Thermal performance of scleractinian corals

Temperature has a fundamental influence on the physiology, biology and ecology of all organisms, and varies over time and space. Organisms evolved different strategies to cope with this spatial and temporal thermal heterogeneity. For instance, organisms that inhabit thermally variable environments will function over a wider range of temperatures than organisms that live in relatively constant thermal environments. Reef-building corals including their algal symbionts generally live in warm, tropical environments close to their upper thermal maxima, however their performance at varying environmental temperatures remains poorly documented. The overarching aim of my thesis is to determine how temporal and spatial heterogeneity of the thermal environment influences coral and symbiont performance. Through a series of controlled thermal experiments in this thesis I quantify the rate of photosynthesis of reef-building corals and their algal symbionts (termed the holobiont) at various temperatures using coral colonies from different thermal environments and geographic regions. This study is the first to quantify and compare the thermal optima and performance breadth for holobiont and symbiont performance from different thermal environments using thermal performance curves and thereby providing new insights into the mechanisms underlying thermal acclimation. Acclimation to environmental change takes time and does not necessarily result in full compensation of an organism's performance. In Chapter 2 I identified the acclimation trajectory of massive Porites spp. for a set of host and symbiont physiological traits during exposure to heat (31 °C) and cold (21 °C) for 30 days. Cold acclimation took approximately two weeks and resulted in 'no' or 'inverse' compensation of the performance. In contrast, I found no evidence of heat acclimation holobiont and symbiont performance declined continuously instead of reaching a steady state. These results show that there is no rapid compensatory acclimation response when massive Porites spp. are exposed to a change in the thermal environment, and that compensation of the performance is unlikely to occur in response to short-term variations in temperature. I then investigated the between-season variation in performance of two coral species with contrasting life-history strategies (Chapter 3). Acclimation to seasonal variation was species-specific, with an increase of the thermal optimum in summer for a fast-growing and thermally sensitive species (A. valenciennesi) and a change of the thermal breadth for a slow-growing and thermally tolerant species (Porites cylindrica). Additionally, the symbiont performance was less plastic than the holobiont performance indicating that the reversible acclimation mostly occurs through the coral host. Comparisons of thermal performance of coral species living in different thermal environments along a latitudinal gradient in the Great Barrier Reef (Chapter 4) demonstrated significant geographic variation in the thermal performance among populations. Acclimation of the thermal optimum to the local environment was more accurate for the symbiont performance than for the holobiont. In general, the thermal optimum for holobiont performance was ~4 – 6 °C below the environmental temperature, which may result from an inherent time lag in the mechanisms of acclimation, or from constraints imposed during early ontogeny (i.e., developmental acclimation). In Chapter 5 I assessed whether the thermal performance of temperate corals is less sensitive to changes in temperature than that of tropical corals due to their history of exposure to more variable thermal environments. To do this I compared the thermal performance of corals sampled along the GBR latitudinal gradient, with the thermal performance of corals from the Mediterranean Sea. Interestingly, despite clear differences in thermal optima, no observable differences occurred between the performance breadths of temperate versus tropical corals at either the holobiont or symbiont level. This result is likely because all of the sampled coral species had a wide thermal tolerance, which fully encompassed the total local annual variation in temperature in each location. Overall, the results of this thesis demonstrate that reef-building corals may be more generalist than previously thought. However, a high degree of inter-colony variability in thermal performance was consistently observed for all of the sampled coral species, even between colonies from the same local population. These findings indicate that despite the mean thermal optima being consistently below the average environmental temperatures for all populations, some individual colonies maintain the capacity to perform well at very high and very low temperatures, which suggest that corals may cope with environmental variability through genetic variation rather than reversible plasticity. Hopefully, such high among-colony variation can contribute to the capacity of coral populations to persist in the face of rapid climate change.