Response Of Corals Research Articles

Thermotolerant coral symbionts modulate heat stress-responsive genes in their hosts.

Some corals may become more resistant to bleaching by shuffling their Symbiodiniaceae communities toward thermally tolerant species, and manipulations to boost the abundance of these symbionts in corals may increase resilience in warming oceans. However, the thermotolerant symbiont Durusdinium trenchii may reduce growth and fecundity in Caribbean corals, and these tradeoffs need to be better understood as this symbiont spreads through the region. We sought to understand how D.trenchii modulates coral gene expression by manipulating symbiont communities in Montastraea cavernosa to produce replicate ramets containing D.trenchii together with paired ramets of these same genets (n=3) containing Cladocopium C3 symbionts. We then examined differences in global gene expression between corals hosting Durusdinium and Cladocopium under control temperatures, and in response to short-term heat stress. We identified numerous transcriptional differences associated with symbiont identity, which explained 2%-14% of the transcriptional variance. Corals with D.trenchii upregulated genes related to translation, ribosomal structure and biogenesis, and downregulated genes related to extracellular structures, and carbohydrate and lipid transport and metabolism, relative to corals with Cladocopium. Unexpectedly, these changes were similar to those observed in Cladocopium-dominated corals in response to heat stress, suggesting that thermotolerant D.trenchii may cause corals to increase expression of heat stress-responsive genes, explaining both the increased heat tolerance and the associated energetic tradeoffs in corals containing D.trenchii. These findings provide insight into the ecological changes occurring on contemporary coral reefs in response to climate change, and the diverse ways in which different symbionts modulate emergent phenotypes of their hosts.

Unravelling the different causes of nitrate and ammonium effects on coral bleaching

Mass coral bleaching represents one of the greatest threats to coral reefs and has mainly been attributed to seawater warming. However, reduced water quality can also interact with warming to increase coral bleaching, but this interaction depends on nutrient ratios and forms. In particular, nitrate (NO3−) enrichment reduces thermal tolerance while ammonium (NH4+) enrichment tends to benefit coral health. The biochemical mechanisms underpinning the different bleaching responses of corals exposed to DIN enrichment still need to be investigated. Here, we demonstrated that the coral Stylophora pistillata underwent a severe oxidative stress condition and reduced aerobic scope when exposed to NO3− enrichment combined with thermal stress. Such condition resulted in increased bleaching intensity compared to a low-nitrogen condition. On the contrary, NH4+ enrichment was able to amend the deleterious effects of thermal stress by favoring the oxidative status and energy metabolism of the coral holobiont. Overall, our results demonstrate that the opposite effects of nitrate and ammonium enrichment on coral bleaching are related to the effects on corals’ energy/redox status. As nitrate loading in coastal waters is predicted to significantly increase in the future due to agriculture and land-based pollution, there is the need for urgent management actions to prevent increases in nitrate levels in seawater. In addition, the maintenance of important fish stocks, which provide corals with recycled nitrogen such as ammonium, should be favoured.

Organic eutrophication increases resistance of the pulsating soft coral Xenia umbellata to warming.

