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2009; Kaushik, 2010; Lipok et al, 2010). Cerdergreen and Olsen (2010) used the isopropylamine salt (1PA) formulation of glyphosate, while Wong (2000) used the [N-(phosphonomethyl) glycine] active ingredient. It was demonstrated that glyphosate-based herbicides containing surfactants, such as polyethoxylated tal Iow aminę (POEA) which facilitates the penetration of the herbicide inside the organisms, had acute toxicity towards water photoautotrophs when compared to IPA and [N-(phosphonomethyl) glycine] acid (Lipok et al., 2010). We did not observe any growth stimulation at Iow glyphosate concentrations found in the commercial formulation Factor540® containing 49% of glyphosate and 51% of unknown ingredients, which might includes surfactants (IPCO, Winnipeg, Manitoba, Canada). It is also possible that glyphosate degradation into aminomethylphosphonic acid (AMPA) occurred, even if the experiment was done in sterile conditions, via algae and cyanobactcria themselves. Indeed, it have been hypothesized by Franz et al. (1997) that plants might be able to degrade a fraction of glyphosate into AMPA, in a similar way than microorganisms degrade glyphosate into AMPA in soils and water. If so, AMPA might had exerced some phytotoxicity, preventing growth stimulation in our conditions.

The inhibitory effects of the glyphosate-based herbicide on the reduction of Q_A in PSII might occur via its inhibitory effects on the shikimate pathway leading to a perturbation of the biosynthesis of aromatic amino acids, especially tyrosine. Firstly, after water photolysis, the manganese cluster reduces a tyrosine residue that is part of the Dl protein: Tyr 161 also named Y_z. The function of the tyrosine-residue is to re-reduce the P680 chlorophyll, by transferring electrons from the Mn-cluster. P680* then reduces a pheophytin moleculc, itself reducing Q_A (Dau, 1994; Strasser et al, 1995; Govindjee et al, 2010). A reduction of the biosynthesis of tyrosine caused by glyphosate inhibition of the shikimate pathway could lead to a decreasc of the number of Q_A' and reduced PSU capacity, when compared to algae and cyanobacteria not exposed to glyphosate-based herbicides. There is also evidence that glyphosate might interfere with manganese (Mn^2f) bioavailability through its chelating effects, in water (Bernards et al., 2005) and in plant lcaves and seeds (Eker et al, 2006; Cakmak et al, 2009). Manganese is an essential co-factor of the oxygen-evolving complex, which reduces Tyr 161 (Kern et Rcnger, 2007; Umena et al, 2011) and a lack of manganese availability might affect the amount of energy trapped by PSII (TRo/RC). It seems that, in