Overabundance of Algal Symbionts Increase Susceptibility of Coral Bleaching

Increasing water temperatures due to global climate change are causing mass declines in live coral cover due to coral bleaching. Understanding the factors that make corals more susceptible to bleaching is important in managing protection efforts. However, not much information is currently known about the relationship between algal symbiont density and coral bleaching. Cunning and Baker (2012) show that symbiont cell ratio density is dependent on both environmental conditions and symbiont type. Their results support previous research suggesting that higher densities of symbionts do not buffer coral’s thermal tolerance and in turn, corals with a higher abundance of symbionts are more susceptible to bleaching events.—Kelsey Waite
Ross Cunning, Andrew C. Baker. Excess algal symbionts increase the susceptibility of reef corals to bleaching. Nature Climate Change, 2012; 3: 259–262.

                  Cunning and Baker collected 53 colonies of Pocillopora damicornis from shallow reefs (3–6 feet depth) at Taboga, Panama. Corals were maintained in an outdoor tank for a six-month period of natural seasonal warming that ended with a bleaching event. Branch tips were collected from each colony three times during the warming period and twice during the bleaching event. Symbiont DNA was then extracted and Symbiodium (algal) types were indentified. Using quantitative PCR, symbiont cell ratio densities—a ratio of the total number of symbionts to the total number of host cells—were calculated for each sample.
                  Cunning and Baker found that symbiont cell ration densities increased with warming but decreased with the onset of acute bleaching. As coral host cells are lost during warming, symbionts are expelled to keep cell ratio densities below a certain threshold. Although both symbiont and host cell densities decrease during warming, there is a greater net loss of host coral cells, resulting in an increased symbiont to host cell ratio. Since symbiont cell ratio density decreases during thermal stress (bleaching events) yet increases during natural warming, Cunning and Baker suggest the cellular and molecular underpinnings of these losses are different.
                  Coral colonies with a high density bleached more severely than colonies with low density, regardless of the symbiont type. This difference is attributed primarily to the fact that corals with more algal symbionts will produce more reactive oxygen species (ROS), which trigger bleaching—suggesting that corals with high symbiont cell ratio densities are more vulnerable to climate-change-induced bleaching. Furthermore, the severity of bleaching events may be influenced by stressors such as ocean acidification and poor water quality, which change symbiont density. Managing these stressors can significantly help corals survive climate change. 

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