Effects of radionuclide contamination on leaf litter decomposition in the Chernobyl exclusion zone

The effects of radioactive contamination on ecosystem processes such as litter decomposition remain largely un- known. Because radionuclides accumulated in soil and plant biomass can be harmful for organisms, the function- ing of ecosystems may be altered by radioactive contamination. Here, we tested the hypothesis that decomposition is impaired by increasing levels of radioactivity in the environment by exposing uncontaminated leaf litter from silver birch and black alder at (i) eleven distant forest sites differing in ambient radiation levels (0.22–15 μGy h−1) and (ii) along a short distance gradient of radioactive contamination (1.2–29 μGy h−1) within a single forest in the Chernobyl exclusion zone. In addition to measuring ambient external dose rates, we estimat- ed the average total dose rates (ATDRs) absorbed by decomposers for an accurate estimate of dose-induced eco- logical consequences of radioactive pollution. Taking into account potential confounding factors (soil pH, moisture, texture, and organic carbon content), the results from the eleven distant forest sites, and from the single forest, showed increased litter mass loss with increasing ATDRs from 0.3 to 150 μGy h−1. This unexpected result may be due to (i) overcompensation of decomposer organisms exposed to radionuclides leading to a higher decomposer abundance (hormetic effect), and/or (ii) from preferred feeding by decomposers on the un- contaminated leaf litter used for our experiment compared to locally produced, contaminated leaf litter. Our data indicate that radio-contamination of forest ecosystems over more than two decades does not necessarily have detrimental effects on organic matter decay. However, further studies are needed to unravel the underlying mechanisms of the results reported here, in order to draw firmer conclusions on how radio-contamination affects decomposition and associated ecosystem processes

Etudes préliminaires sur la production d'acides gras cyclopropanes, par génie génétique, dans les embryons de lin

Les acides gras cycliques forment après une hydrogénation des acides gras méthylés qui confèrent des propriétés lubrifiantes aux huiles végétales. Pour produire des acides gras cycliques dans les graines de lin, le gène Ec-cfas codant pour l'acide gras cyclopropane synthase pourrait être exprimé sous le contrôle d'un promoteur graine-spécifique, le promoteur napin ou le promoteur At-FAEl. La transformation génétique du lin a d'abord été adaptée aux cultivars Oliver et Barbara. Les efficacités de transformation sont restées faibles (<1% ) mais 16 lignées transgéniques ont été régénérées. Ensuite, la fonctionnalité des deux promoteurs a été étudiée via le gène gus. Le promoteur At-FAE1 montre une activité maximale dans les embryons âgés de 16 à 18 jours. Enfin, l'activité de la protéine Ec-CFA a été montrée chez Arabidopsis par la production d'acides gras cyclopropanes. Chez le lin, la synthèse de ces acides gras inhabituels doit dépendre de facteurs à la fois génétiques et métaboliques.Cyclic fatty acids form after hydrogenation of carbon chains methyl-branched fatty acids conferring excellent lubricating properties on vegetable oils. ln order to produce cyclic fatty acids in linseed, the Ec-cfas gene encoding a fatty acid cyclopropan synthase could be expressed under the control of seed-specific promoter, the napin promoter or the At-FAE1 promoter. First, flax transformation was adapted to cultivars Barbara and Oliver. Whereas the efficiencies of genetic transformation remain low (<1%), 16 transgenic lines were regenerated. Then, the functionality of both promoters was studied using the gus gene. Both promoters specifically expressed transgene in embryos. The At-FAE1 promoter showed a maximal activity in 16 to 18 day-old embryos. Finally, the activity of Ec-CFA protein was showed in Arabidopsis by the production of cyclopropan fatty acids. In linseed, the synthesis of such unusual fatty acids could depend on various genetic as well as metabolic factors.COMPIEGNE-BU (601592101) / SudocSudocFranceF

Effects of waterborne uranium on survival, growth, reproduction and physiological processes of the freshwater cladoceran Daphnia magna

International audienceAcute uranium toxicity (48 h immobilisation test) for Daphnia magna was determined in two different exposure media, differing in pH and alkalinity. LC50 varied strongly between media, from 390 +/- 40 mu g L-1 U at pH 7 to 7.8 +/- 3.2 mg L-1 U at pH 8. According to the free ion activity model uranium toxicity varies as a function of free uranyl concentration. This assumption was examined by calculating uranium speciation in our water conditions and in those reported in the literature. Predicted changes in free uranyl concentration could not solely explain observed differences in toxicity, which might be due to a competition or a non-competitive inhibition of H+ for uranium transport and/or the involvement of other bioavailable chemical species of uranium.Chronic effects of uranium at pH 7 on mortality, ingestion and respiration, fecundity and dry mass of females, eggs and neonates were investigated during 21-day exposure experiments. A mortality of 10% was observed at 100 mu g L-1 U and EC50 for reproduction was 14 +/- 7 mu g L-1 U. Scope for growth was affected through a reduction in feeding activity and an increase in oxygen consumption at 25 mu g L-1 U after 7 days of exposure. This had strong consequences for somatic growth and reproduction, which decreased, respectively, by 50% and 65% at 50 mu g L-1 U after 7 days and at 25 mu g L-1 U after 21 days. Uranium bioaccumulation was quantified and associated internal alpha dose rates from 2.1 to 13 mu Gy h(-1) were estimated. Compared to the toxicity of other alpha-emitting radionuclides and stable trace metals, our results confirmed the general assumption that uranium chemical toxicity predominates over its radiotoxicity