Corals select algal partnerships to mitigate environmental stress: study

Corals live in symbiosis with a variety of microscopic algae that provide most of the energy that corals need, and certain algae can make corals more resistant to heat stress. By evaluating one of the main reef builders in Hawai’i, Montipora capitata or rice coral, the researchers found that the community of symbionts in these corals varied greatly in different parts of Kaneohe Bay.

In the study, published recently in the Royal Society Open Science, researchers tagged and collected 600 rice coral colonies in Kane’ohe Bay, off the windward side of O’ahu, Hawai’i. They identified the algal symbionts in the colonies and collected environmental data, such as temperature and sedimentation, in each part of the bay.

While scientists knew that corals harbor a diversity of symbionts, it is unclear if algal species change from area to area and what would drive these changes.

Cladocopium and Durusdinium are the two genera of algae most commonly hosted by corals in the Pacific Ocean. Cladocopium is found widely, while Durusdinium is usually found in shallow corals exposed to high light or sea surface temperature, or in areas with high temperature variability; and is associated with increased resistance to heat stress.

“Coral in the far north and far south of Kane’ohe Bay harbored fewer stress-tolerant symbionts,” said study lead author Mariana Rocha de Souza, who was a graduate student at the Institute. of marine biology from UH Manoa at the time of the study. “It makes sense, as these areas experience less light, less warming and less temperature variation. However, we were surprised to see that the symbiote really responded to these fine environmental differences in certain parts of the bay – something that hadn’t been found in other studies.”

Heat stress is the main threat affecting corals worldwide. Sea temperatures in many tropical regions have risen by almost a degree Celsius over the past 100 years and continue to warm.

“Our fine-scale sampling of coral colonies over a relatively small (~10 km) spatial gradient in Kane’ohe Bay showed that the community structure of algal symbionts can respond to the conditions in which the coral lives,” said Rocha de Souza. “This paves the way for understanding the role of environmental conditions in shaping the distribution of algal communities in space and time.”

Healthy coral reefs are among the most biologically diverse and valuable ecosystems on Earth. However, corals in Hawaii and around the world have been affected by climate change, resulting in coral bleaching and potential coral death.

“Understanding the symbionts present in corals in Hawai’i and what determines symbiont community composition can help us predict how these corals will respond to future heat stress,” Rocha de Souza said.

During the 2019 bleaching event in Kane’ohe Bay, the research team resampled these corals and are currently analyzing the data. Their next step is to study how corals with different symbionts from different parts of the bay reacted to the bleaching event.

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