An investigation from San Diego State University, published in "Nature Microbiology" explores how a process destroys the delicate food chain of coral reefs.
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 chain release a variety of nutrients that support a complex and efficient food chain, but when this system gets out of control, the cycle is broken, jeopardizing the health of the reef's reefs. coral.
Millions of people around the world depend on coral reefs for productive fishing, but overfishing in waters near reefs removes algae eaters from the environment, allowing fleshy algae populations to explode. In areas with large human populations, pollution often exacerbates the problem by stimulating algae.
The fleshy algae on reefs release large amounts of nutrients known as dissolved organic carbon (DOC), which feed on microbes. Researchers theorize that when there are lots of microbial food producing algae, higher levels of potentially harmful microbes are also recorded throughout the reef ecosystem.
In this new abundant population of microbes, selective evolution pushes for microbes that endanger corals, either by depletion of oxygen from the environment or by disease. As corals die, the algae have even more room to take over, producing more DOC and creating a runaway feedback loop that leads to more coral death and microbes take over the ecosystem.
As reefs are dominated by fleshy algae, "most of the energy in the ecosystem goes into the microbes," says the study's lead author, Andreas F. Haas, a biologist at SDSU. "The variety of organisms that make up reefs is no longer compatible with a healthy system," adds the author of the study.
Haas and co-author Mohamed FM Fairoz of the Sri Lanka Oceanic University, along with their colleagues, set out to test this theory by collecting more than 400 water samples from 60 coral reef sites across the Indian Ocean, Pacific and Atlantic. In the lab, they analyzed these samples with evidence of a process called "microbialization" of algal-dominated reefs around the world - that is, with more microbes with the potential to harm reef organisms.
MORE ALGAE, MORE MICROBES
First, the team analyzed the abundance of microbes present in their samples, the results of which supported their hypotheses: They found that reef sites with the highest algal cover also had more microbes. Using metagenomic sequencing techniques, they found that on reefs dominated by algae, the microbial community is more likely to harbor harmful pathogens.
This pattern also has implications for the oceanic carbon cycle. One of the counterintuitive predictions made by this model is that because the microbes driven by algal growth are voracious, they remove DOC from the reef and limit the transfer of organic material to larger organisms like invertebrates and fish.
Indeed, Haas and his team found that on reefs with high algal cover, such as the island of Kiritimati in the central Pacific Ocean, DOC concentrations were very low, while on reefs with low algal cover, such as the Kingman reef in the northern Pacific Ocean, the amount of DOC was higher.
Across 60 sample sites and across three ocean basins, these researchers found this relationship: the higher the algae cover, the lower the DOC. "Algae always release more dissolved organic carbon than corals," says Haas, "but on reefs with more algae we see less DOC."
In summary, the study results support the idea that "microbialization" associated with increased algal cover on coral reefs can decimate reef ecosystems by microbial takeover of the ecosystem.
"Metagenomics shows us that microbes in algae-covered reefs are less efficient recyclers of carbon, shorting out the transfer of organic matter for higher organisms like fish," says co-author Craig Nelson of the Center for Microbial Oceanography. from the University of Hawaii, Manoa, United States.
As overfishing and eutrophication are two of the main causes of increased algal cover, humans must be concerned about how their actions directly and indirectly impact one of the world's most important ecosystems, conclude the authors of this work.
"This well-documented study shows that human activities are affecting coral reefs in very subtle ways," says Dr. David Garrison, program director in the National Science Foundation's Division of Ocean Sciences, which funded the research.