Effect of selenate on growth and photosynthesis of Chlamydomonas reinhardtii.

Algal communities play a crucial role in aquatic food webs by facilitating the transfer of dissolved inorganic selenium (both an essential trace element and a toxic compound for a wide variety of organisms) to higher trophic levels. The dominant inorganic chemical species of selenium in freshwaters are selenite (SeO(3)(2-)) and selenate (SeO(4)(2-)). At environmental concentrations, selenite is not likely to have direct toxic effects on phytoplankton growth [Morlon, H., Fortin, C., Floriani, M., Adam, C., Garnier-Laplace, J., Boudou, A., 2005a. Toxicity of selenite in the unicellular green alga Chlamydomonas reinharditii: comparison between effects at the population and sub-cellular level. Aquat. Toxicol. 73(1), 65-78]. The effects of selenate, on the other hand, are poorly documented. We studied the effects of selenate on Chlamydomonas reinhardtii growth (a common parameter in phytotoxicity tests). Growth inhibition (96-h IC(50)) was observed at 4.5+/-0.2 microM selenate (p<0.001), an effective concentration which is low compared to environmental concentrations. Growth inhibition at high selenium concentrations may result from impaired photosynthesis. This is why we also studied the effects of selenate on the photosynthetic process (not previously assessed in this species to our knowledge) as well as selenate's effects on cell ultrastructure. The observed ultrastructural damage (chloroplast alterations, loss of appressed domains) confirmed that chloroplasts are important targets in the mechanism of selenium toxicity. Furthermore, the inhibition of photosynthetic electron transport evaluated by chlorophyll fluorescence induction confirmed this hypothesis and demonstrated that selenate disrupts the photosynthetic electron chain. Compared to the classical 'growth inhibition' parameter used in phytotoxicity tests, cell diameter and operational photosynthetic yield were more sensitive and may be convenient tools for selenate toxicity assessment in non-target plants.

Determination of photosynthetic and enzymatic biomarkers sensitivity used to evaluate toxic effects of copper and fludioxonil in alga Scenedesmus obliquus.

Modulated PAM fluorometry and Plant Efficiency Analyser methods were used to investigate photosynthetic fluorescence parameters of alga Scenedesmus obliquus exposed to inhibitory effect of fungicides copper sulphate and fludioxonil (N-(4-nitrophenyl)-N'-propyl-uree). The change of those parameters were studied when alga S. obliquus have been exposed during 48 h to different concentrations of fungicides (1, 2 and 3 mgl(-1)). Under the same condition, enzymatic activities of catalase, ascorbate peroxidase, glutathione reductase and glutathione S-transferase were investigated to evaluate antioxidative response to fungicides effects. The change of sensitivity of those parameters was dependent to the mode of fungicide action, their concentration and time of exposure. For copper effects, the most indicative photosynthetic biomarkers were parameters Q(N) as non-photochemical fluorescence quenching, Q(Emax) as the proton induced fluorescence quenching and ABS/RC as the antenna size per photosystem II reaction center. Copper induced oxidative stress was indicated by increased activity of catalase serving as the most sensitive and valuable enzymatic biomarker. On the other hand, fludioxonil effect on photosynthetic parameters was very negligible and consequently not very useful as biomarkers. However, fludioxonil induced strong antioxidative activities associated with cytosol enzymes, as we found for catalase, ascorbate peroxidase and glutathione S-transferase activities. By obtained results, we may suggest for the activation of those enzymes to be sensitive and valuable biomarkers of oxidative stress induced by fludioxonil. Determination of biomarkers sensitivity may offer advantages in providing real criteria to use them for ecotoxicological diagnostic studies.

Comparison of different physiological parameter responses in Lemna minor and Scenedesmus obliquus exposed to herbicide flumioxazin.

The sensitivity of different physiological parameters in Scenedesmus obliquus and Lemna minor exposed to herbicide (flumioxazin) was investigated to indicate the most convenient and sensitive parameter. To assess toxicity of flumioxazin, we used a panel of biomarkers: pigment contents, chlorophyll fluorescence parameters and antioxidative enzyme activities. Algae and duckweed were exposed to 48-h IC50 for growth rate. In L. minor, the sensitivity of the parameters was as follows: QN > Oxygen emmision > phiS(PSII) > QP > phi(PSII) > CAT, GR > Pigment> APO > Growth. For S. obliquus, this ranking was as follows: CAT > Oxygen emission > QP > APO > GR > Pigment > phiS(PSII) > Growth > phi(PSII) > QN (from the greatest to the least sensitive). The results demonstrated that the observed toxicity is related not only to interspecific variations but also to the selected parameter.