Recent research indicates that hard corals in a process that is termed phase shift are often replaced by soft corals in reefs. The simultaneous occurrence of local (i.e. organic eutrophication as highly under-investigated parameter) and global (i.e. ocean warming) factors may facilitate these phase shifts as hard corals are negatively affected by both ocean warming and organic eutrophication. Knowledge about soft coral responses to environmental change remains incomplete, although these organisms are becoming important players in reefs. The present study thus investigated the individual and combined effects of organic eutrophication (as glucose addition) and warming on the ecological data of the pulsating soft coral Xenia umbellata. We assessed health status, growth and pulsation rates of soft corals in a 45 day aquarium experiment, with first manipulation of organic eutrophication (no, low, medium and high glucose addition) over 21 days followed by step-wise increases in water temperature from 26 to 32 °C over 24 days. Findings revealed that glucose addition did not affect health status, growth and pulsation rates of the investigated soft corals. Under simulated ocean warming, soft corals that had experienced organic eutrophication before, maintained significantly higher pulsation rates (averaging 22 beats per minute—bpm) and no mortality compared to the controls that showed a decrease of 56% (averaging 15 bpm) in pulsation rates and mortality of 30% at water temperatures of 32 °C compared to 26 °C. This apparently positive effect of organic eutrophication on the ecological data of soft corals under an ocean warming scenario decreased with increasing water temperature. This study thus indicates that (a) organic eutrophication as additional energy source up to a certain threshold may increase the resistance of soft corals to ocean warming and (b) pulsation rates of soft corals may be used as inexpensive, easily detectable, and non-invasive early warning indicator for ocean warming effects on benthic reef communities. When comparing findings of this study for soft corals with previous results for hard corals, it can be assumed that soft corals under the predicted increases of organic eutrophication and warming gain more and more competitive advantages. This may further facilitate phase shifts from hard to soft corals in warming reefs.

A unique approach to monitor stress in coral exposed to emerging pollutants

Metabolomic profiling of the hexacoral Pocillopora damicornis exposed to solar filters revealed a metabolomic signature of stress in this coral. It was demonstrated that the concentration of the known steroid (3β, 5α, 8α) -5, 8-epidioxy- ergosta- 6, 24(28) - dien- 3- ol (14) increased in response to octocrylene (OC) and ethylhexyl salicylate (ES) at 50 µg/L. Based on the overall coral response, we hypothesize that steroid 14 mediates coral response to stress. OC also specifically altered mitochondrial function at this concentration and above, while ES triggered a stress/inflammatory response at 300 µg/L and above as witnessed by the significant increases in the concentrations of polyunsaturated fatty acids, lysophosphatidylcholines and lysophosphatidylethanolamines. Benzophenone-3 increased the concentration of compound 14 at 2 mg/L, while the concentration of stress marker remained unchanged upon exposition to the other solar filters tested. Also, our results seemed to refute earlier suggestions that platelet-activating factor is involved in the coral inflammatory response.

Prior exposure to elevated pCO2 does not affect calcification of a tropical scleractinian when returned to ambient pCO2

Coral reefs experience biologically-driven pCO2 oscillations that are predicted to become more extreme in magnitude and duration under ocean acidification (OA) regimes. Understanding the plasticity of responses in common reef-building corals to oscillations in pCO2 will allow for better predictions of their function in future seawater conditions. This study explored the effects of variation in seawater pCO2 on coral calcification using experiments conducted over one month between 9 April 2018 and 18 May 2018. Branches (~4-cm long) of Acropora retusa were sampled from colonies at 10-m depth on the fore reef of Mo'orea, French Polynesia (17° 28′ 53.9004" S, 149° 49′ 50.5992" W). We tested the hypothesis that depressed calcification caused by elevated pCO2 (~1000 μatm) is relaxed (i.e., calcification increases) upon return to ambient pCO2 (~400 μatm). Corals first were incubated in ambient or elevated pCO2 for 19 days, with the result that calcification integrated over this period was reduced by 31% under elevated pCO2. The same corals were then incubated at ambient pCO2 for 11 days, during which calcification was independent of the experimental pCO2 exposure history. Our results suggest that a quick relaxation of pCO2-depressed calcification in A. retusa following cessation of high pCO2 indicates that corals are capable of a reversible plastic response of calcification when confronted by pCO2 oscillations.

Fluorescence-Activated Cell Sorting for the Isolation of Scleractinian Cell Populations.

Coral reefs are under threat due to anthropogenic stressors. The biological response of coral to these stressors may occur at a cellular level, but the mechanisms are not well understood. To investigate coral response to stressors, we need tools for analyzing cellular responses. In particular, we need tools that facilitate the application of functional assays to better understand how cell populations are reacting to stress. In the current study, we use fluorescence-activated cell sorting (FACS) to isolate and separate different cell populations in stony corals. This protocol includes: (1) the separation of coral tissues from the skeleton, (2) creation of a single cell suspension, (3) labeling the coral cells using various markers for flow cytometry, and (4) gating and cell sorting strategies. This method will enable researchers to work on corals at the cellular level for analysis, functional assays, and gene expression studies of different cell populations.

