Photochemical responses of three marine phytoplankton species exposed to ultraviolet radiation and increased temperature: role of photoprotective mechanisms.

We carried out experiments using long-term (5-7 days) exposure of marine phytoplankton species to solar radiation, in order to assess the joint effects of ultraviolet radiation (UVR) and temperature on the photochemical responses and photoprotective mechanisms. In the experiments, carried out at Atlantic coast of Patagonia (43°18.7'S; 65°2.5'W) in spring-summer 2011, we used three species as model organisms: the dinoflagellate Prorocentrum micans, the chlorophyte Dunaliella salina and the haptophyte Isochrysis galbana. They were exposed under: (1) two radiation quality treatments (by using different filters): P (PAR, >400 nm) and PAB (PAR+UV-A+UV-B, >280 nm); (2) two radiation intensities (100% and 50%) and (3) two experimental temperatures: 18 °C and 23 °C during summer and 15 °C and 20 °C in spring experiments, simulating a 5 °C increase under a scenario of climate change. In addition, short-term (4h) artificial radiation exposure experiments were implemented to study vertical migration of cells pre- and non-acclimated to solar radiation. We observed species-specific responses: P. micans displayed a better photochemical performance and a lower inhibition induced by UVR than D. salina and I. galbana. In accordance, P. micans was the only species that showed a synthesis of UV-absorbing compounds (UVACs) during the experiment. On the other hand, non-photochemical quenching (NPQ) was activated in D. salina at noon throughout the exposure, while I. galbana did not show a regular NPQ pattern. This mechanism was almost absent in P. micans. Regarding vertical migration, I. galbana showed the most pronounced displacement to deepest layers since the first two hours of exposure in pre- and non-acclimated cells, while only non-acclimated D. salina cells moved to depth at the end of the experiment. Finally, temperature partially counteracted solar radiation inhibition in D. salina and I. galbana, whereas no effect was observed upon P. micans. In particular, significant UVR and temperature interactive effects were found in I. galbana, the most UVR sensitive species. The joint effects on UVR and temperature, and the species-specific photoprotective responses will affect the trophodynamics and production of aquatic ecosystems in a way that is difficult to predict; however the specificity of the responses suggests that not all phytoplankton would be equally benefited by temperature increases therefore affecting the balance and interaction among species in the water column.

Temperature benefits the photosynthetic performance of the diatoms Chaetoceros gracilis and Thalassiosira weissflogii when exposed to UVR.

The aim of this study was to assess the combined effects of temperature and UVR on the photosynthesis performance of two diatoms -Chaetoceros gracilis and Thalassiosira weissflogii. In particular, we evaluated the role of UVR in inducing photoinhibition and the potential mitigation of this negative effect by an increase in temperature. Cultures were pre-acclimated at two temperatures - 18°C and 23°C - and exposed to different radiation treatments - UVR+PAR (280-700nm); UV-A+PAR (315-700nm) and PAR only (400-700nm) under two temperatures: 18°C (local surface summer water temperature) and 23°C (simulating a potential increase estimated by the year 2100). Exposure to natural solar radiation resulted in UVR-induced photoinhibition that was significantly higher in T. weissflogii than in C. gracilis. Both species benefited from the higher temperature (23°C) resulting in a lower photoinhibition as compared to samples exposed at 18°C. Inter-specific differences were determined in regard to the heat dissipation processes (NPQ) which were higher at high temperatures, and much more evident in C. gracilis than in T. weissflogii. The analyses of inhibition and recovery rates under different irradiances indicate that the balance between negative (inhibition) and positive (repair-dissipation) effects shifted towards a more positive balance with increasing temperature. Our results highlight for a beneficial effect of temperature on photosynthesis performance during exposure to UVR, although important inter-specific differences are found, probably due to differences in cell size as well as in their distribution within the oceanic realm (i.e., coastal versus oceanic species).