Nutrient gradients simulate different adjustments of coral-algal symbiosis

Abstract Background: Eutrophication is one of the major causes of coral reef degradation but the effect of eutrophication on coral and its symbiont algae remains unclear, particularly for the larval stage of coral. In the present study, the physiological and transcriptomic responses of the larvae of an ecologically important scleractinian coral Pocillopora damicornis were analyzed after a 5-day exposure to elevated nitrate in order to assess the survival and adaptation of coral-algal symbiosis under elevated nutrients. Results: The results showed that multiple larval transcripts were significantly correlated with Symbiodiniaceae transcripts. The major differentially expressed transcripts in coral/Symbiodiniaceae included those responsible for energy synthesis/comsumption, nitrogen metabolism and stressor response. Slightly elevated nitrate concentration could in fact promote the health of coral meta-organism. With increase in nitrate concentrations, coral larvae showed significant stress response to maintain the coral-algal symbiosis and coral-algal symbiosis was impaired, while Symbiodiniaceae switched photosynthetic states for ATP synthesis, material transport and nitrogen metabolism for symbiosis maintenance under the control of the coral hosts.Conclusions: Our results suggest that adjustment of coral-algal symbiosis via coral control and a shift in Symbiodiniaceae photosynthetic states serves as the basis of coral meta-organism adaptation under eutrophication stresses. The larvae of P. damicornis and Symbiodiniaceae displayed different transcriptomic responses to nitrate enrichment. Coral larva meta-organism can adapt to moderately elevated nutrient concentration while extreme eutrophication can impair coral-algal symbiosis and affect coral larvae survival ultimately.