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New maths model explains how coral dies in warming waters | Scientists at Cornell University, US, have created mathematical models to unveil the bacterial community
dynamics behind afflictions that bleach and kill coral. Warming waters are triggering
coral bleaching and disease in the Caribbean, Indian Ocean and Great Barrier Reef
off the Australian coast. Now, new mathematical 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, according to Laura Jones, Cornell
senior research associate in ecology and evolutionary biology. Jones conducted
the research as an undergraduate in the School of Operations Research and Information
Engineering. The model reveals how a healthy normal microbial community in the
coral surface mucus layer protects corals from disease by preventing invasion
and overgrowth by pathogenic bacteria. But when corals are stressed, for example
by elevated temperatures (a heat spell), 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 the warming conditions that lead to a sudden
shift from beneficial bacteria to pathogens on the coral's surface. "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.
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