As climate change turns up the heat on the world’s oceans, coral are becoming more susceptible to bleaching, the bone-white pallor that results from when a reef’s been overwhelmed by warm water. Coral scientists have been scouring the globe for signs of hope, and they may have found an unlikely candidate in an invasive microbe.
Coral are dependent on their algal symbionts, tiny microbes that live within each coral polyp and provide it with food. When water temperatures get too hot, though, the symbionts start throwing off toxic free radicals, and the coral reflexively eject them. If the polyp goes too long without a symbiont, it dies.
For a polyp, there are essentially two ways to survive. One, wait it out and hope they can outlast the heat wave, or two, adopt a more heat-tolerant symbiont. Luckily, there is one microbe, Symbiodinium trenchii, that fits the bill. S. trenchii is native to the Indian and Pacific Oceans, but recently, it invaded the Caribbean.
In many instances, finding a new invasive species is cause for alarm. They have a knack for unsettling balanced ecosystems. In the Caribbean, after a bleaching event in 2005, it did just that, but the results weren’t all bad. Here’s Ed Yong, reporting for Not Exactly Rocket Science:
That year, the Caribbean experienced exceptionally high temperatures; as a result, more than 80 percent of its corals bleached. It was a catastrophe, but for S.trenchii, it was an opportunity. In the months before the bleaching event, LaJeunesse found this species in less than 1 percent of the corals. During the event, he saw it in 20 percent of them. “We found it in the most severely stressed animals,” he says. “We have never seen it behave like this elsewhere.”
The good news is that S. trenchii lets its host polyps endure water that’s a couple degrees warmer, but at a cost: Colonies with S. trenchii build their calcareous exoskeletons at almost half the normal rate.
That could spell trouble down the line for Caribbean reefs. As warmer waters increase the number of bleaching events, the adoption of S. trenchii is likely to increase. But the new microbes could compound the effects of ocean acidification, which also makes it harder for corals to build their exoskeletons. Either way, Caribbean corals are likely to face a difficult future.