Assessment of dietary Selenium and its role in Mercury fate in cultured fish rainbow trout with two sustainable aquafeeds.

This study enhances the current limited understanding of the interaction between mercury (Hg) and selenium (Se) species in fish. Rainbow trout (Oncorhynchus mykiss), a model aquaculture fish, was exposed to Hg and Se species through controlled dietary conditions. Over a 6-month feeding trial, the impact of dietary Se on Hg bioaccumulation in fish, including flesh, brain, and liver, was tracked. Twelve dietary conditions were tested, including plant-based diets (0.25 µgSe g-1) and tuna byproduct diets (0.25 µgHg g-1, 8.0 µgSe g-1) enriched with methylmercury and/or Se as selenite or selenomethionine. The tuna byproduct diet resulted in lower Hg levels than the plant-based diets, with muscle Hg content below the European Commission's safe threshold. This study highlights the significant impact of specific Se compounds in the diet, particularly from tuna-based aquafeed, on Hg bioaccumulation. These promising results provide a strong recommendation for future use of fisheries byproducts in sustainable aquafeeds.

Biogeochemical behaviour of geogenic As in a confined aquifer of the Sologne region, France.

Arsenic (As) is one of the main toxic elements of geogenic origin that impact groundwater quality and human health worldwide. In some groundwater wells of the Sologne region (Val de Loire, France), drilled in a confined aquifer, As concentrations exceed the European drinking water standard (10 µg L-1). The monitoring of one of these drinking water wells showed As concentrations in the range 20-25 µg L-1. The presence of dissolved iron (Fe), low oxygen concentration and traces of ammonium indicated reducing conditions. The δ34SSO4 was anticorrelated with sulphate concentration. Drilling allowed to collect detrital material corresponding to a Miocene floodplain and crevasse splay with preserved plant debris. The level that contained the highest total As concentration was a silty-sandy clay containing 26.9 mg kg-1 As. The influence of alternating redox conditions on the behaviour of As was studied by incubating this material with site groundwater, in biotic or inhibited bacterial activities conditions, without synthetic organic nutrient supply, in presence of H2 during the reducing periods. The development of both AsV-reducing and AsIII-oxidising microorganisms in biotic conditions was evidenced. At the end of the reducing periods, total As concentration strongly increased in biotic conditions. The microflora influenced As speciation, released Fe and consumed nitrate and sulphate in the water phase. Microbial communities observed in groundwater samples strongly differed from those obtained at the end of the incubation experiment, this result being potentially related to influence of the sediment compartment and to different physico-chemical conditions. However, both included major Operating Taxonomic Units (OTU) potentially involved in Fe and S biogeocycles. Methanogens emerged in the incubated sediment presenting the highest solubilised As and Fe. Results support the hypothesis of in-situ As mobilisation and speciation mediated by active biogeochemical processes.

Recycling and persistence of iodine 127 and 129 in forested environments: A modelling approach.

Differences in the source and behaviour of 129I compared to 127I isotopes have been described for a variety of surface environments, but little is known about the cycling rates of each isotope in terrestrial ecosystems. We developed a compartment model of the iodine cycle in a forest ecosystem, with a labile and non-labile pool to simplify the complex fate of iodine in the forest floor and soil. Simulations were performed using atmospheric 127I and 129I inputs for sites differing in climate, vegetation, and soil. In general, considering dry deposition in addition to wet deposition improved model simulations. Model results support the view that soil is the sink for atmospheric iodine deposited in forest ecosystems, while tree vegetation has little influence on long-term iodine budgets. Modelling also showed that iodine cycling reaches equilibrium after a period of about 5000 years, mainly due to a gradual incorporation of iodine into the bulk stabilised soil organic matter. At steady state, this pool of non-labile iodine in soil can retain about 20% of total deposition with a mean residence time of 900 years, while the labile iodine pool is renewed after 90 years. The proportions of modern anthropogenic 129I in each modelled pool reflect those of stable 127I at least several decades after input to the forest; this result explains why isotopic disequilibrium is common in field data analysis. Volatilisation plays a central role in regulating iodine storage in soil and, therefore, its residence time, while drainage is a minor export pathway, except at some calcareous sites. Dynamic modelling has been particularly helpful for gaining insight into the long-term response of iodine partitioning to continuous, single or even varying deposition. Our modelling study suggested that better estimates of dry deposition of atmospheric iodine, weathering of parent rock, and volatilisation of the deposited iodine from soil and vegetation will be required for reliable predictions of iodine cycling in specific forests, because these processes remain insufficiently explored.

