Like an unhappy landlord, coral kicks out its tenants—symbiotic algae that live in its mineral skeleton—when the weather gets too hot. That’s because as temperatures rise, the food-providing algae start to produce something else: toxic chemicals similar to hydrogen peroxide. The chemicals, which start to secrete at higher temperatures, damage both the corals and the algae, reducing the photosynthetic ability of the algae and physiologically stressing the coral population. Expelling the algae does still more damage: It makes the corals more susceptible to disease, and it takes away their primary food supply, leading to reef bleaching and starvation (above, right). For example, fully 93% of Australia’s Great Barrier Reef is now experiencing some level of coral bleaching, according to Australia’s National Coral Bleaching Taskforce. But that means that 7% of the reef has escaped bleaching. Why? Scientists suspected that their algae houseguests might hold the answers. So a team of researchers compared the genes of algae that did well at high temperatures with those of algae that got sick after a temperature increase from 27°C to 32°C. They found that the heat-resistant algae turned on several genes associated with stress resistance, causing them to produce less of the harmful toxins, they report today in Molecular Biology and Evolution. Now that the scientists know what genes to looks for, they may be able to determine which parts of the reefs are at high risk for coral bleaching—and which parts might be able to escape the deadly consequences of climate change.