The diatom Cylindrotheca closterium was exposed to transient light- and osmotic conditions as occur during its tidal emersion. The objective was to analyze how this simulated emersion contributes to the production of active oxygen species (AOS) and via this, to oxidative cell damage. Light- and salinity conditions were varied in factorial combination: low light (no UVB) or high light (unweighted UVB-dose rates of respectively 0.01; 0.07; 0.24; 1.03 W m-2) at normal (30 psu) or high salinity (60 psu). UVB (0.01–0.24 W m-2) and high salinity had a significant, negative effect on the photosynthetic efficiencies ?F/F m’ (steady-state quantum yield) and F v/F m (maximum yield). UVB at 1.03 W m-2 (15 kJ m-2 d-1) almost arrested electron transport. At ecologically relevant UVB levels, i.e. below 0.24 W m-2 (˜3.4 kJ m-2 d-1) with UVB:PAR0.07 W m-2 and at 60 psu indicated a reversal of the diatom xanthophyll cycle (diminished photoprotection) which may be caused by an enhanced AOS production. Oxidative stress and -damage to C. closterium cells were assessed applying fluorescent indicator dyes, via confocal microscopy and quantitative image analysis. AOS production rates (cellular DCF fluorescence) were stimulated by UV, and were ~50% higher at 60 psu. AOS production decreased with an increasing pre-exposure (0–4 h) to normal UVB (0.24 W m-2), which indicated a stimulation of the antioxidative defence. Non-protein thiols (indicator CMF) and glutathione pools (HPLC-analyzed) decreased with UVB-dose rates (0.01–0.24 W m-2), most likely due to AOS-mediated thiol oxidation. Hypersalinity (60 psu) and UVB (0.01–0.24 W m-2) caused membrane depolarization (dye DIBAC4(3)) and phospholipid hydrolysis (phospholipase A2 dye: bis-BODIPY FL-C11-PC). AOS production may have diminished the membrane polarity, and peroxidized the membrane lipids (HPLC-analyzed malondialdehyde) which enhanced PLA2 activity. The dyes indicated an increased oxidative (lipid) damage at a 15% inhibition of photosynthesis in this diatom, at UVB levels and salinities that can be expected in situ during its periodic tidal emersion.