Towards modelling the future risk of cyclone wave damage to the world's coral reefs.

Tropical cyclones generate extreme waves that can damage coral reef communities. Recovery typically requires up to a decade, driving the trajectory of coral community structure. Coral reefs have evolved over millennia with cyclones. Increasingly, however, processes of recovery are interrupted and compromised by additional pressures (thermal stress, pollution, diseases, predators). Understanding how cyclones interact with other pressures to threaten coral reefs underpins spatial prioritization of conservation and management interventions. Models that simulate coral responses to cumulative pressures often assume that the worst cyclone wave damage occurs within ~100km of the track. However, we show major coral loss at exposed sites up to 800km from a cyclone that was both strong (high sustained wind speeds >=33m/s) and big (widespread circulation >~300km), using numerical wave models and field data from northwest Australia. We then calculate the return time of big and strong cyclones, big cyclones of any strength and strong cyclones of any size, for each of 150 coral reef ecoregions using a global data set of past cyclones from 1985 to 2015. For the coral ecoregions that regularly were exposed to cyclones during that time, we find that 75% of them were exposed to at least one cyclone that was both big and strong. Return intervals of big and strong cyclones are already less than 5years for 13 ecoregions, primarily in the cyclone-prone NW Pacific, and less than 10years for an additional 14 ecoregions. We identify ecoregions likely at higher risk in future given projected changes in cyclone activity. Robust quantification of the spatial distribution of likely cyclone wave damage is vital not only for understanding past coral response to pressures, but also for predicting how this may change as the climate continues to warm and the relative frequency of the strongest cyclones rises.

Different responses of coral and rubble-dwelling coral reef damselfishes (Family: Pomacentridae) to chemosensory cues from coral reef microhabitats

Coral reef fishes are known to respond to chemical cues in the selection of appropriate microhabitats at settlement. Coral- and non-coral-associated species are likely to respond to different stimuli and the cues may change as larvae settle and become familiar with the reef environment. Here, the chemosensory responses of both late-stage larvae and newly settled juvenile damselfishes to microhabitat odours were tested in Kimbe Bay (PNG), including four obligate coral-dwelling species (Dascyllus melanurus, D. reticulatus, Chrysiptera arnazae and Pomacentrus moluccensis) and four rubble-dwelling species (Pomacentrus adelus, P. simsiang, Chrysiptera cyanea and C. rollandi). Damselfishes were subjected to a series of pair-wise chemosensory choice trials using a two-channel choice flume. The responses by late-stage larvae (pre-settled) to chemical cues from their preferred microhabitat type mirrored those exhibited by recently settled juveniles. All four rubble-dwelling damselfish species exhibited a significant aversion toward chemical cues derived from coral microhabitats, preferring to remain either in the water seeded with chemical cues derived from rubble, or the unseeded (control) water. The obligate coral-dwelling damselfish species tended to avoid rubble and select coral cues when tested against seawater, but unexpectedly, exhibited a neutral response when given a choice between coral and rubble odours. These results indicate that coral and rubble-dwelling damselfish differ in the strength of their attraction to preferred or avoidance of non-preferred microhabitats. Both factors are likely to play a role in settlement choices. Our results indicated that for some species, newly settled juveniles could act as suitable substitutes for testing larval behavioural traits.

