Selenium is usually known as the ‘double-edged sword element' for its dual toxic and beneficial character to health. Since the pioneer works by Schwarz and Foltz on the relationships between selenium deficiency and liver, muscle and heart diseases, many efforts have been undertaken to better understand the role of selenium in health. At the same time, an increasing number of publications have appeared during these last years on the selenium physico–chemical interactions within the environment. Both types of research represent ongoing efforts to correctly estimate the bioavailability of selenium species for health and the environment. Redox reactions, diffusion, adsorption and precipitation processes or interactions with organic matter and biota govern the speciation and mobility of selenium in the environment. This review intends to emphasize and collect the important advances made during these last years in the mechanistic understanding of processes which govern selenium cycling and bioavailability, like adsorption at the mineral/water interface, precipitation of elemental selenium, or bioavailability of nanoscaled precipitates. The advent of powerful spectroscopic techniques, like X-ray absorption spectroscopy, has allowed the structural description of adsorption and substitution processes that selenium undergoes in a variety of minerals. These and other structural details about selenium precipitates are reviewed here, together with their relationships to the bioavailability of the element in the environment.

Selenium environmental cycling and bioavailability: a structural chemist point of view

Selenium is a natural trace element that is of fundamental importance to human health. The extreme geographical variation in selenium concentrations in soils and food crops has resulted in significant health problems related to deficient or excess levels of selenium in the environment. To deal with these kinds of problems in the future it is essential to get a better understanding of the processes that control the global distribution of selenium. The recent development of analytical techniques and methods enables accurate selenium measurements of environmental concentrations, which will lead to a better understanding of biogeochemical processes. This improved understanding may enable us to predict the distribution of selenium in areas where this is currently unknown. These predictions are essential to prevent future Se health hazards in a world that is increasingly affected by human activities.

Environmental selenium research: from microscopic processes to global understanding.

Spectroscopic approaches to plant and soil sciences have provided important information for several decades. However, many of these approaches suffered from a number of limitations and drawbacks especially in terms of spatial resolution and requirements for sample preparation. The advent of dedicated synchrotron facilities, that allow the exploitation of the particular qualities of synchrotron radiation as a research tool, has revolutionised the way we approach the investigation of nutrients and contaminants in environmental samples. Various synchrotron-based techniques are currently available that permit such investigations in situ and at the molecular level. The continuous development of these techniques is delivering substantial gains in terms of sensitivity and spatial resolution which allows analyses of diluted samples at the sub-micron scale. This paper aims at providing an introduction to synchrotron radiation and to the fundamentals of some widely used synchrotron-based techniques, in particular X-ray absorption, fluorescence and tomography. Furthermore, examples are provided regarding the applications of synchrotron-based techniques in the field of plant, soil and rhizosphere research. Finally, current limitations and future perspectives of synchrotron techniques are discussed.

Synchrotron-based techniques for plant and soil science: opportunities, challenges and future perspectives

It has been estimated that almost one third of the world’s population lack sufficient Zn for adequate nutrition. This can be alleviated by increasing dietary Zn intakes through Zn-biofortification of edible crops. Biofortification strategies include the application of Zn-fertilisers or the development of crop genotypes that acquire more Zn from the soil and accumulate it in edible portions. Zinc concentrations in roots, leaves and stems can be increased through the application of Zn-fertilisers. Root Zn concentrations of up to 500-5000 mg kg-1 DM, and leaf Zn concentrations of up to 100-700 mg kg-1 dry matter (DM), can be achieved without loss of yield when Zn-fertilisers are applied to the soil. It is possible that greater Zn concentrations in non-woody shoot tissues can be attained using foliar Zn-fertilisers. By contrast, Zn concentrations in fruits, seeds and tubers are severely limited by low Zn mobility in the phloem and Zn concentrations higher than 30-100 mg kg-1 DM are rarely observed. However, genetically modified plants with improved abilities translocate Zn in the phloem might be used to biofortify these phloem-fed tissues. In addition, genetically modified plants with increased tolerance to high tissue Zn concentrations could be used to increase Zn concentrations in all edible produce and, thereby, increase dietary Zn intakes.

/pdf/physiological-limits-to-zinc-biofortification-of-edible-3839m52k16.pdf

Physiological limits to zinc biofortification of edible crops.

