Symbiont Dependent Thermal Bleaching Susceptiblity in Two Reef-building Corals, Stylophora pistillata and Platygyra ryukyuensis.

Abstract

Coral species, one susceptible (Stylophora pistillata) and the other resistant (Platygyra ryukyuensis) to bleaching, were exposed to a sudden elevated temperature (33.5^oC) under dim light (5 μmol quanta m^-2 s^-1) for 10 to 720 min in time course experiments and to temperatures varying from 30^oC to 40^oC at 65 μmol quanta m^-2 s^-1 for 10 min at each  temperature. Chlorophyll α fluorescence measurements in Symbiodinium of both coral species indicated that the maximum electron transport rate (ETR_max) and the maximum quantum yield of photosystem II (PSII) fluorescence (F_v/F_m) were sensitive to thermal stress. The non-stressed Symbiodinium ETR_max value in S. pistillata was halved earlier and at a lower temperature stress than in those of P. ryukyuensis. Denaturing gradient gel electrophoresis (DGGE) analysis of the internal transcribed spacer 2 (ITS2) region of the ribosomal DNA revealed inter-colony  diversity of Symbiodinium in both species, though each species contained genetically distinct Symbiodinium types. Heat dissipation in PSII, through non-photochemical quenching (NPQ), increased in Symbiodinium of P. ryukyuensis irrespective of Symbiodinium genotype (C60 and C55), while in S. pistillata it either increased (C59) or decreased (C1) depending on genotype. Thus distinct Symbiodinium ITS2 types exhibit diverse photo-physiological responses to thermal stress, and may partially explain the variable bleaching susceptibilities of some hermatypic coral species.

Keywords: coral, PAM fluorometry, Symbiodinium, thermal stress