Iodine distribution and volatilization in contrasting forms of forest humus during a laboratory incubation experiment.

Radionuclides 129I (t1/2 = 15.7 × 106 years) and 131I (t1/2 = 8.02 days) are both introduced into the environment as a result of nuclear human activities. Environmental transfer pathways and fluxes between and within ecosystems are essential information for risk assessment. In forest ecosystems, humus degradation over time could result in re-mobilization and then downward migration and/or volatilization of intercepted 129I. In order to estimate the scale of these processes, humus (mull and moder forms) sampled under deciduous and coniferous forests were spiked with 125I- (t1/2 = 59.4 days), as a surrogate for 129I, in order to study the evolution of its water-soluble and organic fractions as well as the volatilization rate during humus degradation at laboratory scale. To our knowledge, this is the first time that interactions between iodine and contrasting forms of forest humus have been investigated. The evolution of native stable iodine (127I) pools in unspiked humus was also studied. The nature of the humus' organic matter appears to be a factor that impacts on the proportions of water-soluble and organic fractions of iodine and on their evolution. Iodine-125 was mainly organically bound (fraction for mulls and moders: ∼54-59 and 41-49%, respectively) and no clear evolution was observed within the 4-month incubation period. A large decrease in 125I water-solubility occurred, being more marked for mull (from ∼14-32 to 3-7%) than for moder (from ∼21-37 to 7-19%) humus. By contrast, a significant fraction was not extractible (∼38-43%) and varied in inverse proportion to the water-soluble fraction, suggesting a stabilization of iodine in humus after wet deposit. The nature of the humus organic matter also impacted on 125I volatilization. Although of the same order of magnitude, the total volatilization of 125I was higher for moders (∼0.039-0.323%) than for mulls (∼0.015-0.023%) within the 4-month incubation period. Volatilization rates for mulls were correlated with the water-soluble fraction, implying that volatilization of 125I could occur from the humus solution. Our results showed that humus is thus a zone of iodine accumulation by association with organic matter and that potential losses by lixiviation are significantly more important compared to volatilization.

First Time Identification of Selenoneine in Seabirds and Its Potential Role in Mercury Detoxification.

Birds are principally exposed to selenium (Se) through their diet. In long-lived and top predator seabirds, such as the giant petrel, extremely high concentrations of Se are found. Selenium speciation in biota has aroused great interest in recent years; however, there is a lack of information about the chemical form of Se in (sea)birds. The majority of publications focus on the growth performance and antioxidant status in broilers in relation to Se dietary supplementation. The present work combines elemental and molecular mass spectrometry for the characterization of Se species in wild (sea)birds. A set of eight giant petrels (Macronectes sp.) with a broad age range from the Southern Ocean were studied. Selenoneine, a Se-analogue of ergothioneine, was identified for the first time in wild avian species. This novel Se-compound, previously reported in fish, constitutes the major Se species in the water-soluble fraction of all of the internal tissues and blood samples analyzed. The levels of selenoneine found in giant petrels are the highest reported in animal tissues until now, supporting the trophic transfer in the marine food web. The characterization of selenoneine in the brain, representing between 78 and 88% of the total Se, suggests a crucial role in the nervous system. The dramatic decrease of selenoneine (from 68 to 3%) with an increase of Hg concentrations in the liver strongly supports the hypothesis of its key role in Hg detoxification.

Selenium distribution and speciation in waters of pristine alpine lakes from central-western Pyrenees (France-Spain).

