Photoacclimation modulates excessive photosynthetically active and ultraviolet radiation effects in a temperate and an Antarctic marine diatom

The influence of photoacclimation on the effects of excessive photosynthetically active (PAR; 400-700 nm) and ultraviolet (UVR; 280-400 nm) radiation was assessed for the marine diatoms Thalassiosira weissflogii (Grunow) Fryxell and Hasle and Thalassiosira antarctica (Comber). Low and high PAR acclimated cultures were subjected to simulated surface irradiance (SSI) that mimicked irradiance around noon, including UVR. PSII efficiency, xanthophyll conversion, superoxide dismutase (SOD) activity, carbohydrate buildup, and lipid peroxidation were investigated after 30 min SSI and during 120 min recovery in low irradiance. Furthermore, viability loss was measured during 4 h SSI. Prior to SSI, the diadino-diatoxanthin pool was increased in high irradiance acclimated cells, compared with cells grown under low irradiance. Thirty-minutes SSI caused a pronounced decline in PSII efficiency. This coincided with de-epoxidation of diadinoxanthin in high irradiance acclimated cells, which was completely reversed during recovery in low irradiance. De-epoxidation was lower for low irradiance acclimated cells. whereas PSII efficiency and carbohydrate buildup were lower during the recovery phase. Furthermore, clear UVR effects on PSII efficiency were observed in low irradiance but not in high irradiance acclimated cells. Although 30 min SSI did not increase cellular SOD activity and lipid peroxidation, prolonged (4 h) SSI caused viability loss in low irradiance acclimated cells, which was enhanced by UVR. Therefore, PAR and UVR-induced PSII inactivation and viability loss were reduced by high irradiance-mediated changes in light harvesting and the xanthophyll pigments. In addition to photoacclimation-modulated differences, minor sensitivity differences were found between species.