Applications of Coral Fluorescence

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Fluorescence is an often overlooked property of reef-building corals that can improve the classification of reef habitats from imagery and provide diagnostic information on corals.

Applications of Coral Fluorescence

Fluorescence is a trait expressed by numerous coral reef denizens, including algae, cnidarians, sponges, polychaetes, fish, crustaceans, and mollusks. The source of the fluorescence is organism-dependent. Organisms that conduct photosynthesis, either directly or via endosymbiotic algae, contain chlorophyll, which fluoresces red. Anemones, reef-building corals, and other cnidarians often contain additional pigments in their epithelial cells that fluoresce at a variety of wavelengths, corresponding to colors from cyan to red. These fluorescent pigments are homologs of the green fluorescent protein (GFP) first isolated in the north Pacific jellyfish Aequorea victoria. The function and ecological significance of GFPs in corals remains an open question and an area of active research. Most efforts have been focused on the diversity, biochemical structure, and possible physiological role of GFPs in corals.

The purpose of this task is to evaluate the potential of fluorescence as an investigative tool for two applications: classifying coral reef habitats and identifying disease-induced physiological stress.

Fluorescence endmember library

Fluorescence endmember library. Based on empirical measurements, most fluorescent signals observed on Caribbean reefs are attributable to one or more of these spectra. (Public domain.)

Fluorescence endmember library

Fluorescence functional groups. These 15 spectra denote the most common fluorescent emissions measured on Caribbean reefs. Note: All of the stony coral functional groups (1-7) also include a chlorophyll component. Fluorescence intensities are plotted in arbitrary units, and the spectra indicate typical peak-to-peak ratios. See PLoS ONE article for details. (Public domain.)

Improving Seabed Classification with Fluorescence

Basic classification of Caribbean seabed features (e.g., live coral, dead coral, rubble, sand, and algae) is difficult with conventional reflectance-based remote sensing. Fluorescence spectra may provide additional distinguishing features to improve classification results. A modeling experiment using a constrained set of fluorescence spectra representing 15 ecological functional groups common to Caribbean reefs yielded an average classification accuracy of 87%. For details, see PLoS ONE article "Fluorescence-Based Classification of Caribbean Coral Reef Organisms and Substrates."

Diagnosing Coral Disease

Mitigation of impacts from coral diseases and syndromes requires better diagnostic techniques to detect the onset of disease. Ideally, these techniques would be rapid and non-destructive. Prior work (Changes in the fluorescence of the Caribbean coral Montastraea faveolata during heat-induced bleaching) has shown that the same GFP fluorescent emissions that may aid classification can be used as a proxy for temperature-induced stress in corals. Analyzing the differences between diseased and healthy specimens, might enable the development of an index relating fluorescence to the onset of disease or precursors to it. Such a diagnostic tool would provide researchers and resource managers with a rapid, non- invasive means to assess coral health.

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Daylight and fluorescent images of a Montastrea cavernosa coral colony growing on the seawall at Ft. Jefferson, Dry Tortugas National Park. The brownish color of the coral in daylight is largely attributable to chlorophyll contained within the symbiotic algae harbored by the coral. The fluorescent image was taken at night using blue LED lights to stimulate fluorescence. The green color is attributable to a compound called green fluorescent protein (GFP) contained in the coral tissue, not the symbiotic algae. 

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Daylight and fluorescen images of a Orbicella faveolata coral colony in Dry Tortugas National Park. In this colony, the GFP fluorescence is so intense that it overwhelms the weaker red fluorescence of chlorophyll. 

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Daylight and fluorescent images of two coral colonies, Diploria clivosa (left) and Orbicella faveolata (right) growing on the seawall at Ft. Jefferson, Dry Tortugas National Park. The two colonies are separated by an area of dead coral that has been colonized by algae. In the fluorescent image, red corresponds to chlorophyll and green to GFP. Note that the GFP is concentrated in the center of each coral polyp.