Surviving in the hot waters of the Arabian Gulf, the corals that live off the coast of the UAE have long been known for their extraordinary resilience.
But the source of their hardiness has been a mystery. Now a collaboration between scientists in Abu Dhabi and Britain is starting to reveal some surprising answers.
In 2010, a team of scientists from New York University Abu Dhabi (NYUAD) and the University of Southampton in the UK, set out to identify the molecular mechanisms behind the coral’s resistance to high temperatures.
Their first results, published this month in the scientific journal Marine Pollution Bulletin, contradict earlier findings that heat-tolerant marine algae that live in symbiosis with the Gulf corals could be a major part of the answer.
The algae, belonging to the genus Symbiodinium, also known as zooxanthellae, live in the coral tissue, which would otherwise be mostly transparent, said Dr John Burt, head of the Marine Biology Laboratory at NYUAD.
Like plants, the coral algae use sunlight to photosynthesise, producing sugars that sustain the coral.
In return, the coral also feed by catching plankton from the water around them, providing additional nutrients to the zooxanthellae.
A decade-old study found that the algae living in Gulf coral belonged to a group known as “clade D”, which is known to be tolerant to high temperatures.
The NYUAD group had expected similar findings when they studied Abu Dhabi reefs in 2010. But they did not.
“Instead what we found was that clade D was virtually non-existent in corals in the reefs of Abu Dhabi,” said Dr Burt. “Nobody, worldwide, would have expected this.”
They took samples from seven coral species common in the Gulf – the mound corals Cyphastrea microphthalma, Favia pallida, Platygyra daedalea, Porites harrisoni, Porites lobata and Porites lutea, and the branching coral Acropora clathrata.
The samples were collected from the Saadiyat and Dhabia reefs in October 2010. They were then subjected to molecular analysis at the Coral Reef Laboratory at the University of Southampton.
It found that rather than clade D, the predominant strains in the Abu Dhabi samples were from clade C.
While clade C is common in corals around the world, it is not usually found in corals exposed to high levels of heat stress.
This suggests clade C strains may be more capable of withstanding heat stress than previously thought.
“The general view is that clade D helps corals to survive in hot water and there is nothing wrong with that,” said Dr Joerg Wiedenmann, head of the Coral Reef Laboratory. “But obviously, clade D is not a mandatory prerequisite for survival in hot water.”
Heat stress disturbs the symbiotic relationship between coral and the algae living within them. Around the world, reefs thrive at temperatures of about 28°C. Higher temperatures can disturb the algal photosynthesis inside the coral tissue, resulting in the production of damaging oxygen radicals.
Above a certain temperature, the corals start expelling the algae from their tissue. Without the pigments of the algae, the white coral skeletons shine through the coral tissue – a phenomenon known as coral bleaching. Bleached corals can sometimes recover but in many cases die.
But the temperature at which corals bleach varies around the world. Australian corals start to bleach if water reaches 31°C, while Gulf corals are quite comfortable at that temperature, and do not start to bleach until the water reaches 35°C and 36°C.
Dr Burt suspects clade D’s previous dominance of Gulf samples could be down to timing. Between 1996 and 1998, an abnormal warming of surface ocean waters caused large-scale coral die-offs around the world, including in the Gulf. It could be that clade D was prevalent in the early 2000s as corals were recovering from heat stress, but was expelled later on.
Besides being extremely hot in summer, the Gulf gets quite cold in winter and clade D organisms are known to be intolerant to cold temperatures. It might be that corals preferentially eject clade D organisms during cold times.
However, Dr Burt believes it is too early to be sure, and further sampling is planned from other corals, and at different times of the year.
Meanwhile, other answers are coming from Dr Wiedenmann’s lab in Southampton, where since mid-2010 he has been growing corals from Saadiyat reef, as well at from other reefs around the world. That allows his team to vary parameter at a time – impossible in the wild.
He has been subjecting samples of P. lobata to controlled heat stress, comparing the Saadiyat culture with one that originated from Fiji.
“What we found is that both strains showed signs of stress, but the corals from Fiji seemed to be more affected,” Dr Wiedenmann said.
Eventually, the Fiji samples died, while the Abu Dhabi ones recovered.
“This reflects that the heat tolerance these corals show in their home environment is indeed memorised by the laboratory corals,” he added.
The two teams plan to continue their work together for at least the next three years, looking not only at how corals and their algal partners contribute to thermal tolerance, but also into the role of environmental factors such as salinity levels in the Gulf corals’ resilience.
And that, said Dr Wiedenmann, should help answer some important questions about the potential of corals to survive in the world’s warming oceans.