Investigating the Photo-Physiology of Symbiodinium Sub-Clades and its Relationship to Coral Bleaching

The identification of diverse genetic types of symbiotic algae within the genus Symbiodinium was established over a decade ago. Since then the resolving power of the genetic identification used has increased to the point where multiple clades and sub-clades are now distinct. Observations of differences in susceptibility within and between coral species in regard to coral bleaching has been equally well documented over the last ten years. The role that different Symbiodinium types play in controlling bleaching susceptibility has gained much focus due to recent research indicating that certain Symbiodinium sub-clades often occur in symbiosis with bleaching tolerant corals. The aim of this thesis was to determine if physiological differences existed between various sub-clades of Symbiodinium and how they may explain the disparity in bleaching susceptibility seen in the field during bleaching events. This involved several experiments on both the intact host and isolated Symbiodinium of various reef building corals under experimental conditions. These varied from increased temperature to the addition of exogenous active oxygen species. The results of these studies suggested that Symbiodinium sub-clades show various physiological differences in both their tolerance to experimental conditions and the capacity of protective measures. The Symbiodinium sub-clades C3 and C15 showed differences in chlorophyll fluorescent measurements with distinct reductions in the dynamic yield of PSII, dark-adapted yield of PSII, relative electron transport rate and non-photochemical quenching at elevated temperatures for both freshly isolated Symbiodinium and for intact corals (in-hospite). Differences were also shown in the capacity of Symbiodinium types to use non-photochemical quenching, measured by chlorophyll fluorescence, and correlated with the size of the xanthophyll pool as well as its cycling, although appearing not to relate to temperature tolerance in this instance. Considerable differences were also found in light acclimation of different isolated Symbiodinium, while light acclimation did not differ significantly in-hospite. The effect of the addition of exogenous active oxygen species correlated with results obtained from experimental heating, suggesting that temperature tolerance to some extent was related to the capacity of Symbiodinium to deal with oxidative stress. From the results presented here it is hypothesised that perhaps damage to the thylakoid membrane occurred during both temperature and oxidative stress and that the observed differences in tolerance between C3 and C15 Symbiodinium sub-clades to both of these stress events could be explained by the disparity of thylakoid damage. Differences between Symbiodinium types may therefore explain the apparent variation in bleaching tolerance in some instances. Although this thesis proposes physiological differences between some Symbiodinium types, more research is needed to determine the full extent to which this controls bleaching susceptibility on a reef-wide scale. The outcomes of this thesis also indicate the considerable involvement that host variability plays in controlling bleaching susceptibility even amongst closely related coral species. Therefore, Symbiodinium type alone can not be used as the sole indicator of bleaching tolerance within reef building corals