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1.	Gutensohn, Mareike Franziska, 1992- (författare) Unraveling the importance of thiol compounds on mercury speciation, uptake and transformation by the iron-reducer Geobacter sulfurreducens 2023 Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract The biogenic methylation of inorganic, divalent mercury (Hg(II)) by methylating microorganisms leads to formation and bioaccumulation of monomethyl mercury (MeHg) in the environment and can cause severe damage to ecosystems and human health. Diverse microorganisms carry the gene sequence hgcAB and are able to methylate Hg(II) intracellularly. The interplay of biological, chemical and physical parameters is driving mercury (Hg) transformation by microorganisms. The chemical speciation of Hg(II) with thiol compounds, both with dissolved low molecular mass (LMM) thiols and thiols present on microbial membrane surfaces, is one key factor for Hg availability and transformation. In this work the role of thiol compounds with respect to Hg speciation, uptake and transformation was studied by the iron-reducing model organisms Geobacter sulfurreducens. The turnover of dissolved thiols and the role of outer and inner membrane thiols was studied with novel experimental strategies.In Paper I and II the formation of thiol compounds was studied under varying nutrient conditions. It was shown that the formation of LMM-thiol compounds was impacted by divalent iron, Fe(II). Furthermore, we showed the turnover of the small LMM-thiol cysteine to the branched LMM-thiol penicillamine, which was further amplified by the addition of exogenous cysteine or nutrients. This turnover of small to branched LMM-thiols impacted the Hg(II) speciation in methylation assays and the relative contribution between cysteine and penicillamine was important for Hg(II) availability, uptake and methylation. In addition, the partition of Hg(II) between the cell-adsorbed and dissolved phase was shifted towards the latter at higher LMM-thiol concentrations. Nutrient concentrations impacted cell physiology due to a shift to an active metabolism and a faster metabolization of LMM-thiols. We concluded that the interplay between thiol metabolism, Hg(II) speciation and cell physiology are key parameters for Hg(II) methylation by G. sulfurreducens. In Paper III The outer and inner membrane was characterized independently by two X-ray absorption spectroscopy techniques. The determination of the Hg speciation by both X-ray absorption spectroscopy techniques showed coherent results for both the outer and inner membrane of G. sulfurreducens. The concentration of thiol membrane groups was higher on the inner compared to the outer membrane. The differences between the outer and inner membrane suggested that thiol concentration and Hg coordination environment likely impact the Hg(II) internalization. The role of membrane thiols for Hg(II) uptake and transformation was further investigated in Paper IV by selectively blocking these functional groups. Partitioning and uptake of Hg was not affected by blocking the outer and inner membrane thiols of whole cell and spheroplast samples, respectively. However, the Hg(II) methylation was decreased by blocking thiols at the outer membrane, but no effect was observed by blocking thiols at the inner membrane. Blocking of membrane surface thiols changed the physiology in whole cells but not in spheroplasts. This result suggested weaknesses of the applied blocking approach. In addition, Hg(II) reduction was studied on the outer and inner membrane and showed the formation of liquid and gaseous elemental Hg, Hg(0), in Paper III and IV, respectively.Overall, this work showed the central role of dissolved and cell-associated thiol compounds for Hg(II) uptake and the transformation reactions. Herby, concentration, compositions and distribution of thiols are crucial and impact the Hg(II) speciation, partitioning, uptake and availability for Hg(II) methylation and reduction. In addition, cell physiology is impacting the methylation potential and the turnover of LMM-thiol compounds. The role of membrane surface thiols for Hg(II) uptake was not fully identified, however such thiols were for the first time characterized selectively for the outer and inner membrane by X-ray absorption spectroscopy.
2.	Gutensohn, Mareike, et al. (författare) Metabolic turnover of cysteine-related thiol compounds at environmentally relevant concentrations by Geobacter sulfurreducens 2023 Ingår i: Frontiers in Microbiology. - : Frontiers Media S.A.. - 1664-302X. ; 13 Tidskriftsartikel (refereegranskat)abstract Low-molecular-mass (LMM) thiol compounds are known to be important for many biological processes in various organisms but LMM thiols are understudied in anaerobic bacteria. In this work, we examined the production and turnover of nanomolar concentrations of LMM thiols with a chemical structure related to cysteine by the model iron-reducing bacterium Geobacter sulfurreducens. Our results show that G. sulfurreducens tightly controls the production, excretion and intracellular concentration of thiols depending on cellular growth state and external conditions. The production and cellular export of endogenous cysteine was coupled to the extracellular supply of Fe(II), suggesting that cysteine excretion may play a role in cellular trafficking to iron proteins. Addition of excess exogenous cysteine resulted in a rapid and extensive conversion of cysteine to penicillamine by the cells. Experiments with added isotopically labeled cysteine confirmed that penicillamine was formed by a dimethylation of the C-3 atom of cysteine and not via indirect metabolic responses to cysteine exposure. This is the first report of de novo metabolic synthesis of this compound. Penicillamine formation increased with external exposure to cysteine but the compound did not accumulate intracellularly, which may suggest that it is part of G. sulfurreducens’ metabolic strategy to maintain cysteine homeostasis. Our findings highlight and expand on processes mediating homeostasis of cysteine-like LMM thiols in strict anaerobic bacteria. The formation of penicillamine is particularly noteworthy and this compound warrants more attention in microbial metabolism studies.
