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

Seasonal acclimation of thermal performance in two species of reef-building corals

Thermal performance curves describe the relationship between temperature and the rate of biological processes. These relationships can vary among species and environments, allowing organisms to acclimatize to their local thermal regime. This study quantified the seasonal variation in the thermal performance of several coral and symbiont-dominated physiological traits for the thermally tolerant coral species Porites cylindrica and the thermally sensitive coral species Acropora valenciennesi. Photosynthesis rates, respiration rates, maximum photosystem II (PSII) quantum yield and electron transport rates were measured in winter and summer on coral fragments exposed to an acute temperature increase and decrease up to 5 degrees C above and below the average seawater temperature in each season. Results showed that colonies of A. valenciennesi acclimated primarily by shifting their optimal temperature to a higher temperature in summer, whereas colonies of P. cylindrica had broader thermal breadth during summer. For symbionts within both species, performance was higher at all temperatures in summer, while the thermal optima and performance breadth remained unchanged. Despite these changes in thermal performance, the thermal optima of most traits did not match the ambient environmental temperature, but fell between the summer and winter temperatures. Overall, these results showed that both coral species were physiologically plastic in response to temperature change, but that there are constraints on the rate or capacity for acclimation that prevent a perfect match between the average temperature of the environment and the thermal optimum of the species

Environmental drivers of variation in bleaching severity of Acropora species during an extreme thermal anomaly

High sea surface temperatures caused global coral bleaching during 2015–2016. During this thermal stress event, we quantified within- and among-species variability in bleaching severity for critical habitat-forming Acropora corals. The objective of this study was to understand the drivers of spatial and species-specific variation in the bleaching susceptibility of these corals, and to evaluate whether bleaching susceptibility under extreme thermal stress was consistent with that observed during less severe bleaching events. We surveyed and mapped Acropora corals at 10 sites (N = 596) around the Lizard Island group on the northern Great Barrier Reef. For each colony, bleaching severity was quantified using a new image analysis technique, and we assessed whether small-scale environmental variables (depth, microhabitat, competition intensity) and species traits (colony morphology, colony size, known symbiont clade association) explained variation in bleaching. Results showed that during severe thermal stress, bleaching of branching corals was linked to microhabitat features, and was more severe at reef edge compared with lagoonal sites. Bleaching severity worsened over a very short time-frame (∼1 week), but did not differ systematically with water depth, competition intensity, or colony size. At our study location, within- and among-species variation in bleaching severity was relatively low compared to the level of variation reported in the literature. More broadly, our results indicate that variability in bleaching susceptibility during extreme thermal stress is not consistent with that observed during previous bleaching events that have ranged in severity among globally dispersed sites, with fewer species escaping bleaching during severe thermal stress. In addition, shaded microhabitats can provide a refuge from bleaching which provides further evidence of the importance of topographic complexity for maintaining the biodiversity and ecosystem functioning of coral reefs

Similar thermal breadth of two temperate coral species from the Mediterranean Sea and two tropical coral species from the Great Barrier Reef

Temperate organisms are generally exposed to a more variable and cooler climate than tropical organisms, and are therefore expected to have broader thermal tolerance and a different thermal performance curve. This study investigated these hypotheses by comparing the thermal performance of two common tropical coral species found in the Great Barrier Reef with the two most common temperate coral species from the Mediterranean Sea. Photosynthesis rates, dark respiration rates, maximum PSII quantum yield (Fv/Fm) and electron transport rates (rETRm) were measured on coral fragments exposed to an acute temperature increase and decrease up to 5 °C above and below the average environmental seawater temperature. Dark respiration rates and Fv/Fm increased linearly with temperature, suggesting broad thermal tolerance. For photosynthesis and rETRm, the performance breadths were surprisingly similar between the tropical and temperate species. However, the thermal optimum for performance was generally below the local average temperature, and only coincided with the prevailing environmental temperature for one of the tropical species. The broad thermal tolerance for photosynthesis displayed in this study supports previous observations that corals can survive short periods of abnormally warm temperatures and suggests that corals adopt thermal generalist strategies to cope with temperature variation in the environment. Nevertheless, current mean temperatures are 10–30% above the thermal optimum for the species studied here, demonstrating that conditions are already pushing the boundaries of coral thermal tolerance

