The effects of high PAR (400–700 nm), UVA (315–400 nm), and UVB (280–315 nm) radiation on viability and photosynthesis were investigated for Chaetoceros brevis Schütt. This Antarctic marine diatom was cultivated under low, medium, and high irradiance and nitrate, phosphate, silicate, and iron limitation before exposure to a simulated surface irradiance (SSI) treatment, with and without UVB radiation. Light‐harvesting and protective pigment composition and PSII parameters were determined before SSI exposure, whereas viability was measured by flow cytometry in combination with a viability stain after the treatment. Recovery of PSII efficiency was measured after 20 h in dim light in a separate experiment. In addition, low and high irradiance acclimated cells were exposed outdoors for 4 h to assess the effects of natural PAR, UVA, and UVB on viability. Low irradiance acclimated cells were particularly sensitive to photo induced viability loss, whereas no viability loss was found after acclimation to high irradiance. Furthermore, nutrient limitation reduced sensitivity to photo induced viability loss, relative to nutrient replete conditions. No additional viability loss was found after UVB exposure. Sunlight exposed cells showed no additional UVB effect on viability, whereas UVA and PAR significantly reduced the viability of low irradiance acclimated cells. Recovery of PSII function was nearly complete in cultures that survived the light treatments. Increased resistance to high irradiance coincided with an increased ratio between protective‐ and light‐harvesting pigments before the SSI treatment, demonstrating the importance of nonphotochemical quenching by diatoxanthin for survival of near‐surface irradiance. We conclude that a sudden transfer to high irradiance can be fatal for low irradiance acclimated C. brevis.

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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.

Estuarine microphytobenthos are frequently exposed to excessively high irradiances. Photoinhibition in microalgae is prevented by various photophysiological responses. We describe here the role of the xanthophyll pigments in photoacclimation. The pigment composition of the microphytobenthos was studied in three European estuaries (Barrow, Ireland; Eden, UK; Tagus, Portugal). Using HPLC-analyses, microscale changes in biomass and pigment composition were monitored over short (hourly) and long (seasonal) time scales. In the Barrow estuary, the biomass of microphytobenthos (measured as chlorophyll a) increased significantly in the top 400-500 mm of the sediment surface within 1 h of emersion; simultaneously, the xanthophyll pool size (diadinoxanthin plus diatoxanthin, dd þ dt) almost doubled. A more gradual conversion of dd into dt was observed, with the dt:dd ratio increasing from <0.1 at the start of emersion to >0.3 after 3 h emersion. Similar trends in the dt:dd ratio were observed in the surface sediments of the Eden and the Tagus estuaries. Higher ratios were recorded in the Tagus estuary, which may be explained by higher incident irradiance. In addition, seasonal studies carried out in the Eden and Tagus estuaries showed that the xanthophyll pool size increased by 10% in the summer months. The pool size was highest in the Tagus estuary. Concurrently, high values for the de-epoxidation state were recorded, with values for dt/(dt þ dd) > 0.35 recorded in the summer. At the Eden, the ratio never exceeded 0.3. The de-epoxidation state was higher in winter than in summer, which was ascribed to the low winter temperatures. During a vertical migration study, a negative relationship between chlorophyll a and the de-epoxidation state was observed. It is suggested that this relationship originates from 'micro-migration' within the biofilm. Migration within the euphotic zone may provide an alternative means for cells to escape photodamage. In this paper, we propose that both xanthophyll cycling and 'micro'-migration play an important role in photoacclimation and it appears that these processes operate in parallel to regulate the photosynthetic response.