Ultraviolet-B radiation stimulates shikimate pathway-dependent accumulation of mycosporine-like amino acids in the coral Stylophora pistillata despite decreases in its population of symbiotic dinoflagellates
Jm. Shick et al., Ultraviolet-B radiation stimulates shikimate pathway-dependent accumulation of mycosporine-like amino acids in the coral Stylophora pistillata despite decreases in its population of symbiotic dinoflagellates, LIMN OCEAN, 44(7), 1999, pp. 1667-1682
Colonies of Stylophora pistillata maintained for four years in indoor aquar
ia in the near absence of ultraviolet radiation (UVR) contained only small
amounts (<5 nmol mg(-1) protein) of 10 identified mycosporine-like amino ac
ids (MAAs, which act as UV sunscreens), the largest number reported in any
organism. The concentrations of most MAAs increased linearly or exponential
ly when colonies were exposed to ultraviolet-A (UVA) and ultraviolet-B (UVB
) for 8 h d(-1) in the presence of photosynthetically active radiation (PAR
). Total MAA concentration reached 174 nmol mg(-1) protein after 30 d, with
palythine and mycosporine-2 glycine constituting more than half of the fin
al total. WE specifically stimulated MAA accumulation: after 15 d, MAA leve
ls in colonies exposed to PAR alone and to PAR and WA did not differ (7 and
5 nmol MAA mg(-1) protein, respectively), while those in colonies exposed
to PAR and UVA + UVB were significantly higher (28 nmol mg(-1) protein). Gl
yphosate, an inhibitor of the shikimate pathway, eliminated or reduced the
UV-induced accumulation of most MAAs during 7 d of exposure, providing the
first experimental evidence of their synthesis via this pathway in a coral
symbiosis. Densities of zooxanthellae in colonies of S. pistillata, Acropor
a sp., and Seriatopora hystrix exposed to UVR for 15 d were only one-third
of those in control colonies unexposed to UVR. This net decrease in the num
ber of zooxanthellae in the corals (bleaching) occurred despite W-stimulate
d increases in algal cytokinesis and in the host cell-specific density of z
ooxanthellae in hospite, increases that apparently destabilized the symbios
is and caused expulsion of the zooxanthellae.