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coral reef and fish
A healthy reef dominated by calcifying corals and coralline algae, Line Islands. (photo credit: Jennifer Smith, Scripps Institution of Oceanography)

Coral reefs—the world’s most productive and diverse marine ecosystems—rely on a masterful recycling program to stay healthy. The corals and algae that form the base of the reef’s food web release a variety of nutrients that support a complex and efficient food chain. But when this system gets out of whack, the cycle breaks down and endangers the coral reef’s health. A new study led by researchers at San Diego State University (SDSU) and co-authored by assistant professor Craig Nelson at the University of Hawaiʻi at Mānoa Center for Microbial Oceanography: Research and Education explores how a process called microbialization destroys links in this delicate food chain.

Millions of people around the world depend on coral reefs to provide productive fisheries and they play an important role in global environmental health. Overfishing the waters near reefs, however, removes the primary consumers of algae, allowing fleshy algae to spread unchecked. In areas with large human populations, pollution often exacerbates the problem by stimulating the algae.

How more algae brings more microbes

Fleshy algae on reefs exude copious amounts of nutrients known as dissolved organic carbon (DOC), which microbes eat. Researchers theorized that when reef ecosystems have elevated levels of algae producing meals for microbes, higher levels of potentially harmful microbes can occur throughout the reef ecosystem. In this newly abundant population of microbes, evolutionary selection pressures favor microbes that endanger corals, either by depleting oxygen from the environment or through disease.

As the corals die off, the algae have even more space to take over, producing more DOC and creating a runaway feedback loop that leads to further coral mortality and microbes taking over the ecosystem, a process the scientists have termed “microbialization.”

“Reefs dominated by algae show a fundamental change in the way carbon and nutrients are recycled by microbes,” said Nelson.

“Most of the energy in the ecosystem goes into the microbes,” said the study’s lead author, Andreas F. Haas, a biologist at SDSU. “It doesn’t support the variety of reef organisms which make up a healthy system anymore.”

Testing the theory

Co-author Forest Rohwer (SDSU professor) collecting benthic microbial samples for metagenomics. (photo credit Zafer Kizilkaya)

Haas, Nelson and the international team of researchers set out to test this theory by collecting more than 400 water samples from 60 coral reef sites across the Indian, Pacific and Atlantic Oceans. In the laboratory, they tested these samples for evidence of microbialization of algae-dominated reefs worldwide: more microbes with more potential to harm reef organisms.

First, they analyzed the abundance of microbes throughout their samples. Supporting their hypotheses, they found that reef sites with higher algal cover also had more microbes. Using metagenomic sequencing techniques, they found that in algae-dominated reefs, the microbial community is more likely to harbor harmful pathogens.

This pattern has geochemical implications for the ocean carbon cycle, as well. One of the counterintuitive predictions made by the model is that the microbes fostered by algal growth are voracious, stripping the reef of DOC and limiting the transfer of organic material to larger organisms like invertebrates and fish.

Sure enough, the team found that in reefs with high algal cover, such as the island of Kiritimati in the central Pacific Ocean, DOC concentrations were very low, whereas in reefs with low algal cover, such as near the Kingman reef in the north Pacific Ocean, DOC was higher. Across the 60 sampling sites and across three ocean basins they found this relationship held true: the higher the algal cover, the lower the DOC.

“Algae always release more dissolved organic carbon than corals,” Haas said, “but in the reefs with more algae you see less DOC.”

close-up of coral
Coral being overgrown by fleshy algae. (photo credit: Jennifer Smith, Scripps Institution of Oceanography)

The big picture

In short, the study’s results support the idea that microbialization associated with increasing algae cover in coral reefs can decimate the reef ecosystem through microbial takeover. The researchers published their findings today in the journal Nature Microbiology.

As overfishing and eutrophication are two of the leading causes of increased algal cover, humans should be concerned about how their actions both directly and indirectly impact one of the world’s most important ecosystems, the researchers concluded.

—By Marcie Grabowski

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