3.	Gutensohn, Mareike, et al. (författare) The combined effect of Hg(II) speciation, thiol metabolism, and cell physiology on methylmercury formation by Geobacter sulfurreducens 2023 Ingår i: Environmental Science and Technology. - : American Chemical Society (ACS). - 0013-936X .- 1520-5851. ; 57:18, s. 7185-7195 Tidskriftsartikel (refereegranskat)abstract The chemical and biological factors controlling microbial formation of methylmercury (MeHg) are widely studied separately, but the combined effects of these factors are largely unknown. We examined how the chemical speciation of divalent, inorganic mercury (Hg(II)), as controlled by low-molecular-mass thiols, and cell physiology govern MeHg formation by Geobacter sulfurreducens. We compared MeHg formation with and without addition of exogenous cysteine (Cys) to experimental assays with varying nutrient and bacterial metabolite concentrations. Cysteine additions initially (0–2 h) enhanced MeHg formation by two mechanisms: (i) altering the Hg(II) partitioning from the cellular to the dissolved phase and/or (ii) shifting the chemical speciation of dissolved Hg(II) in favor of the Hg(Cys)2 complex. Nutrient additions increased MeHg formation by enhancing cell metabolism. These two effects were, however, not additive since cysteine was largely metabolized to penicillamine (PEN) over time at a rate that increased with nutrient addition. These processes shifted the speciation of dissolved Hg(II) from complexes with relatively high availability, Hg(Cys)2, to complexes with lower availability, Hg(PEN)2, for methylation. This thiol conversion by the cells thereby contributed to stalled MeHg formation after 2–6 h Hg(II) exposure. Overall, our results showed a complex influence of thiol metabolism on microbial MeHg formation and suggest that the conversion of cysteine to penicillamine may partly suppress MeHg formation in cysteine-rich environments like natural biofilms.
4.	Hu, Haiyan, et al. (författare) Shifts in mercury methylation across a peatland chronosequence : From sulfate reduction to methanogenesis and syntrophy 2020 Ingår i: Journal of Hazardous Materials. - : Elsevier. - 0304-3894 .- 1873-3336. ; 387 Tidskriftsartikel (refereegranskat)abstract Peatlands are globally important ecosystems where inorganic mercury is converted to bioaccumulating and highly toxic methylmercury, resulting in high risks of methylmercury exposure in adjacent aquatic ecosystems. Although biological mercury methylation has been known for decades, there is still a lack of knowledge about the organisms involved in mercury methylation and the drivers controlling their methylating capacity. In order to investigate the metabolisms responsible for mercury methylation and methylmercury degradation as well as the controls of both processes, we studied a chronosequence of boreal peatlands covering fundamentally different biogeochemical conditions. Potential mercury methylation rates decreased with peatland age, being up to 53 times higher in the youngest peatland compared to the oldest. Methylation in young mires was driven by sulfate reduction, while methanogenic and syntrophic metabolisms became more important in older systems. Demethylation rates were also highest in young wetlands, with a gradual shift from biotic to abiotic methylmercury degradation along the chronosequence. Our findings reveal how metabolic shifts drive mercury methylation and its ratio to demethylation as peatlands age.
