Hydrologie et géochimie isotopique

Cet ouvrage rassemble des travaux récents dans les domaines de l'hydrologie et de la géochimie isotopique. La première partie explore le cycle des eaux naturelles, depuis les précipitations jusqu'aux aquifères profonds, par l'analyse des variations de la composition isotopique de l'hydrogène, de l'oxygène et du carbone. Des types régionaux sont présentés, sous diverses latitudes : Spitzberg, Bassin parisien, bassin soudano-sahélien du Niger. L'intérêt de l'approche isotopique dans l'étude du changement global est illustrée en deuxième partie. Les variations hydro-climatiques de types régionaux, en Afrique essentiellement, sont retracées par l'analyse multidisciplinaire d'enregistrements sédimentaires, dont la géochimie isotopique constitue la clé essentielle. L'homogénéité du refroidissement, aux moyennes et basses latitudes, lors du dernier maximum glaciaire est démontrée par l'analyse isotopique des gaz nobles dans les eaux souterraines. La mise en évidence de l'origine organique du carbone de carbonates sédimentaires remet en question l'interprétation de certaines analyses isotopiques. La troisième partie fait référence aux isotopes d'origine cosmique et à la production in situ de radionucléides. Il est rappelé que la datation par 14C a contribué à la compréhension des mécanismes des changements du climat au cours du dernier maximum glaciaire et depuis lors. Les limites de cette méthode et ses progrès récents font l'objet d'une synthèse amplement documentée. Par l'apport de données isotopiques, la géochimie s'avère fonder la modélisation des conditions de stabilité du réacteur de fission nucléaire fossile d'Oklo. (Résumé d'auteur

Inflammasome Sensor Nlrp1b-Dependent Resistance to Anthrax Is Mediated by Caspase-1, IL-1 Signaling and Neutrophil Recruitment

Bacillus anthracis infects hosts as a spore, germinates, and disseminates in its vegetative form. Production of anthrax lethal and edema toxins following bacterial outgrowth results in host death. Macrophages of inbred mouse strains are either sensitive or resistant to lethal toxin depending on whether they express the lethal toxin responsive or non-responsive alleles of the inflammasome sensor Nlrp1b (Nlrp1bS/S or Nlrp1bR/R, respectively). In this study, Nlrp1b was shown to affect mouse susceptibility to infection. Inbred and congenic mice harboring macrophage-sensitizing Nlrp1bS/S alleles (which allow activation of caspase-1 and IL-1β release in response to anthrax lethal toxin challenge) effectively controlled bacterial growth and dissemination when compared to mice having Nlrp1bR/R alleles (which cannot activate caspase-1 in response to toxin). Nlrp1bS-mediated resistance to infection was not dependent on the route of infection and was observed when bacteria were introduced by either subcutaneous or intravenous routes. Resistance did not occur through alterations in spore germination, as vegetative bacteria were also killed in Nlrp1bS/S mice. Resistance to infection required the actions of both caspase-1 and IL-1β as Nlrp1bS/S mice deleted of caspase-1 or the IL-1 receptor, or treated with the Il-1 receptor antagonist anakinra, were sensitized to infection. Comparison of circulating neutrophil levels and IL-1β responses in Nlrp1bS/S,Nlrp1bR/R and IL-1 receptor knockout mice implicated Nlrp1b and IL-1 signaling in control of neutrophil responses to anthrax infection. Neutrophil depletion experiments verified the importance of this cell type in resistance to B. anthracis infection. These data confirm an inverse relationship between murine macrophage sensitivity to lethal toxin and mouse susceptibility to spore infection, and establish roles for Nlrp1bS, caspase-1, and IL-1β in countering anthrax infection

A dated phylogeny and collection records reveal repeated biome shifts in the African genus Coccinia (Cucurbitaceae)

Background: Conservatism in climatic tolerance may limit geographic range expansion and should enhance the effects of habitat fragmentation on population subdivision. Here we study the effects of historical climate change, and the associated habitat fragmentation, on diversification in the mostly sub-Saharan cucurbit genus Coccinia, which has 27 species in a broad range of biota from semi-arid habitats to mist forests. Species limits were inferred from morphology, and nuclear and plastid DNA sequence data, using multiple individuals for the widespread species. Climatic tolerances were assessed from the occurrences of 1189 geo-referenced collections and WorldClim variables. Results: Nuclear and plastid gene trees included 35 or 65 accessions, representing up to 25 species. The data revealed four species groups, one in southern Africa, one in Central and West African rain forest, one widespread but absent from Central and West African rain forest, and one that occurs from East Africa to southern Africa. A few individuals are differently placed in the plastid and nuclear (LFY) trees or contain two ITS sequence types, indicating hybridization. A molecular clock suggests that the diversification of Coccinia began about 6.9 Ma ago, with most of the extant species diversity dating to the Pliocene. Ancestral biome reconstruction reveals six switches between semi-arid habitats, woodland, and forest, and members of several species pairs differ significantly in their tolerance of different precipitation regimes. Conclusions: The most surprising findings of this study are the frequent biome shifts (in a relatively small clade) over just 6 - 7 million years and the limited diversification during and since the Pleistocene. Pleistocene climate oscillations may have been too rapid or too shallow for full reproductive barriers to develop among fragmented populations of Coccinia, which would explain the apparently still ongoing hybridization between certain species. Steeper ecological gradients in East Africa and South Africa appear to have resulted in more advanced allopatric speciation there

The palaeolimnological record from lake Cullulleraine, lower Murray River (south-east Australia): implications for understanding riverine histories

Australia’s largest river system, the Murray-Darling Basin, is the focus of scientific and political attention, due mainly to the competing issues of economic productivity versus environmental flows. Central to this dialogue is the need to know about the Basin’s natural condition and the degree to which the system has deviated from this pre-disturbance, baseline status. This study examines the patterns of ecological change in Lake Cullulleraine, a permanently connected artificial wetland adjacent to Lock Nine on the Murray River, south-east Australia. A 43-cm sediment core was collected in January 1998 and diatoms were analysed at 1-cm intervals for use as aquatic ecological indicators. The sediment core was dated using ²¹⁰Pb. Changes in the diatom community have occurred since the time of lake formation in 1926, particularly shifts between Aulacoseira subborealis, Staurosira construens var. venter, Aulacoseira granulata, Staurosirella pinnata and Pseudostaurosira brevistriata. An electrical conductivity (EC) transfer function was applied to the fossil diatom assemblages and inferred EC values were compared to long-term, historical EC data from the River. Despite the presence of good analogues between fossil and modern diatom assemblages, inferred EC did not reflect measured EC accurately. In recent decades, patterns in the two data sets were reversed. Despite clear changes in the fossil record, quantitative palaeo-environmental interpretation was limited because the dominant taxa occupy broad ecological niches. Despite these limitations, changes in the Lake Cullulleraine record, particularly in the planktonic taxa, can be interpreted in terms of landscape change. Furthermore, because of the good chronology from the site, the record may be useful for dating changes observed in sites with poor chronological control.Jennie Fluin, John Tibby and Peter Gel