Fluorescence could help diagnose sick corals

Imaging technique offers a new way to monitor reef health

Montipora capitata

GET YOUR GLOW ON  The reef coral Montipora capitata naturally fluoresces in red and cyan, as seen in this confocal microscopy image.

J.M. Caldwell et al/Scientific Reports 2017

Sickness makes some corals lose their glow.

Disease reduces a coral’s overall fluorescence even before any sign of the infection is visible to the naked eye, a new study finds. An imaging technique that illuminates the change could help with efforts to better monitor coral health, researchers report November 6 in Scientific Reports.

Many corals naturally produce fluorescent proteins that glow in a wavelength of light that human eyes can’t see in natural light. Previous studies have shown that heat stress and wounding, among others stressors, can affect coral fluorescence, but the new study is the first to look at the relationship between fluorescence and infectious disease.

Jamie Caldwell, a disease ecologist now at Stanford University, and colleagues used a technique called live-imaging laser scanning confocal microscopy to compare fluorescence in living fragments of healthy  and diseased  Montipora capitata coral. The reef coral, common in Hawaii, fluoresces in red and cyan, and can contract a bacterial infection called Montipora white syndrome, which causes coral lesions and tissue loss.

PICTURE OF HEALTH Healthy Montipora capitata (left) fluoresce more than diseased counterparts. A computer analysis of the image (right) shows that the fluorescence is distributed fairly equally across the coral. J.M. Caldwell et al/Scientific Reports 2017
PICTURE OF HEALTH Healthy Montipora capitata (left) fluoresce more than diseased counterparts. A computer analysis of the image (right) shows that the fluorescence is distributed fairly equally across the coral. J.M. Caldwell et al/Scientific Reports 2017

The diseased bits looked healthy at the macroscopic level, but under the researchers’ microscope, the sick coral’s pallid complexion was pronounced. Computer analyses of the microscopy images  quantified the lost glow (red is the total area of fluorescence, black regions are where fluorescence was lost, and white lines indicate edges between the two zones). Among the samples studied, healthy coral had on average 1.2 times as much fluorescence area as diseased fragments. Diseased coral had disorganized and fragmented patterns of fluorescence — similar to a forest that has been logged extensively, the researchers found. 

SICK LIST A living sample of diseased coral (left) fluoresces less than healthy samples, confocal microscopy revealed. A computer analysis of the image (right) helped researchers quantify the missing glow. J.M. Caldwell et al/Scientific Reports 2017
SICK LIST A living sample of diseased coral (left) fluoresces less than healthy samples, confocal microscopy revealed. A computer analysis of the image (right) helped researchers quantify the missing glow. J.M. Caldwell et al/Scientific Reports 2017

Such research “is transformative in our struggle to visualize the dance between pathogen attack and host response in the initial attack,” says Drew Harvell, a disease ecologist at Cornell University.

Many coral diseases appear to be increasing around the world, even when accounting for increased research effort, Caldwell says. Along with bleaching events and pollution, disease is considered one of the major contributors to reef declines globally. The new technique could be used for other coral species and diseases, she says.

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