Examining the Effect of Heat Stress on Montastraea cavernosa (Linnaeus 1767) from a Mesophotic Coral Ecosystem (MCE)

Coral reefs are under increasing pressure from global warming. Little knowledge, however, exists regarding heat induced stress on deeper mesophotic coral ecosystems (MCEs). Here, we examined the effect of acute (72 h) and chronic (480 h) heat stress on the host coral Montastraea cavernosa (Linnaeus 1767) collected from an upper MCE (~30 m) in Florida, USA. We examined six immune/stress-related genes: ribosomal protein L9 (RpL9), ribosomal protein S7 (RpS7), B-cell lymphoma 2 apoptosis regulator (BCL-2), heat shock protein 90 (HSP90), catalase, and cathepsin L1, as a proxy for coral response to heat stress. Quantitative real-time polymerase chain reaction (qRT-PCR) was performed to evaluate the gene expression. Overall, both acute and chronic heat stress treatments elicited a response in gene expression relative to control samples. Acute heat exposure resulted in up-regulation of catalase, BCL-2, and HSP90 at all time points from hour 24 to 48, suggesting the activation of an oxidative protective enzyme, molecular chaperone, and anti-apoptotic protein. Fewer genes were up-regulated in the chronic experiment until hour 288 (30 °C) where catalase, RpL9, and RpS7 were significantly up-regulated. Chronic heat exposure elicited a physiological response at 30 °C, which we propose as a heat-stress threshold for Montastraea cavernosa (M. cavernosa) collected from an MCE.

Variation in the elemental stoichiometry of the coral–zooxanthellae symbiosis

Organismal elemental stoichiometry is important at all scales of ecological interactions, particularly in symbiosis. Symbiotic partnerships are found extensively in corals, where coral hosts and their photosynthetic dinoflagellate partners trade essential nutrients. Using an ecological stoichiometry framework, we assessed variations in carbon (C), nitrogen (N), and phosphorus (P) concentrations in coral hosts and symbiotic zooxanthellae. Our aim was to assess whether corals and zooxanthellae differ in the stoichiometry of C, N and P, which may indicate that corals regulate their symbionts by restricting their access to nutrients. We also investigated the influence of biological and environmental factors on the elemental stoichiometry of coral–zooxanthellae symbiosis. Analysing field data from four locations on the Great Barrier Reef using boosted regression tree models, we observed that C:P and N:P ratios are significantly higher in zooxanthellae than in their coral hosts, suggesting P limitation for zooxanthellae, although the degree of P or N limitation in zooxanthellae varied among coral taxa. Coral taxonomy was also a significant driver of coral–zooxanthellae stoichiometry variations at the species level. Geographical location had a small influence on the variations in stoichiometry. Our analyses suggest that coral species may differentially control the access of zooxanthellae to nutrients which may lead to variation in reef coral responses to nutrient enrichment.

Physiological stress response of the scleractinian coral Stylophora pistillata exposed to polyethylene microplastics

We investigated physiological responses including calcification, photosynthesis and alterations to polar metabolites, in the scleractinian coral Stylophora pistillata exposed to different concentrations of polyethylene microplastics. Results showed that at high plastic concentrations (50 particles/mL nominal concentration) the photosynthetic efficiency of photosystem II in the coral symbiont was affected after 4 weeks of exposure. Both moderate and high (5 and 50 particles/mL nominal) concentrations of microplastics caused subtle but significant alterations to metabolite profiles of coral, as determined by Nuclear Magnetic Resonance (NMR) spectroscopy. Specifically, exposed corals were found to have increased levels of phosphorylated sugars and pyrimidine nucleobases that make up nucleotides, scyllo-inositol and a region containing overlapping proline and glutamate signals, compared to control animals. Together with the photo-physiological stress response observed and previously published literature, these findings support the hypothesis that microplastics disrupt host-symbiont signaling and that corals respond to this interference by increasing signaling and chemical support to the symbiotic zooxanthellae algae. These findings are also consistent with increased mucus production in corals exposed to microplastics described in previous studies. Considering the importance of coral reefs to marine ecosystems and their sensitivity to anthropogenic stressors, more research is needed to elucidate coral response mechanisms to microplastics under realistic exposure conditions.

