Studies On The Impacts Of Ocean Acidification And UV Radiation On The Photosynthesis Of Diatoms

Atmospheric CO2 concentration has been rising since the Industrial Revolution and is predicted to double by the end of this century. The increasing atmospheric CO2 and subsequent rise of pCO2 in seawater, alters the carbonate system and related chemical reactions, resulting in lower pH and higher HCO3- concentration, which is called ocean acidification (OA), thus affects the photosynthetic processes in phytoplankton in direct and/or indirect ways. Diatoms play a fundamental role in marine biological CO2 pump, accounting for 40% of the marine primary produciton, however, little is known about their physiological responses to OA. Other environmental factors, like solar UV radiation and nutritive salt, could also affect the photosynthetic responses of diatoms, the combination effects of these factors need to be explored. In this study, we used pH/pCO2 perturbation technique to cultivate three diatom species (Skeletonema costatum, Thalassiosira weissflogii and Thalassiosira pseudonana) under two different pCO2 levels, representing the atmospheric pCO2 of present and the year around 2100(pCO2:800 ppmv) to evaluate the physiological responses of the diatoms to OA, and revealed the combined effects of OA and UV or nutritive salt. The main results are as follows:1. After acclimation to high CO2/low pH for 25 generations or more, the specific growth rates of S. costatum and T. weissflogii were decreased by 21% and 19.4% respectively. No extracelluar carbonic anhydrase activity was examined in both diatoms under OA conditions, while the activity of intracelluar carbonic anhydrase was inhibited by 54.8% and 31.3% respectively. OA lowered the maximum photosynthetic oxygen evolution rate, reduced the inorganic carbon affinity and down-regulated the carbon concentrating mechanisms (CCMs) in both diatoms.2. Both high visible light and solar UV radiation could result in the photoinhibition of T. weissflogii and T. pseudonana, the inhibitory effect was accelerated under OA conditions in both diatoms. As compared with T. weissflogii, T. pseudonana was more sensitive to photoinhibition. The turnover of D1 protein played an important role in counteracting the photoinhibition of PSII under OA and non-OA conditions. In addition, OA led to the reduction of rETRmax, a and Ik in T.weissflogii, but increase in T.pseudonana, and the combined effect of UV radiation brought about further decrease of these photosynthetic parameters.3. OA enhanced the Pmax and Ik in T.pseudonana under both phosphorus limited and phosphorus abundant conditions, the highest photosynthetic oxygen evolution rate was achieved at the N/P ratio of 80:1 under OA condition, but OA lowed the Pmax in S.costatum and T.weissflogii, the highest photosynthetic oxygen evolution rate of T.weissflogii was achieved at the N/P ratio of 16:1.