Excess algal symbionts increase the susceptibility of reef corals to bleaching

Rising ocean temperatures associated with global climate change are causing mass coral bleaching and mortality worldwide(1). 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 symbionts(2), the effect of symbiont density, and the factors controlling it, remain poorly understood. By applying a new metric of symbiont density(3) 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 suggested(4). These results indicate that environmental conditions that increase symbiont densities, such as nutrient pollution(5,6), 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.