Coral select algae partnerships to ease environmental stress

Corals live in symbiosis with a variety of microscopic algae that provide most of the energy corals need, and some algae can make coral more resilient to heat stress. In assessing one of the major reef builders in Hawai’i, Montipora capitata (rice coral), researchers at the University of Hawai’i at Mānoa found that the symbiote community in those corals varied significantly in different parts of Kaneohe Bay.

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

Although scientists knew that corals harbor a diversity of symbionts, it’s unclear whether the algae species change from one area to another and what would cause those changes.

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

“Coral in the far north and far south of Kāne’ohe Bay harbored less of the stress-resistant symbiote,” said Mariana Rocha de Souza, the study’s lead author, a graduate student at UH Mānoa’s Hawai’i Institute of Marine Biology. during the time of the study. “This 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 parts of the bay — something that wasn’t in other parts of the bay.” found. study.”

Thermal stress is the biggest threat to corals worldwide. Sea temperatures in many tropical areas have risen by nearly one degree Celsius over the past 100 years and are continuing to rise.

“Our fine-grained sampling of coral colonies over a relatively small spatial gradient (~10 km) in Kāne’ohe Bay showed that the structure of the algal symbiote community can respond to the conditions under which the coral lives,” said Rocha de Souza. “This provides the basis for understanding the role of environmental conditions in shaping how algal communities are distributed in space and time.”

Healthy coral reefs are among the most biologically diverse and economically valuable ecosystems on Earth. However, corals in Hawaii and worldwide have been affected by climate change, leading to coral bleaching and possible coral death.

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

During the 2019 bleaching event in Kāne’ohe Bay, the research team re-sampled these corals and are now analyzing the data. Their next step is to investigate how the corals with different symbionts from different parts of the bay reacted to the bleaching.