Temporal and environmental influences on the early establishment and\ud maintenance of coral-symbiodinium symbioses

Understanding mechanisms underlying the formation and maintenance of coral-\ud Symbiodinium symbioses as well as factors affecting the integrity of these symbioses is\ud critical to predicting how coral holobionts might change in response to warming oceans\ud predicted by climate change. Research reported in this thesis aims to enhance current\ud knowledge of coral-Symbiodinium symbioses by: (1) examining temporal variation in\ud Symbiodinium uptake by coral juveniles, (2) exploring the role of parental effects and\ud ontogenetic stage in determining Symbiodinium associations, (3) assessing the impact of\ud environmental parameters in the establishment of symbioses, and (4) evaluating whether\ud some host-symbiont combinations are more resilient to environmental stress than others.\ud \ud I found that newly settled juveniles of the corals Acropora millepora and A.\ud tenuis do not necessarily take up the Symbiodinium type present in parental colonies,\ud and that a potentially opportunist type D Symbiodinium quickly dominates symbioses in\ud juveniles of both species at three sites in the central Great Barrier Reef. I also found that\ud adult patterns of association may not become established for up to 2.5-3.5 years,\ud suggesting a delay in the expression of symbiont specificity. In A. tenuis, continuing\ud changes in Symbiodinium communities over the first 3.5 years are interpreted as finetuning\ud of specificity mechanisms leading to establishment of the homologous algal\ud symbiont characteristic of adult populations. Algal endosymbioses were much more\ud stable over the same time period in juveniles of A. millepora, although further research\ud is required to distinguish between absence of specificity and delayed expression of\ud specificity. Changes in Symbiodinium communities in A. tenuis juveniles are not linked\ud to the onset of reproductive maturity but may be linked to changes in microenvironmental\ud conditions (possibly light intensity or access to nutrients) associated with\ud growth of the colony.\ud \ud Field studies investigating the role of environmental parameters in the\ud establishment of symbioses revealed that light has little effect on the type of\ud Symbiodinium initially acquired by both A. millepora and A. tenuis. This result was\ud confirmed by experimental manipulations in aquaria where equal amounts of\ud Symbiodinium types C1 and D were offered to newly settled juveniles maintained in two\ud light levels by three temperature treatments. In contrast, I found that temperature has a\ud significant effect on algal symbioses by affecting the type of Symbiodinium acquired by\ud both coral species and by slowing and potentially stopping Symbiodinium uptake and\ud the onset of symbioses at elevated temperatures. Type D Symbiodinium was found in\ud larger proportions in juveniles at elevated temperatures (30 and 31°C), providing further\ud evidence of the infective and potentially opportunistic nature of this Symbiodinium type.\ud The benefits of type D to the host require further investigation as these juveniles had\ud low levels of infection and it is unclear if their survival would depend on other\ud mechanisms, such as a shift towards heterotrophy.\ud \ud Comparisons of the resilience of corals hosting type C1 or D Symbiodinium to\ud environmental stress indicate that A. tenuis juveniles have lower metabolic costs and\ud enhanced physiological tolerance when hosting type C1 Symbiodinium. In other studies,\ud the same D-type has been shown to confer higher thermal tolerance than both C2 in\ud adults and C1 in juveniles of the closely related coral A. millepora. My results challenge\ud speculations that associations with type D are universally most robust to thermal stress\ud and highlight a potential role of host factors in determining the physiological\ud performance of the holobiont. They also show that although the heat tolerance of corals\ud may be contingent on the Symbiodinium strain in hospite, their response to heat and\ud light stress is determined by species-specific interactions between both partners in the\ud association.\ud \ud Keywords: coral symbiosis, climate change, environmental stresses, coral growth, colony growth, heat tolerance, light tolerance, Great Barrier Reef, Australia, algal symbionts, Symbiodinium specie