Surviving harsh environments becomes a death-trap for specialist corals

Researchers from the University of Southampton and the New York University Abu Dhabi found that local adaptation to high salinity levels in the southern Persian/Arabian Gulf (PAG) may prevent coral escaping their fate, as they lose their superior heat tolerance in waters with normal salinity levels.

The research is published this week in The ISME Journal.

Warm water corals depend on a vital partnership with unicellular algae of the genus Symbiodinium. Damage to the algal symbiont through heat stress can result in the breakdown of the association, leading to fatal coral bleaching. Most corals fall victim to bleaching at water temperatures above 32ºC. However, corals from the PAG region survive summer peak temperatures of up to 35ºC on a regular basis.

Senior author of the study, Professor Jörg Wiedenmann from Ocean and Earth Science at the University of Southampton, explains: "It was not clear whether this resilience is related to the presence of a new type of symbiotic alga (Symbiodinium thermophilum) that was recently discovered by our team in this region. Therefore, we used molecular markers to identify the algal partners of three coral species along the coast of the southern Gulf and the adjacent Gulf of Oman. We found that this special symbiont indeed seems to play an integral role for coral survival in the world's hottest sea."

The researchers studied corals along 1,000km of coastline in the southern PAG, a region where the world's warmest coral reef habitats are separated from the wider Indian Ocean by the narrow Strait of Hormuz. Notably, the PAG features not only record temperatures, but the water is also exceptionally salty.

Professor Wiedenmann, who runs the University's Coral Reef Laboratory based at the National Oceanography Centre Southampton, continues: "As soon as you leave the Gulf, corals start to host different symbionts. This 'partner exchange' starts when the salinity of the water approaches normal oceanic levels."

Lead-author Dr Cecilia D'Angelo, Senior Research Fellow at the University of Southampton, says: "We have simulated these conditions in our laboratory and found that corals from the Gulf lose their exceptional heat stress tolerance when they need to cope at the same time with salinity levels commonly found in coral reefs elsewhere. This may explain why the PAG-typical coral-alga associations are rarely found in the less salty water or the Gulf of Oman."

Dr D'Angelo adds: "Some corals may potentially escape their fate in waters heated by global warming by shifting their geographic distribution. However, our findings indicate that in addition to barriers such as landmasses, the lack of suitable substrate for settlement and adverse currents, the dependence on certain local environmental conditions may represent an invisible fence that could trap corals in their endangered habitat."

With rising ocean temperatures anticipated to cause a loss of most warm water reefs within the next 100 years, it has been discussed whether heat tolerant corals adapted to hot environments, such as PAG, could be used to replenish reefs damaged by global warming elsewhere.

Professor Wiedenmann comments: "Our results suggest that the transplantation of corals over large geographic distances is not a straight-forward solution to restore reefs since they may struggle to adjust to different environmental factors apart from the temperature in the new habitat. Efforts to protect coral reefs should rather focus on other measures including the reduction of nutrient enrichment, sedimentation, overfishing and destructive coastal development. At the same time all attempts should be made to reduce CO₂ emissions to prevent further global warming."