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The changing nature of coral reefs

Warming oceans and other environmental disturbances are highlighting key differences in the resistance of coral species to stress

Is biodiversity always a good survival tool? The answer may be a qualified “no.” Ruth Gates, a marine biologist at the Hawai‘i Institute of Marine Biology, has spent a quarter of a century studying corals and coral reefs. Gates focuses on the intimate relationships (symbiosis) between corals and single-celled dinoflagellates (zooxanthellae) that live inside coral tissues. Recent work points to a counter-intuitive finding about this relationship. Pacific corals that host low diversity and highly specific communities of zooxanthellae are more likely to survive environmental disturbances associated with climate change as compared to corals that host diverse and variable zooxanthellae communities.

Confocal Image of Coral
Confocal Image of Coral

At the most basic level, corals are colonial animals composed of thousands of individual polyps. The polyps are connected and function collectively as a colony. The corals provide shelter and some important nutrients to zooxanthellae that live inside the coral’s cells. The zooxanthellae, like all algae, use photosynthesis to harness the power of sunlight and generate food in the form of carbon. The algae feeds the carbon to its coral host. This provides the coral polyps with energy to secrete calcium carbonate skeletons that form the three-dimensional structures we know as coral reefs.

Three decades ago marine biologists believed that there was only one type of zooxanthellae in endo- symbiosis with all corals all over the world. Using DNA fingerprinting techniques, scientists discovered hundreds of different types of zooxanthellae living inside corals. “We learned that not only are there many more types of zooxanthellae than previously thought, but also that there are different combinations of zooxanthellae living in neighboring corals of the same or different species,” says Gates.

The revelations that coral–zooxanthellae relationships might be far more complex and dynamic than was previously thought led to novel and somewhat controversial theories about how variations in coral zooxanthellae unions might influence the capacity of corals to adapt to climate change. In 1993, researchers Bob Buddemeier and Dauphne Fautin hypothesized that coral bleaching, a stress response that reflects the loss of endosymbionts from coral tissues, is not all bad, and potentially provides coral an opportunity to expel less generous zooxanthellae and recruit new types from the environment which provide more support to the coral hosts. The field of coral biology has largely accepted that it is advantageous for corals to be flexible and host different types of zooxanthellae.

Gates and her team of researchers have recently compared the diversity (number) of zooxanthellae types found in different coral species and examined how sensitive or resistant to stress each species is based on how they responded to past disturbances. The results are very surprising. Gates explains, “The diversity of a coral’s zooxanthellae community relates very strongly to stress tolerance, with corals that host highly specific, less diverse communities showing more resistance to stress than corals that host diverse zooxanthellae communities which can change in composition. This is exactly the opposite of what we expected.”

Gates does not know exactly why hosting low diversity zooxanthellae communities makes corals more environmentally resistant. She suspects the specificity and stability of the unions has allowed these corals and zooxanthellae to co-evolve, integrate and communicate so effectively that they essentially function as a single entity. In contrast, the association between coral hosts and diverse zooxanthellae communities is looser. The diverse zooxanthellae challenge the immune systems of the coral, and so maintaining the relationship requires more energy. This is compounded by environmental stress. The combined impacts rapidly push these corals past a tipping point of general health that causes the symbiosis to disintegrate, the coral to bleach, and in severe cases, die. “These are two completely different biological strategies,” says Gates.

There are even hints that some zooxanthellae can act like parasites in some corals. Two years ago Gates and her research team collected corals from the remote and pristine Northwest Hawaiian Islands. “We found that a few of the corals we analyzed were sick,” says Gates. “When we examined the sick corals, we found a type of zooxanthellae that had never been found in corals in Hawai‘i before. Further testing revealed that this zooxanthellaetype was not providing sufficient food to its host corals so they were essentially starving, and were more susceptible to disease agents.”

Unfortunately, corals that host stable and specific zooxanthellae communitites and resist environmental disturbances are the massive corals resembling large rocks or boulders. In contrast, the corals that host diverse and variable zooxanthellae communities are topologically complex branching corals which serve as home to the majority of the flora and fauna living in coral reef environments. Gates believes that these findings suggest a blanket coral die-off is unlikely. “Corals are not going to go away,” says Gates. “There is hope for persistence through time. However, future reefs will probably be very will probably be very different.”

In all likelihood, the branching corals will become scarcer. Reefs of the future will be flatter, more akin to vast expanses of low-lying boulders, and support far less diversity of sea life. There is no doubt that any changes to the structure and nature of coral reefs will have an outsized impact on life on Earth. Globally, these “Rainforests of the Sea” cover less than 1% of world’s ocean surface but provide habitat for approximately one-quarter of all known marine species. These shifts will profoundly impact organisms that rely on the globally important coastal marine ecosystems, from humans to apex predators to the tiny single-celled animals that make a living on coral reefs.”

Coral Confocal Video

About the Researcher
Ruth Gates is a researcher at UH Mānoa’s Hawai’i Institute of Marine Biology. She is an author or co-author of more than 80 scholarly articles and has received research grants from the NSF, NOAA, and the World Bank, among others.

This article originally appeared in the Summer 2011 print edition of Kaunānā.