Abstract
High sea surface temperatures often lead to coral bleaching wherein reef-building corals lose significant numbers of their endosymbiotic dinoflagellates (Symbiodiniaceae). These increasingly frequent bleaching events often result in large scale coral mortality, thereby devasting reef systems throughout the world. The reef habitats surrounding Palau are ideal for investigating coral responses to climate perturbation, where many inshore bays are subject to higher water temperature as compared with offshore barrier reefs. We examined fourteen physiological traits in response to high temperature across various symbiotic dinoflagellates in four common Pacific coral species, Acropora muricata, Coelastrea aspera, Cyphastrea chalcidicum and Pachyseris rugosa found in both offshore and inshore habitats. Inshore corals were dominated by a single homogenous population of the stress tolerant symbiont Durusdinium trenchii, yet symbiont thermal response and physiology differed significantly across coral species. In contrast, offshore corals harbored specific species of Cladocopium spp. (ITS2 rDNA type-C) yet all experienced similar patterns of photoinactivation and symbiont loss when heated. Additionally, cell volume and light absorption properties increased in heated Cladocopium spp., leading to a greater loss in photo-regulation. While inshore coral temperature response was consistently muted relative to their offshore counterparts, high physiological variability in D. trenchii across inshore corals suggests that bleaching resilience among even the most stress tolerant symbionts is still heavily influenced by their host environment.
Highlights
The mutualistic symbioses between scleractinian corals and their endosymbiotic dinoflagellates, Symbiodiniaceae, is critical to the continual growth of coral reef systems
We examined the relationship between symbiont identity to coral stress response in four coral species from inshore Rock Island and offshore barrier reef habitats during a short-term exposure to high temperature (32 °C)
Analysis of molecular variance (AMOVA) suggested that some clustering of variance was attributed to the host coral species (e.g. C. chalcidicum)
Summary
The mutualistic symbioses between scleractinian corals and their endosymbiotic dinoflagellates, Symbiodiniaceae, is critical to the continual growth of coral reef systems. As climate projections suggest a continued increase in seasonal water temperatures and more frequent bleaching events[12,14], efforts are underway to understand the biology of reef corals in inshore and offshore habitats[11,15]. Some of this resistance to thermal stress could be attributed to the dominance of a thermally tolerant endosymbiont, Durusdinium trenchii, among inshore corals[8]. Resistance to thermal stress may correspond to local inshore acclimatization or a higher proportion of adapted coral genotypes that have successfully proliferated over many generations[21,22]
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