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This work aims at determining early effects of uranium on the green microalga Chlamydomonas reinhardtii. First, the global effect on growth rate inhibition of exponentially-growing cultures was assessed on favourable conditions for uranium bioavailability (e.g. pH=5), EC50-24hrs equals roughly to 150 nM whatever the uranium isotopic composition considered (depleted U or 233U). Then, the sensitivity of different parameters representative of (i) oxidative stress (GSH/[GSH + 0.5 GSSG] ratio) (ii) metal detoxifying (phytochelatins production) and (iii) photosynthetic activity (chlorophyll fluorescence) was tested. Setting assay of different forms of glutathione and phytochelatins by HPLC was firstly optimised with cadmium-contaminated cells. This assay completed by chlorophyll fluorescence and algal growth was subsequently applied on samples contaminated by 150 nM of depleted uranium or 233U. No phytochelatin was produced in our experimental conditions. No difference of GSH/[GSH + 0.5 GSSG] ratio was shown between control and contaminated algae. This result suggests that the algae could be stressed before contamination due to culture condition. Chlorophyll fluorescence measurement showed photosynthetic activity inhibition after 24 hrs, in the same way for depleted uranium and 233U. Thus, the effect observed on the photosynthetic activity could be mainly attributed to the chemical toxicity of the metal.