Fluorescence 3D de la matière organique dissoute du fleuve amazone: (Three-dimensional fluorescence of the dissolved organic carbon in the Amazon river)

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.