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think that resistant species were best adapted to the test conditions (light and temperaturę), explaining their dominance in the community when a stressor (herbicide) was added to the microcosms. Indeed, cyanobacteria, for example, were almost absent of the sampled phytoplankton community, a phenomenon that could be explained by the Iow temperaturę at the sampling period (15°C), sińce they prefer warmer waters, while diatoms generally prefer cold waters. Berard and Benninghoff (2001) found that, in addition to atrazine, seasonal factors would influence the species composition in phytoplankton community, cyanobacteria being morę present in summer and diatoms in spring.

An inerease of the shikimate content was verified after 96 hours of exposure to glyphosate-based herbicide, at high glyphosate concentrations of 500 and 1000 pg I¹, meaning that the shikimate pathway was inhibited by glyphosate, leading to an accumulation of shikimate-3-phosphate in chloroplasts. This result also proves that the shikimate content can be used as a biomarker of the effects of glyphosate-based herbicides on phytoplankton communities. To our knowledge, the accumulation of shikimate in phytoplankton cells was never verify experimentally.

Along with this specific inhibition target site, the glyphosate action also leads to oxidative events in plants which are most probably the secondary effect of the blocked shikimate pathway (Ahsan et al2008). Among the enzymatic Systems, ROS-scavenging enzymes are frequently used as indicators of oxidative stress (Gunes et al., 2007), along with MDA (Mittler, 2002). Although changes in these oxidative stress markers have been reported in . plants exposed to glyphosate (Kielak et al, 2011; Miteva et al, 2010), little information is currently available conceming the effects of glyphosate on phytoplankton oxidative events. We found that, at high concentrations, glyphosate induced oxidative stress, as measured by lipid peroxidation. However, we noticed that some phytoplankton species (Nitzschia palea, Kateblepharis ovalis, Chromulina elegans and Chlamydomonas dinobryoni) were present in these samples indicating that these species developed some tolerance which could be associated to the activation of antioxidant systems. In fact, we verificd inereased activity of antioxidant enzymes (SOD, CAT and APX) in phytoplankton exposed to high glyphosate concentrations. SOD is the major scavenger of supcroxide anions (0₂*‘) and provides the first linę of defense against celi injury by environmental factors (Gratao et al., 2005). Hydrogen