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Environmental Biochemistry
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Environmental
biochemistry
Photobiological
Chemistry of Coral Symbiosis
Walter C
Dunlap
Australian Institute of Marine Science
J Malcolm Shick
Department of Biological Sciences,
University of Maine
Synopsis
| Shallow-water environments of
tropical coral reefs are characterised by
high levels of ultraviolet-A (UVA,
320-400 nm) and ultraviolet-B (UVB,
280-320 nm) radiation. This is due to the
thinness of the earth’s ozone layer
near the equator and to the
UV-transparency of tropical ocean waters.
All shallow-water marine organisms have
natural features and behaviors that can
reduce exposure to UV radiation and limit
the amount of photodamage to functional
biomolecules and organelles. |
Shallow-water
coral reef
|
In marine organisms this
protection often includes the production
of natural UV-absorbing compounds
("sunscreens") and related
antioxidants. This protection is
phylogenically widespread among marine
organisms occurs in all shallow-water
habitats at the global scale.
Photobiology
| The clear waters surrounding
tropical coral reefs are typically
oligotrophic (nutrient-poor), yet these
reefs sustain high productivity by
supporting dense populations of marine
organisms. This paradox is resolved by
many coral reef invertebrates by
accommodating unicellular, endosymbiotic
algae (Symbiodinium spp., commonly
referred to "zooxanthellae")
within their tissues. This
photoautotrophic symbiosis allows for a
beneficial exchange of nutrients between
the algae and animal host. |
Elkhorn
coral
|
Organic carbon produced by the
algal partner is released to the host for
nutrition while inorganic metabolic
wastes are recycled to fertilise algal
photosynthesis.
| Such algal-animal symbioses are
especially common in the shallow-water
environment of coral reefs where they are
typically exposed to high levels of
visible and ultraviolet radiation. In
clear water, UV radiation can penetrate
to ecologically significant depths
(<20 m) and this presents an
evolutionary challenge to symbiosis.
Since algal symbionts reside within coral
tissues, overlying tissues of the animal
host must be transparent to facilitate
the penetration of the visible
wavelengths of light required for algal
photosynthesis. |
Coral
symbiotic Zooxanthellae
algae
|
The photosynthetic requirement
for symbiosis thus precludes the
morphological development of a protective
covering (hair, scales and feathers in
higher vertebrates) to intercept
potentially harmful wavelengths of UV
radiation from reaching vulnerable
biochemical sites in both partners. This
problem is exacerbated by the release of
photosynthetic oxygen within the host
tissues which, in combination with high
light intensities, is a potential cause
of photooxidative stress to the
symbiosis.
| Having evolved at low latitudes
where exposure to high UV intensities are
part of the normal environment, it is not
unexpected that corals have developed an
efficient defence against the potential
damage of long-term solar irradiation.
Shibata in 1969 was first to discover the
presence of a UV-absorbing substance
("S-320") in the aqueous
extracts of shallow-water corals.
Concentrations of S-320 in corals were
subsequently observed to vary inversely
with depth, presumably in compensation to
the ambient levels of solar UV radiation
penetrating to their habitat. |
Photomicrograph
of a coral polyp showing
clusters of endosymbiotic algae
|
This correlation may explain the
observation by Siebeck that corals
collected from depths of 1-2 m were found
to be more resistant to artificial
near-UV exposure than corals of the same
species growing at depths greater than
5-6 m.
| S-320 has since been identified
in corals to be comprised of a family of
mycosporine-like amino acids (MAAs)
having absorption maxima in the range
310-360 nm and were originally discovered
as common metabolites found in a diverse
range of marine organisms. The parent
class of mycosporines was prior
discovered as fungal metabolites
occurring in the developing mycelium
associated with light-induced
sporulation. Their reproductive function
has later been postulated to provide UV
protection to fungal during propagation
by atmospheric transport spores while
exposed to direct solar irradiation. The deleterious effects of solar
UV radiation in the coral reef
environment was first demonstrated by
Jokiel in 1980 by observing that cryptic
reef epifauna were killed when relocated
and acutely exposed to ambient levels of
UV radiation in shallow water.
|
Typical
MAAs found
in marine organisms
|
From these observations Jokiel
suggested that the community structure of
coral reefs was profoundly affected by
the relative UV tolerances of their
constituent species. Jokiel and York were
later to provide physiological evidence
that enhanced UV exposure reduces
skeletal growth in a reef-building coral,
Pocillopora damicornis, and
reduces the photosynthetic capacity of
its isolated endosymbiotic zooxanthellae
measured in vitro. However,
analyses of MAAs in coral tissues reveal
that concentrations are predominantly
associated with the coral tissue, and
this protective barrier would account for
why photosynthesis by symbionts in
hospite (within the host) under
equivalent conditions is largely
unaffected by UV exposure under
prevailing environmental conditions.
