Coral being overgrown by fleshy algae. Photo by Jennifer Smith, Scripps Institution of Oceanography
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