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

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 & 20^oC; Acidification: 900 µatm & 20^oC; Warming: 450 µatm & 25^oC; Greenhouse: 900 µatm & 25^oC). 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.