Bleaching and mortality of a photosymbiotic bioeroding sponge under future carbon dioxide emission scenarios
The bioeroding sponge Cliona orientalis is photosymbiotic with dinoflagellates of the genus Symbiodinium and is pervasive on the Great Barrier Reef. We investigated how C. orientalis responded to past and future ocean conditions in a simulated community setting. The experiment lasted over an Austral summer under four carbon dioxide emission scenarios: a pre-industrial scenario (PI), a present-day scenario (PD; control), and two future scenarios of combined ocean acidification and ocean warming, i.e., B1 (intermediate) and A1FI (extreme). The four scenarios also simulated natural variability of carbon dioxide partial pressure and temperature in seawater. Responses of C. orientalis generally remained similar between the PI and PD treatments. C. orientalis under B1 displayed a dramatic increase in lateral tissue extension, but bleached and displayed reduced rates of respiration and photosynthesis. Some B1 sponge replicates died by the end of the experiment. Under A1FI, strong bleaching and subsequent mortality of all C. orientalis replicates occurred at an early stage of the experiment. Mortality arrested bioerosion by C. orientalis under B1 and A1FI. Overall, the absolute amount of calcium carbonate eroded by C. orientalis under B1 or A1FI was similar to that under PI or PD at the end of the experiment. Although bioerosion rates were raised by short-term experimental acidification in previous studies, our findings from the photosymbiotic C. orientalis imply that the effects of bioerosion on reef carbonate budgets may only be temporary if the bioeroders cannot survive long-term in the future oceans.
KeywordsCliona orientalis Symbiodinium Warming Acidification Great Barrier Reef
The experiment was co-funded by the Great Barrier Reef Foundation (to OHG), the Australian Research Council (ARC) Centre of Excellence for Coral Reef Studies CE0561435 (to SD and OHG), ARC Linkage LP0775303 (to SD and OHG), a Queensland Smart State Fellowship (to OHG) and an International Society for Reef Studies Graduate Fellowship (to JKHF). We thank the two anonymous reviewers for their valuable comments on this article, and Aaron Chai, Giovanni Bernal Carrillo, Annamieke Van Den Heuvel and Collette Bagnato for their assistance in maintaining the experiment. We also acknowledge the technical support provided by the staff at Heron Island Research Station. All experimental cores were collected under a permit provided by the Great Barrier Reef Marine Park Authority (G10/33917.1).
Author contribution statement
JKHF, CHLS, OHG and SD conceived and designed the experiment. JKHF, MAMA, OHG and SD performed the experiment. JKHF and CHLS wrote the manuscript with the contribution from MAMA, MA, OHG and SD.
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