Seawater carbonate chemistry and the physiological performance of the chlorophyte Dunaliella tertiolecta

The effects of increased CO2 and irradiance on the physiological performance of the chlorophyte Dunaliella tertiolecta were studied at different PAR and UVR (UVA+UVB) irradiances, simulating the solar radiation at different depths, under present (390 ppmv, LC) and predicted CO2 levels for the year 2100 (1000 ppmv, HC). Elevated CO2 resulted in higher optimum and effective quantum yields (Fv/Fm and Phi PSII, respectively), electron transport rates (ETR) and specific growth rates (µ). Cell stress was alleviated in HC respect to LC as evidenced by a decrease in reactive oxygen species (ROS) accumulation. DNA damage showed a 42-fold increase in cyclobutane-pyrimidine dimers (CPDs) formation under the highest irradiance (1100 µmol quanta/m2/s) in LC with respect to the lowest irradiance (200 µmol quanta/m2/s). Photolyase (CII-PCD-PL) gene expression was upregulated under HC resulting in a drastic decrease in CPDs accumulation to only 25% with respect to LC. Proliferating cell nuclear antigen (PCNA) accumulation was always higher in HC and the accumulation pattern indicated its involvement in repair or growth depending on the irradiance dose. The repressor of silencing (ROS1) was only marginally involved in the response, suggesting that photoreactivation was the most relevant mechanism to overcome UVR damage. Our results demonstrate that future scenarios of global change result in alleviation of irradiance stress by CO2–induced photoprotection in D. tertiolecta.