Reward and punishment systems in coral-algae relationship

15 January 2023

Symbiosis is a source of evolutionary innovation that occurs throughout the natural world, from microbial to macro-faunal systems. These partnerships can expand access to food and provide protection from enemies or perhaps even the changing climate. Marine ecologists in Hong Kong hone in on the complex nutrient dynamics that occur between corals and the algae that sustain them.

In the coral-algae relationship, the coral host and the algal symbiont share and recycle nutrients they cannot access on their own. But this relationship is open to abuse. Some algae may practice non-cooperation and hold back on their part of the resource-sharing deal with the coral as part of their symbiosis, thus disrupting what would typically be a mutually beneficial relationship. In such a case, corals may ‘punish’ the algae that live inside them by cutting off their food supply. These are the findings of Dr Shelby E. McIlroy and her team at the Swire Institute of Marine Science and the School of Biological Sciences at the University of Hong Kong.

Corals can host several species of symbiotic algae at the same time. Some algae will take advantage of their host by keeping more nutrients for their own needs instead of passing them to the coral. In this way, the selfish algae gain an advantage in growth over species that share their nutrients more generously. The actions of the self-serving algae ultimately undermines the long-term health of the coral itself.

Microalgae abundance and proportions affect nutrient exchange in a coral symbiotic relationship. Credit: Shelby E. McIlroy, University of Hong Kong, Research Square.

McIlroy used stable isotope techniques to follow the flow of nutrients between different species of algae in their host, a Caribbean coral species.

"Stable isotopes combined with genetic techniques allow us to track the currency exchange between the partners," McIlory said. "In this case, the currency is nutrients, in the form of carbon and nitrogen. ... Our study showed that corals seem to limit the supply of nutrients to the symbiotic algae that are less beneficial to them, as a way of fostering more beneficial algal symbionts."

Understanding how corals control and manipulate their symbiotic algae is now crucially important for coral survival. Because of climate change, the seas are becoming too hot for the algae living inside the coral. When water temperatures spike, algae die, and so does the coral itself, a phenomenon known as bleaching. Episodes of bleaching are now becoming increasingly common, and most of the world’s coral reefs are now threatened by it.

If scientists could get the coral to host the species of algae that can handle higher temperatures, it could prevent bleaching and buy more time for corals threatened by the warming of the oceans. McIlroy explains this as a form of ‘coral probiotics'.

"We may have the ability to intervene and help corals resist bleaching by exposing them to more thermally tolerant partners. But we need to understand the biology of corals and how they might react to these interventions so that we can work effectively and efficiently. There’s no time to lose." McIlroy concluded.

The results of project has been published in Microbiome.