Responses of urban reef corals during the 2016 mass bleaching event

Predicting the bleaching responses of corals is crucial in light of frequent heat stress events to manage further losses of biodiversity and ecosystem functioning, especially for reefs impacted by urbanisation. We examined if the coral cover and community at various Singapore sites changed during the 2016 global coral bleaching event. Bleaching prevalence varied widely among sites in June 2016, and was best explained by site and coral species. While some sites were minimally impacted, others registered significant decreases in coral cover and community changes persisting till March 2017, when normal colouration was mostly regained by corals. Bleaching susceptibility was associated with larger corallites in hermaphrodites and smaller corallites in gonochores (probably due to the cost of maintaining dual sexual functions in hermaphrodites), and with increasing proximity between polyps (likely because thermal damage would be less contained among polyps with greater physiological integration). However, bleaching resilience—the capacity to regain baseline pigmentation—was poorly explained by the traits studied. Our findings suggest that the interplay between local conditions and species composition strongly affects bleaching outcomes on urbanised reefs, and underscore the utility of coral traits for predicting bleaching responses to help in formulating appropriate management strategies.

Endosymbiont diversity and community structure in Porites lutea from Southeast Asia are driven by a suite of environmental variables

Many corals depend upon the highly specialised and intricate relationship they form with Symbiodiniaceae algal symbionts. Porites lutea is a massive reef-building coral found throughout Southeast Asia that hosts these endosymbionts obligately. Yet despite the prevalence and importance of P. lutea as one of the most dominant corals here, its associated Symbiodiniaceae communities have not been precisely characterised. In this study, we used high-throughput DNA amplicon sequencing of the nuclear internal transcribed spacer 2 (ITS2) to characterise the diversity, community structure and biogeographic distribution of Symbiodiniaceae in P. lutea throughout Singapore and Peninsular Malaysia. Consistent with previous studies, we found that Cladocopium was the most dominant genus among all samples, and Cladocopium C15 was the most dominant type (or subclade) with 100% occurrence in all samples from every study site. Results also revealed numerous Symbiodiniaceae types associated with P. lutea that were previously undetected in Southeast Asia. Endosymbiont diversity and community variation are driven by a combination of site-specific mean monthly cloud cover and variance in monthly sea surface temperature. This study contributes baseline data toward understanding differences in Symbiodiniaceae assemblages hosted by P. lutea, shedding light on how they might be indicative of particular environmental conditions and coral responses.

Responses of corals to chronic turbidity

Dredging increases suspended sediment concentrations (SSCs), causing elevated water turbidity (cloudiness) and light attenuation. Close to dredging, low light periods can extend over many days, affecting phototrophic epibenthic organisms like corals. To improve the ability to predict and manage dredging impacts, we tested the response of corals to an extended period of elevated turbidity using an automated sediment dosing system that precisely controlled SSCs and adjusted light availability accordingly. Replicates of four common species of corals encompassing different morphologies were exposed to turbidity treatments of 0–100 mg L−1 SSC, corresponding to daily light integrals of 12.6 to 0 mol quanta m−2 d−1, over a period of ∼7 weeks. Symbiotic dinoflagellate density and algal pigment concentration, photosynthetic yields, lipid concentrations and ratios and growth varied among the turbidity treatments, with corals exhibiting photoacclimation within low turbidity treatments. A range of physiological responses were observed within the high turbidity treatments (low light), including bleaching and changes in lipid levels and ratios. Most corals, except P. damicornis, were capable of adjusting to a turbidity treatment involving a mean light level of 2.3 mol photons m−2 d−1 in conjunction with a SSC of 10 mg L−1 over the 7 week period.

Endosymbiotic dinoflagellates pump iron: differences in iron and other trace metal needs among the Symbiodiniaceae

