Response Of Corals Research Articles

Host\u2013symbiont combinations dictate the photo-physiological response of reef-building corals to thermal stress

High sea surface temperatures often lead to coral bleaching wherein reef-building corals lose significant numbers of their endosymbiotic dinoflagellates (Symbiodiniaceae). These increasingly frequent bleaching events often result in large scale coral mortality, thereby devasting reef systems throughout the world. The reef habitats surrounding Palau are ideal for investigating coral responses to climate perturbation, where many inshore bays are subject to higher water temperature as compared with offshore barrier reefs. We examined fourteen physiological traits in response to high temperature across various symbiotic dinoflagellates in four common Pacific coral species, Acropora muricata, Coelastrea aspera, Cyphastrea chalcidicum and Pachyseris rugosa found in both offshore and inshore habitats. Inshore corals were dominated by a single homogenous population of the stress tolerant symbiont Durusdinium trenchii, yet symbiont thermal response and physiology differed significantly across coral species. In contrast, offshore corals harbored specific species of Cladocopium spp. (ITS2 rDNA type-C) yet all experienced similar patterns of photoinactivation and symbiont loss when heated. Additionally, cell volume and light absorption properties increased in heated Cladocopium spp., leading to a greater loss in photo-regulation. While inshore coral temperature response was consistently muted relative to their offshore counterparts, high physiological variability in D. trenchii across inshore corals suggests that bleaching resilience among even the most stress tolerant symbionts is still heavily influenced by their host environment.

Uncovering a mitochondrial unfolded protein response in corals and its role in adapting to a changing world.

The Anthropocene will be characterized by increased environmental disturbances, leading to the survival of stress-tolerant organisms, particularly in the oceans, where novel marine diseases and elevated temperatures are re-shaping ecosystems. These environmental changes underscore the importance of identifying mechanisms which promote stress tolerance in ecologically important non-model species such as reef-building corals. Mitochondria are central regulators of cellular stress and have dedicated recovery pathways including the mitochondrial unfolded protein response, which increases the transcription of protective genes promoting protein homeostasis, free radical detoxification and innate immunity. In this investigation, we identify a mitochondrial unfolded protein response in the endangered Caribbean coral Orbicella faveolata, by performing in vivo functional replacement using a transcription factor (Of-ATF5) originating from a coral in the model organism Caenorhabditis elegans. In addition, we use RNA-seq network analysis and transcription factor-binding predictions to identify a transcriptional network of genes likely to be regulated by Of-ATF5 which is induced during the immune challenge and temperature stress. Overall, our findings uncover a conserved cellular pathway which may promote the ability of reef-building corals to survive increasing levels of environmental stress.

Response of Corals Acropora pharaonis and Porites lutea to Changes in pH and Temperature in the Gulf

Coral reefs are harboring a large part of the marine biodiversity and are important ecosystems for the equilibrium of the oceans. As a consequence of anthropogenic CO2 emission, a drop in pH and an increase in seawater temperature is observed in the Gulf coastal waters that potentially threaten coral assemblages. An experimental study was conducted on two species of corals to assess the effect of ocean warming and ocean acidification on the net calcification rate. Two pH conditions 8.2 and 7.5 and three temperatures, 22.5, 27.5 and 32.5 °C, were considered. Net calcification rates were measured using 45Ca radiotracer. Both temperature and pH had a significant effect on net calcification rates following a similar pattern for both species. The highest calcification rate was observed at low temperature and high pH. Increased temperature and decreased pH led to a decrease in net calcification rates. An interactive effect was observed as the effect of pH decreased with increasing temperature. However, the two species of coral were able to calcify in all the tested combination of temperature and pH suggesting that they are adapted to short term changes in temperature and pH. Ability to calcify even at a high temperature of 32.5 °C that is identical to the summertime Gulf seawater temperature under both the ambient and low pH condition with no mortalities, raises a question: are these corals adapted to high seawater temperatures and low pH? More in-depth assessments will be required to confirm if this is an adaptation to higher temperatures in Persian Gulf corals.

Proteomic Basis of Symbiosis: A Heterologous Partner Fails to Duplicate Homologous Colonization in a Novel Cnidarian- Symbiodiniaceae Mutualism.

