Corals have weaker immunity as the climate warms

The balance of bacteria within coral mucus is very important because it serves as a type of artificial immune system, keeping corals healthy by preventing bad bacteria from entering their systems. Researchers at Ohio State University have identified two effects of climate change which can disrupt the vital balance in coral microbe populations and allow bad bacteria to take over their bodies.

“Just like we need good bacteria to be healthy, so do coral,” said Professor Andrea Grottoli. “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 primary objective of this study was to help guide the development of effective coral conservation strategies before the anticipated rise in ocean temperature and acidity, which has been predicted by the Intergovernmental Panel on Climate Change (IPCC) to happen by the end of this century.

“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,” said Professor Grottoli.

The research team focused its study on two species of coral which are found commonly across the globe, including staghorn coral and yellow scroll coral.

Symbiotic algae within the coral’s cells produce the animal’s colors. Stress causes the corals to expel the algae and turn white, an event known as coral bleaching. In recent years, microbes have also emerged as an important component of coral ecology.

Yellow scroll coral is much more resistant to bleaching in rising temperatures than staghorn coral. Researchers expected that the yellow scroll coral would also have the advantage regarding microbes, because this species makes more mucus. Most microbes in the coral microbiome live in the mucus that covers the outside of their bodies.

The researchers exposed both species of coral to a rise in temperature from 26.5 degrees Celsius to 29 degrees Celsius over the course of 24 days. The team also gradually increased the acidity of the water until it was about 80 percent more acidic.

The study revealed that the yellow scroll coral maintained a stable microbiome under stress, while the staghorn coral experienced a decline in microbial diversity and a rise in bad bacteria populations. The staghorn was much more sensitive to temperatures and bleached in response to stress, showing signs of an overall health decline.

On the other hand, the yellow scroll coral had the strongest microbiome and did not bleach. This indicates that the dynamic between this particular coral, its algae, and its microbe communities makes it exceptionally durable.

The study, which is the first of its kind to investigate how the coral microbiome responds to the strain of both rising ocean temperatures and acidification, is published in the journal PLOS ONE.

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By Chrissy Sexton, Earth.com Staff Writer