1. Supplementary Material Figure 1 Calibration curve of the EPR signal using an H2O2 solution Dependence of the EPR signals of H2O2-derived hydroxylradicals on H2O2 concentration. A given concentration of H2O2wasadded to TAP medium and incubated for 5 min with50 mM 4-POBN, 4% ethanol, 50 μM Fe-EDTA. Table 1: H2O2 concentration in the cultures as determined by the spin trappingassay. Chlorophyll concentration of the culture: 10 µg ml-1. Results are expressed as mean bars represent ± S.D. (3-6 independent experiments).

2. Figure 2 Photosynthesis after a dark to high light switch in wild type C. reinhardtii and in catalase knock-down mutants. wild type amiRNAcat40 amiRNAcat15 Photosynthesis was measured in the presence (circles) or absence (diamonds) of 1 mMNaHCO3 in wild type and catalase knock-down mutants exhibiting 40% or 15% residual catalase activity following a dark to high light (700 µmol quanta m-2s-1) shift, and during an additional 30 min recovery at 70 µmol quanta m-2s-1. Typical O2 evolution curves are shown, representative of at least three independent experiments. Measurements of photosynthesis were performed in a Liquid-Phase Oxygen Electrode Chamber (Hansatech Instruments, Norfolk, England). Photosynthetic activity was measured in TAP medium under saturating white light.