Three-dimensional fluorescence of the dissolved organ carbon in the AmazonRiver

Natural organic matter is an important pool that is not yet totally describ ed. Two types of compounds are found: some chemically well characterised mo lecules (biopolymers) and uncharacterised humic substances (geopolymers). T he spectroscopic properties of this pool of organic matter have recently be en advanced by the excitation emission fluorescence matrix (EEFM) [Coble, G . P. (1996) Characterization of marine and terrestrial DOM in seawater usin g excitation-emission matrix spectroscopy. Marine Chemistry 51, 325-346.]. Three types of fluorophores are described by their excitation/emission wave length at maximum intensity (lambda(ex)/lambda(em))(max): the tyrosin and t ryptophan like structures, not examined here, and the humic like fluorescen t structures of type A (lambda(260)/lambda(445)) and type C (lambda(330)/la mbda(445)) The EEFM applied to sequential tangential ultrafiltered (UFTS) a mazonian fresh waters give spectroscopic information on the fluorescent pro perties of particulate (>0.22 mu m), colloidal and dissolved (< 5 kDa) orga nic matter. Chromophores A and C are present in all sized fraction samples. Their (lambda(ex)/lambda(em))(max) are in the same domains as those of ter restrial 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 us ing excitation-emission matrix spectroscopy, Marine Chemistry 51, 325-346.] . Differentiation of the type A and type C peaks in 3D diagrams are based o n their (lambda(ex)/lambda(em))(max) position and the I-a/I-c ratio. Differ ences are observed between humic material extracted by hydrophobic resins o r concentrated sample from tangential sequential ultrafiltration (UFTS). Sp ectroscopic properties of the humic material are not modified by the ultraf iltration process. A particular attention is given on the differentiation b etween black water (Rio Negro River) and white water (Rio Solimoes and Rio Madeira River). Black waters are generally known as humic rich and low mine ral content waters. EEFM analysis on these two types of water lead to the f ollowing conclusions: (i) the humic like fluorescent compounds of type C ar e preferentially retained by membranes with 5 kDa cut off. These compounds have the larger molecular weight. (ii) In the presence of copper cation, th e type C compounds are divided in two groups according to their (lambda(ex) /lambda(em))(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 f ulvic acids while C fluorophores seems to be more related to humic acid. (C ) 1999 Elsevier Science Ltd. All rights reserved.