3-D fluorescence spectroscopy; A lexandrium catenella ;exudation;fluorescent dissolved organic matter; PARAFAC ;trace metals
We investigated the effects of zinc or lead on growth and on exudation of fluorescent dissolved organic matter (FDOM) by the marine toxic dinoflagellate Alexandrium catenella (Whedon & Kofoid) Balech. The species was exposed to increasing free zinc (1.34 × 10−7 M–3.98 × 10−6 M) or lead (5.13 × 10−9 M–1.82 × 10−7 M) concentra-tions. Low metal levels ([Zn2+] = 1.34 × 10−7 M; [Pb2+] = 5.13 × 10−9 M) had no effect on cell growth. Toxic effects were observed from higher metal contamination ([Zn2+] = 3.98 × 10−6 M; [Pb2+] = 6.54 × 10−8 M), as a conversion of vegetative cells into cysts. Analysis of the released FDOM by three-dimensional (3-D) fluorescence spectroscopy was achieved, using the parallel factor analysis (PARAFAC). The PARAFAC modeling revealed four components associated with two contributions: one related to the biological activity; the other linked to the organic matter decomposition in the culture medium. The C1 component combined a tryptophan peak and characteristics of humic substances, whereas the C2 component was considered as a tryptophan protein fluorophore. The two others C3 and C4 components were associated with marine organic matter production. Relea-sed fluorescent substances were induced by low ([Zn2+]= 1.34 × 10−7 M; [Pb2+] = 5.13 × 10−9 M) and moderate ([Zn2+] = 6.21 × 10−7 M; [Pb2+] = 2.64× 10−9 M) metal concentrations, suggesting the activation of cellular mechanisms in response to metal stress, to exudate FDOM that could complex metal cations and reduce their toxicity toward A. catenella cells.