Some corals that have survived near death experiences pair up with different algal species when they recover. Three papers in leading journals have explained this previously mysterious behavior, which could prove key to coral survival in a warmer world.
Most tropical corals depend for their survival on a symbiotic relationship with photosynthetic dinoflagellates, known as zooxanthellae. The corals provide the zooxanthellae with places to grow and in return receive nutrients. Without these colorful single-celled organisms, corals are pure white.
When stressed, however, corals expel their zooxanthellae. Stresses can come in many forms, but in recent years heatwaves have dominated. Although bleached corals don't grow and are more prone to disease, they can recover if the bleaching does not last for too long.
However, marine biologists have noticed an odd feature of the recovery process. Some corals become repopulated with the same zooxanthellae that they had before, but others take on new species in a process known as symbiont shuffling. One theory holds that bleaching occurs to give corals a chance to shuffle symbionts.
Dr Ross Cunning and colleagues at the University of Miami placed the endangered star coral Orbicella faveolata in tanks and heated the water to temperatures that mimicked bleaching events. Their results were published in the Proceedings of the Royal Society B.
The authors report, “Increases in heat-tolerant symbionts after thermal bleaching can reduce coral susceptibility to future stress. [...] The proportion of heat-tolerant symbionts dramatically increased following severe experimental bleaching, especially in a warmer recovery environment, but tended to decrease if bleaching was less severe.”
“We discovered that partner switching in Caribbean star corals is dependent upon the severity of the bleaching event and the temperature during recovery," said Cunning.
Cunning was also lead author of a paper in Ecology last month that found that corals adjust the number of zooxanthellae with which they partner to match different conditions, with high diversity in the natural environment falling when coral are maintained under constant conditions.
A third paper, published by the same team earlier this year in Global Change Biology, showed that the symbionts most tolerant of heat also survive exposure to herbicide better than more sensitive species. The same paper also revealed that prolonged exposure to warm (but not bleaching) water does not give corals a better capacity to cope with heat waves.
The findings give hope that global warming will not be as devastating for coral reefs as feared, but shuffling is an imperfect solution. The authors observed that “Higher proportions of heat-tolerant symbionts linearly increased bleaching resistance but reduced photochemical efficiency, suggesting that any change in community structure oppositely impacts performance and stress tolerance.” Consequently, even corals that adapt to a warmer world will have less efficient zooxanthellae, and will therefore get less food.
Moreover, symbiont shuffling does not appear to provide any protection against ocean acidification from increased carbon dioxide, which may be even more of a threat to coral than rising temperatures.