Photosynthetic responses to Solar UV radiation of Gracilaria lemaneiformis cultured under different temperatures and CO2 concentrations

Abstract

The marine red macroalga Gracilaria lemaneiformis (Bory) Weber-van Bosse (Gigartinales, Rhodophyta) is an economically important species for cultivation in China. The cultivation of this species has been extensively spread which could be found in both northern and southern parts of China. This species could support as food resources for both human beings and aquaculture, and could also be commonly used in agar industry and biofuel production. The growth condition of G. lemaneiformis in natural environment varied frequently (especially the temperature, light intensity and quality, pCO2), which would potentially affect the physiology and photosynthetic production. Though the physiological and ecological effects induced by these environmental variations have been extensively concerned and studied, multi-factor coupling effects to marine macroalgae are still less documented up to now. To study the physiological responses of G. lemaneiformis to multiple stressors of ocean acidification, rising temperature and Ultraviolet (UV) radiation, the thalli of G. lemaneiformis cultured under different temperature (20 °Cas control, and high temperature group where it was increased by 4 °Cto 24 °C) and different CO2 concentrations (ambient atmosphere CO2 concentration, 390 μL/ L, and elevated CO2 concentration set at 1000 μL/ L which expected attain in the end of this century according to IPCC report of A1F1 scenario) for two weeks, and then the algae were treated with three levels of radiations (Photosynthetically active radiation, PAR, 400-700 nm; Photosynthetically active radiation+Ultraviolet A, PA, 320-700 nm; Photosynthetically active radiation+Ultraviolet A + Ultraviolet B, PAB, 280-700 nm) respectively, in the short-period. The photosynthetic pigments and chlorophyll fluorescene characteristics of the thalli of G. lemaneiformis were determined. The results showed that ocean acidification, rising temperature and UV irradiance alone did not affect the concentrations of chlorophyll a and carotenoid of G. lemaneiformis, and also no interactive effects were found among the treatments. Elevated CO2 in culture generally did not have a significant effect on the value of maximum relative electron transport rates (rETRmax). Temperature had no effect on the photosynthetic light-use efficiencies for thalli of G. lemaneiformis. However, exposure to UV radiation strong reduced the values of light-use efficiencies. Moreover, exposure of UV radiation significantly lowered the effective quantum yield in both G. lemaneiformis thalli grown 20 and 24 °C, with the inhibition rate being more pronounced in the algae grown under high CO2 condition together with increased temperature than the algae grown at control conditions (ambient CO2 concentration and 20 °C). Combine effects of ocean acidification and rising temperature enhanced the sensitivity of G. lemaneiformis to UV radiation, which could be reflected by the increased damage rate (k), decreased repair rate (r), and finally decreased ratio of repair to damage (r / k) in the thalli grown at high CO2 concentration together with increased temperature. Taken together, we proposed that under the background of global change, intensified UV radiation (ozone hole continue exist), increased surface seawater temperature and enhanced ocean CO2 absorption (ocean acidification) will synergistically exert negative effects on photosynthetic performance of G. lemaneiformis, and could thereby potentially decrease the yield and affect the aquaculture of this species.