5.	Högberg, Mona N, et al. (författare) Does ectomycorrhiza have a universal key role in the formation of soil organic matter in boreal forests? 2020 Ingår i: Soil Biology and Biochemistry. - : Elsevier BV. - 0038-0717 .- 1879-3428. ; 140 Tidskriftsartikel (refereegranskat)abstract Forest soil organic matter (SOM) is an important dynamic store of C and N, which releases plant available N and the greenhouse gases CO2 and N2O. Early stages of decomposition of recent plant litters are better known than the formation of older and more stable soil pools of N and C, in which case classic theory stated that selective preservation of more resistant plant compounds was important. Recent insights heighten that all plant matter becomes degraded and that older SOM consists of compounds proximally of microbial origin. It has been proposed that in boreal forests, ectomycorrhizal fungi (ECMF), symbionts of trees, are actively involved in the formation of slowly-degrading SOM.We characterized SOM in the mor-layer along a local soil N supply gradient in a boreal forest, a gradient with large variations in chemical and biological characteristics, notably a decline in the biomass of ECMF in response to increasing soil N supply.We found contrasting and regular patterns in carbohydrates, lignin, aromatic carbon, and in N-containing compounds estimated by solid-state C-13 and N-15 nuclear magnetic resonance (NMR) spectroscopy. These occurred along with parallel changes in the natural abundances of the stable isotopes C-13 and N-15 in both bulk SOM and extracted fractions of the SOM. The modelled "bomb-C-14" age of the lower layers studied ranged between 15 years at the N-poor end, to 70 years at the N-rich end of the gradient. On average half the increase in delta C-13 with soil depth (and hence age) of the mor-layer can be attributed to soil processes and the other half to changes in the isotopic composition of the plant C inputs. There was a decrease in carbohydrates (O-alkyl C) with increasing depth. This supports the classical hypothesis of declining availability of easily decomposable substrates to microorganisms with increasing soil depth and age. The observed increase in delta C-13 with depth, however, speaks against the idea of selective preservation of more resistant plant compounds like lignin. Furthermore, from the N-poor to the N-rich end the difference between N-15 in plant litter N and N in the deeper part of the mor-layer, the H-layer, decreased in parallel with a decline in ECMF.The latter provides evidence that the role of ECMF as major sink for N diminishes, and hence their potential role in SOM stabilization, when the soil N supply increases. At the N-rich end, where bacteria dominate over fungi, other agents than ECMF must be involved in the large build-up of the H-layer with the slowest turnover rate found along the gradient.
6.	Jonsson, Sofi, 1984-, et al. (författare) Geochemical and Dietary Drivers of Mercury Bioaccumulation in Estuarine Benthic Invertebrates 2022 Ingår i: Environmental Science and Technology. - : American Chemical Society (ACS). - 0013-936X .- 1520-5851. ; 56:14, s. 10141-10148 Tidskriftsartikel (refereegranskat)abstract Sediments represent the main reservoir of mercury (Hg) in aquatic environments and may act as a source of Hg to aquatic food webs. Yet, accumulation routes of Hg from the sediment to benthic organisms are poorly constrained. We studied the bioaccumulation of inorganic and methylmercury (HgII and MeHg, respectively) from different geochemical pools of Hg into four groups of benthic invertebrates (amphipods, polychaetes, chironomids, and bivalves). The study was conducted using mesocosm experiments entailing the use of multiple isotopically enriched Hg tracers and simulation of estuarine systems with brackish water and sediment. We applied different loading regimes of nutrients and terrestrial organic matter and showed that the vertical localization and the chemical speciation of HgII and MeHg in the sediment, in combination with the diet composition of the invertebrates, consistently controlled the bioaccumulation of HgII and MeHg into the benthic organisms. Our results suggest a direct link between the concentration of MeHg in the pelagic planktonic food web and the concentration of MeHg in benthic amphipods and, to some extent, in bivalves. In contrast, the quantity of MeHg in benthic chironomids and polychaetes seems to be driven by MeHg accumulation via the benthic food web. Accounting for these geochemical and dietary drivers of Hg bioaccumulation in benthic invertebrates will be important to understand and predict Hg transfer between the benthic and the pelagic food web, under current and future environmental scenarios.
7.	Larson, Johannes, et al. (författare) What happens to trees and soils during five decades of experimental nitrogen loading? 2024 Ingår i: Forest Ecology and Management. - 0378-1127 .- 1872-7042. ; 553 Tidskriftsartikel (refereegranskat)abstract High deposition of nitrogen was postulated to drive losses of NO3 - and nutrient base cations, causing soil acidification, nutrient deficiencies reducing tree growth and ultimately tree mortality. We tested these predictions in a uniquely long-term study involving three NH4NO3 addition treatments (N1-N3) in a boreal Pinus sylvestris forest. The lowest level (N1), 30 kg N ha− 1 yr− 1 was applied during 50 years. Twice this rate (N2) was added 38 years, followed by 12 years of recovery, while thrice this rate (N3) was added 20 years followed by 30 years of recovery. We compared tree growth, changes in foliar and soil chemistry among treatments including control plots without N additions. As predicted, the N treatments lowered soil pH and reduced soil base saturation by around 50 %. They also lowered foliar levels of Ca, Mg, K, P and B initially, but after 50 years only Ca and Mg remained lower than in the control. Lack of B motivated a single addition of 2.5 kg ha− 1 after ten years of N treatment. Tree stem growth became and then remained higher in N1 than in the other treatments through the 50 years of treatments. In N2 and N3, foliar δ15N increased during the N-loading phase, but declined during the recovery phase, indicating a return of ectomycorrhizal fungi and their role in tightening the N cycle in N-limited forests. In the terminated, initially highest N treatments, N2 and N3, the trees even show signs of returning to Nlimitation. In these treatments, the soil base saturation remains lower, while the pH was only lower at 0–10 depth in the mineral soil, but not in the 10–20 cm depth horizon or in the superficial organic mor-layer. Accurately documenting the effect of N additions on forest growth required a long-term approach, where reasonable rates of application could be compared with extreme rates. Such long-term experiments are necessary to support forest management in achieving goals for developing future forests as they shift in response to major, global-scale changes.
