Hydrogen peroxide production protects Chlamydomonas reinhardtii against light-induced cell death by preventing singlet oxygen accumulation through enhanced carotenoid synthesis

The effect of hydrogen peroxide (H2O2) on carotenoid synthesis in Chlamydomonas reinhardtii under light-induced stress at 3000 μmol m−2 s−1 has been investigated. This very high light (VHL) illumination triggered a transient increase in H2O2 production during the initial 30 min of light stress, followed by singlet oxygen (1O2) production, growth inhibition and necrotic cell death. The carotenoid content was slightly reduced during the first 30 min of VHL illumination and strongly diminished after 60 min, while the expression of the transcripts of enzymes involved in carotenoid biosynthesis, including phytoene synthase (PSY), phytoene desaturase (PDS), and lycopene ɛ-cyclase (LCYE), initially increased and then decreased. Lycopene β-cyclase (LCYB) transcripts did not change. Treatment with dimethylthiourea, a H2O2 scavenger, under VHL conditions reduced H2O2 production and PSY and PDS transcript levels and accelerated the reduction of carotenoids, the production of 1O2, viability loss and necrotic cell death. Pretreatment with 0.1 μM methyl viologen or 0.2 mM H2O2 under 50 μmol m−2 s−1 low light for 60 min increased VHL tolerance, carotenoid content, and PSY and PDS transcripts, while LCYB and LCYE transcripts were not affected. These results suggest that H2O2, produced under VHL stress, ameliorates the 1O2-mediated oxidative damage to C. reinhardtii through a reduction in the degree of carotenoid breakdown by activation of de novo carotenoid synthesis.