Rôle des acides humiques dans le transfert du pyrène entre les minéraux argileux et l'eau

Les substances humiques se fixent en surface des minéraux argileux et modifient les sites d'adsorption des polluants organiques hydrophobes (POH). Parallèlement, les substances humiques dissoutes complexent les POH non ioniques selon des mécanismes de liaison réversible généralement évalués par le coefficient de partage Koc. Les solutés humiques seraient ainsi responsables du transport des POH dans les écosystèmes aquatiques par solubilisation de la fraction adsorbée sur le sédiment. Notre étude a pour objectif la quantification et la modélisation de ces phénomènes pour des composés modèles disponibles commercialement : kaolinite, pyrène et acide humique Aldrich purifié.L'isotherme d'adsorption de l'acide humique (AH) sur la kaolinite à pH 6,5 et à force ionique 10-2 M suit une loi de Freundlich, dont les valeurs du modèle sont KF =0,79 et nF =1,90. Dans les mêmes conditions expérimentales, le pyrène, composé modèle à quatre noyaux du groupe des hydrocarbures aromatiques polycycliques (HAP), semble s'adsorber en formant des microcristaux à la surface de l'argile. La présence d'acide humique fixé sur la kaolinite modifie l'adsorption du pyrène, qui se modélise par une isotherme de Freundlich de paramètres K'F =0,30 et n'F =2,00.Les interactions entre le pyrène et l'acide humique dissous ont été quantifiées par spectroscopie d'extinction de fluorescence et relation de Stern-Volmer. La fraction humique dissoute non adsorbée sur le minéral argileux se caractérise par de plus fortes valeurs de Koc que l'acide humique initial (2,95.105 L.kg-1 contre 1,86.105 L.kg-1). L'hypothèse de fractionnement des substances humiques lors de l'adsorption sur l'argile est confirmée par les variations du poids moléculaire moyen des AH, déterminé par chromatographie d'exclusion stérique, et des variations des intensités de fluorescence des fractions chromatographiées.Les résultats obtenus pour la quantification du pyrène désorbé du solide argileux dans l'eau pure et dans une solution d'AH (50 mg.L-1) tendent à minimiser le rôle de la matière organique naturelle dissoute dans le transfert des polluants hydrophobes du milieu solide au milieu liquide.Humic substances become fixed to the surface of clay minerals and modify the adsorption sites of hydrophobic organic pollutants (HOP). The dissolved humic substances complex the HOP according to reversible binding mechanisms normally assessed by the Koc distribution coefficient. The humic solutes could thus be responsible of transporting HOP in aquatic ecosystems by solubilising the adsorbed fraction on sediment. Our study aims to quantify and model these phenomena using commercially available compounds:- pyrene, a polycyclic aromatic hydrocarbon (PAH) generally used as fluorescence probe and able to bind strongly to humic acid (AH); - a commercially available humic acid (Aldrich), with a high molecular weight and high aromaticity, which can easily adsorb onto colloidal matter and strongly bind PAH; - kaolinite clay, a ubiquitous mineral in aquatic systems, with a particle size distribution in aqueous solution similar to that observed in lake waters.We also have chosen physico-chemical conditions representative of freshwater: pH (6.5) and ionic strength (10-2 M). The adsorption isotherm of the humic acid (HA) on kaolinite at pH=6.5 and at ionic strength 10-2 M follows a Freundlich isotherm, for which model values are fixed at KF =0.79 and nF =1.90. The carboxylic groups of the dissolved HA, which represented more than half the total acidity, were not protonated under these conditions. Despite the negative surface charge of the particles, kaolinite was able to adsorb about 0.2 mgC.g-1 of HA. A decrease in the solution pH occurred as the adsorbed HA quantity increased, suggesting a chemisorption of HA onto clay. The high-pressure size exclusion chromatogram (HPSEC) of the non-adsorbed HA, isolated after centrifugation of the colloidal solution, reflects the selective adsorption of the HA. The higher the molecular weight of the HA (>70 000 Da), the better the adsorption. The hypothesis of the selective adsorption of the humic substances onto clay was also confirmed by the fluorescence variations of the chromatographed fractions.Using the same experimental conditions, pyrene seemed to adsorb and generate micro-crystals on the surface of the clay. The first part of the isotherm fits the pyrene monomer adsorption onto the hydrophobic siloxane surface of the clay. In the second part, the dramatic increase in the adsorption isotherm could be attributed to the formation of pyrene micro-crystals on the solid surface. For a residual pyrene concentration equal to 40 nmol.L-1, the PAH quantity adsorbed onto kaolinite clay (7 nmol.g-1) was about twenty times lower than the quantity adsorbed onto montmorillonite clay (150 nmol.g-1). This could be explained by the inaccessibility of the interlayer cavity of kaolinite to any solute.Natural colloids are often coated by a humic layer. This coating can thus modify HOP adsorption onto the particles. The presence of humic acid fixed on kaolinite effectively modifies pyrene adsorption. This adsorption can be modelled by a Freundlich isotherm with the parameters K'F =0.30 and n'F =2.00. The adsorbed HA molecules seem to limit the formation of pyrene micro-crystals, even if the pyrene adsorption is not competitive with the HA fixation. Indeed, no HA desorption occurs during pyrene fixation onto the organic coated kaolinite.The interactions between pyrene and dissolved humic acid have been quantified by extinction fluorescence spectroscopy and the Stern-Volmer relationship. The unabsorbed, dissolved humic fraction is characterised by Koc values higher than the initial humic acid (2.95 x 105 L.kg-1 compared to 1.86 x 105 L.kg-1). These two values are of the same magnitude as the published value obtained for the non-purified Aldrich HA (2.3±0.3 x 105). The Koc value obtained for the non-adsorbed fraction of HA is unexpected because of the low molecular weight of these molecules. This Koc value can be related either to a better accessibility of the HA hydrophobic cavity for pyrene, or rather to a more efficient fluorescence quenching of pyrene by the humic solutes.The dissolved humic acid may complex the non-ionic PAH according to reversible bonding mechanisms. Therefore, HA can act as a carrier of hydrophobic contaminants if it is able to desorb HOP from the surface of the particles. The results obtained for the quantification of the desorbed pyrene in pure water and in a HA solution (50 mg.L-1) tend to minimise the role of dissolved natural organic matter in the transport of hydrophobic pollutants from the solid to the liquid medium. The values of pyrene concentration in solution obtained after 24 hours contact with kaolinite clay sorbed pyrene with pure water or with the HA solution were very low (respectively 2.4 nM and 12.9 nM) and below the water solubility of pyrene (802 nM). It has been previously demonstrated that the desorption kinetics of pyrene from quartz material are not influenced by the organic matter content of the solution (SCHMITT, 1999). Our results demonstrate that the HA content of the solution did not greatly modify the quantity of pyrene desorbed from clay minerals