8.	Li, Chuxian, et al. (författare) Mercury deposition and redox transformation processes in peatland constrained by mercury stable isotopes 2023 Ingår i: Nature Communications. - : Nature Publishing Group. - 2041-1723. ; 14:1 Tidskriftsartikel (refereegranskat)abstract Peatland vegetation takes up mercury (Hg) from the atmosphere, typically contributing to net production and export of neurotoxic methyl-Hg to downstream ecosystems. Chemical reduction processes can slow down methyl-Hg production by releasing Hg from peat back to the atmosphere. The extent of these processes remains, however, unclear. Here we present results from a comprehensive study covering concentrations and isotopic signatures of Hg in an open boreal peatland system to identify post-depositional Hg redox transformation processes. Isotope mass balances suggest photoreduction of HgII is the predominant process by which 30% of annually deposited Hg is emitted back to the atmosphere. Isotopic analyses indicate that above the water table, dark abiotic oxidation decreases peat soil gaseous Hg0 concentrations. Below the water table, supersaturation of gaseous Hg is likely created more by direct photoreduction of rainfall rather than by reduction and release of Hg from the peat soil. Identification and quantification of these light-driven and dark redox processes advance our understanding of the fate of Hg in peatlands, including the potential for mobilization and methylation of HgII.
9.	Liem-Nguyen, Van, et al. (författare) Methylmercury formation in boreal wetlands in relation to chemical speciation of mercury(II) and concentration of low molecular mass thiols 2021 Ingår i: Science of the Total Environment. - : Elsevier. - 0048-9697 .- 1879-1026. ; 755, s. 1-9 Tidskriftsartikel (refereegranskat)abstract Methylmercury (MeHg) is a neurotoxin formed from inorganic divalent mercury (Hg-II) via microbial methylation, and boreal wetlands have been identified as major sources of MeHg. There is however a lack of studies investigating the relationship between the chemical speciation of Hg-II and MeHg formation in such environments, in particular regarding to role of thiol compounds. We determined Hg-II methylation potentials, k(meth), in boreal wetland soils using two Hg-II isotope tracers: Hg-198(OH)(2)(aq) and Hg-II bonded to thiol groups in natural organic matter, Hg-200(II)-NOM(ads), representing Hg-II sources with high and low availability for methylation. The Hg-198(OH)(2)(aq) tracer was consistently methylated to a 5-fold higher extent than Hg-200(II)-NOM(ads), independent of environmental conditions. This suggests that the concentration of Hg-II in porewater was a decisive factor for Hg-II methylation. A comprehensive thermodynamic speciation model (including Hg-II complexes with inorganic sulfide (H2S), polysulfides (H2Sn), thiols associated with natural organic matter (NOM-RSH) and specific low molecular mass thiols (LMM-RSH) provided new insights on the speciation of Hg-II in boreal wetland porewaters, but did not demonstrate any clear relationship between k(meth) and the calculated chemical speciation. In contrast, significant positive relationships were observed between k(meth) and the sum of LMM thiol compounds of biological origin. We suggest two possible mechanisms underlying these correlations: 1) LMM thiols kinetically control the size and composition of the HgII pool available for microbial uptake, and/or 2) LMM thiols are produced by microbes such that the correlation reflects a relation between microbial activity and MeHg formation. (C) 2020 The Author(s). Published by Elsevier B.V.
