Abstract
Tropical reef-building coral stress levels will intensify with the predicted rising atmospheric CO2 resulting in ocean temperature and acidification increase. Most studies to date have focused on the destabilization of coral-dinoflagellate symbioses due to warming oceans, or declining calcification due to ocean acidification. In our study, pH and temperature conditions consistent with the end-of-century scenarios of the Intergovernmental Panel on Climate Change (IPCC) caused major changes in photosynthesis and respiration, in addition to decreased calcification rates in the coral Acropora millepora. Population density of symbiotic dinoflagellates (Symbiodinium) under high levels of ocean acidification and temperature (Representative Concentration Pathway, RCP8.5) decreased to half of that found under present day conditions, with photosynthetic and respiratory rates also being reduced by 40%. These physiological changes were accompanied by evidence for gene regulation of calcium and bicarbonate transporters along with components of the organic matrix. Metatranscriptomic RNA-Seq data analyses showed an overall down regulation of metabolic transcripts, and an increased abundance of transcripts involved in circadian clock control, controlling the damage of oxidative stress, calcium signaling/homeostasis, cytoskeletal interactions, transcription regulation, DNA repair, Wnt signaling and apoptosis/immunity/ toxins. We suggest that increased maintenance costs under ocean acidification and warming, and diversion of cellular ATP to pH homeostasis, oxidative stress response, UPR and DNA repair, along with metabolic suppression, may underpin why Acroporid species tend not to thrive under future environmental stress. Our study highlights the potential increased energy demand when the coral holobiont is exposed to high levels of ocean warming and acidification.
Highlights
Changes in atmospheric CO2 are likely to fundamentally alter ocean ecosystems through their influence on sea temperature and carbonate ion chemistry [1,2]
Our results show that the Symbiodinium population of A. millepora holobiont decreased by 50% when exposed to the future scenario Representative Concentration Pathway, RCP8.5 compared to corals exposed to present day conditions (PD) (Fig 1A, Table 2)
This was reflected in chlorophyll a cm-2 levels, which were halved. In concert with these changes, the photosynthetic rates of corals exposed to RCP8.5 future conditions were reduced to 33–41% compared to corals under Present Day (PD) conditions (Fig 1B and 1C, Table 2)
Summary
Changes in atmospheric CO2 are likely to fundamentally alter ocean ecosystems through their influence on sea temperature and carbonate ion chemistry [1,2]. Coral reef ecosystems are among the major oceanic systems that are likely to be detrimentally affected by global warming and ocean acidification (OA) [3,4]. These highly productive and biologically diverse ecosystems provide important goods and services to more than 450 million people in coastal communities around the world [5]. Over the past few decades, mass coral bleaching events have increased both in frequency and intensity [7,8,9] The result of these events is usually high coral mortality and for colonies that survive, decreased colony growth and depressed reproductive output is common [10,11]. There are differences in bleaching susceptibility and severity among coral species and it has been shown that fast-growing branching genera such as Acropora are more susceptible to severe bleaching [8,12]
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
