Cellular responses to elevated light levels in Fucus spiralis embryos during the first days after fertilization

Cellular responses of 1-, 2- and 4-d-old Fucus spiralis embryos subjected t o a single dose of elevated photosynthetically active photon flux density ( PPFD), with or without ultraviolet (U-V) radiation, were investigated by me asuring the effects on the effective quantum yield of photosystem II (Delta F/F-m') and intracellular production of active oxygen species (AOS). Produc tion of AOS was determined by the in vivo conversion of 5-(and-6)-chloromet hyl-2',7 -dichlorodihydrofluorescein diacetate (CM-DCFH2-DA) to the fluores cent compound dichlorofluorescein (DCF) using confocal laser scan microscop y (CLSM) and image analysis. The role of xanthophyll cycle pigments in phot oprotection was also assessed. A rapid decline in DeltaF /F-m' was observed under all elevated fight conditions. A correlation was found between non-p hotochemical quenching and the de-epoxidation ratio zeaxanthin/(zeaxanthin + violoxanthin). Active oxygen formation increased with PPFD and was higher in older embryos and when UVB was present. Two photoinhibition responses w ere recognized: (i) a rapid decline of the PSII yield due to the violoxanth in-zeaxanthin cycle (photoprotection), and (ii) a slower second-phase decli ne, correlated with active oxygen production. Electron transport rate (ETR) increased with embryo age, and was correlated with AOS production. As a re sult of enhanced AOS production, there was a slow recovery of the PSH yield , in particular with increased effective UV dose. In general, embryos were able to recover from the imposed light conditions, but UVB had a more damag ing effect. Overall, our data suggest that under natural conditions, embryo s of F spiralis are susceptible to elevated light levels, and that UVB radi ation is an important stress factor.