Bacillus anthracis diversity in Kruger National Park [South Africa]

The Kruger National Park (KNP), South Africa, has a recorded history of periodic anthrax epidemics causing widespread disease among wild animals. Bacillus anthracis is the causative agent of anthrax, a disease primarily affecting ungulate herbivores. Worldwide there is little diversity among B. anthracis isolates, but examination of variable-number tandem repeat (VNTR) loci has identified six major clones, with the most dissimilar types split into the A and B branches. Both the A and B types are found in southern Africa, giving this region the greatest genetic diversity of B. anthracis worldwide. Consequently, southern Africa has been hypothesized to be the geographic origin of B. anthracis. In this study, the genotypic types of 98 KNP B. anthracis isolates were identified using multiple-locus VNTR analysis. Two major types are evident, the A branch and the B branch. The spatial and temporal distribution of the different genotypes indicates that anthrax epidemic foci are independent, though correlated through environmental cues. Kruger B isolates were found on significantly higher-calcium and higher-pH soils than were Kruger type A. This relationship between genotype and soil chemistry may be due to adaptive differences among divergent anthrax strains. While this association may be simply fortuitous, adaptation of A types to diverse environmental conditions is consistent with their greater geographic dispersal and genetic dissimilarity

Reassessment of the evolution of wheat chromosomes 4A, 5A, and 7B.