Reef corals and sea anemones form symbioses with unicellular symbiotic dinoflagellates. The molecular circumventions that underlie the successful intracellular colonization of hosts by symbionts are still largely unknown. We conducted proteomic analyses to determine molecular differences of Exaiptasia pallida anemones colonized by physiologically different symbiont species, in comparison with symbiont-free (aposymbiotic) anemones. We compared one homologous species, Symbiodinium linucheae, that is natively associated with the clonal Exaiptasia strain (CC7) to another heterologous species, Durusdinium trenchii, a thermally tolerant species that colonizes numerous coral species. This approach allowed the discovery of a core set of host genes that are differentially regulated as a function of symbiosis regardless of symbiont species. The findings revealed that symbiont colonization at higher densities requires circumvention of the host cellular immunological response, enhancement of ammonium regulation, and suppression of phagocytosis after a host cell in colonized. Furthermore, the heterologous symbionts failed to duplicate the same level of homologous colonization within the host, evidenced by substantially lower symbiont densities. This reduced colonization of D. trenchii correlated with its inability to circumvent key host systems including autophagy-suppressing modulators, cytoskeletal alteration, and isomerase activity. The larger capability of host molecular circumvention by homologous symbionts could be the result of a longer evolutionary history of host/symbiont interactions, which translates into a more finely tuned symbiosis. These findings are of great importance within the context of the response of reef corals to climate change since it has been suggested that coral may acclimatize to ocean warming by changing their dominant symbiont species.

Differential coral response to algae contact: Porites tissue loss, praise for Halimeda interaction at southeast coast of India

Worldwide, reef building corals are being degraded due to increasing anthropogenic pressure, and as a result, macroalgal cover is being increased. Hence, mechanism of coral-algal interaction, differential coral response to algal overgrowth, is critical from every geographical location to predict future coral dynamics. This paper documents the frequency of coral-algal (Halimeda) interactions, differential coral response to algal interaction. We found difference in susceptibility among coral genera to competitive effects. Out of 970 coral colonies surveyed, 36.7% were in contact with Halimeda sp. Most frequent contact was observed in Porites (57%) followed by Favites 28% (n = 60), Acropora 26% (n = 48), Platygyra 5% (n = 5) and Symphyllia 4.2% (n = 3). Frequent discoloration and tissue loss were only observed in Porites. Continuous monitoring revealed that long-term algal physical contact prevents light required for polyp for photosynthesis and stops coral feeding ability. In this study, we also found mutual exclusion between Halimeda and coral recruit. Out of 180 coral colonies (size class between 5 and 15cm) comprised of Favites (n =74), Acropora (n = 20), Favia (n =79) and Porites (n =7) surveyed, none of them were found in Halimeda-dominated sites. The documented effects of recruitment exclusion and tissue mortality followed by algal interaction on major reef building corals (Porites) could affect replenishing process and health of the remaining healthy corals in the Palk Bay reef if algal proliferation rate is not controlled through proper management strategies.

The Whole-Genome Sequence of the Coral Acropora millepora.

[Excerpt] Reef-building corals are iconic animals that are in global decline as a consequence of increasing anthropogenic pressure, but the development of strategies to ensure their conservation is constrained by our limited understanding of the molecular bases of many aspects of coral biology. Some coral genera are particularly sensitive to stress and, among these, Acropora is of particular significance because this is the dominant genus of reef-building corals in the Indo-Pacific. These factors have led to members of this genus often being the subjects of investigation into coral responses to various physical and biological stressors. Fittingly, the first coral genome to be sequenced was Acropora digitifera; the availability of this whole-genome sequence (Shinzato et al. 2011) allowed substantial progress in several areas of coral biology, including the molecular underpinnings of symbiosis and calcification (Hamada et al. 2013; Ramos-Silva et al. 2013). Here we report the whole-genome sequence of a second Acropora species, A. millepora, which has been the most extensively studied Acropora species at the molecular level (reviewed in Miller et al. 2011) by virtue of its wide distribution (Carpenter et al. 2008; Madin et al. 2016) and the ease with which it can be identified in what is a highly speciose genus.