Iron (Fe) is essential to the physiology and growth of marine phytoplankton. It remains unclear how important iron is to the functional ecology of symbiotic dinoflagellates in the family Symbiodiniaceae, and whether limitations in iron ultimately affect the health and productivity of coral hosts, especially during episodes of ocean warming. Five Symbiodiniaceae species (spanning three genera) were used to investigate the effects of reduced iron availability on cell growth and the acquisition of other trace metals. When grown under iron replete conditions, intracellular iron quotas (content) reflected a large biochemical demand and ranged from 7.8 to 23.1 mmol Fe mol Phosphorus−1. Symbiodinium necroappetens was the only species that acclimated and maintained high growth rates while subjected to the lowest iron treatment (250 pM Fe′). Cultures surviving under low iron concentrations experienced changes in cellular concentrations (and presumably their use as cofactors) of other trace metals (e.g., zinc, copper, cobalt, manganese, nickel, molybdenum, vanadium), in ways that were species-specific, and possibly related to the natural ecology of each species. These changes in trace metal contents may have cascading effects on vital biochemical functions such as metalloenzyme activities, photosynthetic performance, and macronutrient assimilation. Furthermore, these species-specific responses to iron limitation provide a basis for investigations on how iron availability effects cellular processes among species and genera of Symbiodiniaceae, and ultimately how metal shortages modulate the response of coral–algal mutualisms to physiological stressors.

Meta-Analysis Reveals Reduced Coral Calcification Under Projected Ocean Warming but Not Under Acidification Across the Caribbean Sea

Ocean acidification and warming are two of the many threats to coral reefs worldwide, and Caribbean reef-building corals are especially vulnerable. However, even within the Caribbean, experimental acidification and warming studies reveal a wide array of coral calcification responses across reef systems and among species, complicating efforts to predict how corals will respond to these global-scale stressors. We conducted a meta-analysis to investigate the calcification responses of Caribbean corals to experimentally induced seawater ocean acidification, ocean warming, and the combination of both stressors. Calcification rates were reduced for corals reared under warming alone, but acidification and the combination of both stressors did not clearly reduce calcification rates. Calcification responses of corals collected from the Florida Keys and Belize were compared for regional differences since a greater number of studies were performed on corals collected from these two regions. Notably, corals from the Florida Keys did not exhibit reduced calcification under acidification, warming, or the combination of both stressors, while corals from Belize exhibited reduced calcification under warming alone. Further investigation of these regional trends suggests that the warming and acidification treatments employed dictated calcification responses, rather than collection region. Results from this meta-analysis are constrained by the very few studies that have been conducted within the Caribbean to assess ocean acidification and warming and the large variation in experimental procedure among studies. This meta-analysis reveals existing gaps in our understanding of how corals will likely respond to projected acidification and warming and highlights ways to improve comparability among experimental studies conducted on corals within the same region to better predict coral calcification response under global change.

Photophysiology of a mesophotic coral 3 years after transplantation to a shallow environment

With shallow coral reefs suffering from an ongoing rapid decline in many regions of the world, the interest in studies on mesophotic coral ecosystems (30–150 m) is growing rapidly. While most photoacclimation responses in corals were documented within the upper 30 m of reefs, in the present study we transplanted fragments of a strictly mesophotic species from the Red Sea, Euphyllia paradivisa, from 50 m to 5 m for a period of 3 years. Following the retrieval of the corals, their physiological and photosynthetic properties of the corals were tested. The transplanted corals presented evidence of photosynthetic acclimation to the shallow habitat, lower sensitivity to photoinhibition, and a high survival percentage, while also demonstrating a reduced ability to utilize low light compared to their mesophotic counterparts. This long-term successful transplantation from a mesophotic depth to a shallow habitat has provided us with insights regarding the ability of mesophotic corals and their symbionts to survive and withstand shallow environments, dominated by a completely different light regime. The extensive characterization of the photobiology of E. paradivisa, and its photoacclimation response to a high-light environment also demonstrates the plasticity of corals and point out to mechanisms different than those reported previously in shallower corals.

Exposure to the Florida red tide dinoflagellate, Karenia brevis, and its associated brevetoxins induces ecophysiological and proteomic alterations in Porites astreoides.

