Coral Reef, © USFWS Pacific

Another Potential Threat To Corals?

Dr. Nathan Kirk 
Dr. Daniel Thornhill, Coral Reef Marine Scientist

Corals are remarkable animals that build massive and complex reef structures throughout the tropical oceans. For example, the structure of Australia’s Great Barrier Reef has grown so large that it is visible from outer space! Much of the success of corals, and therefore of coral reef ecosystems, results from tiny symbiotic algae, known as Symbiodinium or zooxanthellae, that live within the coral’s cells. These symbiotic algae act like tiny plants, giving the coral energy and promoting coral growth in exchange for nutrients and a safe place to call home.

Each coral has its own symbiotic micro-algae (©Todd C LaJeunesse)

Each coral has its own symbiotic micro-algae (©Todd C LaJeunesse)

Symbiodinium are not the only organisms that make their home within corals. In fact, coral colonies contain many species, like bacteria, fungi, flat worms, annelid worms, mussels and barnacles, to name just a few. These symbiotic organisms can hurt, help or just coexist with corals. In some cases these organisms can cause corals to sicken and die, so it is important to understand how these organisms affect corals in order to protect our imperiled reefs.

One enigmatic group of microorganisms sometimes found within corals is called apicomplexans. These microbes are closely related to the Symbiodinium that help power corals and build reefs. However, unlike their algal brethren, apicomplexans are almost always parasites. The species most familiar to us is malaria (Plasmodium falciparum), which causes severe illness in people throughout the world. Apicomplexans also include the human pathogens Toxoplasma and Cryptosporidium, both of which can cause severe sickness, especially in people with weakened immune systems. Though the effects of Apicomplexans on humans and other animals are well documented, what apicomplexans are doing to corals is still anyone’s guess.

In order to better understand how common apicomplexans are within corals, researchers at Auburn University, the University of Georgia, and Defenders of Wildlife examined four different coral species in the Florida Keys and the Bahamas over several years. To our surprise, we found that nearly all coral colonies, regardless of health, were associated with apicomplexans year-round.

Bleached coral (©Oregon State University)

Bleached coral (©Oregon State University)

So what does this mean for corals that are already under stress from pollution, overfishing, ocean acidification and global climate change? In an earlier study, Drs. Steve Upton and Esther Peters found apicomplexans in corals that were undergoing partial bleaching, which can be a sign of stress. But they also found apicomplexans in healthy-looking coral tissue. Our study found something similar: we detected apicomplexans seasonally within the same coral colonies, some of which are several hundred years old and none of which showed signs of disease.

With so much wildlife depending on coral reefs – thousands of species, from fish and sponges to anemones and sea turtles – we need to know if this organism could be a threat to corals as well. We will need to learn more before we’ll know whether or not they truly are parasites of corals, and if so, how to protect coral reefs from them. This research can help us understand the threats facing coral reef wildlife, and may even help us cope with our own apicomplexan parasites.

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