Effect of hydroxyl radical on intact microalgal photosynthesis

Photosynthetic CO 2 fixation with microalgae for new energy and chemical sources is one of the potential method to mitigate CO 2 emission. To gain the more productivities for CO 2 mitigation, the enhancement of photosynthetic productivity is required. We focused on the active oxygen that is supposed to be produced in algal cells and causes harmful effects on photosynthesis under high irradiation that is the case of outdoor cultivation circumstances. In this report, we have challenged to detect the active oxygens in microalgal cells, and then the results were described. The levels of free radical species generated in the living cells of Chlorella vulgaris var. vulgaris (IAM C-534) were detected with electron spin resonance (ESR) spectrometer with 5,5 dimethyl-1-pyrroline N-oxide (DMPO) as a spin-trapping reagent. Before detecting the free radicals in the living Chlorella cells, the influence of DMPO concentration toward the photosynthetic growth rate of the cells was measured. Since the growth rate was not influenced by up to 130 mM DMPO, the DMPO concentration was adjusted to 90 mM during the measurement. Only one DMPO adduct, which is assigned as the hydroxyl radical (DMPO-OH; aN=1.49mT, aH=1.49mT) was detected in the solution of intact cells of Chlorella vulgaris irradiated with visible light (1). Moreover, the production of DMPO-OH adducts was inhibited by some hydroxyl radical ( −OH) scavengers such as KI and ethanol. It has been estimated that −OH is one of the photoinhibition factors of the photosynthetic organisms. But, −OH has never been detected in vivo. In this report, −OH was detected in vivo, and the −OH was increased according to the light intensity.