Abundant betaines in giant clams (Tridacnidae) and western Pacific reef corals, including study of coral betaine acclimatization

A large literature documents that betaines play significant roles in protecting photosynthesis in the face of multiple stresses, including heat and photon stresses, in terrestrial plants and free-living algae. Betaines therefore can be expected to defend against photosystem stresses (e.g. photoinhibition and bleaching) in reef-building corals and tridacnid clams (both symbiotic with algae) in addition to functioning as osmolytes employed in osmotic stress defense. None-theless, the presence of betaines has just started to be studied in corals and has never before been investigated in tridacnids. The present research demonstrates the following. (1) Betaines, especially aminovaleric acid betaine and glycine betaine (GlyB), are abundant metabolites in all 4 major tissues of 5 tridacnid species studied. (2) Pacific corals have at least 9 betaines rather than only 1 as previously reported. (3) In regards to concentrations of betaines in Pacific corals, GlyB and proline betaine (ProB) typically dominate. Taxa differ in betaine profiles, however, including that Acropora spp. are exceptionally low in total betaines and Porites spp. have (in addition to GlyB and ProB) relatively high concentrations of alanine betaine, hydroxyproline betaine, and taurine betaine. (4) Genus-specific betaine profiles in corals may well be consistent across the Pacific basin. (5) During a year of laboratory acclimatization, coral species studied declined in bulk skeletal density and underwent both increases and decreases in betaine concentrations.