Chemical characterization of exudates, and study of the proteomic response from two marine phytoplankton species exposed to trace metals : the diatom Skeletonema costatum and the toxic dinoflagellate Alexandrium catenella
Mpact of several trace metals (lead, zinc, copper or cadmium) on the growth of two marine phytoplankton species: the diatom Skeletonema costatum and the toxic dinoflagellate Alexandrium catenella, has been studied from cultures grown under physiological and contaminated conditions. The results obtained revealed growth perturbations in S. costatum and in A. catenella, when these species were exposed to high metal concentrations. The lethal effects appeared through a strong cell death, coming, in A. catenella, with the conversion of vegetative cells into resistance cysts. Then, exudation of dissolved organic matter (DOM) by S. costatum and by A. catenella in response to metal stress has been characterized. Exudation of dissolved organic carbon (DOC) varied according to the growth phase, and was associated with the level of metal contamination and the nature of the tested metal. The maximal values ​​of DOC exudated per cell were measured in response to contamination by copper 16 µM during the adaptation phase, and in response to contamination by cadmium 200 µM during the exponential phase of growth. More specifically, the fluorescent dissolved organic matter (FDOM) exudated by S. costatum and by A. catenella has been analyzed by 3D fluorescence spectroscopy combined with PARAFAC algorithm. The PARAFAC treatments revealed four components, which were attributed to two main contributions, one related to the biological activity of the species, and the other related to the decomposition of organic matter. The C1 and C2 components were combined with tryptophan peaks and with humic substances, while the C3 and C4 components were generally associated with the production of marine organic matter. Moreover, glucose and galactose were the main aldoses among the polysaccharides exudated in the DOM. Proteomic changes occurred in the proteomes of A. catenella in response to the various metal stresses. Stress proteins expressed by A. catenella were obtained by comparison of the protein expression profiles (PEPs) achieved by two-dimensional electrophoresis (2D - electrophoresis) under physiological and contaminated conditions. Stress proteins are involved in many functional categories: response to oxidative stress (superoxide dismutase, proteasome sub - units), photosynthesis (ribulose 1,5- bisphosphate carboxylase, peridinin chlorophyll protein complex, ferredoxin -NADP reductase), carbohydrate metabolism (triosephosphate isomerase, ribose 5-phosphate isomerase, malate dehydrogenase), photorespiration and phosphorus metabolism (phosphoglycolate phosphatase), energy metabolism (ATP - synthase), cell signalling (calmodulin), chaperone activity (HSP 70, HSP 90) and bioluminescence of A. catenella (luciferin - binding protein). Over - expression of phosphoglycolate phosphatase (PGP) and of ATP – synthase, occurring in response to metal contamination by lead, could represent a strategy of defence set up by A. catenella (i) to protect itself from the oxidative stress due to metal contamination, PGP being involved in the dissipation of excess energy (ii) to produce more reserves of energy (ATP) so as to satisfy the increased requirements resulting from the metal stress, in order to adapt to it.