MEPS 295:135-156 (2005)  -  doi:10.3354/meps295135

ABSTRACT: This study addresses several unresolved questions regarding the biosynthesis, metabolism, regulation, and diversity of MAAs in zooxanthellate scleractinian corals. Starved colonies of Stylophora pistillata accumulated the same concentrations of mycosporine-like amino acids (MAAs) as fed corals after 28 d of exposure to photosynthetically available radiation (PAR) and ultraviolet radiation (UVR), suggesting that dietary MAAs are of little quantitative importance in this phototrophic symbiosis. Starved corals continued to accumulate MAAs and conserved them disproportionally compared with declining protein and chlorophyll a, indicating the priority placed on maintaining this UV-sunscreen defense. In 2 different colonies (SP1 and SP2) exposed to enriched (10 µM) ammonium and PAR + UVR during starvation, the final concentrations of MAAs and chlorophyll a were identical. However, under ambient ammonium (<0.4 µM), SP2 produced MAAs at the expense of chlorophyll a, whereas SP1 maintained chlorophyll a levels but synthesized less MAA. Ammonium consistently affected only the accumulation of primary, Symbiodinium-MAAs (mycosporine glycine, shinorine, porphyra-334, and mycosporine-2 glycine) and not secondary MAAs derived from the former, probably in the host’s tissues. Mycosporine-2 glycine and palythine (a secondary MAA) were synthesized by SP1 but not SP2, suggesting (1) genotypic differences between the zooxanthellae in SP1 and SP2, and (2) a biosynthetic relationship between these 2 MAAs that we proposed previously. Exposure to UVR alone did not support large-scale biosynthesis of MAAs in S. pistillata, and accumulation of the full suite of MAAs required PAR + UVR; together with an inhibitory effect of DCMU, this indicates that photosynthesis is required for the UV-stimulated, de novo biosynthesis of MAAs. Cultures of zooxanthellae isolated from S. pistillata and exposed to PAR + UVR showed increased levels of shinorine, and some production of mycosporine-glycine. Host extract had no qualitative effect on the MAAs produced by these zooxanthellae.

KEY WORDS: Mycosporine-like amino acids (MAAs) · Coral · Zooxanthellae · Ultraviolet radiation (UVR) · Dissolved ammonium · Photosynthesis