 2 adsorption data.

Energy & Fuels↗

USGS2019

Nanoscale molecular fractionation of organic matter within unconventional petroleum source beds

Fractionation of petroleum during migration through sedimentary rock matrices has been observed across lengths of meters to kilometers. Selective adsorption of specific chemical moieties at mineral surfaces and/or the phase behavior of petroleum during pressure changes typically are invoked to explain this behavior. Such phenomena are of interest as they impact both the quality and recoverability of petroleum resources. Given the current emphasis on unconventional (continuous) resources, there is a need to understand petroleum fractionation occurring during expulsion and migration at the nanometer to micron scale, due to the fine-grained nature of petroliferous mudrocks. Here, organic matter compositional differences observed within kukersites (petroleum source beds containing acritarch Gloeocapsomorpha prisca) and the overlying carbonate reservoir layer from the Ordovician Stonewall Formation are explored using a suite of spectroscopic methods, primarily through atomic force microscopy based infrared spectroscopy (AFM-IR). AFM-IR is capable of providing spatial resolutions approaching 50 nm and allows for assessment of the molecular fingerprint of kukersite organic matter across transition zones from organic-rich ‘source’ layers into neighboring carbonate ‘reservoir’ layers ~150 μm away. Results indicate that organic matter composition begins to vary immediately following expulsion from source layers, with loss of carbonyl groups and a concomitant decrease in alkyl chain-length, as migration distance increases. These chemical transitions correlate with a decrease in fluorescence intensity, increase in solid bitumen reflectance, and increase in Raman aromaticity proxies (D-G band separation) in the organic matter. Our findings are consistent with the retention of polar compounds onto mineral grains during expulsion and migration, following primary cracking and bituminization of the Gloeocapsomorpha prisca kerogen.

Energy and Fuels↗

USGS2021

Demethylation of methylmercury in bird, fish, and earthworm

Toxicity of methylmercury (MeHg) to wildlife and humans results from its binding to cysteine residues of proteins, forming MeHg-cysteinate (MeHgCys) complexes that hinder biological functions. MeHgCys complexes can be detoxified in vivo, yet how this occurs is unknown. We report that MeHgCys complexes are transformed into selenocysteinate (Hg(Sec)4) complexes in multiple animals from two phyla (a waterbird, freshwater fish, and earthworms) sampled in different geographical areas and contaminated by different Hg sources. In addition, high energy-resolution X-ray absorption spectroscopy and chromatography-ICP mass spectrometry of the waterbird liver support the binding of Hg(Sec)4 to selenoprotein P and biomineralization of Hg(Sec)4 to chemically inert nanoparticulate mercury selenide (HgSe). The results provide a foundation for understanding mercury detoxification in higher organisms, and suggest that the identified MeHgCys to Hg(Sec)4 demethylation pathway is common in nature.

Environmental Science & Technology↗

USGS2015

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 ([H2AsIIIS−IIO2] −), formed as an intermediate by abiotic reaction of arsenite with sulfide. We now report that this species is monothioarsenate ([HAsVS−IIO3] 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 (AsV) 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.

Environmental Science & Technology↗

USGS2025

Examining the compositional selectivity of hydrocarbon oxidation products using liquid–liquid extraction and solid-phase extraction techniques

The effect of extraction methods on detecting hydrocarbon oxidation products (HOPs) in groundwater remains unclear. HOPs are polar, water-soluble byproducts of petroleum biodegradation. Our previous work showed that liquid–liquid extraction (LLE), a method commonly used in regulatory monitoring, has a significantly lower extraction efficiency for HOPs compared to solid-phase extraction (SPE). In this study, we evaluate the analytical limitations and compositional selectivity of LLE and SPE using groundwater samples from the Bemidji, MN, crude oil spill site. Optical properties were characterized using excitation–emission matrix spectroscopy (EEMs), and a three-component PARAFAC model was validated, showing consistent trends across both extracts and whole water samples. Ultrahigh-resolution mass spectrometry (UHR-MS) revealed that LLE selectively recovered aliphatic-like compounds but underrepresented more polar oxygenated HOPs. In contrast, SPE methods were more effective at isolating highly oxidized compound classes. These differences were consistent across a gradient of contamination. Overall, the LLE was less precise and less representative of polar HOPs, introducing bias in the characterization of HOPs. This study is the first to quantitatively demonstrate the compositional selectivity and analytical bias of LLE versus SPE for HOPs using combined EEM-PARAFAC and UHR-MS techniques, with implications for long-term monitoring and site assessment protocols.

