Spatial and temporal dynamics of photosynthesis regulation of genetically defined coral/algal symbiosis associations

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Photosynthetic capacity of scleractinian corals relies predominantly on the productivity of single-celled endosymbiotic dinoflagellates of the genus Symbiodinium, known as zooxanthellae, residing intracellularly within coral endoderm tissue. The regulation of photosynthesis of zooxanthellae is in turn dependent on light and temperature. This thesis explores the genetic basis for variation in photosynthesis capacity of zooxanthellae by examining the photo-physiology of genetically characterised Symbiodinium communities at a range of spatial and temporal scales. In situ and manipulative experiments were conducted to improve our understanding of metabolic responses of zooxanthellae under climate change scenarios. Fine scale measurements of irradiance and photosynthesis allowed the assessment of photo-physiological changes across individual colonies of Pocillopora damicornis and Acropora valida. Pocillopora damicornis generally contain genetically homogeneous populations of Symbiodinium, whilst genetically diverse Symbiodinium communities exist within Acropora valida. Measurements of light absorption in P. damicornis were conducted using a scalar irradiance microprobe and it was found that light absorption was greatest in shade-adapted polyp tissue and smallest in sun-adapted coenosarc tissue. Genetic heterogeneities, found at the scale of individual polyps in A. valida, correlated with O2 concentration at the surface of the colony which was greater in polyps that harboured the two clades (A + C) than in polyps that only harboured clade C. In both corals, measurements using an O2 microelectrode and a fibre-optic microprobe yielded dissimilar results when used at moderate to high irradiances. Seasonal changes in photosynthetic capacity suggested that P. damicornis is more sensitive to combined effects of relatively higher temperature and irradiance in summer than A. valida suggesting that the symbiont community of A. valida may not be physiologically compromised possibly due to phylogenetic changes of Symbiodinium. Furthermore, thermal tolerances of conspecific corals were examined at narrow and wide spatial scales across the length of the Great Barrier Reef. Pocillopora damicornis, which harboured Symbiodinium type Cl, thus bleached in correlation with latitude, whereas Turbinaria reniformis bleached in correlation with the presence and absence of the known thermo-tolerant Symbiodinium clade D. The results, integrating over spatial and temporal scales suggest that the acclimatisation capacity of corals to light and temperature is determined by i) history of light and temperature exposure and in cases where corals associate with multiple Symbiodinium types ii) the distribution of Symbiodinium.
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