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Miraculous Red Sea corals that are resisting climate change

Romain Savary
Romain Savary
Researcher in genetics at Ecole Polytechnique Fédérale de Lausanne (EPFL) and AXA post-doctoral research fellow

Romain Savary, a biol­o­gist spe­cial­is­ing in sym­bio­sis at the École Poly­tech­nique Fédérale de Lau­sanne (EPFL), recent­ly pub­lished a study in the pres­ti­gious jour­nal PNAS1, on the corals of the Red Sea. He explores the genet­ic mech­a­nisms that give them the excep­tion­al abil­i­ty of resis­tance to cli­mate change, at a time when many reefs are under threat around the world. 

Why did you become inter­est­ed in the corals of the Red Sea?

Since 2014, the sci­en­tif­ic com­mu­ni­ty has dis­cov­ered that corals in the north­ern Red Sea are resis­tant to cli­mate change and, in par­tic­u­lar, to ris­ing water tem­per­a­tures. As a result, Aqa­ba Bay, near Eilat in Israel, has been dubbed a “ther­mal refuge”. 

It is impor­tant to know that corals do not gen­er­al­ly tol­er­ate tem­per­a­ture changes. This is what the Amer­i­can Ocean­ic Insti­tute has shown: beyond a cer­tain tem­per­a­ture vari­a­tion, corals ‘bleach’. That is to say that the sym­bio­sis between the coral and the algae with­in them breaks down, lead­ing to the lat­ter being expelled. With­out its algae, the coral is unable to access cer­tain essen­tial nutri­ents, so it dies. But the corals in Aqa­ba Bay can sur­vive in much warmer water than sci­ence predicts.

All corals have a mar­gin of tol­er­ance, an accept­able dif­fer­ence between their life tem­per­a­ture and the tem­per­a­ture beyond which they bleach. This tol­er­ance is esti­mat­ed at 1°C. For the corals of the north­ern Red Sea that we stud­ied, which belong to the Sty­lopho­ra pis­til­la­ta – a species con­sid­ered else­where to be par­tic­u­lar­ly vul­ner­a­ble to cli­mate change – this gap is much larg­er. They can with­stand a 5°C rise in water tem­per­a­ture. Hence, while their aver­age tem­per­a­ture is 27°C, these corals sur­vive up to 32°C. This is unheard of!

The Red Sea is an open-air exper­i­ment for corals. The fur­ther south you go, the warmer the water tem­per­a­ture becomes. It is as if the south­ern part of the sea allows us to see in advance what is going to hap­pen in the north.

It is thought that the corals of Eilat inher­it­ed this resis­tance dur­ing a dry spell in its ancient past. The north­ern corals must have died, and when the south­ern corals grad­u­al­ly colonised the area, as the water rose, they must have dif­fused their ther­mal resilience, accord­ing to the bot­tle­neck principle.

It is as if the south of the sea can show in advance what will hap­pen in the north.

By study­ing their genet­ics, what did you discover?

We didn’t just study their genet­ics, we also analysed the genet­ics of their sym­bi­ot­ic algae and asso­ci­at­ed bac­te­ria, i.e., their micro­bio­me. With these three ele­ments, we con­sid­ered the whole holo­biont [an envi­ron­men­tal sci­ence con­cept that describes the ensem­ble formed by a host and all the species liv­ing in its eco­log­i­cal niche]. 

When we raise the tem­per­a­ture of the water in which these corals and their holo­biont are liv­ing, we observe a rapid change in gene expres­sion. It is not the sequence of the genes that adapts to the change in tem­per­a­ture but their expres­sion pro­file, that is to say the type and quan­ti­ty of pro­teins pro­duced, which trans­forms the bio­log­i­cal machinery. 

We test­ed two sit­u­a­tions. In the first, the heat stress is rapid, the tem­per­a­ture rise lasts three hours and the holo­biont returns to a nor­mal expres­sion lev­el very quick­ly if we do not exceed 32°C. In the sec­ond con­fig­u­ra­tion, we imposed a long heat stress of 12 days. Again, two days after return­ing to a tem­per­a­ture of 27°C, gene expres­sion returned to nor­mal. So, regard­less of the dura­tion of the stress, the tem­per­a­ture lim­it is suf­fi­cient to pre­dict the sur­vival of these corals. 

