Stressed-Out Coral.

• Author: Gorder, Pam Frost
• Position: THE ENVIRONMENT

If you ever have been stressed out and fighting a sinus infection, then you know something of what coral will endure in the face of climate change. They do not have sinuses, but these colorful aquatic animals do actually make mucus, and the balance of different species of bacteria living in their mucus is very important, because it functions as an ad hoc immune system, ensuring that the coral stay healthy by keeping unfriendly bacteria at bay.

In a study appearing in the journal PLOS ONE, researchers at Ohio State University and their colleagues have demonstrated how two separate effects of climate change combine to destabilize different populations of coral microbes--that is, unbalance the natural coral "microbiome"--opening the door for bad bacteria to overpopulate corals' mucus and their bodies as a whole.

"Just like we need good bacteria to be healthy, so do coral," says Andrea Grottoli, professor of earth sciences. "Coral don't have immune systems like humans do, but the microbes living in and on their bodies can impart immune-like function. When that falls apart, they can become sickly."

The goal of the study, she indicates, is to help guide conservation efforts in advance of the expected rise in ocean temperature and acidity by the end of this century, as forecast by the Intergovernmental Panel on Climate Change.

"If we want to make good decisions about which coral populations are more resilient and which ones need more help, this study suggests that we have to take their associated microbial communities into account."

Many questions remain about how coral immunity works. Researchers still are piecing together the complex role that microbes in and on human bodies play in human immunity, and how those microbes respond to stress, but this study is the first to probe how the coral microbiome and physiology respond to simultaneous stresses of temperature and acidification.

Grottoli's team tested two species of coral that are extremely common around the world, Acropora millepora, or staghorn coral, and Turbinaria reniformis, or yellow scroll coral. Staghorn is a branching coral, while yellow scroll is a wavy coral resembling cabbage or lettuce leaves.

Some of the colors of both species come from symbiotic algae that live inside the animals' cells. Many researchers have studied how stress causes coral to expel their algae and turn white, a phenomenon called bleaching. In recent years, microbes have emerged as a third component of coral...