Depth-dependent temperature variability in the Southern California bight with implications for the cold-water gorgonian octocoral Adelogorgia phyllosclera

Cold-water corals are highly sensitive to changes in water temperature, as it is an important determinant of their distribution. In recent years, several heatwave events have occurred in multiple marine ecosystems, including the northeast Pacific Ocean. However, the effects of elevated ocean temperatures on cold-water corals are largely unknown. Determining the upper thermal limits of cold-water octocorals is an important first step in identifying if warm-water events pose a potential threat. Temperature data were obtained from loggers placed in the Channel Islands National Marine Sanctuary (CINMS) at 20, 50, 100, and 200 m prior to the 2015–2016 El Niño Southern Oscillation (ENSO) event and used to characterize warm-water anomalies. Live colonies of the common gorgonian octocoral, Adelogorgia phyllosclera, were collected from the CINMS using a remotely operated vehicle (ROV) and transported to laboratory aquaria where they were maintained. A laboratory study was conducted to investigate the upper thermal limit of A. phyllosclera during a series of temperature assays using coenenchyme health scores, polyp activity, and estimated survival. Results of in-situ temperature analyses indicated that warm-water anomalies occurred frequently at 50 and 100 m, with most of these anomalies occurring during strong ENSO months. Based on the laboratory temperature assays, the upper thermal limit of A. phyllosclera was estimated to be 20 °C, with a time to effect of 96 hours. During the 2015–2016 ENSO event, this upper thermal limit was exceeded (21.1 °C) by warm-water anomalies at 20 m that lasted up to 14.1 hours, and approached (18.3 °C) by warm-water anomalies at 50 m that lasted up to 52.7 hours. Projections for future warm-water events suggest that the upper thermal limit of A. phyllosclera and other cold-water corals will likely be reached more frequently in the coming years. Thus, understanding the responses of cold-water corals to thermal stress will help predict the resilience of these species to future ocean warming.

Spatial variation in the biochemical and isotopic composition of corals during bleaching and recovery.

Ocean warming and the increased prevalence of coral bleaching events threaten coral reefs. However, the biology of corals during and following bleaching events under field conditions is poorly understood. We examined bleaching and postbleaching recovery in Montipora capitata and Porites compressa corals that either bleached or did not bleach during a 2014 bleaching event at three reef locations in Kāne‘ohe Bay, O‘ahu, Hawai‘i. We measured changes in chlorophylls, tissue biomass, and nutritional plasticity using stable isotopes (δ 13C, δ 15N). Coral traits showed significant variation among periods, sites, bleaching conditions, and their interactions. Bleached colonies of both species had lower chlorophyll and total biomass, and while M. capitata chlorophyll and biomass recovered 3 months later, P. compressa chlorophyll recovery was location dependent and total biomass of previously bleached colonies remained low. Biomass energy reserves were not affected by bleaching, instead M. capitata proteins and P. compressa biomass energy and lipids declined over time and P. compressa lipids were site specific during bleaching recovery. Stable isotope analyses did not indicate increased heterotrophic nutrition in bleached colonies of either species, during or after thermal stress. Instead, mass balance calculations revealed that variations in δ 13C values reflect biomass compositional change (i.e., protein : lipid : carbohydrate ratios). Observed δ 15N values reflected spatiotemporal variability in nitrogen sources in both species and bleaching effects on symbiont nitrogen demand in P. compressa. These results highlight the dynamic responses of corals to natural bleaching and recovery and identify the need to consider the influence of biomass composition in the interpretation of isotopic values in corals.

Common Caribbean corals exhibit highly variable responses to future acidification and warming

We conducted a 93-day experiment investigating the independent and combined effects of acidification (280-3300 µatm pCO2) and warming (28°C and 31°C) on calcification and linear extension rates of four key Caribbean coral species ( Siderastrea siderea, Pseudodiploria strigosa, Porites astreoides, Undaria tenuifolia) from inshore and offshore reefs on the Belize Mesoamerican Barrier Reef System. All species exhibited nonlinear declines in calcification rate with increasing pCO2. Warming only reduced calcification in Ps. strigosa. Of the species tested, only S. siderea maintained positive calcification in the aragonite-undersaturated treatment . Temperature and pCO2 had no effect on the linear extension of S. siderea and Po. astreoides, and natal reef environment did not impact any parameter examined. Results suggest that S. siderea is the most resilient of these corals to warming and acidification owing to its ability to maintain positive calcification in all treatments, Ps. strigosa and U. tenuifolia are the least resilient, and Po. astreoides falls in the middle. These results highlight the diversity of calcification responses of Caribbean corals to projected global change.

