Alleviation of solar ultraviolet radiation (UVR)-induced photoinhibition in diatom Chaetoceros curvisetus by ocean acidification

The study aimed to unravel the interaction between ocean acidification and solar ultraviolet radiation (UVR) in Chaetoceros curvisetus. Chaetoceros curvisetus cells were acclimated to high CO2 (HC, 1000 ppmv) and low CO2 concentration (control, LC, 380 ppmv) for 14 days. Cell density, specific growth rate and chlorophyll were measured. The acclimated cells were then exposed to PAB (photosynthetically active radiation (PAR) + UV-A + UV-B), PA (PAR + UV-A) or P (PAR) for 60 min. Photochemical efficiency (phi PSII), relative electron transport rate (rETR) and the recovery of ?PSII were determined. HC induced higher cell density and specific growth rate compared with LC. However, no difference was found in chlorophyll between HC and LC. Moreover, phi PSII and rETRs were higher under HC than LC in response to solar UVR. P exposure led to faster recovery of phi PSII, both under HC and LC, than PA and PAB exposure. It appeared that harmful effects of UVR on C. curvisetus could be counteracted by ocean acidification simulated by high CO2 when the effect of climate change is not beyond the tolerance of cells.

Growth and photosynthesis of a diatom Cylindrotheca closterium grown under elevated CO2 in the presence of solar UV radiation

The combination of elevated CO2 and the increased acidity in surface oceans is likely to have an impact on photosynthesis via its effects on inorganic carbon speciation and on the overall energetics of phytoplankton. Exposure to UV radiation (UVR) may also have a role in the response to elevated CO2 and acidification, due to the fact that UVR may variously impact on photosynthesis and because of the energy demand of UVR defense. The cell may gain energy by down-regulating the CO2 concentrating mechanism, which may lead to a greater ability to cope with UVR and/or higher growth rates. In order to clarify the interplay of cell responses to increasing CO2 and UVR, we investigated the photosynthetic response of the marine and estuarine diatom Cylindrotheca closterium f. minutissima cultured at either 390 (ambient) or 800 (elevated) ppmv CO2, while exposed to solar radiation with or without UVR (UVR, 280-400 nm). After a 6 day acclimation period, the growth rate of cells was little affected by elevated CO2 and no obvious correlation with the radiation dose (for both PAR and PAR + UV treatments) could be detected. However, the relative electron transport rate was reduced and was more sensitive to UVR in cells main - tained at elevated CO2 as compared to cells cultured at ambient CO2. The CO2 concentrating mechanism was down regulated at 800 ppmv CO2, but was apparently not completely switched off. These data are discussed with respect to their significance in the context of global climate change.