Symbiodinium morphology data and statistics

Coral bleaching is the dysfunction of the coral-algal endosymbiosis and is characterized as a loss of <i>Symbiodinium</i>cells 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 <i>Symbiodinium</i>sp. (C3) within <i>Acropora aspera</i>exposed to thermal stress. Thermally stressed corals experienced significant declines in <i>Symbiodinium</i>density 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 <i>Symbiodinium</i>morphology 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 <i>Symbiodinium</i>were 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 <i>Symbiodinium</i>populations, their coral hosts and subsequently reefs worldwide.