SOURCES AND FUNCTIONS OF MYCOSPORINE-LIKE AMINO ACIDSIN MARINE INVERTEBRATES

J. Malcolm Shick and Walter C. Dunlap
Dept. of Biological Sciences, Univ. of Maine, 5751 Murray Hall, Orono,ME 04469-5751, U.S.A., and Australian Inst. of Marine Science, PMB No.3, Townsville, Q. 4810, Australia

Mycosporine-like amino acids (MAAs) are phylogenetically widespreadamong marine organisms. MAAs consist of a conjugated cyclohexenone(or cyclohexenimine) substituted in the 3-position by glycine, taurine,or methylamine, with the carbonyl (1-position) bonded to O, NH, an iminoacid, or imino alcohol. The high molar extinction coefficient ofMAAs in the range 309–360 nm, and the frequently-observed correlation oftheir concentration with solar exposure suggests a role as UV sunscreens. Likely products of the shikimic acid pathway, MAAs probably are not synthesizedby metazoans, which may obtain them from algae, bacteria, or fungi, eitherby digestion or in symbiotic association. Some species of zooxanthellaesymbiotic with cnidarians synthesize MAAs but others do not. Deposit-feedingsea cucumbers ingest benthic microalgae and cyanobacteria from which theyabsorb MAAs. Marine bacteria metabolically convert certain MAAs tothe simplest MAA, mycosporine-glycine, a conversion which can occur duringpassage of MAAs through the digestive tract. Controlled experimentson sea urchins eating macroalgae conclusively show that these consumersremove>99% of the MAAs from their diet and concentrate them in their eggs. Fertilized eggs rich in MAAs are less subject to UV-induced cleavage delaythan are MAA-poor eggs. UV-sensitivity of photosynthesis in coralcolonies, and of mitosis in their zooxanthellae, are inversely relatedto the concentration of MAAs, which are located primarily in the animaltissue and which may shield the algal endosymbionts. Mycosporine-glycinehas antioxidant activity and may also protect against oxidative stressin photoautotrophic symbioses where oxygen concentrations are high. Survival of coral larvae during exposure to solar UV is positively relatedto their mycosporine-glycine concentration.