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Coral reefs face harmful vicious circle

by Kurt Kleiner Jun 16 / 16

When coral reefs begin to degrade, they enter a feedback loop in which runaway algal growth feeds harmful bacteria that further harm the corals. New research confirms that this feedback loop occurs in degraded reefs all around the world.

Forest Rohwer (San Diego State University), a senior fellow in CIFAR’s Integrated Microbial Biodiversity program, published the paper with co-authors in Nature Microbiology.

“Once coral reefs begin their decline a process of microbialization sets in and begins to snowball, and the health of the reefs just keeps getting worse,” Rohwer says.

Coral reefs are the most productive and diverse marine ecosystems in the world, and are home to a quarter of all ocean species. But they are under pressure from overfishing, warming waters, acidification and other environmental stresses. The new work shows how stressed reefs become even unhealthier as the ecosystem is colonized by new types of microbes.

Corals are small animals that create hard shells around themselves, and often co-exist with an algae called zooxanthellae. In a healthy coral reef, the corals and the algae form the base of a food web for a balanced and self-sustaining community.

But human overfishing can lead to an increase of grass-like turfing algae, and later seaweed-like fleshy macroalgae, which are normally controlled by feeding fish.

Rohwer and his colleagues tested the idea that this increased algal growth provided extra nutrients that led to an overabundance of harmful bacteria. Paradoxically, these bacteria then use up the nutrients and oxygen around the coral, and even attack the corals directly. This leads to a feedback loop in which corals continued to die and algae continued to multiply.

Rohwer and his colleagues collected more than 400 samples from 60 coral reef sites around the world and tested them. They found that the process of microbialization occurred all over the world. Wherever reefs were experiencing greater algal growth they also showed higher levels of bacteria, and lower levels of oxygen and nutrients in the water. The samples also showed greater numbers of pathogenic bacteria.

“Healthy coral reef communities exist in a delicate balance. Once you disturb that balance you can see a number of negative effects that reinforce one another. This work emphasizes the danger coral reefs face, and we hope will help in the fight to save the reefs,” Rohwer says.