Global changes caused by industrial activities are known as enhanced solar ultra-violet radiation (UVR), increased atmospheric CO2 concentration and ocean acidification. How such environmental changes would influence marine ecosystems is an important issue for marine scientists. One of the most interested questions is to see how UVR and ocean acidification affect marine primary producers. Therefore, we investigated the effects of UVR and ocean acidification on the photosynthesis and growth of the diatom Skeletonema costatum and the cocolithorphore Emiliania huxleyi, two cosmopolitan and ecologically important phytoplankton. The main results are as follows:
Solar UVR inhibited the photochemical activity and photosynthetic carbon fixation of the S. costatum cells. The extent of inhibition depends on light history of the cells, doses and intensity of irradiation. In contrast to the cells of naturally grown S. costatum cells (WS), indoor grown strain (IS), which had been grown under low PAR for decades, was more sensitive to solar UVR and slower in repairing the damage of PSII caused by UVR. Even under changing levels of solar radiation simulated for vertical mixing, solar UVR still induced higher inhibition in IS than in WS. During long-term exposures to solar radiation, the specific growth rate was much lower in IS than in WS at the beginning, then increased 3 days later to reach an equivalent level as that of WS until the end of experiment (10th day). It indicated that the repair processes for UVR-induced damages must have been accelerated for IS, however, the contents of UV-absorbing compounds was not increased during the long-term acclimation to solar radiation.
E. huxleyi, as a calcifying phytoplankton, operates two kinds of carbon fixation: photosynthesis and calcification. Both processes were inhibited by UVR, but UVR-induced inhibition was higher in calcification than in photosynthesis. However, changing levels (in contrast to constant one) of radiation simulated for vertical mixing decreased the deleterious effects of UVR. During a long-term experiment, the cells became acclimated to solar radiation, showing higher resistance to UVR. During such an acclimation, the cells became larger with thicker cocolith layer and more UV-absorbing compounds. On the other hand, when E. huxleyi cells were grown under simulated oceanic acidification conditions, calcification was inhibited due to of the chemical changes in the seawater carbonate system. At the same time, photosynthetic affinity to inorganic carbon decreased under the acidified conditions.