MAAs are assumed to be produced
by the algal partner in coral symbiosis
since biosynthesis involves the shikimic
acid pathway, a biochemical route
unavailable to invertebrate synthesis.
The major distribution of MAAs in coral
symbiosis, however, resides within the
animal tissues suggesting that the algal
partner provides UV protection to the
whole of the symbiosis via MAA
translocation. A protective function for
these compounds is inferred by their
efficient UVA- and UVB-absorbing
properties, together with the often
observed correlation between MAA
concentrations and natural or
experimental levels of UV exposure. It
has also been determined that
oxy-carbonyl mycosporines (e.g.
mycosporine-glycine), but not the
imino-carbonyl MAAs, may also function as
a physiological antioxidant, whereas the oxidative
robustness of imino-mycosporines is in
keeping with their primary function as a
stable sunscreen to provide long-term UV
protection.
Sources and
functions of MAAs
| MAAs are nature’s sunscreen
in the living marine environment. They
are typically found in high
concentrations in algal producers, but
are also common in higher invertebrates
and are often found together with several
biochemically related gadusols having
strong antioxidant properties [see A
novel antioxidant derived from seaweed]. MAAs have been identified in
taxonomically diverse marine organisms,
including a heterotrophic bacterium,
cyanobacteria, microalgae (phytoplankton)
and macroalgae (seaweeds). |
Crown-of-Thorns
starfish feeding
on a coral colony
|
Within non-symbiotic marine
invertebrates MAAs have been identified
in echinodems (starfish and sea urchins),
a mussel, a sea hare, brine shrimp
and an ascidian. MAAs are also found in
the eyes, skin and reproductive tissues
of tropical and temperate fishes. They
are common in microalgal-invertebrate
symbioses on coral reefs and elsewhere,
including: sponges, scleractinians (hard
corals) including their eggs and mucus,
sea anemones, octocorals (soft
corals), a zoanthid, a jellyfish,
tridacnid clams and ascidians. MAAs
occur in organisms from tropical and
subtropical coral reefs to the Antarctic
Ocean where they may protect benthic
species and the planktonic food
web of the southern oceans from extreme
fluctuations of UV exposure under the
"ozone hole" caused by the
depletion stratospheric ozone [see Antarctic
research].
| Assuming that invertebrates are
unable to synthesize MAAs, dietary
accumulation of MAAs by non-symbiotic
reef consumers may be a significant
pathway for UV protection. This pathway
was hypothesized on observing high
concentrations of MAAs (mostly
pathythine) in the dermal tissues of the
Crown-of-Thorn Starfish feeding on the
MAA-rich tissues of corals. |
A
coral reef holothurian
|
MAAs are also trophically
acquired by holothurians (sea cucumbers)
from ingesting sediment epiflora whereby
algal MAAs are relocation to epidermal
tissues and gonads suggesting a
photoprotective function against topical
UV exposure and during reproduction; MAAs
are also present in the enteric contents
and gut tissues consistent with a dietary
origin. This trophic pathway of MAA
accumulation has been confirmed in sea
urchins by controlled feeding
experiments, and the photoprotective
function of MAAs was clearly demonstrated
by the improved reproductive success of
fertilized eggs rich in MAAs after
exposure to simulated levels of full
solar radiation.
Commercial
sunscreen development
| Shallow-water corals produce
large quantities of MAAs and their
apparent ability to withstand long-term,
environmental UV exposure suggests
utilisation of the unique properties of
their natural UV-absorbing chromophore
for human and industrial sunscreen
applications.
See Sunscreen
research
|
The
next wave of sunscreens
is surfacing now
|
Reviews (with
references)
- Dunlap WC and
Shick JM (1998). Ultraviolet
radiation-absorbing
mycosporine-like amino acids in
coral reef organisms: a
biochemical and environmental
perspective. J. Phycol.
34: 418-430 (1998).
- Dunlap WC, Chalker
BE, Banderanayake WM and WuWon,
JJ (1998). Nature’s
sunscreen from the Great Barrier
Reef, Australia. Int. J.
Cosmet. Sci. 20: 41-51.
Current areas
of collaborative research
- Acclimatization
and affects of UV light on coral
reproduction and recruitment.
- Photooxidative
stress and coral bleaching via
destruction of photosynthetic
pigments and/or loss of
endosymbionts.
- Trophic transfer
of MAAs in the
phytoplankton-zooplankton
food-chain and validation of the
concept of UV-induced trophic
co-adaptation.
- Affects of
environmental change on coral
reef and other marine organisms.
For further information
contact
Email: Dr Walter C Dunlap - AIMS
Townsville
Telephone:
+61 (07) 47534365
AIMS home page
web@aims.gov.au
Last updated - December 18, 2008
Copyright ©1996-2002 Australian
Institute of Marine Science
URL
http://www.aims.gov.au
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