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A microscopic mystery at the heart of mass-coral bleaching

2020; National Academy of Sciences; Volume: 117; Issue: 5 Linguagem: Inglês

10.1073/pnas.1921846117

1091-6490

Amy McDermott,

Marine and fisheries research

In the summer of 2017, a small plane hummed over Australia’s Great Barrier Reef. Corals far below gleamed pale white in the sunlight, a stark contrast to the cerulean sea. The scene might have been gorgeous, if it wasn’t so bleak. At least two-thirds of the Great Barrier Reef has been bleached under the extreme stress of marine heat waves. Image credit: The Ocean Agency/XL Catlin Seaview Survey. Aerial surveys by the Australian Research Council Centre of Excellence for Coral Reef Studies in Townsville, Australia, revealed that two-thirds of the Great Barrier Reef had severely paled in 2016 and 2017, “bleaching” under the extreme stress of marine heat waves that can kill corals (1, 2). Summer 2017 marked the finale of the worst mass-bleaching event on record worldwide, three consecutive years of feverish ocean temperatures, driven by climate change, which affected more than 75% of reefs (3, 4). Newspaper headlines frequently reference bleaching events. It’s no secret that reefs are in trouble. But for all the attention to bleaching, researchers are still puzzling over the cellular mechanisms that cause it. What’s clear is that bleaching is the breakup of the tenuous relationship between a coral and the photosynthetic algae that live inside its cells. Heat stress can disrupt this relationship, causing the coral to expel its algae and to pale. New research suggests that algae, too, can be disruptive, turning on their hosts at high temperatures by switching from symbionts to parasites, which may also lead to bleaching. Coral algae have a reputation “as this friendly, only do-good kind of hero,” because they provision the coral with nutrients, says reef ecologist David Baker at the University of Hong Kong. “And I think that’s a misguided sentiment.” New efforts in genomics are helping to further explicate the basic …