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

Abstract

Cellular responses of 1-, 2- and 4-d-old Fucus spiralis embryos subjected to a single dose of elevated photosynthetically active photon flux density (PPFD), with or without ultraviolet (U-V) radiation, were investigated by measuring the effects on the effective quantum yield of photosystem II (DeltaF/F-m') and intracellular production of active oxygen species (AOS). Production of AOS was determined by the in vivo conversion of 5-(and-6)-chloromethyl-2',7 -dichlorodihydrofluorescein diacetate (CM-DCFH2-DA) to the fluorescent compound dichlorofluorescein (DCF) using confocal laser scan microscopy (CLSM) and image analysis. The role of xanthophyll cycle pigments in photoprotection was also assessed. A rapid decline in DeltaF /F-m' was observed under all elevated fight conditions. A correlation was found between non-photochemical 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 were recognized: (i) a rapid decline of the PSII yield due to the violoxanthin-zeaxanthin cycle (photoprotection), and (ii) a slower second-phase decline, correlated with active oxygen production. Electron transport rate (ETR) increased with embryo age, and was correlated with AOS production. As a result 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 damaging effect. Overall, our data suggest that under natural conditions, embryos of F spiralis are susceptible to elevated light levels, and that UVB radiation is an important stress factor. [KEYWORDS: active oxygen; chloromethyl dichlorodihydrofluorescein diacetate (CM-DCFH2-DA); confocal laser scan; oxidative stress; xanthophyll cycle Alga dictyota-dichotoma; ultraviolet-b radiation; free-radical production; marine-phytoplankton; chlorella-vulgaris; oxidative stress; uv-b; chlorophyll fluorescence; hydroxyl radicals; oxygen production]