Study explores evolutionary stability of coral photosymbiosis

Coral Photosymbiosis Photo 1
UOG alumnus Jordan Gault wrote “Lineage-specific variation in the evolutionary stability of coral photosymbiosis,” which was published in September 2021 by the journal Science Advances.

A study by University of Guam researchers has examined the evolutionary stability of photosymbiosis in scleractinian corals. The study, which was funded by the university’s National Science Foundation EPSCoR grant, was published in September in the peer-reviewed Science Advances journal.  

Scleractinian corals, also called stony corals, are the hard corals that are typically seen as reef-building corals found in shallow, tropical waters that receive nutrients from the photosymbiotic algae living in their tissues. In exchange for nutrients, the algae support the calcification of coral skeletons, encouraging the growth of expansive reefs in shallow tropical and subtropical waters. Photosymbiosis is a type of symbiotic relationship between two organisms that includes one that is capable of photosynthesis.  

However, half of the order’s members are non-photosymbiotic and tend to be small, not colonial, and are found in deep waters.  

“The origin of the order has been shrouded in mystery. When scleractinian corals first appeared in the fossil record, they were already highly diversified,” said lead author Jordan Gault, a UOG alumnus who wrote the paper for his master’s thesis. “There’s evidence that some of them were photosymbiotic, but where did they all come from? If they’re diversified already, there’s evolutionary history that goes further back that you cannot see in the fossil record yet. That’s one thing we’ve set out to understand with this study.”  

Coral Photosymbiosis Photo 2
Scleractinian corals, also called stony corals, are the hard corals that are typically seen as reef-building corals found in shallow, tropical waters that receive nutrients from the photosymbiotic algae living in their tissues. This photo of a scleractinian coral was taken in Apra Harbor by David Burdick, Guam NSF EPSCoR’s Biorepository collections manager.

The study reconstructed the evolutionary history of photosymbiosis in Scleractinia by applying mathematical models to phylogenetic trees, which are diagrams that show evolutionary relationships. The phylogenetic trees included 1471 of the 1619 recognized species in Scleractinia.  

“There are certain groups where the association seems to be almost irreversibly stable. Those two partners are bound to each other for the whole group and they thrive and die together while others may be more flexible,” said UOG Associate Professor Bastian Bentlage, the co-author of this study. “There may be some lineages – if they’re not as tightly integrated with the photosymbionts – that may be less susceptible to a breakdown of these relationships. That’s really cool in terms of understanding the dynamics of what we see on our reefs in a changing climate.”  

At first, the project faced delays because the initial simulation studies took a long time to run on the computational resources that were available at the time. To address these issues, the research team used the Open Science Grid, a network of computers spread nationally that allows open access to high throughput computing for research in the U.S.  

“Facilitating this study meant relying on this grid that was able to run hundreds of thousands of individual simulations,” said Bentlage. “That wouldn’t have been possible with a desktop computer. Having access to this high-speed computing grid was very essential to finishing it off.”  

As part of the Guam NSF EPSCoR’s strategic plan, the program is working on establishing a computation hub at UOG.  

Prior to pursuing a doctoral degree at the University of Oldenburg, Gault spent eight years at the UOG Marine Laboratory pursuing his thesis research and working for the long-term coral reef monitoring program. He said that getting the paper published feels like closing a chapter in his life.   

“It’s nice and a little bittersweet. I’m proud of the work that we did and I’m happy to have it out there. The question is now: is it useful for other scientists? Does it matter going forward? The best outcome is if it somehow shapes some research down the road. If people address our results and ask questions further down the line, I think that would be excellent,” said Gault.