Selenium Toxicity to Aquatic Organisms

Ralstonia metallidurans CH34, a soil bacterium resistant to a variety of metals, is known to reduce selenite to intracellular granules of elemental selenium (Se(0)). We have studied the kinetics of selenite (Se(IV)) and selenate (Se(VI)) accumulation and used X-ray absorption spectroscopy to identify the accumulated form of selenate, as well as possible chemical intermediates during the transformation of these two oxyanions. When introduced during the lag phase, the presence of selenite increased the duration of this phase, as previously observed. Selenite introduction was followed by a period of slow uptake, during which the bacteria contained Se(0) and alkyl selenide in equivalent proportions. This suggests that two reactions with similar kinetics take place: an assimilatory pathway leading to alkyl selenide and a slow detoxification pathway leading to Se(0). Subsequently, selenite uptake strongly increased (up to 340 mg Se per g of proteins) and Se(0) was the predominant transformation product, suggesting an activation of selenite transport and reduction systems after several hours of contact. Exposure to selenate did not induce an increase in the lag phase duration, and the bacteria accumulated approximately 25-fold less Se than when exposed to selenite. Se(IV) was detected as a transient species in the first 12 h after selenate introduction, Se(0) also occurred as a minor species, and the major accumulated form was alkyl selenide. Thus, in the present experimental conditions, selenate mostly follows an assimilatory pathway and the reduction pathway is not activated upon selenate exposure. These results show that R. metallidurans CH34 may be suitable for the remediation of selenite-, but not selenate-, contaminated environments.

https://hal.archives-ouvertes.fr/hal-00022629/document

Chemical forms of selenium in the metal-resistant bacterium Ralstonia metallidurans CH34 exposed to selenite and selenate.

http://ftp.esrf.eu/pub/smis2.esrf.fr/20533_A.pdf

Zinc distribution and speciation in roots of various genotypes of tobacco exposed to Zn

Mycorrhizae are symbiotic associations between soil fungi and plant roots, which enhance mineral nutrition for the plant, and might play an important role in metals acquisition and accumulation. The processes allowing metals mobilization in the soil, absorption by the root and/or the fungus, transfer or bioaccumulation are still poorly understood. However, the properties of mycorrhizal fungi could be used for phytoremediation, a soft technique using plants for the clean-up of metal polluted soils. In this work, mycorrhized roots of tomato plants grown in a Zn-contaminated soil were investigated. The distribution of metals and the speciation of Zn were studied at the micron scale using micro synchrotron-based X-ray fluorescence (pSXRF) and micro X-ray absorption spectroscopy (μEXAFS). Zn associated to the root was Zn malate and/or Zn citrate, and Zn associated to the fungus was Zn phyllosilicate. This study illustrates the great potential of X-ray microbeams for the study of biological samples containing various amounts of metals.

Localization an speciation of Zn in mycorrhized roots by μSXRF and μEXAFS

Mycorrhizae are symbiotic associations between soil fungi and plant roots, which enhance mineral nutrition for the plant, and might play an important role in metals acquisition and accumulation. The processes allowing metals mobilization in the soil, absorption by the root and/or the fungus, transfer or bioaccumulation are still poorly understood. However, the properties of mycorrhizal fungi could be used for phytoremediation, a soft technique using plants for the clean-up of metal polluted soils. In this work, mycorrhized roots of tomato plants grown in a Zn-contaminated soil were investigated. The distribution of metals and the speciation of Zn were studied at the micron scale using micro synchrotron-based X-ray fluorescence (μSXRF) and micro X-ray absorption spectroscopy (μEXAFS). Zn associated to the root was Zn malate and/or Zn citrate, and Zn associated to the fungus was Zn phyllosilicate. This study illustrates the great potential of X-ray microbeams for the study of biological samples containing various amounts of metals.

https://hal.archives-ouvertes.fr/hal-00105731/document

Localization and speciation of Zn in mycorrihizd roots by μSXRF and μEXAFS.

http://ftp.esrf.eu/pub/UserReports/22956_B.pdf

Nicolas Geoffroy

Papers

Chemical forms of selenium in the metal-resistant bacterium Ralstonia metallidurans CH34 exposed to selenite and selenate.

Zinc distribution and speciation in roots of various genotypes of tobacco exposed to Zn

Localization an speciation of Zn in mycorrhized roots by μSXRF and μEXAFS

Localization and speciation of Zn in mycorrihizd roots by μSXRF and μEXAFS.

Names and affiliations of applicants (* indicates experimentalists):