The speciation of both redox reactive and volatile selenium (Se) compounds, barely reported in pristine aquatic environments, has never been investigated in remote alpine lakes, considered as sensitive ecosystems to detect the effect of global change. This work presents an integrated investigation on Se distribution and speciation conducted in 20 high altitude pristine lakes from the central-western Pyrenees. Five seasonal sampling campaigns were carried out after snowmelt (June/July) and in early fall (October) for the period 2017-2019. Concentrations of total dissolved Se (TDSe) ranged from 7 to 78 ng L-1, with selenate being ubiquitously observed in most cases (median of 61% of TDSe). Selenite was only occasionally detected up to 4 ng L-1, therefore a fraction of TDSe was presumably in the forms of elemental Se(0) and/or selenides. Depth profiles obtained in different lakes showed the occurrence of such Se(-II, 0) pools in bottom hypoxic to anoxic waters. The production of volatile Se compounds presented a low median total concentration (TVSe) of 33 pg L-1 (range 3-120 pg L-1), mainly in the form of dimethylselenide in subsurface samples (median of 82% of TVSe). The Se concentration in lake waters was significantly correlated with the sulphate concentration (ρ = 0.93, p < 0.0001), demonstrating that it is influenced by erosion and dissolution of Se and S-enriched parent bedrocks. In addition, for Se depleted alpine lake-bedrock systems, long-range transport and wet atmospheric depositions represent a major source of Se for lake waters.

Influence of tree species on selenium and iodine partitioning in an experimental forest ecosystem.

Storage of selenium and iodine can greatly vary between forest ecosystems, but the influence of tree species on partitioning and recycling of those elements remains elusive. In this study, contents of Se and I were measured in tree compartments, litterfall, humus, and soil horizons in monospecific stands of Douglas fir, pine, spruce, beech, and oak under identical climatic and edaphic conditions. The cycle of each element was characterized in terms of stocks and fluxes. Lowest concentrations were in wood (Se: 8-13 µg kg-1; I: <16.5 µg kg-1). Senescing organs had higher Se and I content, than the living parts of trees due to direct exposure to atmospheric deposition, with some variation between coniferous and deciduous trees. For all stands, low amounts of Se and I were involved in biological cycle as reflected by low root uptake. In humus, the enrichment of elements greatly increased with the stage of organic matter (OM) degradation with average factors of 10 and 20 for Se and I. OM degradation and element persistence in humus was influenced by tree species. Deciduous trees, with low biomass, and fast degradation of OM stored less Se and I in humus compared to fir and spruce with high humus biomass. Interestingly, tree species did not affect soil reserves of Se and I. Concentration ranges were 331-690 µg Se kg-1 and 4.3-14.5 mg I kg-1. However, the divergent vertical profiles of the elements in the soil column indicated greater mobility of I. Selenium concentrations regularly decreased with depth in correlation with OM and Fe oxides content. For iodine, the maximum iodine concentration at a soil depth of 15 to 35 cm was caused by a parallel precipitation/sorption behavior of aluminium and organic iodine dissolved in the topsoil.

Selenium distribution in French forests: Influence of environmental conditions.

Selenium is a trace element and an essential nutrient. Its long-lived radioisotope, selenium 79 is of potential radio-ecological concern in surface environment of deep geological repository for high-level radioactive waste. In this study, the influence of environmental, climatic and geochemical conditions on stable Se (as a surrogate of 79Se) accumulation was statistically assessed (PCA analysis, Kruskall-Wallis and Spearman tests) based on the analysis of its concentration in litterfall, humus, and soil samples collected at 51 forest sites located in France. Selenium concentrations were in the ranges: 22-369, 57-1608 and 25-1222 µg kg-1 respectively in litterfall, humus, and soil. The proximity of the ocean and oceanic climate promoted Se enrichment of litterfall, likely due to a significant reaction of wet deposits with forest canopy. Se content was enhanced by humification (up to 6 times) suggesting that Se concentrations in humus were affected by atmospheric inputs. Selenium stock in humus decreased in the order of decreasing humus biomass and increasing turnover of organic matter: mor > moder > mull. Positive correlations between Se content and geochemical parameters such as organic carbon content, total Al and total Fe confirmed the important role of organic matter (OM) and mineral Fe/Al oxides in Se retention in soils.

Tissue localization of selenium of parental or dietary origin in rainbow trout (Oncorhynchus mykiss) fry using LA-ICP MS bioimaging.

