Photo: Pexels/Francesco Ungaro

Coral reefs around the tropics are feeling the heat and scientists have been sounding the alarm about their prospects as temperatures continue to rise. Yet not all of them may be doomed. Temperature-resilient corals might weather climate change, it has been found by scientists.

Now new research indicates that the symbiosis between corals and certain heat-tolerant algae could also help save reefs, which are among the most biodiverse places in the world.

“Our findings refute the general perception that reef-building corals with thermally tolerant algal symbionts grow poorly,” says Todd LaJeunesse, a professor of biology at Penn State University.

“Instead, these warm-adapted partnerships better tolerate severe marine heatwaves and are likely to expand ecologically and dominate reef ecosystems in the future,” the scientist explains. “While reefs of the future may not look pretty with low diversity and greatly diminished ecosystem services the resilient animals left behind will likely continue to provide food and habitat for other animals, and some reef growth to the ecosystems they’ve created.”

Coral reefs are structures created by coral colonies of tiny individual sea-anemone-like polyps whose tissues contain dense populations of photosynthetic algae called dinoflagellates. These dinoflagellates called symbionts vary in their ability to tolerate high temperatures, the scientists explain.

“When the ocean gets too warm, many symbiont species die, and their coral hosts die along with them. And when coral reefs collapse, fisheries, tourism and ecosystem services, such as hurricane buffers, are also at risk,” they note.

Following the 1997-1998 El Niño Southern Oscillation event in the Eastern Pacific Ocean when water temperatures rose by 2 to 4°C, native corals that hosted Durusdinium glynnii symbionts survived while corals with Cladocopium latusorum symbionts perished.

“Clearly, corals that associated with D. glynnii were at an advantage during that extreme heat event, but does hosting these temperature-resistant symbionts come at a cost?” says Mark Warner, a professor of marine science and policy at the University of Delaware.

“Previous research has suggested that the costs of thermal tolerance manifest as reduced nutrient translocation from symbiont to host and significant negative physiological tradeoffs, such as reduced growth and reproductive success. We wanted to know if a similar tradeoff could occur in corals and whether this could affect the fate of coral reef ecosystems,” he elucidates.

The researchers compared the growth and reproduction of Pocillopora corals hosting the thermally tolerant D. glynnii symbiont and the more sensitive C. latusorum symbiont, both of which are common throughout the Indian and Pacific oceans.

“We found that D. glynnii provided the capacity to endure water temperatures that compromise most coral-dinoflagellate mutualisms without noticeable tradeoffs,” saus Kira Turnham, a scientist at Penn State who led the research. “This partner combination grows and reproduces just as well as the more temperature-sensitive partnership.”

The findings highlight “the importance and incredible biology of coral symbioses,” Turnham stresses.

“By investigating the coevolutionary history of the symbioses, providing a contextual lens and using improved symbiont species recognition, we can make more meaningful predictions about the persistence of corals as oceans continually warm from climate change,” the scientist says.

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