“Taste” and “Smell” of Coral Reefs Provide Insights into Dynamic Ecosystem

Coral reefs are hotspots of biodiversity and are amazingly productive with a vast number of organisms interacting simultaneously. Hundreds of molecules that are made by important members of the coral reef community were recently discovered by a team of scientists led by researchers at UC San Diego’s Scripps Institution of Oceanography.

Together, the molecules—modified amino acids, vitamins and steroids—comprise the “smell” or “taste” of corals and algae in a tropical reef, and will help scientists understand both the food web dynamics and the chemical ecology of these ecosystems.

This study provides the first snapshot of the diversity of dissolved chemicals floating among coral reefs and a window into the interactions among organisms that scientists are just beginning to understand.

“There were several surprises with our findings,” said Scripps Oceanography marine microbiologist 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 percent 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.

The study was co-led by the University of Hawai‘i at Mānoa and the NIOZ Royal Netherlands Institute for Sea Research. It was published Feb. 1 in the Proceedings of the National Academy of Sciences.

Although corals and limu (seaweeds) are fixed to the seafloor, these organisms interact via chemicals dissolved in the water. Despite knowing the importance of these molecules built during photosynthesis and released into the seawater environment, biologists have struggled to understand their quantity, energy content, and structural diversity.

The science 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 co-author Andreas Haas of the NIOZ Royal Netherlands Institute for Sea Research.

In the reefs surrounding Mo’orea, one of the Society Islands of French Polynesia, the scientists collected specimens from two reef-building corals (boulder coral and cauliflower coral), one calcified red alga (crustose coralline algae), one brown alga, and one algal turf (a mix of microscopic filamentous algae). Then, they isolated and analyzed the molecules that each organism released into the seawater during photosynthesis in the daytime and, separately, at night when photosynthesis ceases.

They found that these organisms release hundreds of different kinds of molecules which ultimately influence the chemistry of the seawater. These chemical 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.

Another key finding was the demonstration that the molecules released by corals contained many more nutrients than those made by algae, which may have major implications for the availability of nitrogen, phosphorus, and sulfur in these reef ecosystems in the face of environmental change. Perhaps more importantly for reef food webs, the work showed that the combination of molecules released into the water by seaweeds were more chemically reduced.

“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,” said Haas.

Reefs worldwide are changing and degrading under local pressures from human misuse and overuse as well as global threats of 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 University of Hawai‘i researcher Craig Nelson, co-lead author of the work. “Understanding what shifts like this mean to the chemistry of an ecosystem is critical for managers, and this work demonstrates differences in the chemical exudates of corals and algae that can help us understand what changes in corals and algae mean for the ecosystem.”

In future work, the science team will observe how the diverse array of molecules behaves on reefs including 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.

Additional co-authors of the study from Scripps Oceanography include graduate students Irina Koester and Zachary Quinlan and chemical oceanographer Lihini Aluwihare. Pieter Dorrestein from the Skaggs School of Pharmacy and Pharmaceutical Sciences at UC San Diego contributed to the study as did researchers from the University of Tübingen in Germany, San Diego State University, and UC Santa Barbara.
 

Adapted from University of Hawaii