10.	Schaefer, Jeffra K., et al. (författare) Anaerobic guilds responsible for mercury methylation in boreal wetlands of varied trophic status serving as either a methylmercury source or sink 2020 Ingår i: Environmental Microbiology. - : John Wiley & Sons. - 1462-2912 .- 1462-2920. ; 22:9, s. 3685-3699 Tidskriftsartikel (refereegranskat)abstract Wetlands are common sites of active Hg methylation by anaerobic microbes; however, the amount of methylmercury produced varies greatly, as Hg methylation is dependent upon both the availability of Hg and the composition and activity of the microbial community involved. In this study, we identified the major microbial guilds responsible for Hg methylation along a trophic gradient composed of two sites and three different types of wetlands: a bog–fen peatland gradient and a black alder swamp, serving as net sources and a sink for methylmercury respectively. Iron‐reducing bacteria in the Geobacteraceae were important Hg methylators across all wetlands and seasons examined, as evidenced by abundant 16S rRNA and hgcA transcripts clustering with this family. Molybdate inhibited Hg methylation more efficiently in the peatlands than in the swamp, suggesting an increasing role of sulfate‐reducing bacteria and/or related syntrophs in the methylation of Hg with decreasing trophic status. Sulfate addition failed to increase Hg methylation rates in the peatlands, suggesting that SRBs/syntrophs were instead likely metabolizing alternative substrates such as syntrophic fermentation of organic compounds with methanogens. These results highlight the interconnectivity of anaerobic metabolism and importance of community dynamics on the methylation of Hg in wetlands with different trophic status.
11.	Seelen, Emily, et al. (författare) Dissolved organic matter thiol concentrations determine methylmercury bioavailability across the terrestrial-marine aquatic continuum 2023 Ingår i: Nature Communications. - : Springer Nature. - 2041-1723. ; 14:1 Tidskriftsartikel (refereegranskat)abstract The most critical step for methylmercury (MeHg) bioaccumulation in aquatic food webs is phytoplankton uptake of dissolved MeHg. Dissolved organic matter (DOM) has been known to influence MeHg uptake, but the mechanisms have remained unclear. Here we show that the concentration of DOM-associated thiol functional groups (DOM-RSH) varies substantially across contrasting aquatic systems and dictates MeHg speciation and bioavailability to phytoplankton. Across our 20 study sites, DOM-RSH concentrations decrease 40-fold from terrestrial to marine environments whereas dissolved organic carbon (DOC), the typical proxy for MeHg binding sites in DOM, only has a 5-fold decrease. MeHg accumulation into phytoplankton is shown to be directly linked to the concentration of specific MeHg binding sites (DOM-RSH), rather than DOC. Therefore, MeHg bioavailability increases systematically across the terrestrial-marine aquatic continuum as the DOM-RSH concentration decreases. Our results strongly suggest that measuring DOM-RSH concentrations will improve empirical models in phytoplankton uptake studies and will form a refined basis for modeling MeHg incorporation in aquatic food webs under various environmental conditions.
12.	Shakeri Yekta, Sepehr, et al. (författare) Sulfide in engineered methanogenic systems - Friend or foe? 2023 Ingår i: Biotechnology Advances. - : PERGAMON-ELSEVIER SCIENCE LTD. - 0734-9750 .- 1873-1899. ; 69 Forskningsöversikt (refereegranskat)abstract Sulfide ions are regarded to be toxic to microorganisms in engineered methanogenic systems (EMS), where organic substances are anaerobically converted to products such as methane, hydrogen, alcohols, and carboxylic acids. A vast body of research has addressed solutions to mitigate process disturbances associated with high sulfide levels, yet the established paradigm has drawn the attention away from the multifaceted sulfide interactions with minerals, organics, microbial interfaces and their implications for performance of EMS. This brief review brings forward sulfide-derived pathways other than toxicity and with potential significance for anaerobic organic matter degradation. Available evidence on sulfide reactions with organic matter, interventions with key microbial metabolisms, and interspecies electron transfer are critically synthesized as a guidance for comprehending the sulfide effects on EMS apart from the microbial toxicity. The outcomes identify existing knowledge gaps and specify future research needs as a step forward towards realizing the potential of sulfide-derived mechanisms in diversifying and optimizing EMS applications.
