Compromising Coral Immunity
Increasing oceanic temperatures are driving epidemics of coral disease.
In the summer of 1983, hundreds of square meters of graceful elkhorn coral on the floor of the Caribbean Sea began collapsing from disease and became overrun by a green carpet of algae. Within a few years, only rubble remained. "It wiped out most of the population of the Acropora," which are elkhorn and staghorn, says Ernesto Weil, a professor of coral reef biology and ecology at the University of Puerto Rico. Weil has been working with coral reefs in the Caribbean for 30 years.
Coral disease is a tricky problem to solve....
Scientists have linked global warming and increased water temperatures to coral bleaching, which occurs when corals release their algal symbionts into the water column. No one is quite sure why that happens, but it might be because the coral releases nitric oxide in response to the stress, which causes the algae to be released with a layer of the corals' own endothelium. Global warming is also causing increases in coral diseases such as black band. Based on anecdotal evidence, researchers think these two phenomena are related: Opportunistic infections take over when corals are weakened by stressful events such as bleaching.
To do so, those infections must overcome the well-developed innate immune defenses of coral. Corals produce highly toxic chemicals and are normally "so hard to kill," says Harvell. Like those of other marine invertebrates, cnidarian immune systems comprise three elements: antimicrobial, oxidative chemical, and even a cellular immunity. In corals, cellular immunity is based on amoebocytes, a motile phagocytic cell type that aids in tissue repair. These immune systems also rely on bacterial symbionts. Each coral "is really a complex multiorganism system," or holobiont, says Weil. Aside from the zooanthellae (photosynthetic algae), corals are also supported by hundreds of different bacterial strains that live within the coral tissue and in the coral's mucous coat. "The number of bacteria that is really there is astounding," says Ritchie.
Increasing temperature seems to affect that bacterial mix. In her studies of coral symbionts, Kimberly Ritchie, a manager of the microbiology program at the Mote Marine Laboratory in Sarasota, Fla., found that the mucous taken from healthy corals inhibited the growth of pathogenic bacteria up to ten times more than mucous from temperature-stressed corals. As much as 20% of the bacteria found in the healthy mucus showed antibiotic activity. Mucus collected from corals during a bleaching event, however, was filled with pathogenic bacteria, rather than the strains that produced protective chemicals.
The story becomes more complex. Harvell looked at one of the major reef-building corals, the sea fan (Gorgonia ventalina), and the fungal pathogen (Aspergillus) that creates purple-rimmed holes in the coral. When she experimentally infected the healthy corals with the Aspergillus and turned up the water temperature, the coral increased its production of antifungal compounds in response to the increased temperature. The fungal pathogen also replicated faster at higher temperatures and outpaced increases in coral resistance over the same temperature range.
The results make sense in the field, says Weil. Aspergillus is a common infective agent, but the sea fans he's seen can survive the disease. This isn't the case for more deadly diseases such as white plague and black band. So, understanding the dynamics of one coral-pathogen interaction doesn't easily translate to other species.
The Global Environmental Fund is now supporting a study of coral destruction to develop strategies for curbing damage to reef systems. Harvell leads a group on coral diseases that is gathering information about the infectious agents and how they affect corals around the world, one outbreak at a time. "It's kind of like the CDC, but we're not nearly that well organized."
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