Environmental Science and Technology↗

USGS2026

Widespread occurrence of Magnéli phases in wildland-urban interface fire ashes

The increasing activity of wildland–urban interface (WUI) fires has raised concerns regarding the potential environmental and human health impacts of residual ash remaining after burning. In this study, we investigated the concentration and speciation of titanium in WUI fire ash. Total titanium concentrations in WUI fire ash ranged from 0.53 to 80 g kg –1 . Synchrotron-based macro- and microscale X-ray absorption near-edge structure (XANES and μXANES, respectively) spectroscopy were used to quantify the relative abundance of major Ti phases in the fire ash, and the results were corroborated by high resolution-transmission electron microscopy (HR-TEM) measurements. Rutile (α-TiO 2 ), anatase (β-TiO 2 ), ilmenite (FeTiO 3 ), and titanium(III) oxide (Ti 2 O 3 ) were detected in all 20 ashes investigated by XANES and accounted for 0.26–0.83, 0.19–0.83, 0.33, and 0.17–0.72 of the spectral weight, respectively. Deeper analysis by μXANES of one sample demonstrated that Ti-bearing particles occurred as a mixture of rutile (α-TiO 2 ), anatase (β-TiO 2 ), ilmenite (FeTiO 3 ), and titanium(III) oxide Ti 2 O 3, with the absence of a pure Ti 2 O 3 phase. The presence of Ti 2 O 3 in the WUI fire ash is ascribed to the reduction of rutile and anatase to Magnéli titania (Ti n O 2 n– 1 , n = 4–9), which is estimated to be the dominant phase of titanium in the 20 WUI fire ashes investigated by XANES. The occurrence of Magnéli titania was corroborated by HR-TEM. Our findings demonstrate the impact of WUI fires on titanium speciation; fires convert titanium dioxides (e.g., rutile and anatase) to reduced titanium phases (e.g., Magnéli titania). Based on HR-TEM analyses, most of the titanium-bearing particles were less than 500 nm in size. Magnéli particles have been shown to be more toxic than rutile and anatase and have been linked to reduced lung function. Therefore, this study provides critical insights into the pollution characteristics and potential health risks of WUI fire ashes and associated particles, which are currently poorly understood.

California↗

USGS2017

Spatial dependence of reduced sulfur in Everglades dissolved organic matter controlled by sulfate enrichment

Sulfate inputs to the Florida Everglades stimulate sulfidic conditions in freshwater wetland sediments that affect ecological and biogeochemical processes. An unexplored implication of sulfate enrichment is alteration of the content and speciation of sulfur in dissolved organic matter (DOM), which influences the reactivity of DOM with trace metals. Here, we describe the vertical and lateral spatial dependence of sulfur chemistry in the hydrophobic organic acid fraction of DOM from unimpacted and sulfate-impacted Everglades wetlands using X-ray absorption spectroscopy and ultrahigh-resolution mass spectrometry. Spatial variation in DOM sulfur content and speciation reflects the degree of sulfate enrichment and resulting sulfide concentrations in sediment pore waters. Sulfur is incorporated into DOM predominantly as highly reduced species in sulfidic pore waters. Sulfur-enriched DOM in sediment pore waters exchanges with overlying surface waters and the sulfur likely undergoes oxidative transformations in the water column. Across all wetland sites and depths, the total sulfur content of DOM correlated with the relative abundance of highly reduced sulfur functionality. The results identify sulfate input as a primary determinant on DOM sulfur chemistry to be considered in the context of wetland restoration and sulfur and trace metal cycling.

Florida↗

USGS2017

Modifications to EPA Method 3060A to Improve Extraction of Cr(VI) from Chromium Ore Processing Residue-Contaminated Soils

It has been shown that EPA Method 3060A does not adequately extract Cr(VI) from chromium ore processing residue (COPR). We modified various parameters of EPA 3060A toward understanding the transformation of COPR minerals in the alkaline extraction and improving extraction of Cr(VI) from NIST SRM 2701, a standard COPR-contaminated soil. Aluminum and Si were the major elements dissolved from NIST 2701, and their concentrations in solution were correlated with Cr(VI). The extraction fluid leached additional Al and Si from the method-prescribed borosilicate glass vessels which appeared to suppress the release of Cr(VI). Use of polytetrafluoroethylene vessels and intensive grinding of NIST 2701 increased the amount of Cr(VI) extracted. These modifications, combined with an increased extraction fluid to sample ratio of ≥900 mL g –1 and 48-h extraction time resulted in a maximum release of 1274 ± 7 mg kg –1 Cr(VI). This is greater than the NIST 2701 certified value of 551 ± 35 mg kg –1 but less than 3050 mg kg –1 Cr(VI) previously estimated by X-ray absorption near edge structure spectroscopy. Some of the increased Cr(VI) may have resulted from oxidation of Cr(III) released from brownmillerite which rapidly transformed during the extractions. Layered-double hydroxides remained stable during extractions and represent a potential residence for unextracted Cr(VI).

