Computer model helps biologists understand how coral dies in warming waters

Warming waters are triggering coral bleaching and disease in the Caribbean, Indian Ocean and Great Barrier Reef off the Australian coast. The new models explain for the first time how beneficial bacteria on coral suddenly give way to pathogens when waters warm.

"Before this study, we just had observations but little understanding of the mechanism" for what causes coral disease and bleaching, said Laura Jones, Cornell senior research associate in ecology and evolutionary biology and a co-author of a paper that appears in PLoS Biology, published by the Public Library of Science.

Justin Mao-Jones '08, who conducted the research as an undergraduate in the School of Operations Research and Information Engineering, is the paper's lead author. Stephen Ellner, Cornell professor of ecology and evolutionary biology, is the paper's senior author.

The model reveals how a healthy, normal microbial community in the coral surface-mucus layer protects corals from disease by preventing the invasion and overgrowth of pathogenic bacteria. But when corals are stressed by warmer temperatures (a heat spell), for example, the community of microbes suddenly switches. Species associated with a healthy coral organism -- "resident species" -- decline as pathogens associated with coral disease take their place.

The researchers used models to simulate bacterial community dynamics within the surface coral mucus under normal conditions and under warmer conditions.

"There's a critical threshold where the system jumps to a pathogen-dominated state," said Jones.

They also found that the models replicated a pattern others have observed: Once the disease-causing microbes establish themselves, they persist even if the water cools down enough to favor the beneficial bacteria. The coral is then often too damaged to recover, and the reefs begin to die.

Preventing oceans from warming will require people to curb climate change, and may be unavoidable in the short term, said Jones. But reducing poor water quality, which stresses the coral and makes the oceans more hospitable to pathogens, could perhaps ward off the sudden shift to pathogens dominating the coral surface, she added.

Kim Ritchie, a marine biologist at the Mote Marine Laboratory in Sarasota, Fla., also co-authored the paper.

The study was funded by an Emerging Infectious Diseases grant from the National Science Foundation.