In relation to the decrease of selenium (Se) content in aquafeeds, the impact of level and form of parental and dietary Se supplementation was investigated in rainbow trout fry using laser ablation-inductively coupled plasma mass spectrometry (LA-ICP MS) bioimaging. The offspring of rainbow trout broodstock, fed either a control diet without any Se supplementation (0.3 mg Se/kg diet) or a diet supplemented with Se (0.6 mg Se/kg diet) either as sodium selenite or hydroxy-selenomethionine, were sampled at swim-up fry stage or after 11 weeks of cross-feeding. Total body Se levels were influenced by parental Se nutrition in swim-up fry and by direct Se feeding in 11-week fry with higher levels in the Se-supplemented groups compared with the control and the highest levels in the hydroxy-selenomethionine treatment. The Se retention was lower for dietary sodium selenite. Selenomethionine levels increased when Se was provided as hydroxy-selenomethionine. LA-ICP MS maps revealed yolk in swim-up fry and intestine, liver, and kidney in 11-week fed fry as tissues with high Se abundance. In swim-up fry, muscle Se was the highest abundant when parents were fed hydroxy-selenomethionine. In 11-week fed fry, muscle Se abundance was higher in the head part of fry fed both Se-supplemented diets, but only in the tail part of fry fed hydroxy-selenomethionine. Liver Se abundance was higher in fry fed sodium selenite compared with the control diet supporting the hypothesis that tissue Se distribution can be influenced by parental and dietary Se forms and levels.

Atmospheric iodine, selenium and caesium depositions in France: I. Spatial and seasonal variations.

The spatial distribution and seasonal variations of atmospheric iodine (I), selenium (Se) and caesium (Cs) depositions remain unclear and this precludes adequate inputs for biogeochemical models. We quantified total concentrations and fluxes of these elements in rainfalls from 27 monitoring sites in France with contrasted climatic conditions; monthly measurements were taken over one year (starting in 2016/09). Since speciation of I and Se can impact their behaviour in the environment, analysis of their inorganic compounds was also conducted. Our results showed that annual I concentrations in rainfall were much higher than those of Se and Cs (annual means = 1.56, 0.044 and 0.005 µg L-1, respectively). The annual iodine concentrations were highly positively correlated with those of marine elements (i.e. Na, Cl and Mg), involving higher I concentrations under oceanic climate than for transition, continental and mountainous ones. Furthermore, common patterns were found between Se concentrations and both marine and terrestrial components consistent with the various sources of Se in atmosphere. The association of Cs with two anthropogenic components (i.e. NH4+ and NO3-) used in agriculture supports the hypothesis of its terrestrial origin (i.e. from atmospheric dusts) in rainfall. We found higher rainfall concentrations of I during the warmest months for all climates. However, no specific seasonal trend occurred for Se and Cs. On annual average, rainfall contained mostly unidentified selenium compounds (inorganic Se proportions = 25-54%) and equal proportions of inorganic and unidentified I compounds. Concentrations of iodate were higher under oceanic climate consistent with an iodine marine-origin.

Atmospheric iodine, selenium and caesium depositions in France: II. Influence of forest canopies.

Estimation of the canopy influence on atmospheric inputs of iodine (I), selenium (Se) and caesium (Cs) in terrestrial ecosystems is an essential condition for appropriate biogeochemical models. However, the processes involved in rain composition modifications after its passage through forest canopy have been barely studied for these elements. We monitored I, Se and Cs concentrations in both rainfall and throughfall of fourteen French forested sites throughout one year, and estimated dry deposition and canopy exchange fluxes for these elements, as well as speciation of I and Se. Comparison of rainfall and throughfall elemental composition highlighted an important impact of forest canopy on both (i) concentrations and fluxes of I, Se and Cs, and (ii) I and Se species. For the three elements, most of their throughfall concentrations were higher than corresponding rainfall. The increase of throughfall elemental fluxes was mostly due to dry deposition for I and Se although the canopy exchange model revealed some sorption within the canopy in most cases; for Cs, foliage leaching was most influencing. Regarding speciation, iodine species in rainfall were highly modified by forest canopy with an important increase of unidentified I proportion in throughfall (on average 49 and 82% in rainfall and throughfall, respectively), possibly due to washoff of dry deposition and/or to transformation into organic forms. Similarly, while rainfall was composed of 26-54% of inorganic Se, inorganic species were undetectable in throughfall. This dataset represents key information to improve modelling of I, Se and Cs cycling within forest ecosystems.

