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Excess algal symbionts increase the susceptibility of reef corals to bleaching

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Abstract

Rising ocean temperatures associated with global climate change are causing mass coral bleaching and mortality worldwide1. Understanding the genetic and environmental factors that mitigate coral bleaching susceptibility may aid local management efforts to help coral reefs survive climate change. Although bleaching susceptibility depends partly on the genetic identity of a coral’s algal symbionts2, the effect of symbiont density, and the factors controlling it, remain poorly understood. By applying a new metric of symbiont density3 to study the coral Pocillopora damicornis during seasonal warming and acute bleaching, we show that symbiont cell ratio density is a function of both symbiont type and environmental conditions, and that corals with high densities are more susceptible to bleaching. Higher vulnerability of corals with more symbionts establishes a quantitative mechanistic link between symbiont density and the molecular basis for coral bleaching, and indicates that high densities do not buffer corals from thermal stress, as has been previously suggested4. These results indicate that environmental conditions that increase symbiont densities, such as nutrient pollution5,6, will exacerbate climate-change-induced coral bleaching, providing a mechanistic explanation for why local management to reduce these stressors will help coral reefs survive future warming.

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Figure 1: Cell ratio densities of clade C and D symbionts.
Figure 2: Changes in temperature and mean symbiont cell ratio density.
Figure 3: Clade- and density-dependence of density changes during warming and bleaching.

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Acknowledgements

We thank J. Maté and the Smithsonian Tropical Research Institute Naos Marine Laboratory for assistance with coral collections and export permits (ANAM SE/A-117-08, CITES SEX/A-140-08), the STRI Marine Environmental Program for the use of temperature data, M. Schmale for microscope use, C. Hurt and the University of Miami Molecular Core Facility for sequencing assistance, and J. Ault for statistical advice. Further support was provided by the National Science Foundation (OCE-0527184 to A.C.B. and OCE-0526361 to P. Glynn) and a Pew Fellowship in Marine Conservation to A.C.B. R.C. was supported by a University of Miami Fellowship and a National Science Foundation Graduate Research Fellowship.

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A.C.B. conceived the study and collected the corals. R.C. designed and validated the qPCR assays, conducted the repetitive sampling and analysed the data. The authors jointly interpreted the data and wrote the manuscript.

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Correspondence to Ross Cunning.

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The authors declare no competing financial interests.

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Cunning, R., Baker, A. Excess algal symbionts increase the susceptibility of reef corals to bleaching. Nature Clim Change 3, 259–262 (2013). https://doi.org/10.1038/nclimate1711

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