Are
corals a shining beacon at night? Corals are not just a wonder to observe
during the day, at night they glow. This isn’t just for our viewing benefit; it
plays a vital role in the long term survival of coral.
Fluorescence of Porites cylindrica |
Due to the richness of life they
create, corals are often described as the rainforests of the ocean. Their
structural complexity supports one of the world’s most productive ecosystems
providing ecological diversity and outstanding beauty. The coral animal (polyp)
co-habitats its calcium carbonate skeleton with photosynthetic algae called
zooxanthellae. These algae harness energy from solar radiation and provide the
polyp with 95% of its food. Coral is therefore limited to the habitat range of
the algae, which in turn is limited by the penetration of the suns ray into the
ocean; both the intensity and spectral diversity of light dramatically decreases
with increasing depth. Although the blue/green portion of sunlight reaches
depths of around 200m the algae requires the higher light levels found in the
upper 30m of the ocean. Corals are therefore limited to the upper portion of
the ocean; aptly named the sunlight ocean.
![]() |
Spectral diversity of white light (sunlight) and the depth that the light waves penetrate. Image credit to:http://ksuweb.kennesaw.edu |
The corals exposure to high light
levels is crucial for its survival, but this is not without consequence. The high
light intensity that corals are subjected to everyday can damage coral and zooxanthellae
– similar to our skin and sunburn. Shallow water corals have a solution to this:
fluorescence. The coral contains special pigments (green fluorescent pigments (GFP)
and non-fluorescent chromoproteins (CP) which act as sunblock. The fluorescent
pigments are in particularly high concentrations and contribute to the beautiful
rainbows of colours which can be observed on the reef. When the coral is
subjected to high sun exposure the pigment concentration increases, hence limiting
the damage experienced by the algae when under stress from sunlight. The
pigments are also involved in growth related activities, including repair. Injured
coral will produce colourful patches concentrating these pigments around their
injury site which prevents further cell damage. Some corals have been found to distribute
fluorescent pigments around their tentacles and mouth to attract prey.
We are able to observe the fluorescent
pigments when corals are illuminated at specific wavelengths (generally blue
light). In high pigment concentrations corals can become shining beacons at night.
Light is absorbed by the pigments and then re-emitted. During this process some
energy is lost resulting in a different colour being observed – generally green.
During our blue light night snorkel it is possible to see corals glowing on the
house reef at Gili Lankanfushi.
Fluorescence of Porites cylindrica |
It
is now widely accepted that fluorescent pigments aid in sun protection, so why
do corals below 30m still have these pigments? In shallow reefs generally only
green fluorescence is observed, whereas in the mesophotic zone (between 30 –
100m) corals shine green, orange, yellow and red. Fluorescent pigments are
energetically costly to create, therefore the pigments must have a biological
purpose, or else they would not exist at this depth. A study carried out by the
University of Southampton found that deeper corals produce fluorescence without
light exposure, which suggests that these corals are not producing pigments for
sun protection. It is suspected that the corals are producing pigments to
transform short light wavelengths received into longer wavelengths to enhance
algae photosynthesis, thus producing more food for the polyp. It has also been
suggested that it may link to behaviour of reef fish, although more studies are
required. Next time you are night diving take a look. Harnessing these
fluorescent pigments could pose significant advances for medical, commercial
and ecological purposes.
![]() |
Many Acropora species also have fluorescent pigments. Credits to: Reef Works |
Marine biologists at the Scripps
Institution of Oceanography at San Diego have suggested that monitoring
fluorescence could be an easy and less invasive way to monitor reef health. Scientists
measured the fluorescence levels after corals were exposed to cold and heat
stress. The levels were reduced when exposed to both stresses, although coral
subjected to cold stress adapted and fluorescence levels returned to normal. Corals
subjected to heat stress lost their algae and starved. Therefore, if high
fluorescence levels are observed it suggests that the reef has a healthy coral
population. Additionally there are many medical benefits that can be gained through
the understanding and utilisation of coral fluorescence.
There are promising applications for biomedical imaging, for example pigments can be used to tag certain cells e.g. cancer cells which can then be easily viewed under the microscope. The fluorescent pigments also have the potential to be used in sun screen. Fish feeding on coral benefit from the fluorescent pigments which suggests that the pigments move up the tropic levels (food chain). Senior lecturer from King’s College London and project leader of coral sunscreen research, Paul Long and his team have suggested that if the transportation pathway up the food chain is identified it may be possible to use this to protect our skin against UV rays in the form of a tablet. This could a break-through in terms of reef safe sun screen.
There are promising applications for biomedical imaging, for example pigments can be used to tag certain cells e.g. cancer cells which can then be easily viewed under the microscope. The fluorescent pigments also have the potential to be used in sun screen. Fish feeding on coral benefit from the fluorescent pigments which suggests that the pigments move up the tropic levels (food chain). Senior lecturer from King’s College London and project leader of coral sunscreen research, Paul Long and his team have suggested that if the transportation pathway up the food chain is identified it may be possible to use this to protect our skin against UV rays in the form of a tablet. This could a break-through in terms of reef safe sun screen.
Next time you are night snorkeling
shine a blue light on the corals and view this natural wonder yourself!
No comments:
Post a Comment