Contrasting bloom potential of green-tide forming alga Ulva ohnoi under ocean acidification and warming

Eun Ju Kang1, A-Reum Han2, Il-Nam Kim1, Sukyeon Lee2 and Ju-Hyoung Kim2, (1)Incheon National University, Department of Marine Science, Incheon, South Korea, (2)Kunsan National University, Faculty of Marine Applied Biosciences, Gunsan, South Korea
The occurrence of green algal bloom causes enormous economic losses, and the bloom potential is determined by various human activities and biogeochemical process. In this study, we investigated ecophysiology of Ulva ohnoi under mimicked future climate conditions to predict bloom potential using chlorophyll a fluorescence, oxygen evolution, and stable isotope analysis. Mesocosm system simulates four experimental conditions along the individual and combination treatments of elevated CO2and temperature (Control: 450 µatm & 20oC; Acidification: 900 µatm & 20oC; Warming: 450 µatm & 25oC; Greenhouse: 900 µatm & 25oC). Photosynthetic electron transport efficiency (rETR) was significantly increased under acidification although net photosynthesis and growth is not affected. rETR, net photosynthesis, and growth were significantly reduced under elevated temperature conditions. These results represent unbalance of energy between electron transport and O2 production under acidification. The unbalanced energy metabolism supposes be related to carbon and nitrogen assimilation of U. ohnoi. Based on the 13C and 15N discrimination data, U. ohnoi prefers CO2 and NH4+ as a carbon and nitrogen sources, and increase the N content in the thallus under acidification. In conclusion, bloom potential of U. ohnoi could be increased under ocean acidification by N accumulation, but elevated temperature could offset bloom potential with a decrease in photosynthesis and growth.