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UH study: Hawaiian corals select algae partnerships to help survive climate change

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Teamwork is even more important for coral trying to survive an increasingly warming ocean.

A new study by researchers at the University of Hawaiʻi at Mānoa found that the diversity of algae that lives symbiotically with corals in Kāne‘ohe Bay on the windward side of Oʻahu varies significantly in different parts of the bay.

Reefscape in Kāne‘ohe Bay with snorkeler. Credit: Mariana Rocha de Souza

“Understanding the symbionts present in corals in Hawaiʻi and what is driving the symbiont community composition can help us predict how these corals will respond to future heat stress,” Mariana Rocha de Souza, lead author of the study and a graduate student at UH-Mānoa’s Hawaiʻi Institute of Marine Biology at the time of the study, said in a press release.

Corals work together with a variety of microscopic algae that provide most of the energy they require. Some algae also can make coral more resilient to heat stress. While scientists have known corals host a diversity of symbionts, it has been unclear if the algae species change from one area to another and what drives those changes.

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In the study, published in Royal Society Open Science, researchers tagged and collected 600 rice coral colonies throughout Kāne‘ohe Bay. They identified the algal symbionts in the colonies and collected environmental data in each part of the bay, including temperature and sedimentation. Rice coral, or Montipora capitata, is one of the main reef builders in Hawaiʻi.

Rice coral, or Montipora capitata, was the coral used in the study in Kāne‘ohe Bay. Credit: Mariana Rocha de Souza

Cladocopium and Durusdinium are the two genera of algae most commonly hosted by corals in the Pacific Ocean. Cladocopium is found broadly, while Durusdinium is usually found in shallow corals exposed to elevated light or sea surface temperature or in areas with high temperature variability. It is also associated with increased resilience to thermal stress.

“Coral in the extreme north and extreme south of Kāneʻohe Bay hosted less of the stress resilient symbiont,” Rocha de Souza said in the press release. “This makes sense, as these areas experience less light, less warming and less temperature variation. However, we were surprised to see that the symbiont really responded to these fine environmental differences in parts of the bay—something that had not been found in other studies.”

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Thermal stress is the main threat to corals worldwide. Sea temperatures in many tropical regions have increased by almost 1 degree Celsius during the past 100 years and are continuing to warm.

“Our fine-scale sampling of coral colonies across a relatively small spatial gradient (~10 km) within Kāneʻohe Bay showed that algal symbiont community structure can respond to the conditions under which the coral is living,” Rocha de Souza said in the press release. “This sets the stage for understanding the role of environmental conditions in shaping how algal communities are distributed in space and time.”

Healthy coral reefs are some of the most biologically diverse and economically valuable ecosystems on Earth. However, corals in Hawai‘i and worldwide have been impacted by climate change, which leads to coral bleaching and potential coral death.

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During a 2019 bleaching event in Kāneʻohe Bay, the research team re-sampled these corals and is now analyzing the data. The next step is to investigate how the corals with different symbionts from distinct parts of the bay responded to the bleaching event.

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