Abstract
Symbiotic scleractinian corals are particularly affected by climate change stress and respond by bleaching (losing their symbiotic dinoflagellate partners). Recently, the energetic status of corals is emerging as a particularly important factor that determines the corals’ vulnerability to heat stress. However, detailed studies of coral energetic that trace the flow of carbon from symbionts to host are still sparse. The present study thus investigates the impact of heat stress on the nutritional interactions between dinoflagellates and coral Stylophora pistillata maintained under auto- and heterotrophy. First, we demonstrated that the percentage of autotrophic carbon retained in the symbionts was significantly higher during heat stress than under non-stressful conditions, in both fed and unfed colonies. This higher photosynthate retention in symbionts translated into lower rates of carbon translocation, which required the coral host to use tissue energy reserves to sustain its respiratory needs. As calcification rates were positively correlated to carbon translocation, a significant decrease in skeletal growth was observed during heat stress. This study also provides evidence that heterotrophic nutrient supply enhances the re-establishment of normal nutritional exchanges between the two symbiotic partners in the coral S. pistillata, but it did not mitigate the effects of temperature stress on coral calcification.
Highlights
IntroductionOcean warming is causing a decrease in the nutrient enrichment of surface waters, a decline in zooplankton abundance[29,30,31] and direct shifts in zooplankton composition[30]
The percentage of carbon incorporated by the symbionts, significantly increased during the heat stress, both in fed and unfed corals
The present study reveals the impact of heat stress on the nutritional interactions within a coral-dinoflagellate symbiosis
Summary
Ocean warming is causing a decrease in the nutrient enrichment of surface waters, a decline in zooplankton abundance[29,30,31] and direct shifts in zooplankton composition[30] All together, these changes in food abundance and symbiont concentration within coral tissue will impact the energetic capacities of corals and thereby affect corals’ resistance and resilience to environmental stressors. Our working hypothesis was that carbon acquisition, exchange between the two partners of the symbiosis and retention within the symbiont and coral host will change with heat stress and/or heterotrophy, as these two factors impact the symbiont concentration as well as their rates of photosynthesis and respiration. Knowledge of the factors that determine whether bleaching is likely to result in coral mortality will provide a better understanding of how coral reefs, and the ecosystem services, they provide to human societies are likely to change in the future
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