INFLUENCE OF THE POOL SIZE OF THE XANTHOPHYLL CYCLE ON THE EFFECTS OFLIGHT STRESS IN A DIATOM - COMPETITION BETWEEN PHOTOPROTECTION AND PHOTOINHIBITION

In a study of the relationship between nonphotochemical quenching of f luorescence and the xanthophyll cycle, we show that the diatom Phaeoda ctylum tricornutum exhibits several interesting characteristics. This xanthophyll cycle consists of only one reversible epoxidating/deepoxid ating step (diadinoxanthin/diatoxanthin). Diadinoxanthin, which increa ses from 8 to 17 molecules/100 chlorophyll a (Chl a) during the ageing of the culture, was present as two separate pools, with a portion (of about 5 molecules/100 Chl a) which was never deepoxidated. Under a de fined irradiance, the time necessary to abolish net photosynthesis inc reases with the pool size of diadinoxanthin available for deepoxidatio n. A close correlation is found between nonphotochemical quenching and the relative ratio of diatoxanthin until the photosytem II center is inactivated. The photoprotective effect of diadinoxanthin deepoxidatio n is limited to the phase during which quenching of the minimum fluore scence (F-0) develops.