The relationship between coral UV-absorbing compounds (mycosporine-like amino
acids, or MAAs) and solar UV radiation in the shallow coral reef environment
has been much debated. The presence of these compounds in the mucus of
Fungiidae allowed the measurement of weekly compensatory changes in the amount
and composition of these compounds together with shifts in the wavelength of
maximum absorbance (λ max. ) in the mucus of 11
specimens of Fungia repanda (Scleractinia:Fungiidae)
during 18 months of monitoring in situ in Tahiti. The
corals received full sunlight within a topless cage (specimen depth, 1 m)
located at the Arue lagoon (northern coast of Tahiti). The amount of
UV-absorbing compounds was found to be significantly and positively related to
solar UV radiation, with a lag time of one week and with a corresponding
long-wavelength shift in the maximum absorbance without lag, but was not
significantly related to sea-water temperature or to the volume of mucus
secreted. Analysis of mucus MAAs by high-performance liquid chromatography
showed that the shift in λ max. could be related to
an increase in three components: mycosporine-2-glycine, palythine and
mycosporine-glycine.
Specimens of six scleractinian species were gathered during the austral spring
(October–November) 1994 on the external slope of the barrier reef of
Arue, Tahiti, and in the lagoon of Arutua, a Tuamotu island. Mucus of each
specimen was collected and the optical density and volume excreted were
measured. After treatment, mucus was analysed for mycosporine-like amino acids
(MAAs) by high-performance liquid chromatography (HPLC). Nine UV-absorbing
compounds were present in coral mucus at concentrations between 1 and 500 ng
g-1 mucus. Palythine and mycosporine–gly were
found in all mucus studied. Mycosporine–2glycine was recovered in
71% of specimens and shinorine in 28%. Porphyra-334 and
palythinol were identified as minor MAAs. Three recently identified MAAs,
palythine–serine, mycosporine–methylamine:serine and
mycosporine–methylamine:threonine, were also found in mucus from
Pocillopora. Within a genus, there was a qualitative
similarity in MAAs determined by HPLC, irrespective of locality. Values for
optical density of the mucus showed the ability of MAAs to protect the animal
host and endosymbiotic algae from UV-solar flux and, as inferred from the
recent literature, from oxidative forms of oxygen (HO2 .
, O 2 - ,
HO.) derived from photosynthesis.
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