Evaluating bloom potential of the green-tide forming alga Ulva ohnoi under ocean acidification and warming.

The occurrence of green-tides, whose bloom potential may be increased by various human activities and biogeochemical process, results in enormous economic losses and ecosystem collapse. In this study, we investigated the ecophysiology of the subtropical green-tide forming alga, Ulva ohnoi complex (hereafter: U. ohnoi), under simulated future ocean conditions in order to predict its bloom potential using photosynthesis and growth measurements, and stable isotope analyses. Our mesocosm system included four experimental conditions that simulated the individual and combined effects of elevated CO and temperature, namely control (450 μatm CO & 20 °C), acidification (900 μatm CO & 20 °C), warming (450 μatm CO & 25 °C), and greenhouse (900 μatm CO & 25 °C). Photosynthetic electron transport rates (rETR) increased significantly under acidification conditions, but net photosynthesis and growth were not affected. In contrast, rETR, net photosynthesis, and growth all decreased significantly under elevated temperature conditions (i.e. both warming and greenhouse). These results represent the imbalance of energy metabolism between electron transport and O production that may be expected under ocean acidification conditions. This imbalance appears to be related to carbon and nitrogen assimilation by U. ohnoi. In particular, C and N discrimination data suggest U. ohnoi prefers CO and NH over HCO and NO as sources of carbon and nitrogen, respectively, and this results in increased N content in the thallus under ocean acidification conditions. Together, our results suggest a trade-off in which the bloom potential of U. ohnoi could increase under ocean acidification due to greater N accumulation and through the saving of energy during carbon and nitrogen metabolism, but that elevated temperatures could decrease U. ohnoi's bloom potential through a decrease in photosynthesis and growth.