Molecular insights into Darwin’s coral conundrum

Published online 24 March 2023

Lab experiments show how symbiotic sea anemones and tiny algae recycle nutrients.

Letizia Diamante

Nitrogen incorporation across the tissues of a sea anemone tentacle. Magenta indicates an increase in nitrogen incorporation level compared to the baseline in blue.
Nitrogen incorporation across the tissues of a sea anemone tentacle. Magenta indicates an increase in nitrogen incorporation level compared to the baseline in blue.
Guoxin Cui
Both sea anemones and algae contribute to the critical conservation and recycling of nutrients, in particular nitrogen, which is a limited resource in coral reefs. This new evidence provides insights into a phenomenon known as Darwin’s Paradox.

A century and a half ago, Charles Darwin noticed highly productive and biodiverse coral reefs thriving in nutrient-poor ocean waters. To understand why this paradox exists, scientists need to uncover the molecular details of how ‘cnidarian’ marine animals, such as corals and anemones, and the ‘dinoflagellate’ algae symbionts that live inside their gastric cavities can efficiently distribute and recycle nutrients. 

Generally, this symbiotic relationship depends on algal photosynthesis supplying the energy needs of the cnidarians, while the cnidarians protect the algae from grazing. But it hasn’t been clear how energy-containing fixed carbon moves from the algae to its host’s cells. Scientists also don’t know how these symbiotic organisms assimilate growth-enhancing nitrogen from the surrounding nitrogen-poor environment. Until now, it has been thought that algae are the main contributors to nitrogen acquisition and assimilation. But there has been some evidence suggesting that the cnidarian hosts also play a role.

To address this, an international team led by researchers at King Abdullah University of Science and Technology (KAUST) studied the sea anemone Aiptasia in the laboratory. They compared the differences in gene expression and in the localization of glucose and ammonium transporters – proteins that channel nutrients in and out of cells – in the sea anemone living with or without its symbiotic algae. 

The study revealed that Aiptasia has transporters that can move glucose produced by the algae into the anemone. It also has transporters that absorb glucose from the environment. Additionally, the team reported two Aiptasia transporters that uptake and move ammonium, and found enzymes for nitrogen recycling. These discoveries provide further evidence that the dinoflagellate algae are not the sole contributors to nitrogen assimilation. 

Furthermore, the team verified that glucose triggers changes in the gene expression of Aiptasia’s nitrogen recycling enzymes, but does not influence the gene expression of the ammonium transporters. Thus, there must be another algal trigger signal, in addition to glucose, that is still to be determined. 

Paola Furla, professor of animal biology, and PhD student, Keyla Plichon, from Université Côte d’Azur, France, who were not involved in this study, wrote to Nature Middle East saying, “This manuscript represents a significant advancement towards unravelling and comprehending the mechanisms of symbiosis between cnidarians and dinoflagellates. Improved insights into the carbon and nitrogen exchange between the host and the symbiotic alga are expected to enhance our comprehension and preparedness for the impacts of forthcoming conditions on coral reefs. Moreover, the techniques employed in this investigation are highly promising, opening new avenues for future research to further advance our understanding of the functioning of symbiosis.”

“Revealing the molecular mechanism in sea anemones is only the first step. We are actively testing the model in similar animals, including corals. We are aiming to achieve a better understanding of this symbiotic relationship across different cnidarian taxa and ecological contexts,” says study co-author, Guoxin Cui, from KAUST.


Cui G., et al. Molecular insights into the Darwin paradox of coral reefs from the sea anemone Aiptasia. Sci. Adv. 9, eadf7108 (2023).