Ultraviolet radiation induces negative effects on photoprotective mechanisms in a marine diatom (Phaeodactylum tricornutum)

HALAC S., GARCÍA-MENDOZA E. AND A. BANASZAK

PHOTOCHEMISTRY AND PHOTOBIOLOGY

WILEY-BLACKWELL PUBLISHING, INC

Año: 2009 vol. 85 p. 807 - 815

0031-8655

This study demonstrates that ultraviolet radiation (UVR) reduces the photoprotective capacity of the diatom Phaeodactylum tricornutum by affecting xanthophyll cycle (XC) activity. The short-term reduction of photosystem II (PSII) maximum efficiency of charge separation (Fv/Fm) in cells exposed to UVR could be explained mainly by a reduced photoprotective capacity under this condition. P. tricornutum cells acclimated to two different PAR intensities, high light (HL, 200 ìmol quanta m-2 s-1) and low light (LL, 50 ìmol quanta m-2 s-1), were exposed to saturating irradiance (1100 ìmol quanta m-2 s-1) in the presence (PAR+UVR) and absence of UVR (PAR). HL cells exhibited a greater reduction in Fv/Fm in PAR+UVR when compared to the PAR treatment that was related to a reduction in the de-epoxidation of XC pigments. In contrast, in LL cells, UVR did not considerably affect XC de-epoxidation even though the reduction in Fv/Fm was greater than in HL cells. The negative effect of UVR on photoprotection was more pronounced in HL cells because they synthesized more XC pigments than LL cells. This was confirmed when XC activity was blocked with dithiothreitol (DTT) and when PSII repair was inhibited with chloramphenicol (CAP). The differential reduction of Fv/Fm between PAR+UVR and PAR treatments disappeared when XC was blocked in HL cells. A higher reduction and an incomplete recovery of Fv/Fm were observed in cells incubated with CAP in the presence of UVR. Such responses confirm that UVR had a negative effect on photoprotective mechanisms causing an enhancement of damage by PAR, especially in HL-acclimated cells in which heat dissipation is important for PSII regulation.