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

Highlights

Mesocosm was conducted to evaluate bloom potential of U. ohnoi in the future ocean.

Bloom potential was higher in ocean acidification with improved C and N metabolism.

Positive metabolic change in ocean acidification were offset by elevated temperature.

The bloom potential decreases when acidification and warming are combined.

Abstract

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 CO2 and temperature, namely control (450 μatm CO2 & 20oC), acidification (900 μatm CO2 & 20oC), warming (450 μatm CO2 & 25oC), and greenhouse (900 μatm CO2 & 25oC). 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 O2 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, 13C and 15N discrimination data suggest U. ohnoi prefers CO2 and NH4+ over HCO3 and NO3 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.

Kang E. J., Han A.-R., Kim J. H., Kim I. N., Lee S., Min J. O., Nam B. R., Choi Y.-J., Edwards M. S., Diaz-Pulido G., Kim C., in press. Evaluating bloom potential of the green-tide forming alga Ulva ohnoi under ocean acidification and warming. Science of The Total Environment. Article (subscription required).
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Ocean acidification in the IPCC AR5 WG II

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