Miraculous Red Sea corals that are resisting climate change

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