Data from: High pCO2 promotes coral primary production

While research on ocean acidification (OA) impacts on coral reefs has focused on calcification, relatively little is known about effects on coral photosynthesis and respiration, despite these being among the most plastic metabolic processes corals may use to acclimatise to adverse conditions. Here, we present data collected between 2016 and 2018 at three natural CO2 seeps in Papua New Guinea where we measured the metabolic flexibility (i.e., in hospite photosynthesis and dark respiration) of 12 coral species. Despite some species-specific variability, metabolic rates as measured by net oxygen flux tended to be higher at high pCO2 (ca. 1200 µatm), with increases in photosynthesis exceeding those of respiration, suggesting greater productivity of Symbiodiniacea photosynthesis in hospite, and indicating the potential for metabolic flexibility that may enable these species to thrive in environments with high pCO2. However, lab and field observations of coral mortality under high CO2 conditions associated with coral bleaching suggests that this metabolic subsidy does not result in coral higher resistance to extreme thermal stress. Therefore, the combined effects of OA and global warming may lead to a strong decrease in coral diversity despite the stimulating effect on coral productivity of OA alone

Data from: High pCO2 promotes coral primary production

While research on ocean acidification (OA) impacts on coral reefs has focused on calcification, relatively little is known about effects on coral photosynthesis and respiration, despite these being among the most plastic metabolic processes corals may use to acclimatise to adverse conditions. Here, we present data collected between 2016 and 2018 at three natural CO2 seeps in Papua New Guinea where we measured the metabolic flexibility (i.e., in hospite photosynthesis and dark respiration) of 12 coral species. Despite some species-specific variability, metabolic rates as measured by net oxygen flux tended to be higher at high pCO2 (ca. 1200 µatm), with increases in photosynthesis exceeding those of respiration, suggesting greater productivity of Symbiodiniacea photosynthesis in hospite, and indicating the potential for metabolic flexibility that may enable these species to thrive in environments with high pCO2. However, lab and field observations of coral mortality under high CO2 conditions associated with coral bleaching suggests that this metabolic subsidy does not result in coral higher resistance to extreme thermal stress. Therefore, the combined effects of OA and global warming may lead to a strong decrease in coral diversity despite the stimulating effect on coral productivity of OA alone

Triple infection with HIV-1, HTLV-1 and Strongyloides stercoralis, rendering CD4+ T-cell counts a misleading entity

We report the case of a Gabonese HIV-patient who presented with haemoptysis, weight loss, fulminant diarrhoea and subsequent ileus and elevated CD4+ T-cell counts. He was diagnosed with Strongyloides stercoralis and human T-lymphotrophic virus type-1 infection. After treatment of the strongyloides hyperinfection syndrome, his CD4+ T-cell counts dropped greatly. The initially elevated CD4+ T-cell counts were misleading to the clinicians with regard to decision-making on antiretroviral therapy initiatio

Conceptual model of dark calcification impairment by heterotrophy.

<p>Feeding increases metabolic rates, CO₂ production, and as a result proton production in calicoblastic cells. In light, these protons are titrated by photosynthetically generated hydroxide ions in the coelenteron. In darkness, protons accumulate in the calicoblastic ectoderm, increasing the proton gradient between the calicoblastic ectoderm and the calcifying medium (CM). This causes a temporary decrease of the CM pH and aragonite saturation state, shifting the calcification reaction to the left. CC: calicoblastic cell. CM: calcifying medium. M: mitochondrion. CA: carbonic anhydrase. Model based on <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0052702#pone.0052702-Furla1" target="_blank">[10]</a> and <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0052702#pone.0052702-AlHorani2" target="_blank">[17]</a>.</p