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Researchers capture first snapshot of dissolved chemicals floating among coral reefs

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Scientists studied the floating chemicals of the reefscape in the waters off the island of Moorea in French Polynesia. Photo credit: Shayle Matsuda/UH.

A team of international researchers from the University of Hawaiʻi at Mānoa and five other institutions published a study that provides the first snapshot of the diversity of dissolved chemicals floating among coral reefs — and a window into the interactions among organisms, which scientists are just beginning to understand.

The study was published in the Proceedings of the National Academy of Sciences.

Decades of research have shown coral reefs are hotspots of biodiversity and are amazingly productive with a vast number of organisms interacting simultaneously. But only recently have a team of scientists discovered hundreds of floating molecules that are made by important members of the coral reef community.

Together, the floating compounds — modified amino acids, vitamins and steroids — comprise the “smell” or “taste” of corals and algae in a tropical reef.

Despite knowing the importance of these molecules built during photosynthesis and released into the seawater environment, their quantity, energy content and structural diversity always have been a mystery to biologists.

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Although fixed to the seafloor, coral and seaweeds (limu) interact via chemicals dissolved in the water. Learning about these dissolved chemicals will help scientists understand the food web dynamics and the chemical ecology of these ecosystems.

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Global impact

Reefs worldwide are changing and degrading under local pressures from human misuse and overuse, ocean warming and acidification.

“One common global shift is a change from coral dominance to increasing biomass of limu, associated with a shift in the structure and function of the ecosystem and the quantity and types of fish and invertebrates that thrive there,” said the study’s co-lead author Craig Nelson, an associate research professor in the UH Mānoa School of Ocean and Earth Science and Technology.

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“Understanding what shifts like this mean to the chemistry of an ecosystem is critical for managers,” Nelson said. “This work demonstrates differences in the chemical exudates of corals and algae that can help us understand what changes in coral and algae mean for the ecosystem.”

Thousands of molecules 

The team of researchers was led by Scripps Institution of Oceanography at UC San Diego, UH at Mānoa and the NIOZ Royal Netherlands Institute for Sea Research. Co-authors on the study include researchers from University of Tübingen (Germany), San Diego State University and University of California at Santa Barbara.

Graduate students from the University of Hawaiʻi and UC San Diego sample coral reef exometabolites. Photo Credit: Craig Nelson, UH

The team applied a cutting-edge analytical technique, known as untargeted tandem mass spectrometry, to characterize the thousands of small molecules that organisms use for growth, communication and defense.

“We have known for years that organic molecules play a big role in the fate of coral reef systems, but until now we did not have the analytical capabilities to analyze the dynamics of thousands of different molecules that make up the coral reef ‘exometabolome,’” said Andreas Haas, senior author on the work.

In the reefs surrounding the FrenchPolynesia island of Moʻorea, the team collected specimens from two reef-building corals (boulder and cauliflower), one calcified red alga, one brown alga and one algal turf. They isolated and analyzed the molecules each organism released into the seawater during photosynthesis in the daytime and at night when photosynthesis ceases.

They found these organisms release large amounts of hundreds of different compounds, which influence the chemistry of the seawater. The compounds determine nutrient concentrations, the growth of decomposers, and the availability of vitamins and minerals essential to the plants and animals which inhabit coral reefs.

Snapshot of the diversity

“There were several surprises with our findings,” said Linda Wegley Kelly, co-lead author of the work. “First, very few molecules were universal to all five of the organisms we studied. Even the two species of corals made few of the same molecules — more than 85% of the molecules we measured were unique to just one specific organism.” 

The study demonstrated the release of more than 1,000 distinct molecules with diverse structures, pointing the way forward for new explorations into marine natural products.

Another key finding was demonstrating that the molecules released by corals contained many more nutrients than those made by algae, which may have strong implications for the availability of nitrogen, phosphorus and sulfur in these reef ecosystems.

Perhaps more importantly for reef food webs, the study showed the combination of molecules released into the water by seaweeds were more chemically reduced. 

Haas explained: “Algae potentially provide more energy to bacteria in the reef than do corals, with implications for how increasing algae on reefs alters the transfer of energy through microbes into larger organisms in the reef ecosystem.” 

In future work, the team will observe how the diverse array of compounds behaves on the reef. This includes which molecules disappear rapidly, which build up, and whether any of the molecules are taken up directly by other plants and animals that make up the reef community.

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