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Science has found out why some corals are better at avoiding bleaching

The coral Montastraea cavernosa. Image: John Reed

Researchers have identified the genes that make some algae in corals more resilient to higher ocean temperatures and subsequent bleaching.

Tropical corals can’t survive without the photosynthetic organisms Symbiodinium algae living inside them and providing them with more than 90% of their food.

Higher water temperatures stress the algae, causing them to produce an excess of toxic chemicals which damage both the algae and the corals.

The corals then expel the heat-stressed algae, become bleached white and starve to death unless they are recolonised by new algae.

“For the first time, we have uncovered the mechanism that explains why some algae can withstand higher temperatures and avoid bleaching,” says study first author and University of NSW PhD student Rachel Levin.

“We found they can switch on genes to produce proteins that neutralise the toxic chemicals.”

The study is published in the journal Molecular Biology and Evolution.

The research team includes professor Peter Steinberg, director of the Sydney Institute of Marine Science, and professor Madeleine van Oppen of the Australian Institute of Marine Science and the University of Melbourne.

The researchers compared two cultures of algae originally isolated from coral at a warm location and at a cooler area on the Great Barrier Reef.

The algae from the cooler area becomes damaged and are expelled by corals under heat stress, but the algae isolated from the warmer location remain healthy under heat stress.

The team studied the algal genes to uncover those that are activated or de-activated over the course of a heat-stress experiment. The microscopic algae have more genes than humans do.

“We found that only the algae from the warmer reef can activate specific types of genes when under heat stress to counter the damaging effects of the reactive oxygen species,” says Levin.

“The risk of coral bleaching in different areas on the Great Barrier Reef could be assessed by using these genes as markers.”

Professor van Oppen says the study produced another surprising result.

“We also discovered that, under stress, both types of algae may switch from their normal asexual mode of reproduction to sexual reproduction,” she says.

“Sexual reproduction helps speed up evolution and may allow some algae to adapt quickly enough to tolerate the rise in sea surface temperature. This could be a natural ‘golden ticket’ that allows some corals to survive a bleaching event.”

Large areas of the northern Great Barrier Reef have been bleached. This is said to be due to unusually high sea surface temperatures.

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