13.	Skyllberg, Ulf, et al. (författare) Chemical speciation of mercury, sulfur and iron in a dystrophic boreal lake sediment, as controlled by the formation of mackinawite and framboidal pyrite 2021 Ingår i: Geochimica et Cosmochimica Acta. - : Elsevier BV. - 0016-7037 .- 1872-9533 .- 0046-564X. ; 294, s. 106-125 Tidskriftsartikel (refereegranskat)abstract The chemical speciation of mercury (Hg), methyl mercury (MeHg), sulfur and iron was investigated in the sediment and porewater of Lake Angessjon, a boreal, shallow (maximum depth 2.5 m), oligo-/dystrophic lake in northern Sweden. The lake receives terrestrial stream runoff from surrounding coniferous forest soils and peatlands having a low pH (4.6) and high concentrations of dissolved organic matter (DOM, annual average: 45 mg L-1), Fe (60 mu M), sulfate (105 mu M), inorganic Hg (1200 pM) and MeHg (250 pM). Sulfur K-edge XANES and Hg L-m-edge EXAFS spectroscopic measurements were used to characterize and quantify the sulfur speciation in the lake sediment at nine occasions, covering different seasons in the years of 2007 and 2009. In the surface sediment (0-3 cm) sulfate is reduced to zero-valent S and inorganic sulfide, that in turn reacts with Fe to form FeSm (mackinawite) and FeS2 (framboidal pyrite). The latter mineral becomes increasingly dominant by depth in the sediment. Thermodynamic modeling successfully predicted measured porewater concentrations of Hg in the sediment. Metacinnabar (beta-HgS) and Hg(NOM-RS)(2) complexes (the latter formed as a reaction between Hg(II) and thiol groups associated with natural organic matter, NOM-RSH) were the dominant forms of Hg(II) in the solid phase of sediments and Hg(II)-polysulfides (aq) dominated in the porewater. We argue that FeS m is a key component that indirectly controlled the Hg (II) speciation in the sediment by keeping the aqueous phase concentration of inorganic sulfide in the 0.5-2 mu M range throughout the year. Besides providing a pool of readily soluble inorganic sulfide for formation of beta-HgS(s), as demonstrated by previously reported EXAFS experiments, we further suggest FeS m may serve as a precursor for the formation of a more crystalline (less soluble) beta-HgS(s) phase than present in environments devoid of FeSm. Support for this was provided by comparing our results with previously reported thermodynamic modelling results of Hg(II) and MeHg solubility in organic soils devoid of FeSm. In more general terms, we suggest the presence or absence of FeSm, through its influence on the chemical speciation of Hg and MeHg, may be a key factor behind the variability in rates of Hg(II) and MeHg transformation processes, such as methylation, reduction and demethylation, reported for different environmental settings.
14.	Song, Yu, et al. (författare) Toward an Internally Consistent Model for Hg(II) Chemical Speciation Calculations in Bacterium-Natural Organic Matter-Low Molecular Mass Thiol Systems 2020 Ingår i: Environmental Science and Technology. - : American Chemical Society (ACS). - 0013-936X .- 1520-5851. ; 54:13, s. 8094-8103 Tidskriftsartikel (refereegranskat)abstract To advance the scientific understanding of bacteria-driven mercury (Hg) transformation processes in natural environments, thermodynamics and kinetics of divalent mercury Hg(II) chemical speciation need to be understood. Based on Hg LIII-edge extended X-ray absorption fine structure (EXAFS) spectroscopic information, combined with competitive ligand exchange (CLE) experiments, we determined Hg(II) structures and thermodynamic constants for Hg(II) complexes formed with thiol functional groups in bacterial cell membranes of two extensively studied Hg(II) methylating bacteria: Geobacter sulfurreducens PCA and Desulfovibrio desulfuricans ND132. The Hg EXAFS data suggest that 5% of the total number of membranethiol functionalities (Mem-RStot = 380 ± 50 μmol g–1 C) are situated closely enough to be involved in a 2-coordinated Hg(Mem-RS)2 structure in Geobacter. The remaining 95% of Mem-RSH is involved in mixed-ligation Hg(II)-complexes, combining either with low molecular mass (LMM) thiols like Cys, Hg(Cys)(Mem-RS), or with neighboring O/N membrane functionalities, Hg(Mem-RSRO). We report log K values for the formation of the structures Hg(Mem-RS)2, Hg(Cys)(Mem-RS), and Hg(Mem-RSRO) to be 39.1 ± 0.2, 38.1 ± 0.1, and 25.6 ± 0.1, respectively, for Geobacter and 39.2 ± 0.2, 38.2 ± 0.1, and 25.7 ± 0.1, respectively, for ND132. Combined with results obtained from previous studies using the same methodology to determine chemical speciation of Hg(II) in the presence of natural organic matter (NOM; Suwannee River DOM) and 15 LMM thiols, an internally consistent thermodynamic data set is created, which we recommend to be used in studies of Hg transformation processes in bacterium–NOM–LMM thiol systems.
