A Coral Story

– June 5, 2023

Out of all the pictures you can find of underwater life, coral reefs are always one of the highlights. Coral reefs are actually one of the most diverse ecosystems. They are not only important for nature but also for the economy of local communities.

Coral Symbiosis

Corals are simple animals that belong to the Anthozoa class (flower animals). There are different types of coral, but most reefs are mainly made up of stony coral. These corals are connected to each other by a strong (often colorful) calcium skeleton. Corals have a special relationship with small algae called dinoflagellates, which are taken up into their cells. This is called a "symbiotic relationship." Once inside the coral, the algae provide nutrients to the coral through photosynthesis. That's why corals usually thrive in shallow, tropical waters with few nutrients and lots of sunlight.

Coral Bleaching

Due to human activities, the fragile ecosystem of coral reefs is under high pressure. Pollution and climate change, among other things, contribute to the decline of the reefs. Rising temperatures have a specific consequence: coral bleaching. At higher temperatures, the dinoflagellates start releasing substances that are toxic to the coral. The coral expels the algae they have a symbiotic relationship with, causing them to become as pale as a skeleton and eventually die. Without their algae, corals are unable to photosynthesize and they die due to a lack of nutrients.

Molecule Marking

To limit and prevent the negative effects of climate change on corals, a lot of research is being done. Researchers have studied the molecular and cellular processes involved in the cooperation between corals and algae. They discovered that corals secrete a certain molecule to mark algae for uptake. Through this marking, corals can select which algae they will take in or not. Different coral species have been found to be capable of secreting this molecule. By further studying the selection process, researchers hope to find a way for corals to select algae that are more resistant to warmer temperatures. This way, they aim to limit the decline of coral reefs.


Hu, M., Bai, Y., Zheng, X., & Zheng, Y. (2023). Coral–algal endosymbiosis characterized using RNAi and single-cell RNA-seq. Nature Microbiology, 1-12.