In oceans around the world, heat-resistant algae are offering the prospect of a colourful future for corals. The reef-forming animals are upgrading their symbiotic algae so that they can survive the bleaching that occurs in waters warming under climate change.
“The most exciting thing was discovering live, healthy corals on reefs already as hot as the ocean is likely to get 100 years from now,” says Stephen Palumbi of Stanford University.
Corals have a symbiotic relationship with tiny algae called zooxanthellae. The corals give the algae a home and, in exchange, the algae provide the corals with food. When water temperatures get too hot, the corals expel the algae. This is what is known as coral bleaching and it is expected to kill coral reefs around the world as global temperatures rise.
In the past few years, biologists have discovered that some zooxanthellae can live at warmer temperatures than others, making the corals that host them naturally heat-resistant. What’s more, during a heatwave on the Great Barrier Reef in 2006, an Australian team found that many corals that survived the hot period had swapped their algae for more heat-resistant ones.
To see how widespread this algae upgrading is, Palumbi and Stanford colleague Tom Oliver sampled coral colonies from tidal pools that are naturally at different temperatures on the island of Ofu in American Samoa. They found that the proportion of corals that hosted heat-tolerant algae was directly related to how hot the pools were, suggesting that they are able to adapt to their local conditions.
“From reef to reef, the number of corals that have tolerant algae varies with the local temperature regime,” says Palumbi.
The heat-tolerant algae allow corals to survive 1.5 °C rises in temperature above their usual range. In some regions, this may be enough to survive through to the end of the century despite global warming.
Palumbi says that other experiments in American Samoa suggest corals may have more tricks to survive in warmer seas. His team is currently teasing these results apart.
Ultimately, the aim is to determine which reefs will be able to survive warmer seas and which will not, so that conservation efforts can be targeted.
Journal reference: Marine Ecology Progress Series (vol. 378, p 93)
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