Symbiodinium morphology data and statistics

Coral bleaching is the dysfunction of the coral-algal endosymbiosis and is characterized as a loss of Symbiodiniumcells from host tissues or the loss of photosynthetic pigments. Symbiosis breakdown occurs as a result elevated temperature beyond the meta-organism’s thermal threshold. Despite the impacts of coral bleaching we have not resolved the tipping points and the mechanisms underlying bleaching responses. This study characterized the cellular responses of Symbiodiniumsp. (C3) within Acropora asperaexposed to thermal stress. Thermally stressed corals experienced significant declines in Symbiodiniumdensity and dark-adapted yield following exposure to bleaching at 34 °C for four consecutive days, equating to an ecological thermal stress of 4-degree heating weeks (DHW). However large impacts on Symbiodiniummorphology and cell division were seen following exposure to temperatures between 2-3 °C (2 DHW) below the bleaching threshold, and by 4 DHW over 90% of Symbiodiniumwere degraded. While these sub-bleaching levels will not trigger bleaching alerts it is vital that we begin to quantify how these significant sub-bleaching levels will impact the fitness of Symbiodiniumpopulations, their coral hosts and subsequently reefs worldwide.