15.	Thyrel, Mikael, et al. (författare) Phase transitions involving Ca - The most abundant ash forming element - In thermal treatment of lignocellulosic biomass 2021 Ingår i: Fuel. - : Elsevier. - 0016-2361 .- 1873-7153. ; 285 Tidskriftsartikel (refereegranskat)abstract Torrefaction, pyrolysis and gasification are of interest to convert lignocellulosic biomass into fuels and chemicals. These techniques involve thermal treatment at low partial pressures of oxygen. However, little is known about the transformation of ash elements during these processes. The phase transition of the major ash element calcium (Ca) was therefore studied with powder from pine as biomass model treated at temperatures 300-800 degrees C under atmospheres of 100% N-2, 3% O-2 and 6% O-2 and thermodynamic equilibrium modelling. For evaluation, Xray powder diffraction and synchrotron Ca K-edge X-ray absorption near edge structure (XANES) spectroscopy in combination with linear combination fitting and reference compounds was used. The results indicated that the most abundant Ca-containing species in the untreated material was thermally unstable Ca oxalate (CaC2O4) primarily decomposing into Ca phases dominated by carbonates at temperatures up to 600 degrees C. Double carbonates of calcium and potassium were observed in the form of fairchildiite/butscheliite (K2Ca(CO3)(2)), and these phases were stable over the low temperature range studied. Hydroxyapatite (Ca-5(PO4)(3)OH) was expected to be present and thermally stable over the entire temperature interval and was found in untreated material. At temperatures above 600 degrees C calcium oxide (CaO) was formed. The amount of oxygen had little effect on the phase transitions. The results of thermodynamic modeling were in agreement with XANES showing that this is a versatile technique that can be applied to systems as complex as Ca phase transitions in thermally treated lignocellulosic biomass at low partial pressures of oxygen.
16.	Wang, Baolin, et al. (författare) Biogeochemical influences on net methylmercury formation proxies along a peatland chronosequence 2021 Ingår i: Geochimica et Cosmochimica Acta. - : Elsevier. - 0016-7037 .- 1872-9533 .- 0046-564X. ; 308, s. 188-203 Tidskriftsartikel (refereegranskat)abstract A geographically constrained chronosequence of peatlands divided into three age classes (young, intermediate and old) was used to explore the role of biogeochemical influences, including electron donors and acceptors as well as chemical speciation of inorganic mercury (Hg(II)), on net formation of methylmercury (MeHg) as approximated by the fraction of MeHg to total mercury (THg) in the peat soil. We hypothesized that removing vascular plants would reduce availability of electron donors and thus net MeHg formation. However, we found no effect of the vascular plant removal. The sum of the potential electron donors (acetate, lactate, propionate and oxalate), the electron donation proxy organic C/Organic N, and the potential electron acceptors (Fe(III), Mn and sulfate) in porewater all showed significant correlations with the net MeHg formation proxies in peat soil (MeHg concentration and %MeHg of THg). Thus differences in both electron donor and acceptor availability may be contributing to the pattern of net MeHg formation along the chronosequence. In contrast, Hg(II) concentrations in peat porewater showed small differences along the gradient. A chemical speciation model successfully predicted the solubility of Hg and MeHg in the porewater. The modeling pointed to an enhanced concentration of Hg-polysulfide species in the younger peatlands as a potential factor behind increased Hg(II) solubility and methylation in the more nutrient-rich peatlands. This work contributes to the understanding of Hg and MeHg cycling in peatlands which can help guide mitigation measures to reduce aquatic MeHg biomagnification in peatland dominated landscapes.
17.	Wang, Baolin, et al. (författare) Microbial communities mediating net methylmercury formation along a trophic gradient in a peatland chronosequence 2023 Ingår i: Journal of Hazardous Materials. - : Elsevier BV. - 0304-3894 .- 1873-3336. ; 442 Tidskriftsartikel (refereegranskat)abstract Peatlands are generally important sources of methylmercury (MeHg) to adjacent aquatic ecosystems, increasing the risk of human and wildlife exposure to this highly toxic compound. While microorganisms play important roles in mercury (Hg) geochemical cycles where they directly and indirectly affect MeHg formation in peatlands, potential linkages between net MeHg formation and microbial communities involving these microorganisms remain unclear. To address this gap, microbial community composition and specific marker gene transcripts were investigated along a trophic gradient in a geographically constrained peatland chronosequence. Our results showed a clear spatial pattern in microbial community composition along the gradient that was highly driven by peat soil properties and significantly associated with net MeHg formation as approximated by MeHg concentration and %MeHg of total Hg concentration. Known fermentative, syntrophic, methanogenic and iron-reducing metabolic guilds had the strong positive correlations to net MeHg formation, while methanotrophic and methylotrophic microorganisms were negatively correlated. Our results indicated that sulfate reducers did not have a key role in net MeHg formation. Microbial activity as interpreted from 16S rRNA sequences was significantly correlated with MeHg and %MeHg. Our findings shed new light on the role of microbial community in net MeHg formation of peatlands that undergo ontogenetic change.
