When calibrating the DPD-POD assay, we found a linear response of the absorbance at 551 nm to H2O2 concentration between 0 -0.4 mg L -1 H2O2 (see Supplementary Figure ). After appropriate dilution of the dark and irradiated samples so that their H2O2 concentration falls within this range, they can be analyzed using the assay. The detailed experimental procedure is described in Supplementary Section 4.