Preserving the viscous coral surface mucus layer using low-acid glycol methacrylate (GMA) resin

The surface mucus layer (SML) is of critical importance in health, disease, and stress responses of corals; however, visualising the intact SML is challenging. Dehydration during histological preparation causes shrinkage and deformation of the mucus gel layer, while fragile, unattached mucus exudates are typically lost altogether. Here, we describe a novel technique using water-soluble glycol methacrylate resin embedding that more accurately preserves the in situ SML. Thickness of the preserved SML is similar to that previously measured on live corals using a glass microprobe. The technique allows microscopic visualisation of the SML structure, as well as thickness and continuity measurements, which are important indicators of SML function in health and disease.

Recent progress on signalling molecules of coral-associated microorganisms

Coral reefs have high primary productivity and are one of the most important ecosystems in the ocean. However, the health and stability of coral reefs are constantly threatened by climate change and human activities. The coral holobiont is a complex symbiosis between the coral animal, zooxanthellae, and the associated community of microorganisms including bacteria, archaea, viruses, etc. Coral-associated microorganisms are found to be important for the maintenance of coral health, and they are proposed to contribute to the acclimatization and adaptation of reef-building corals under rapid climate change. The coral-microbe interaction mediated by chemical signalling molecules is an important driving force for shaping the microbial communities. Herein, we summarize our current knowledge of the chemical signalling molecules involved in the interaction of the coral holobiont. Specifically, the cooperation and competition among microbes mediated by N -acyl homoserine lactones (AHLs), the interrelationship between microbes and hosts mediated by dimethylsulfoniopropionate (DMSP) and nitric oxide (NO), and the response of corals and microorganisms to reactive oxygen species (ROS) under environmental stresses are highlighted in this review. We further discuss the potential of manipulating the coral microbiome using signalling molecules to restore and protect coral reefs.

The Draft Genome of an Octocoral, Dendronephthya gigantea.

Coral reefs composed of stony corals are threatened by global marine environmental changes. However, soft coral communities of octocorallian species, appear more resilient. The genomes of several cnidarians species have been published, including from stony corals, sea anemones, and hydra. To fill the phylogenetic gap for octocoral species of cnidarians, we sequenced the octocoral, Dendronephthya gigantea, a nonsymbiotic soft coral, commonly known as the carnation coral. The D. gigantea genome size is ∼276 Mb. A high-quality genome assembly was constructed from PacBio long reads (29.85 Gb with 108× coverage) and Illumina short paired-end reads (35.54 Gb with 128× coverage) resulting in the highest N50 value (1.4 Mb) reported thus far among cnidarian genomes. About 12% of the genome is repetitive elements and contained 28,879 predicted protein-coding genes. This gene set is composed of 94% complete BUSCO ortholog benchmark genes, which is the second highest value among the cnidarians, indicating high quality. Based on molecular phylogenetic analysis, octocoral and hexacoral divergence times were estimated at 544 MYA. There is a clear difference in Hox gene composition between these species: unlike hexacorals, the Antp superclass Evx gene was absent in D. gigantea. Here, we present the first genome assembly of a nonsymbiotic octocoral, D. gigantea to aid in the comparative genomic analysis of cnidarians, including stony and soft corals, both symbiotic and nonsymbiotic. The D. gigantea genome may also provide clues to mechanisms of differential coping between the soft and stony corals in response to scenarios of global warming.

Reef‐Scale‐Dependent Response of Massive Porites Corals From the Central Indian Ocean to Prolonged Thermal Stress: Evidence From Coral Sr/Ca Measurements

AbstractHere we present coral Sr/Ca data of biweekly resolution from three modern coral cores drilled from living Porites corals from two different reef settings at Chagos (tropical Indian Ocean). Chagos lies at the eastern margin of the Seychelles‐Chagos thermocline ridge and features open ocean upwelling. In situ temperatures have been recorded by temperature loggers since 2006. High‐resolution satellite temperatures closely track the logger data. Two cores were collected from a patch reef in the lagoon of Peros Banhos, a site characterized by high mean temperatures and low‐temperature variability. An open ocean core was collected at the outer reef slope of Diego Garcia, which experiences larger temperature fluctuations related to open ocean upwelling. The open ocean core shows clear seasonal cycles in Sr/Ca that closely track the satellite temperatures. The Sr/Ca records from the two lagoon corals show good reproducibility. Between 2007 and 2010, Sr/Ca tracks satellite temperatures. However, between 2003 and 2006 the Sr/Ca curves are almost flat and annual coral growth rates are low. During these years, warm sea surface temperatures and coral bleaching have been observed at Chagos. It is likely that the observed reduction in coral growth rates is a response to prolonged warming events, adding to the growing evidence that seemingly healthy and surviving corals are affected by thermal stress. Further, the high correlation between the Sr/Ca records of the two lagoon cores suggests that this effect is not limited to single coral colonies but may affect the entire reef setting.

