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precursor which is necessary for the biosynthesis of chlorophylls, but this hypotliesis has not yet been verified cxperimentally. In higher terrestrial plants, the first visible signs of glyphosate exposure arc the chlorosis of the leavcs, created by oxidative events, which damages chloroplasts membranes (Kitchen et al, 1981; Zaidi et al, 2005). In our study, , increasing concentrations of glyphosate resulted in reduccd phytoplankton community chlorophyll a and carotenoid content, but triggered no detectable effect on the chlorophyll b contcnt. The destruction of the membranę by reactive oxygcn species, thus rcducing the chloroplast integrity and function, would have also reduccd the chlorophyll b conccntration sińce these pigments are locatcd in the thylakoid membranes of chloroplast along willi the other photosynthetic pigments. In our study there was no significant decrease of the total chlorophyll b concentration in the phytoplankton community.

However, lipid peroxidation did occur at the highest glyphosate exposure and the antioxidant response was activated as shown by the inereased amount of antioxidant enzymes and activity, expressing the presence of oxidative events affecting the cells integrity, but this phenomenon cannot explain by itself the changes of concentrations of total pigments. Wong (2000) determined that a glyphosate concentration of 20 mg I*¹ and higher could affect the chlorophyll a synthesis in the green alga Scenedesmus ąuadricauda Berb 614. Our study highlights the effects of increasing concentrations of glyphosate on the phytoplankton community, not only on chlorophyll a concentrations, but also on carotenoid content, at Iow glyphosate concentrations. The global chlorophyll a and carotcnoids concentrations were significantly reduced when exposed to glyphosate concentrations of 1 pg l'¹ and above, while there was no change of the global concentration of chlorophyll b.

The phytoplankton community structure was also different than from the control treatment, at glyphosate-based herbicide concentrations as Iow as 5 pg I'¹ with the apparition of new dominant species, such as Nitzschia palea, Kateblepharis ovalis, Chromulina elegans and Chlamydomonas dinobryoni, from 50 pg I*¹. The dominant phytoplankton groups of the community were also modified, sińce the dominance of the Chlorophyceae class in the control treatment was replaced by the Cryptophyceae, Crysophyceae and Bacillariophyceue classes at higher glyphosate concentrations. These results imply that some phytoplankton species might be resistant to glyphosate-based herbicides. However, it is also possible to

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