Corals don’t live solitary lives. Their existence depends on one-celled algae called zooxanthellae that take up residence inside those ornate structures. The tiny algae give corals oxygen and other nutrients (as well as their beautiful colors), and in return, the corals give the algae carbon dioxide—a symbiotic arrangement.
With global waters warming and increasing in acidity, it’s well known coral reefs are in trouble. Warmer waters cause corals to expel the life-enabling symbiotic algae that they normally pair with, triggering a suicidal process referred to as coral bleaching. Increasing acidity, on the other hand, prevents corals from absorbing the calcium carbonate they need to maintain their skeletons.
Given all these dire findings, it’s no surprise that coral reef research is a hot topic these days (so to speak). Most studies reveal fascinating portents of doom, such as the fact that stressed corals glow brightly before they die, or that sperm and embryonic cell banks might be many coral species’ last hope. A few, however, offer more promising results—such as the fact that one species of coral, at least, seems to be able to tolerate toastier conditions than previously thought.
Now, a new study published in Global Change Biology joins the coral literature, this one offering a mix of good and bad news. The good news is that some corals—specifically, fatty corals that are less discriminating about which algae they pair with—fare better when confronted with warming waters. But the overall message, unfortunately, remains unchanged: Worldwide, global warming will almost certainly cause a decline in coral diversity and reefs.
Researchers from The Ohio State University decided to see what would happen to Caribbean corals that they subjected to warm waters for two years in a row. Other studies have only tested coral bleaching as a single rather than recurring event, reflecting the fact that bleaching normally occurs in nature only rarely. But some studies predict that by 2025, it might be an annual event in the Caribbean.
The researchers collected three types of corals—finger corals, mustard hill corals and boulder corals—from Puerto Morelos Reef National Park in Mexico. They brought the corals back to an outdoor lab, where they increased the water temperature until the delicate organisms bleached. Then, they put the stressed corals back into the ocean to let them recover naturally. To quantify that recovery, they measured things such as the number of algae present in corals cells; the type of algae that came back; and how much fat those cells contained. A year later, they repeated the same process.