Iodine budget in forest soils: Influence of environmental conditions and soil physicochemical properties.

Due to its longevity, radioisotope 129I is a health concern following potential releases in the environment which raises questions about residence and exposure times relevant for risk assessments. We determined 127I concentrations (as a surrogate for 129I) in a series of French forest soils (i.e. litters, humus and mineral soils) under different vegetation and climate conditions in order to identify the major processes affecting its accumulation and persistence in the soil column. The input fluxes linked to rainfall, throughfall and litterfall were also characterized. Main results obtained showed that: (i) rainfall iodine concentrations probably influenced those of litterfall through absorption by leaves/needles returning to the ground; (ii) throughfall was the major iodine input to soils (mean = 83%), compared to litterfall (mean = 17%); (iii) humus represented a temporary storage of iodine from atmospheric and biomass deposits; (iv) iodine concentrations in soils depended on both the iodine inputs and the soil's ability to retain iodine due to its organic matter, total iron and aluminium concentrations; (v) these soil properties were the main factors influencing the accumulation of iodine in the soil column, resulting in residence times of 419-1756 years; and (vi) the leaching of iodine-containing organic matter dissolved in soil solution may be an important source of labile organic iodine for groundwater and streams.

Dissolved Organic Matter Controls Seasonal and Spatial Selenium Concentration Variability in Thaw Lakes across a Permafrost Gradient.

Little is known about the sources and processing of selenium, an important toxicant and essential micronutrient, within boreal and sub-arctic environments. Upon climate warming and permafrost thaw, the behavior of Se in northern peatlands becomes an issue of major concern, because a sizable amount of Se can be emitted to the atmosphere from thawing soils and inland water surfaces and exported to downstream waters, thus impacting the Arctic biota. Working toward providing a first-order assessment of spatial and temporal variation of Se concentration in thermokarst waters of the largest frozen peatland in the world, we sampled thaw lakes and rivers across a 750-km latitudinal profile. This profile covered sporadic, discontinuous, and continuous permafrost regions of western Siberia Lowland (WSL), where we measured dissolved (<0.45 µm) Se concentration during spring (June), summer (August), and autumn (September). We found maximum Se concentration in the discontinuous permafrost zone. Considering all sampled lakes, Se exhibited linear relationship ( R2 = 0.7 to 0.9, p < 0.05, n ≈ 70) with dissolved organic carbon (DOC) concentration during summer and autumn. Across the permafrost gradient, the lakes in discontinuous permafrost regions demonstrated stronger relationship with DOC and UV-absorbance compared to lakes in sporadic/isolated and continuous permafrost zones. Both seasonal and spatial features of Se distribution in thermokarst lakes and ponds suggest that Se is mainly released during thawing of frozen peat. Mobilization and immobilization of Se within peat-lake-river watersheds likely occurs as organic and organo-Fe, Al colloids, probably associated with reduced and elemental Se forms. The increase of active layer thickness may enhance leaching of Se in the form of organic complexes with aromatic carbon from the deep horizons of the peat profile. Further, the northward shift of permafrost boundaries in WSL may sizably increase Se concentration in lakes of continuous permafrost zone.

Iodine distribution and cycling in a beech (Fagus sylvatica) temperate forest.

Radioiodine is of health concerns in case of nuclear events. Possible pathways and rates of flow are essential information for risk assessment. Forest ecosystems could influence the global cycle of long-lived radioiodine isotope (129I) with transfer processes similar to stable isotope (127I). Understanding iodine cycling in forest involves study of the ecosystem as a whole. In this context, we determined the 127I contents and distribution in soil, tree compartments and atmospheric inputs during a three years in situ monitoring of a temperate beech forest stand. The iodine cycle was first characterized in terms of stocks by measuring its concentrations in: tree, litterfall, humus, soil, rainfall, throughfall, stemflow and soil solutions. Main annual fluxes (requirement, uptake and internal transfers) and forest input-output budget were also estimated using conceptual model calculations. Our findings show that: (i) soil is the main I reservoir accounting for about 99.9% of ecosystem total stock; (ii) iodine uptake by tree represents a minor fraction of the available pool in soil (<0.2%); (iii) iodine allocation between tree compartments involves low immobilization in wood and restricted location in the roots; (iv) translocation of excess iodine towards senescing foliage appears as an elimination process for trees, and (v) litterfall is a major pathway in the I biological cycling. In our soil conditions, the input - output budget shows that the ecosystem behaves as a potential source of I for groundwater.