And these changes con­cern both the coral genomes, the algal genomes and those of the asso­ci­at­ed bac­te­ria. Above 32°C, the com­po­si­tion of bac­te­ria (of which there are ~8,000 types) changes. It is still unknown what mech­a­nisms organ­ise these changes.

If we look in more detail at the changes in gene expres­sion, our study also shows that all genes per­turbed by heat stress return to nor­mal after a recov­ery time. The resilience of coral gene expres­sion is rapid. It is pos­si­ble that the resis­tance mech­a­nisms vary between long and fast stress, but the result is the same.

Can we then expect this reef to be safe?

It is true that no cli­mate change sce­nario pre­dicts that the tem­per­a­ture of the north­ern Red Sea will exceed 32°C. But we must also be aware that oth­er fac­tors influ­ence the resis­tance of corals, such as pol­lu­tion. It is thought that the sym­bio­sis with the algae is affect­ed by an imbal­ance of nutri­ents in the envi­ron­ment. Thus, pol­lu­tion can lead to the ejec­tion of the sym­bi­ot­ic alga and bleach­ing. High con­cen­tra­tions of nitrate in the water, linked to inten­sive agri­cul­ture, or the pro­lif­er­a­tion of algae on the coast, also affect the health of the corals. 

If we want to keep this reef as a stock of corals to help recolonise reefs that have suf­fered from cli­mate change, we must pre­serve this area. This is the pur­pose of the Transna­tion­al Red Sea Research Cen­ter that Anders Mei­bom, the direc­tor of my lab­o­ra­to­ry at the Swiss Fed­er­al Insti­tute of Tech­nol­o­gy in Lau­sanne, has cre­at­ed. It aims at facil­i­tat­ing the diplo­mat­ic com­mit­ment nec­es­sary for this research around the Red Sea.

Could these corals in prin­ci­ple be used to repop­u­late dead reefs? 

These reefs in the north­ern Red Sea, Gulf of Aqa­ba, are indeed a very good hope for the future of coral reefs because they may be the last sur­vivors of this ecosys­tem if glob­al warm­ing is not con­trolled in the short term. It is there­fore nec­es­sary to pro­tect the reefs of this region against oth­er more local threats such as pol­lu­tion or mechan­i­cal destruc­tion due to tourism.

Once pro­tect­ed, they will con­sti­tute an impor­tant stock for a nat­ur­al repop­u­la­tion of the areas where the reefs will have dis­ap­peared. But this will only be pos­si­ble if envi­ron­men­tal con­di­tions improve. 

A man­u­al repop­u­la­tion of dis­tant reefs with these corals by man would be a waste of time and a huge under­tak­ing. Pri­mar­i­ly because corals, although very resis­tant to tem­per­a­ture and oth­er vari­ables of their envi­ron­ment, the Red Sea has oth­er char­ac­ter­is­tics that make these corals sur­vive very well in its waters. 

It would be wrong to think that we could replant the Great Bar­ri­er Reef with Red Sea corals, because they might not be adapt­ed for oth­er rea­sons. More­over, the num­ber of corals in the world that would have to be replant­ed would cor­re­spond to sev­er­al bil­lion invidi­vals, an unre­al­is­tic under­tak­ing that must be left to nature. Only a more local repop­u­la­tion in the Red Sea would seem more realistic. 

In con­clu­sion, these corals rep­re­sent a hope for the future, with at least one liv­ing coral reef by the end of the cen­tu­ry that will con­sti­tute a nat­ur­al stock for repopulation. 

Propos recueillis par Agnès Vernet
1https://​www​.pnas​.org/​c​o​n​t​e​n​t​/​1​1​8​/​1​9​/​e​2​0​2​3​2​98118

Contributors

Romain Savary

Romain Savary

Researcher in genetics at Ecole Polytechnique Fédérale de Lausanne (EPFL) and AXA post-doctoral research fellow