New ways warming water and bleaching damage reefs

Warmer ocean waters that bleach reef corals can fundamentally change the makeup of the reef, maybe permanently.

That change will dictate what our reefs will be like in the future, according to researchers writing in the journal Science.

(Image: Starfish surrounded by decomposing coral on the Great Barrier Reef. Credit: XL Catlin Seaview Survey)

Reef-building corals are complex communities, and the more we study them, the deeper the complexity goes.

A reef coral head isn’t a single entity, but a colony of coral animals called polyps. They lay down a calcium skeleton, which forms much of the rocky part of the reef. Each polyp is host to marine plants called zooxanthellae, which conduct photosynthesis and help feed the polyp. But there’s more. It turns out corals also support a community of smaller life forms, a microbiome of bacteria that, once again, both support and are supported by the coral community.

Researchers Tracy Ainsworth of James Cook University in Australia and Ruth Gates  of the Hawai`i Institute of Marine Biology at the University of Hawai`i, write in the June 24 issue of Science that climate change and coral bleaching events dramatically change that microbiome. Their article is entitled, “Corals’ microbial sentinels: The coral microbiome will be key to future reef health.”

“Corals that survive the multiple impacts of climate change and local disturbance will form the basis of future reefs that will differ in fundamental ways from those considered healthy today,” they write.

Corals are accustomed to a range of ocean temperatures. When the water temperature rises beyond that range, the corals lose their algal partners, which leaves them looking white and “bleached.”

That process also causes changes in the collection of bacteria that form part of the reef’s life, and that can further weaken coral polyps.

“The drastic impact of bleaching on the coral animals and, ultimately, its microbiome, can influence the immune system, alter the metabolic capacity and impair the stress resistance of the surviving corals,” Ainsworth and Gates write.

Since some of those bacteria are critical to the health of the corals, their disappearance can increase things like tissue death and disease. The community of corals, alga and bacteria may reach a new steady stage, but it may be a very different community after significant bleaching events occur, the authors write.

“The emergency of new ecosystem norms on coral reefs will be underpinned by changes to the microbiome and the microbial contribution to organism health and stress resistance, under new environmental norms,” they write.

© Jan TenBruggencate 2016