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Coral reefs exude myriad chemicals, fueling dynamic microbial recycling of nutrients

New research reveals the remarkable chemical diversity of substances exuded by coral reefs and demonstrates that thousands of different chemicals derived from tropical corals and seaweeds are available for microbes to decompose and utilize.

The study, published in by an international team led by the Scripps Institution of Oceanography (SIO) and University of Hawai'i (UH) at Mānoa scientists, provides crucial insights into the intricate relationships between coral reefs, marine microorganisms, and the carbon cycle.

In dynamic ecosystems, and especially in the nutrient-limited environments where coral reefs grow, not much will go to waste. Microbes dominate when it comes to decomposing, recycling, and transforming what other organisms discard.

"We've known that some of the substances exuded on coral reefs, termed exometabolites, are available for microbial metabolism," said Craig Nelson, professor at the UH Mānoa School of Ocean and Earth Science and Technology.

"However, in this study, we discovered that the number and variety of exometabolites that microbes find useful is much higher than previously considered, and includes hundreds of compounds spanning most of the broad chemical classifications."

"We were especially surprised to discover that exometabolites belonging to chemical families traditionally thought to be harder for microbes to break down, such as benzene rings, terpenoids, and steroids, were among those that are able to be utilized," said Zachary Quinlan, lead author, postdoctoral researcher at the Hawai'i Institute of Marine Biology in SOEST, and former graduate student at SIO.

"Our results paint a highly dynamic picture of ecosystem production of bioavailable substrates and their effects on microbial metabolism relevant to carbon cycling in coastal marine environments."

Carbon cycle and reef resilience

Combined, all of the dissolved organic material in the ocean, including the chemicals exuded by coral reefs, contain an amount of carbon comparable to the amount of carbon dioxide in the atmosphere. So, the study authors point out, how microbes utilize this organic material has a major influence on the global carbon cycle.

When there is a shift in the types of organisms living on a reef, that is, stony corals versus fleshy seaweeds, the chemistry of the seawater also changes. In addition to their detailed study of what chemicals are being exuded on the reef, the research team also conducted experiments to determine whether microbes preferred to use substances from stony corals or seaweed.

"We observed that coral and algae can selectively facilitate the growth of specific microbial communities by exuding distinct chemicals that can be used by specific types of microbes," said Linda Wegley Kelly, senior author on the study and associate researcher at SIO.

"Our results highlight how shifting from coral-dominated to algae-dominated reefs can alter reef ecosystem function and impact the resilience of the system, potentially making it more susceptible to disease or bleaching."

In the future, the team aims to continue discovering how chemical features can inform coral reef management and be used to advance coral restoration success.