Since 1980, 94 percent of coral reefs have experienced at least one episode of severe bleaching. They think probiotics have a real chance to reverse some damage that has already been done, enabling once threatened corals to flourish and strengthening new human-bred corals that are transplanted onto ailing reefs. On Florida’s reefs, corals the Mote team has grown from fragments merged into larger colonies that in 2020 began successfully spawning in the wild. For skeptics of coral probiotics, this complexity and unpredictability is a source of unease. Peixoto will dose corals with a multistrain cocktail that has a much broader aim—to fortify corals against bleaching and the ill health that follows.