Key messageComparison of genome sequences of wild emmer wheat and Aegilops tauschii suggests a novel scenario of the evolution of rearranged wheat chromosomes 4A, 5A, and 7B. Past research suggested that wheat chromosome 4A was subjected to a reciprocal translocation T(4AL;5AL)1 that occurred in the diploid progenitor of the wheat A subgenome and to three major rearrangements that occurred in polyploid wheat: pericentric inversion Inv(4AS;4AL)1, paracentric inversion Inv(4AL;4AL)1, and reciprocal translocation T(4AL;7BS)1. Gene collinearity along the pseudomolecules of tetraploid wild emmer wheat (Triticum turgidum ssp. dicoccoides, subgenomes AABB) and diploid Aegilops tauschii (genomes DD) was employed to confirm these rearrangements and to analyze the breakpoints. The exchange of distal regions of chromosome arms 4AS and 4AL due to pericentric inversion Inv(4AS;4AL)1 was detected, and breakpoints were validated with an optical Bionano genome map. Both breakpoints contained satellite DNA. The breakpoints of reciprocal translocation T(4AL;7BS)1 were also found. However, the breakpoints that generated paracentric inversion Inv(4AL;4AL)1 appeared to be collocated with the 4AL breakpoints that had produced Inv(4AS;4AL)1 and T(4AL;7BS)1. Inv(4AS;4AL)1, Inv(4AL;4AL)1, and T(4AL;7BS)1 either originated sequentially, and Inv(4AL;4AL)1 was produced by recurrent chromosome breaks at the same breakpoints that generated Inv(4AS;4AL)1 and T(4AL;7BS)1, or Inv(4AS;4AL)1, Inv(4AL;4AL)1, and T(4AL;7BS)1 originated simultaneously. We prefer the latter hypothesis since it makes fewer assumptions about the sequence of events that produced these chromosome rearrangements

Electric-Field-Induced Mott Insulating States in Organic Field-Effect Transistors

We consider the possibility that the electrons injected into organic field-effect transistors are strongly correlated. A single layer of acenes can be modelled by a Hubbard Hamiltonian similar to that used for the kappa-(BEDT-TTF)(2)X family of organic superconductors. The injected electrons do not necessarily undergo a transition to a Mott insulator state as they would in bulk crystals when the system is half-filled. We calculate the fillings needed for obtaining insulating states in the framework of the slave-boson theory and in the limit of large Hubbard repulsion, U. We also suggest that these Mott states are unstable above some critical interlayer coupling or long-range Coulomb interaction.Comment: 9 pages, 7 figure

Microtubules in Bacteria: Ancient Tubulins Build a Five-Protofilament Homolog of the Eukaryotic Cytoskeleton

Microtubules play crucial roles in cytokinesis, transport, and motility, and are therefore superb targets for anti-cancer drugs. All tubulins evolved from a common ancestor they share with the distantly related bacterial cell division protein FtsZ, but while eukaryotic tubulins evolved into highly conserved microtubule-forming heterodimers, bacterial FtsZ presumably continued to function as single homopolymeric protofilaments as it does today. Microtubules have not previously been found in bacteria, and we lack insight into their evolution from the tubulin/FtsZ ancestor. Using electron cryomicroscopy, here we show that the tubulin homologs BtubA and BtubB form microtubules in bacteria and suggest these be referred to as “bacterial microtubules” (bMTs). bMTs share important features with their eukaryotic counterparts, such as straight protofilaments and similar protofilament interactions. bMTs are composed of only five protofilaments, however, instead of the 13 typical in eukaryotes. These and other results suggest that rather than being derived from modern eukaryotic tubulin, BtubA and BtubB arose from early tubulin intermediates that formed small microtubules. Since we show that bacterial microtubules can be produced in abundance in vitro without chaperones, they should be useful tools for tubulin research and drug screening