18.	Wang, Baolin, et al. (författare) Opposing spatial trends in methylmercury and total mercury along a peatland chronosequence trophic gradient 2020 Ingår i: Science of the Total Environment. - : Elsevier BV. - 0048-9697 .- 1879-1026. ; 718 Tidskriftsartikel (refereegranskat)abstract Peatlands are abundant elements of boreal landscapes where inorganic mercury (IHg) can be transformed into bioaccumulating and highly toxic methylmercury (MeHg). We studied fifteen peatlands divided into three age lasses (young, intermediate and old) along a geographically constrained chronosequence to determine the role of biogeochemical factors and nutrient availability in controlling the formation of MeHg. In the 10 cm soil layer just below the average annual growing season water table, concentrations of MeHg and %MeHg (of total Hg) were higher in younger, more mesotrophic peatlands than in older, more oligotrophic peatlands. In contrast, total mercury (THg) concentrations were higher in the older peatlands. Partial least squares (PLS) analysis indicates that the net MeHg production was positively correlated to trophic demands of vegetation and an increased availability of potential electron acceptors and donors for Hg methylating microorganisms. An important question for further studies will be to elucidate why there is less THg in the younger peatlands compared to the older peatlands, even though the age of the superficial peat itself is similar for all sites. We hypothesize that ecosystem features which enhance microbial processes involved in Hg methylation also promote Hg reduction that makes previously deposited Hg more available for evasion back to the atmosphere.
19.	Zhu, Wei, et al. (författare) Mercury Isotope Fractionation during the Exchange of Hg(0) between the Atmosphere and Land Surfaces: Implications for Hg(0) Exchange Processes and Controls 2022 Ingår i: Environmental Science and Technology. - : American Chemical Society (ACS). - 0013-936X .- 1520-5851. ; 56, s. 1445-1457 Tidskriftsartikel (refereegranskat)abstract Atmosphere-surface exchange of elemental mer-cury (Hg(0)) is a vital component in global Hg cycling; however, Hg isotope fractionation remains largely unknown. Here, we report Hg isotope fractionation during air-surface exchange from terrestrial surfaces at sites of background (two) and urban (two) character and at five sites contaminated by Hg mining. Atmospheric Hg(0) deposition to soils followed kinetic isotope fractionation with a mass-dependent (MDF) enrichment factor of -4.32 parts per thousand, and negligible mass-independent fractionation (MIF). Net Hg(0) emission generated average MDF enrichment factors (epsilon Hg-202) of -0.91, -0.59, 1.64, and -0.42 parts per thousand and average MIF enrichment factors ((EHg)-Hg-199) of 0.07, -0.20, -0.14, and 0.21 parts per thousand for urban, background, and Hg mining soils and cinnabar tailing, respectively. Positive correlations between epsilon Hg-202 and ambient Hg(0) concentration indicate that the co-occurring Hg(0) deposition (accounting for 10-39%) in a regime of net soil emission grows with ambient Hg(0). The MIF of Hg(0) emission from soils ((EHg)-Hg-199 range -0.27 to 0.14%o, n = 8) appears to be overall controlled by the photochemical reduction of kinetically constrained Hg(II) bonded to O ligands in background soils, while S ligands may have been more important in Hg mining area soils. In contrast, the small positive MIF of Hg(0) emission from cinnabar ore tailing (mean (EHg)-Hg-199 = 0.21 parts per thousand) was likely controlled by abiotic nonphotochemical reduction and liquid Hg(0) evaporation. This research provides critical observational constraints on understanding the Hg(0) isotope signatures released from and deposited to terrestrial surfaces and highlight stable Hg isotopes as a powerful tool for resolving atmosphere-surface exchange processes.
20.	Åkerblom, Staffan, et al. (författare) Formation and mobilization of methylmercury across natural and experimental sulfur deposition gradients 2020 Ingår i: Environmental Pollution. - : Elsevier BV. - 0269-7491 .- 1873-6424. ; 263 Tidskriftsartikel (refereegranskat)abstract We investigated the influence of sulfate (SO42-) deposition and concentrations on the net formation and solubility of methylmercury (MeHg) in peat soils. We used data from a natural sulfate deposition gradient running 300 km across southern Sweden to test the hypothesis posed by results from an experimental field study in northern Sweden: that increased loading of SO42- both increases net MeHg formation and redistributes methylmercury (MeHg) from the peat soil to its porewater. Sulfur concentrations in peat soils correlated positively with MeHg concentrations in peat porewater, along the deposition gradient similar to the response to added SO42- in the experimental field study. The combined results from the experimental field study and deposition gradient accentuate the multiple, distinct and interacting roles of SO42- deposition in the formation and redistribution of MeHg in the environment.

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