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Fluorescence 3D de la matière organique dissoute du fleuve amazone

1999; Elsevier BV; Volume: 33; Issue: 6 Linguagem: Inglês

10.1016/s0043-1354(98)00347-9

1879-2448

Stéphane Mounier, Nikhil Patel, L. Quilici, J.Y Benaïm, C Benamou,

Isotope Analysis in Ecology

Les eaux de fleuve, fractionnées par ultrafiltration tangentielle séquentielle (UFTS), sont analysées par des matrices d'excitation et d'émission de fluorescence (MEEF). Les substances humiques fluorescentes provenant des eaux du bassin amazonien, sont caractérisées par les longueurs d'onde d'excitation et d'émission des intensités de fluorescence maximum sur des diagrammes 3D (λex/λem) et du rapport des intensités de fluorescence (ra,c=Ia/Ic) de deux fluorophores (type humique A et C) dans les eaux naturelles. Les résultats obtenus montrent que: (i) des espèces fluorescentes provenant d'eaux riches en matériel humique (eaux noires) sont préférentiellement retenues par le seuil de coupure de 5 kDa (type humique C). Ces composés de type C ont une masse moléculaire plus élevée que les composés fluorescents de type humique A. (ii) Les réactions de complexation avec les ions cuivriques (Cu2+) séparent les composés humiques de type C en deux catégories: l'une ne variant pas, l'autre élargissant le domaine des longueurs d'onde d'excitation efficaces de 20 nm. La photolyse produit des effets similaires sur les composés humiques de type C. A partir des données spectroscopiques et des masses moléculaires, une tentative d'identification est proposée pour les fluorophores A (λ260/λ445) et C (λ350/λ445). Les composés de type A et C manifestent des caractéristiques qui les rapprochent respectivement des acides fulviques et des acides humiques. Natural organic matter is an important pool that is not yet totally described. Two types of compounds are found: some chemically well characterised molecules (biopolymers) and uncharacterised humic substances (geopolymers). The spectroscopic properties of this pool of organic matter have recently been advanced by the excitation emission fluorescence matrix (EEFM) [Coble, G. P. (1996) Characterization of marine and terrestrial DOM in seawater using excitation-emission matrix spectroscopy. Marine Chemistry 51, 325–346.]. Three types of fluorophores are described by their excitation/emission wavelength at maximum intensity (λex/λem)max: the tyrosin and tryptophan like structures, not examined here, and the humic like fluorescent structures of type A (λ260/λ445) and type C (λ330/λ445). The EEFM applied to sequential tangential ultrafiltered (UFTS) amazonian fresh waters give spectroscopic information on the fluorescent properties of particulate (>0.22 μm), colloidal and dissolved (<5 kDa) organic matter. Chromophores A and C are present in all sized fraction samples. Their (λex/λem)max are in the same domains as those of terrestrial humic substances [Seritti, A., Morelli, E., Nannicini, L. and Del Vecchio, R. (1994) Production of hydrophobic fluorescent organic matter by marine diation Pheodactylum tricornutum. Chemosphere 28(1), 117–129; Coble, G. P. (1996) Characterization of marine and terrestrial DOM in seawater using excitation-emission matrix spectroscopy. Marine Chemistry 51, 325–346.]. Differentiation of the type A and type C peaks in 3D diagrams are based on their (λex/λem)max position and the Ia/Ic ratio. Differences are observed between humic material extracted by hydrophobic resins or concentrated sample from tangential sequential ultrafiltration (UFTS). Spectroscopic properties of the humic material are not modified by the ultrafiltration process. A particular attention is given on the differentiation between black water (Rio Negro River) and white water (Rio Solimões and Rio Madeira River). Black waters are generally known as humic rich and low mineral content waters. EEFM analysis on these two types of water lead to the following conclusions: (i) the humic like fluorescent compounds of type C are preferentially retained by membranes with 5 kDa cut off. These compounds have the larger molecular weight. (ii) In the presence of copper cation, the type C compounds are divided in two groups according to their (λex/λem)max: the first one is invariant, the other one expands (20 nm) its efficient excitation wavelength domain. (iii) Photochemical reactions induced by UV irradiation also lead to a 20 nm expansion of the efficient excitation wavelength domain. From these spectroscopic and molecular weight complementary data, it is proposed that A type fluorophores are close to fulvic acids while C fluorophores seems to be more related to humic acid.