Environmental Science & Technology↗

USGS2017

Effects of sulfide concentration and dissolved organic matter characteristics on the structure of nanocolloidal metacinnabar

Understanding the speciation of divalent mercury (Hg(II)) in aquatic systems containing dissolved organic matter (DOM) and sulfide is necessary to predict the conversion of Hg(II) to bioavailable methylmercury. We used X-ray absorption spectroscopy to characterize the structural order of mercury in Hg(II)–DOM–sulfide systems for a range of sulfide concentration (1–100 μM), DOM aromaticity (specific ultraviolet absorbance (SUVA 254 )), and Hg(II)–DOM and Hg(II)–DOM–sulfide equilibration times (4–142 h). In all systems, Hg(II) was present as structurally disordered nanocolloidal metacinnabar (β-HgS). β-HgS nanocolloids were significantly smaller or less ordered at lower sulfide concentration, as indicated by under-coordination of Hg(II) in β-HgS. The size or structural order of β-HgS nanocolloids increased with increasing sulfide abundance and decreased with increasing SUVA 254 of the DOM. The Hg(II)–DOM or Hg(II)–DOM–sulfide equilibration times did not significantly influence the extent of structural order in nanocolloidal β-HgS. Geochemical factors that control the structural order of nanocolloidal β-HgS, which are expected to influence nanocolloid surface reactivity and solubility, should be considered in the context of mercury bioavailability.

Environmental Science & Technology↗

USGS2018

Phosphorus speciation and solubility in aeolian dust deposited in the interior American West

Aeolian dust is a significant source of phosphorus (P) to alpine oligotrophic lakes, but P speciation in dust and source sediments and its release kinetics to lake water remain unknown. Phosphorus K-edge XANES spectroscopy shows that calcium-bound P (Ca−P) is dominant in 10 of 12 dust samples (41−74%) deposited on snow in the central Rocky Mountains and all 42 source sediment samples (the fine fraction) (68−80%), with a lower proportion in dust probably because acidic snowmelt dissolves some Ca−P in dust before collection. Iron-bound P (Fe−P, ∼54%) dominates in the remaining two dust samples. Chemical extractions (SEDEX) on these samples provide inaccurate results because of unselective extraction of targeted species and artifacts introduced by the extractions. Dust releases increasingly more P in synthetic lake water within 6−72 h thanks to dissolution of Ca−P, but dust release of P declines afterward due to back adsorption of P onto Fe oxides present in the dust. The back sorption is stronger for the dust with a lower degree of P saturation determined by oxalate extraction. This work suggests that P speciation, poorly crystalline minerals in the dust, and lake acidification all affect the availability and fate of dust-borne P in lakes.

Arizona, Colorado, New Mexico, Utah↗

USGS2018

Effect of calcium on the bioavailability of dissolved uranium(VI) in plant roots under circumneutral pH

We integrated field measurements, hydroponic experiments, microscopy, and spectroscopy to investigate the effect of Ca(II) on dissolved U(VI) uptake by plants in 1 mM HCO 3 – solutions at circumneutral pH. The accumulation of U in plants (3.1–21.3 mg kg –1 ) from the stream bank of the Rio Paguate, Jackpile Mine, New Mexico served as a motivation for this study. Brassica juncea was the model plant used for the laboratory experiments conducted over a range of U (30–700 μg L –1 ) and Ca (0–240 mg L –1 ) concentrations. The initial U uptake followed pseudo-second-order kinetics. The initial U uptake rate ( V 0 ) ranged from 4.4 to 62 μg g –1 h –1 in experiments with no added Ca and from 0.73 to 2.07 μg g –1 h –1 in experiments with 12 mg L –1 Ca. No measurable U uptake over time was detected for experiments with 240 mg L –1 Ca. Ternary Ca–U–CO 3 complexes may affect the decrease in U bioavailability observed in this study. Elemental X-ray mapping using scanning transmission electron microscopy–energy-dispersive spectrometry detected U–P-bearing precipitates within root cell walls in water free of Ca. These results suggest that root interactions with Ca and carbonate in solution affect the bioavailability of U in plants. This study contributes relevant information to applications related to U transport and remediation of contaminated sites.

Environmental Science & Technology↗

USGS2021

Cyprosulfamide: Analysis of the herbicide safener and two of its degradates in surface water and groundwater from the Midwestern United States

Herbicide safeners are commonly included in herbicide formulations to selectively protect crops from herbicide toxicity but are poorly understood in terms of their environmental occurrence and fate. This study established an analytical method for a newer safener, cyprosulfamide, and two of its degradates, cyprosulfamide desmethyl and N -cyclopropyl-4-sulfamoylbenzamide, in water via solid-phase extraction and liquid chromatography with tandem mass spectroscopy. To evaluate the potential for off-field transport and transformation of cyprosulfamide, the method was used to analyze groundwater and surface water samples collected near cornfields in the midwestern United States where cyprosulfamide had been applied. All three compounds were detected in surface water samples ( N = 34); N -cyclopropyl-4-sulfamoylbenzamide was most frequently detected (56%), followed by cyprosulfamide (25%) and cyprosulfamide desmethyl (19%). Maximum concentrations ranged from 22.0 to 5185.9 ng/L, with the highest concentrations and detection rates during the growing season. None of our target analytes were detected in groundwater.