As reef-building corals are increasingly being exposed to persistent threats that operate on both regional and global scales, there is a pressing need to better understand the complex processes that diminish coral populations. This study investigated the impacts of the Florida red tide dinoflagellate Karenia brevis and associated brevetoxins on selected facets of coral biology using Porites astreoides as a model system. When provided with choice assays, P. astreoides larvae were shown to actively avoid seawater containing red tide (5×105 cells L-1–7.6×106 cells L-1) or purified brevetoxins (0.018 μg mL-1 brevetoxin-2 and 0.0018 μg mL-1 brevetoxin-3). However, forced exposure to similar treatments induced time-dependent physiological and behavioral changes that were captured by PAM fluorometry and settlement and survival assays, respectively. Adult fragments of P. astreoides exposed to red tide or associated brevetoxins displayed signs of proteomic alterations that were characterized by the use of an iTRAQ-based quantitative proteomic analysis. The novel use of this technique with P. astreoides demonstrated that protein regulation was highly contingent upon biological versus chemical treatment (i.e. live K. brevis vs. solely brevetoxin exposure) and that several broad pathways associated with cell stress were affected including redox homeostasis, protein folding, energy metabolism and reactive oxygen species production. The results herein provide new insight into the ecology, behavior and sublethal stress of reef-building corals in response to K. brevis exposure and underscore the importance of recognizing the potential of red tide to act as a regional stressor to these important foundation species.

Cold-water coral (Lophelia pertusa) response to multiple stressors: High temperature affects recovery from short-term pollution exposure

There are numerous studies highlighting the impacts of direct and indirect stressors on marine organisms, and multi-stressor studies of their combined effects are an increasing focus of experimental work. Lophelia pertusa is a framework-forming cold-water coral that supports numerous ecosystem services in the deep ocean. These corals are threatened by increasing anthropogenic impacts to the deep-sea, such as global ocean change and hydrocarbon extraction. This study implemented two sets of experiments to assess the effects of future conditions (temperature: 8 °C and 12 °C, pH: 7.9 and 7.6) and hydrocarbon exposure (oil, dispersant, oil + dispersant combined) on coral health. Phenotypic response was assessed through three independent observations of diagnostic characteristics that were combined into an average health rating at four points during exposure and recovery. In both experiments, regardless of environmental condition, average health significantly declined during 24-hour exposure to dispersant alone but was not significantly altered in the other treatments. In the early recovery stage (24 hours), polyp health returned to the pre-exposure health state under ambient temperature in all treatments. However, increased temperature resulted in a delay in recovery (72 hours) from dispersant exposure. These experiments provide evidence that global ocean change can affect the resilience of corals to environmental stressors and that exposure to chemical dispersants may pose a greater threat than oil itself.

Physiological effects of heat and cold exposure in the common reef coral Acropora millepora

Reef-forming corals are under threat globally from climate change, leading to changes in sea temperatures with both hot and cold events recorded and projected to increase in frequency and severity in the future. Tolerance to heat and cold exposure has been found to be mutually exclusive in other marine invertebrates, but it is currently unclear whether a trade-off exists between hot and cold thermal tolerance in tropical corals. This study quantified the changes in physiology in Acropora millepora from the central Great Barrier Reef subjected to three temperature treatments; sub-lethal cold, ambient and sub-lethal heat (23.0 °C, 27.0 °C and 29.5 °C, respectively). After 10 weeks, pigment content and Symbiodiniaceae density increased in cold-treated corals but decreased in heat-treated corals relative to corals at ambient conditions. Heat-treated corals gained less mass relative to both ambient and cold-treated corals. These results indicate that the physiological condition of A. millepora corals examined here improved in response to mild cold exposure compared to ambient exposure and decreased under mild heat exposure despite both these temperatures occurring in situ around 15% of the year. The energetic condition of corals in the hotter treatment was reduced compared to both ambient and cooler groups, indicating that corals may be more resilient to mild cold exposure relative to mild heat exposure. The results indicate that the corals shifted their resource allocation in response to temperature treatment, investing more energy into skeletal extension rather than maintenance. No evidence of thermal tolerance trade-offs was found, and cold thermal tolerance was not lost in more heat-tolerant individuals. An enhanced understanding of physiological responses of corals at both ends of the thermal spectrum is important for predicting the resilience of corals under projected climate change conditions.