The toxicity of selenium in aquatic ecosystems is mainly linked to its uptake and biotransformation by micro-organisms, and its subsequent transfer upwards into the food chain. Thus, organisms at low trophic level, such as algae, play a crucial role. The aim of our study was to investigate the biological effects of selenite on Chlamydomonas reinhardtii, both at the sub-cellular level (effect on ultrastructure) and at the population level (effect on growth). The cells were grown under batch culture conditions in well-defined media and exposed to waterborne selenite at concentrations up to 500 microM; i.e. up to lethal conditions. Based on the relationship between Se concentration and cell density achieved after a 96 h exposure period, an EC50 of 80microM with a 95% confidence interval ranging between 64 and 98 microM was derived. No adaptation mechanisms were observed: the same toxicity was quantified for algae pre-contaminated with Se. The inhibition of growth was linked to impairments observed at the sub-cellular level. The intensity of the ultrastructural damages caused by selenite exposure depended on the level and duration of exposure. Observations by TEM suggested chloroplasts as the first target of selenite cytotoxicity, with effects on the stroma, thylakoids and pyrenoids. At higher concentrations, we could observe an increase in the number and volume of starch grains.
For cells collected at 96 h, electron-dense granules were observed. Energy-dispersive X-ray microanalysis revealed that these granules contained selenium and were also rich in calcium and phosphorus. This study confirms that the direct toxicity of selenite on the phytoplankton biomass is not likely to take place at concentrations found in the environment. At higher concentrations, the link between effects at the sub-cellular and population levels, the overaccumulation of starch, and the formation of dense granules containing selenium are reported for the first time in the literature for a phytoplankton species after exposure to selenite.
a - Laboratoire de Radioecologie et Ecotoxicologie, Institut de Radioprotection et Surete Nucleaire, Cadarache,
Bat 186, BP 3, 13115 Saint-Paul-lez-Durance cedex, France
b - INRS-Eau, Terre et Environnement, Universite du Quebec, 490, rue de la Couronne, Que., Canada G1K 9A9
c - Laboratoire d’Ecophysiologie et Ecotoxicologie des Systemes Aquatiques, Universite Bordeaux 1, 33120 Arcachon, France