Divers conduct research on thriving coral reef in Saudi Arabia
Scuba diving researchers investigates a coral reef in the Red Sea. Photo: Florian Roth

Coral reefs are crucial for many ecosystem services, but the rapid loss of corals due to heat stress has threatened these ecosystems for decades. Yet, the process behind coral bleaching, where food and pigment providing symbiotic algae are expelled from its coral host, have remained cryptic. Until now.

In a recent study, a research team lead by Nils Rädecker from École Polytechnique Fédérale de Lausanne (Switzerland) provides new insights into the causes of this collapse. With novel and refined techniques, the researchers show how corals start to suffer and degrade long before the bleaching is visible. The findings are now published in the prestigious journal PNAS.

- The hypothesis has long been that corals die from starvation after bleaching, but in fact, the symbiosis breaks down way earlier. Before you have any visual signs of bleaching, the breakdown of the metabolic exchange in the symbiosis has already started. Thanks to improved methods, we were able to assess this in a novel way, says Florian Roth, researcher in the Baltic Bridge collaboration and co-author of the publication.

New tools to visualize starvation

In lab experiments, the team observed corals at different temperature conditions, and by using nanoscale secondary ion mass spectrometry (NanoSIMS) and isotope markers, they tracked how the nutrient transfer between the coral and its symbionts was affected by heat stress. The results indicated quite some drama in this relationship.

- When it gets warm, the coral host has a higher metabolic energy demand to cope with the heat stress. As the algae can’t provide enough food at this point, the coral basically starts to use its own reserves, Florian Roth explains.

The breakdown of energy-rich amino acid reserves results in a high availability of ammonium for the algal symbiont, which, in turn, grow immensely and becomes such a disturbance that it eventually is expelled by the host.

Lab experiments with multiple containers of with corals in different water temperatures.
In-situ measurements of heat stress. Photo: Nils Rädecker

New questions for reef management

According to Roth and his colleagues, this new way of exploring metabolism and nutrient cycling within the coral may have important implications for practical management measures of coral reefs. While further research is needed, the findings are a critical step to understand and adjust the idea of when reef ecosystems collapse.

- Previously, we though that bleaching is the point where the system collapses. Actually, this happens already way before. If you want to act on it or assist these corals in stopping the bleaching, you need to start weeks earlier. When bleaching occurs, it may be too late, he concludes.

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Scientific publication: Rädecker, N., Pogoreutz, C., Gegner, H. M., Cárdenas, A., Roth, F., Bougoure, J., ... & Voolstra, C. R. (2021). Heat stress destabilizes symbiotic nutrient cycling in corals. Proceedings of the National Academy of Sciences118(5).