Indo-Pacific corals more resilient to climate change than Atlantic corals

In the face of global warming and other environmental changes, corals in the Atlantic Ocean have declined precipitously in recent years, while corals in the Pacific and Indian Oceans are faring better.

By describing several species of symbiotic algae that these corals need to grow, an international team led by Penn State scientists has found that these mutualistic relationships in the Indo-Pacific may be more flexible and ultimately resilient to higher ocean temperatures than those in the Atlantic.

Coral reefs are vast geological structures made of calcium carbonate produced by coral animals whose colonies possess dense populations of photosynthetic algae from the family Symbiodiniaceae — herein referred to as "symbionts" — within their tissues. Coral bleaching occurs when environmental conditions, such as rising ocean temperatures, cause the relationship between the algae and the coral animals to breakdown, resulting in a white, or bleached, colony. While corals can recover, bleaching may result in coral mortality, depending on the intensity and duration of the stress.

"Coral bleaching not only affects the corals themselves, but also entire ecosystems of organisms — from invertebrates, like sea urchins and spiny lobsters, to vertebrates, like fish and sea turtles," said Penn State biologist Todd LaJeunesse. "It's important to study the biology of corals and their symbionts so we can predict how they will respond to future environmental changes, especially ocean warming."

But, LaJeunesse said, not all corals and symbionts respond in the same way. That's because the world's oceans contain thousands of species of corals, each with their own unique attributes. And, until recently, he said, no one really appreciated the vast diversity of symbiont species and their importance to coral survival.

As LaJeunesse and his colleagues began to describe symbiont species, they learned that some are specialists — meaning they can only associate with one or a few species of coral hosts — whereas others are generalists — meaning they can associate with many species of coral hosts. In addition, they found that some corals, especially from the Caribbean, rely on specialist symbionts, whereas corals from the Indo-Pacific associate with generalists.

The lack of flexibility among Caribbean corals may make them more sensitive to environmental changes while Indo-Pacific corals with more flexible partnerships may withstand greater environmental change. "It's possible that these species may come to dominate coral communities as Earth's oceans warm and more sensitive symbionts die out," said LaJeunesse.

The U.S. National Science Foundation-supported research, published in the Journal of Phycology, provides formal descriptions for several host-generalist symbiont species in the Indo-Pacific region. To conduct their study, the researchers collected samples of coral from across the Indo-Pacific, including the reefs of Palau, Thailand, Zanzibar of Tanzania, the Phoenix Islands, Australia's Great Barrier Reef and New Caledonia.

"This study demonstrates the complexity of the coral-symbiont relationship and may help shed light on why some coral species are faring better than others in a changing ocean," said Cynthia Suchman, a program director in NSF's Division of Ocean Sciences.