Scientists discover new clues to devastating coral disease

Biologists at The University of Texas at Arlington have reported findings from a study to learn how different coral species respond to a devastating disease and which species are more vulnerable.

The project examines the effects of stony coral tissue loss disease (SCTLD), which first appeared in 2014 in the waters around Florida and began spreading to the rest of the Caribbean in 2018-2019. The research was conducted by Laura Mydlarz, a UTA biologist, and her colleagues in the U.S. Virgin Islands and elsewhere.

The U.S. National Science Foundation-supported paper is published in Nature Communications. Kelsey Beavers, also of UTA, is the lead author of the study. 

SCTLD is a rapidly spreading disease characterized by rapid tissue loss and high mortality rates in coral. It has affected corals along the entire 350-plus miles of Florida's coral reef zone and corals in 22 Caribbean countries and territories, including the Virgin Islands and Puerto Rico. Corals are under stress and more susceptible to disease because of climate change.

The team examined gene expression in five coral species to understand which species are more susceptible to the disease, which are affected more severely and how fast the disease spreads in specific coral. 

"We really wanted to figure out what are some responses to the disease and whether that could give us an idea of what the disease is, because even after all this time, we don't know what is causing stony coral tissue loss disease," Beavers said. 

"We did see a difference in species susceptibility that provided a framework for gene expression and found a new potential mechanism for this disease — the coral getting rid of its own symbionts," Mydlarz said. "The symbionts provide the food and the energy for the coral."

A symbiont is an organism living in a mutually beneficial relationship with another organism. In this case, the symbiont is Symbiodiniaceae, which are algae that live inside coral. The coral provides a safe home for the Symbiodiniaceae, and the Symbiodiniaceae use sunlight to produce food for the coral.

The researchers found that SCTLD infection causes an increased expression of the gene rab7, which is involved in a process called symbiophagy; the coral digest dead or dysfunctional symbionts.

"This is the first disease where we've found that the coral may be mounting a response against this symbiont instead of a general immune response," Beavers said. "This indicates that the symbionts that typically live happily inside the coral and give it energy could actually be the source of the disease."

Data gathered during the study showed that in addition to trying to digest their own symbionts to get rid of them, the corals are also undergoing some measure of starvation.