ACS Agricultural Science and Technology↗

USGS2020

The effects of phosphatization on the mineral associations and speciation of Pb in ferromanganese crusts

The older layers of thick ferromanganese (FeMn) crusts from the central Pacific Ocean have undergone diagenetic phosphatization, during which carbonate fluorapatite (CFA) filled fractures and pore space and replaced carbonates. The effects of phosphatization on individual trace metal concentrations, speciation, and phase associations in FeMn crusts remain poorly understood yet may be important to metal enrichment mechanisms, paleoceanography, and extractive metallurgy. This study examines the concentrations, speciation, and mineral phase associations of Pb in phosphatized and nonphosphatized layers within three Pacific Ocean crusts using standard chemical and mineralogical techniques, in addition to bulk X-ray absorption spectroscopy (XAS) and microfocused X-ray fluorescence (μ-XRF) mapping. Our findings challenge the conclusions of previous works, which reported that most Pb in phosphatized crusts was associated with the CFA-dominated residual phase after sequential leaching experiments. These results were interpreted as an indication of Pb transfer from the oxide phases to diagenetic CFA during phosphatization. However, our results reveal an inverse correlation of Pb with Ca and P in the μ-XRF mapping of in situ FeMn crust sections and bulk chemistry. Furthermore, XAS measurements reveal that Pb speciation in bulk phosphatized FeMn crust layers is very similar to nonphosphatized layers, indicating only minor, if any, change in speciation during diagenetic phosphatization. Small differences in the EXAFS spectra for one highly phosphatized layer indicate that minor amounts of Pb may have been altered during phosphatization, but the new Pb-bearing phase was not unequivocally identified. Taken together, our results demonstrate that Pb speciation is not significantly affected by phosphatization.

Earth and Space Chemistry↗

USGS2021

Isotope fractionation from In Vivo methylmercury detoxification in waterbirds

The robust application of stable mercury (Hg) isotopes for mercury source apportionment and risk assessment necessitates the understanding of mass-dependent fractionation (MDF) due to internal transformations within organisms. Here, we used high energy-resolution XANES spectroscopy and isotope ratios of total mercury (δ202THg) and methylmercury (δ202MeHg) to elucidate the chemical speciation of Hg and the resultant MDF due to internal MeHg demethylation in waterbirds. In three waterbirds (Clark’s grebe, Forster’s tern, south polar skua), between 17-86% of the MeHg was demethylated to inorganic mercury (iHg) species primarily in the liver and kidneys as Hg-tetraselenolate (Hg(Sec)4) and minor Hg-dithiolate (Hg(SR)2) complexes. Tissular differences between δ202THg and δ202MeHg correlated linearly with %iHg (Hg(Sec)4 + Hg(SR)2), and were interpreted to reflect a kinetic isotope effect during in vivo MeHg demethylation. The product-reactant isotopic enrichment factor (εp/r) for the demethylation of MeHg  Hg(Sec)4 was −2.2 ± 0.1‰. δ202MeHg values were unvarying within each bird regardless of Hg(Sec)4 abundance, indicating fast internal cycling or replenishment of MeHg relative to demethylation. Our findings document a universal selenium-dependent demethylation reaction in birds, provide new insights on the internal transformations and cycling of MeHg and Hg(Sec)4, and allow for mathematical correction of δ202THg values due to the MeHg  Hg(Sec)4 reaction.

ACS Earth and Space Chemistry↗

USGS2011

A novel photosynthetic strategy for adaptation to low-iron aquatic environments

Iron (Fe) availability is a major limiting factor for primary production in aquatic environments. Cyanobacteria respond to Fe deficiency by derepressing the isiAB operon, which encodes the antenna protein IsiA and flavodoxin. At nanomolar Fe concentrations, a PSI−IsiA supercomplex forms, comprising a PSI trimer encircled by two complete IsiA rings. This PSI−IsiA supercomplex is the largest photosynthetic membrane protein complex yet isolated. This study presents a detailed characterization of this complex using transmission electron microscopy and ultrafast fluorescence spectroscopy. Excitation trapping and electron transfer are highly efficient, allowing cyanobacteria to avoid oxidative stress. This mechanism may be a major factor used by cyanobacteria to successfully adapt to modern low-Fe environments.

Biochemistry↗