Effects of periodic, low UVA radiation on cell characteristics and oxidative stress in the marine planktonic diatom Ditylum brightwellii

A continuous culture experiment was conducted with the marine diatom Ditylu m brighiwellii to assess the photo-oxidative effects of transition from dim light to high Light at three successive levels. The focus was on temporary stress by ambient WA at the water surface, via a simulation of the periodi c ascent of diatoms caused by vertical mixing. Within a 14 h dim-light peri od, cells were exposed daily to 4 h of: (A) moderate PAR (100 mu mol photon m(-2) s(-1)), (B) high PAR (400 mu mol photon m(-2) s(-1)) and (C) high PA R (400-700 nm) plus 67 pmol photon m(-2) s(-1) (2.2 W m(-2)) UVA. Cell shap e, life cycle and sinking rates were not affected by high PAR per se (mode B). The daily 4 h high-irradiance mode C, with additional UVA (WA:PAR = 0.0 16), caused an increase in cell diameter, a decrease in length axes, and a production of vegetative resting stages. Superoxide dismutase (SOD) activit ies, oxidized glutathione (GSSG) pools and malondialdehyde (MDA) contents i ncreased as well, which indicates that this biologically effective WA dose (0.78 W m(-2)) promoted active oxygen production, oxidative stress and lipi d peroxidation. Sinking rates in WA-exposed D. brightwellii were higher tha n in those grown without UVA. A close correlation between sinking rates and MDA contents suggests that UVA-induced lipid peroxidation has altered the membrane functions which regulate buoyancy in D. brightwellii. Although UVA -induced formation of resting cells will contribute to accelerated sinking, it is not evident that this can be considered as a stress avoidance mechan ism under adverse light (WA) conditions. D. brightwellii proved to be sensi tive to transitions from shade to natural surface irradiance conditions. In coastal waters, such large diatom species may experience oxidative stress after a few hours' residence near the water surface at ambient UVA levels.