MEPS prepress abstract  -  doi: 10.3354/meps07360

ABSTRACT: Seasonal variation in the composition of the algal endosymbiont community and photo- physiology was determined in the corals Pocillopora damicornis, which show high local fidelity to one symbiont type, and Acropora valida, with a mixed Symbiodinium community. Pocillopora damicornis associated with Symbiodinium C1 throughout the sampling period, while high intra-colony diversity of symbionts was found in A. valida comprising members of both Symbiodinium clades A and C. The relative abundances of Symbiodinium types varied over time. A significant decline in symbiont density in the summer of 2005 coinciding with a NOAA ‘hotspot’ warning for Heron Island was observed in both species. This also coincided with a relative increase in the presence and dominance of clade A in A. valida, particularly in sun-adapted surfaces. Pocillopora damicornis showed greater xanthophyll cycling than A. valida, irrespective of branch position and sampling time, a possible mechanism to compensate for their fidelity to Symbiodinium C1. Further, symbionts in both species showed greater xanthophyll de- epoxidation in sun-adapted than in shade-adapted surfaces correlating with non- photochemical quenching (NPQ_RLC). The effective quantum yield (Φ_PSII) of PSII suggested that sun-adapted surfaces are more sensitive than shade-adapted surfaces of P. damicornis to combined effects of relatively higher temperature and irradiance in summer. No variation was found in A. valida indicating that resident symbiont communities may not have been physiologically compromised possibly due to changes in the composition of the Symbiodinium community.