Deproteinization assessment using isotopically enriched compounds to trace the coprecipitation of low-molecular-weight selenium species with proteins.

Studies have shown that information related to the presence of low-molecular-weight metabolites is frequently lost after deproteinization of complex matrices, such as blood and plasma, during sample preparation. Therefore, the effect of several deproteinization reagents on low-molecular-weight selenium species has been compared by species-specific isotope labeling. Two isotopically enriched selenium tracers were used to mimic models of small inorganic anionic (77Se-selenite) and organic zwitterionic (76Se-selenomethionine) species. The results presented here show that the use of a methanol-acetonitrile-acetone (1:1:1 v/v/v) mixture provided approximately two times less tracer loss from plasma samples in comparison with the classic procedure using acetonitrile, which may not be optimal as it leads to important losses of low-molecular-weight selenium species. In addition, the possible interactions between selenium tracers and proteins were investigated, revealing that both coprecipitation phenomena and association with proteins were potentially responsible for selenite tracer losses during protein precipitation in blood samples. However, coprecipitation phenomena were found to be fully responsible for losses of both tracers observed in plasma samples and of the selenomethionine tracer in blood samples. This successfully applied strategy is anticipated to be useful for more extensive future studies in selenometabolomics.

Influence of Se concentrations and species in hydroponic cultures on Se uptake, translocation and assimilation in non-accumulator ryegrass.

The success of biofortification and phytoremediation practices, addressing Se deficiency and Se pollution issues, hinges crucially on the fate of selenium in the plant media in response to uptake, translocation and assimilation processes. We investigate the fate of selenium in root and shoot compartments after 3 and 6 weeks of experiment using a total of 128 plants grown in hydroponic solution supplied with 0.2, 2, 5, 20 and 100 mg L-1 of selenium in the form of selenite, selenate and a mixture of both species. Selenate-treated plants exhibited higher root-to-shoot Se translocation and total Se uptake than selenite-treated plants. Plants took advantage of the selenate mobility and presumably of the storage capacity of leaf vacuoles to circumvent selenium toxicity within the plant. Surprisingly, 28% of selenate was found in shoots of selenite-treated plants, questioning the ability of plants to oxidize selenite into selenate. Selenomethionine and methylated organo-selenium amounted to 30% and 8% respectively in shoots and 35% and 9% in roots of the identified Se, suggesting that selenium metabolization occurred concomitantly in root and shoot plant compartments and demonstrating that non-accumulator plants can synthesize notable quantities of precursor compound for volatilization. The present study demonstrated that non-accumulator plants can develop the same strategies as hyper-accumulator plants to limit selenium toxicity. When both selenate and selenite were supplied together, plants used selenate in a storage pathway and selenite in an assimilation pathway. Plants might thereby benefit from mixed supplies of selenite and selenate by saving enzymes and energy required for selenate reduction.

Field study of time-dependent selenium partitioning in soils using isotopically enriched stable selenite tracer.

A better understanding of selenium fate in soils at both short and long time scales is mandatory to consolidate risk assessment models relevant for managing both contamination and soil fertilization issues. The purpose of this study was thus to investigate Se retention processes and their kinetics by monitoring time-dependent distribution/speciation changes of both ambient and freshly added Se, in the form of stable enriched selenite-77, over a 2-years field experiment. This study clearly illustrates the complex reactivity of selenium in soil considering three methodologically defined fractions (i.e. soluble, exchangeable, organic). Time-dependent redistribution of Se-77 within solid-phases having different reactivity could be described as a combination of chemical and diffusion controlled processes leading to its stronger retention. Experimental data and their kinetic modeling evidenced that transfer towards less labile bearing phases are controlled by slow processes limiting the overall sorption of Se in soils. These results were used to estimate time needed for (77)Se to reach the distribution of naturally present selenium which may extend up to several decades. Ambient Se speciation accounted for 60% to 100% of unidentified species as function of soil type whereas (77)Se(IV) remained the more abundant species after 2-years field experiment. Modeling Se in the long-term without taking account these slow sorption kinetics would thus result in underestimation of Se retention. When using models based on Kd distribution coefficient, they should be at least reliant on ambient Se which is supposed to be at equilibrium.

