Why corals bleach

An ecosystem at its limit

Coral reefs are important marine ecosystems, providing habitat, food, and shelter for countless species. Many coral species live in symbiosis with photosynthetic algae. The algae provide nutrients, while the corals offer protection and carbon dioxide in return. However, this symbiosis is extremely sensitive to rising sea temperatures.

Photosynthesis takes place in special membranes in algae, known as thylakoid membranes. When sea temperatures rise, this system becomes unbalanced because the excess energy can no longer be processed sufficiently. As a result, the corals reject the algae, losing both their colour and their most important source of energy: the corals bleach. They can survive short-term bleaching events, but if the loss of algae persists, the corals die.

Looking inside living corals with neutrons

Previous analytical methods often required complex sample preparation, which can damage or kill the cells. The researchers therefore turned to small-angle neutron scattering, a non-destructive method that allowed them to study the algae and, for the first time, analyse their structure within the living host. “Neutrons enable us to observe the membranes directly during active photosynthesis,” explains Dr Christopher Garvey, co-author of the study and instrument scientist at FRM II.

The experiments conducted at the KWS-2 small-angle scattering instrument from Forschungszentrum Jülich at FRM II in Garching revealed that changes in the distance between algae membranes provide important clues about their physiological stress. “On the one hand, this highlights the strength of neutron scattering for investigations on living cells and, on the other hand, it can make an important contribution to protecting coral reefs in a changing climate,” says Dr Christopher Garvey.