Determination of the distribution and speciation of selenium in an argillaceous sample using chemical extractions and post-extractions analyses: application to the hydrogeological experimental site of Poitiers.

To better understand selenium's dynamics in environmental systems, the present study aims to investigate selenium speciation and distribution in black argillaceous sediments, partially fulfilling karstic cavities into the Hydrogeological Experimental Site of Poitiers. These sediments are suspected to be responsible for selenium concentrations exceeding the European Framework Directive's drinking water limit value (10 µg L(-1)) in some specific wells. A combination of a sequential extractions scheme and single parallel extractions was thus applied on a representative argillaceous sample. Impacts of the extractions on mineral dissolution and organic matter mobilization were followed by quantifying major cations and total organic carbon (TOC) in the aqueous extracts. The nature of the released organic matter was characterized using thermochemolysis coupled with gas chromatography-mass spectrometry (GC-MS). About 10 % of selenium from the black argillaceous studied matrix could be defined as 'easily mobilizable' when the majority (around 70 %) revealed associated with the aliphatic and alkaline-soluble organic matter's fraction (about 20 %). In these fractions, selenium speciation was moreover dominated by oxidized species including a mixture of Se(VI) (20-30 %) and Se(IV) (70-80 %) in the 'easily mobilizable' fraction, while only Se(IV) was detected in alkaline-soluble organic matter fraction.

Organotin persistence in contaminated marine sediments and porewaters: In situ degradation study using species-specific stable isotopic tracers.

This paper provides a comprehensive study of the persistence of butyltins and phenyltins in contaminated marine sediments and presents the first data on their degradation potentials in porewaters. The study's aim was to explain the different degradation efficiencies of organotin compounds (OTC) in contaminated sediments. The transformation processes of OTC in sediments and porewaters were investigated in a field experiment using species-specific, isotopically enriched organotin tracers. Sediment characteristics (organic carbon content and grain size) were determined to elucidate their influence on the degradation processes. The results of this study strongly suggest that a limiting step in OTC degradation in marine sediments is their desorption into porewaters because their degradation in porewaters occurs notably fast with half-lives of 9.2 days for tributyltin (TBT) in oxic porewaters and 2.9±0.1 and 9.1±0.9 days for dibutyltin (DBT) in oxic and anoxic porewaters, respectively. By controlling the desorption process, organic matter influences the TBT degradation efficiency and consequently defines its persistence in contaminated sediments, which thus increases in sediments rich in organic matter.

Stable isotope tracing: a powerful tool for selenium speciation and metabolic studies in non-hyperaccumulator plants (ryegrass Lolium perenne L.).

Selenium is both essential and toxic for mammals; the range between the two roles is narrow and not only dose-dependent but also related to the chemical species present in foodstuff. Unraveling the metabolism of Se in plants as a function of Se source may thus lead to ways to increase efficiency of fertilization procedures in selenium deficient regions. In this study, stable-isotope tracing was applied for the first time in plants to simultaneously monitor the bio-incorporation of two inorganic Se species commonly used as foodstuff enrichment sources. Occurrence and speciation of Se coming from different Se sources were investigated in root and leaf extracts of ryegrass (Lolium perenne L.), which had been co-exposed to two labeled Se species ((77)SeIV and (82)SeVI). Although the plant absorbed similar amounts of Se when supplied in the form of selenite or selenate, the results evidenced marked differences in speciation and tissues allocation. Selenite was converted into organic forms incorporated mostly into high molecular weight compounds with limited translocation to leaves, whereas selenate was highly mobile being little assimilated into organic forms. Double-spike isotopic tracer methodology makes it possible to compare the metabolism of two species-specific Se sources simultaneously in a single experiment and to analyze Se behavior in not-hyperaccumulator plants, the ICP-MS sensitivity being improved by the use of enriched isotopes.