Transcriptomic analysis reveals protein homeostasis breakdown in the coral Acropora millepora during hypo-saline stress

BackgroundCoral reefs can experience salinity fluctuations due to rainfall and runoff; these events can have major impacts on the corals and lead to bleaching and mortality. On the Great Barrier Reef (GBR), low salinity events, which occur during summer seasons and can involve salinity dropping ~ 10 PSU correlate with declines in coral cover, and these events are predicted to increase in frequency and severity under future climate change scenarios. In other marine invertebrates, exposure to low salinity causes increased expression of genes involved in proteolysis, responses to oxidative stress, and membrane transport, but the effects that changes in salinity have on corals have so far received only limited attention. To better understand the coral response to hypo-osmotic stress, here we investigated the transcriptomic response of the coral Acropora millepora in both adult and juvenile life stages to acute (1 h) and more prolonged (24 h) exposure to low salinity.ResultsDifferential gene expression analysis revealed the involvement of both common and specific response mechanisms in Acropora. The general response to environmental stressors included up-regulation of genes involved in the mitigation of macromolecular and oxidative damage, while up-regulation of genes involved in amino acid metabolism and transport represent specific responses to salinity stress.ConclusionsThis study is the first comprehensive transcriptomic analysis of the coral response to low salinity stress and provides important insights into the likely consequences of heavy rainfall and runoff events on coral reefs.

Multi-colony coral skeletal Ba/Ca from Singapore’s turbid urban reefs: Relationship with contemporaneous in-situ seawater parameters

Abstract The ratio of barium to calcium in coral skeletons (Ba/Cacoral) is broadly used as a proxy for tracking terrestrial/river runoff. There are, however, inconsistencies in Ba/Cacoral records that have prompted caution in its reliability as an environmental proxy. Direct comparisons between in-situ seawater measurements and coral Ba/Ca are therefore needed to ensure accurate proxy calibration and interpretation. The current study represents the first to test Ba/Cacoral against years-long monthly-resolution contemporaneous measurements of several in-situ seawater parameters i.e. dissolved seawater barium (BaSW), temperature, salinity, suspended sediments, sedimentation rate and photosynthetically active radiation. We analysed the Ba/Cacoral of six Porites lutea corals sampled from two turbid reefs in Singapore, and explored relationships with in-situ seawater parameters over the period 2008–2015. Our study found poor agreement in Ba/Cacoral from replicate corals sampled from the same reef, and only one of the six colonies analysed showed significant but weak Ba/Cacoral–BaSW and Ba/Cacoral–Ba/CaSW relationships. There was also no clear relationship between Ba/Cacoral and skeletal luminescence G/B (a coral proxy strongly linked with salinity and river runoff). This implies that (1) the incorporation of terrestrially derived humic-like substances into the coral is independent of Ba, and (2) it is likely factors other than freshwater discharge/flood events are driving the poor reproducibility of Ba/Cacoral and disconnect with BaSW at our study sites. We found a positive relationship between Ba/Cacoral and organic suspended solids for 4 of the 6 colonies analysed that suggests biological mechanisms such as feeding could be driving skeletal Ba incorporation. A negative relationship between Ba/Cacoral and total sedimentation rate was also found, suggesting that within reef sediment fluxes may be influencing spatio-temporal variability of the Ba-supply. Ba incorporation into coral skeleton, especially in dynamic, turbid settings such as those found in Singapore, is likely more complex than previously thought. Our results highlight the possibility of high heterogeneity in coral responses to environmental conditions, and the need for careful selection of colonies and a site-specific, replicated approach when attempting to apply Ba/Cacoral as a trace element proxy.

Effects of moderate thermal anomalies on Acropora corals around Sesoko Island, Okinawa.

Over the past several decades, coral reef ecosystems have experienced recurring bleaching events. These events were predominantly caused by thermal anomalies, which vary widely in terms of severity and spatio-temporal distribution. Acropora corals, highly prominent contributors to the structural complexity of Pacific coral reefs, are sensitive to thermal stress. Response of Acropora corals to extremely high temperature has been well documented. However, studies on the effects of moderately high temperature on Acropora corals are limited. In the summer of 2016, a moderate coral bleaching event due to moderately high temperature was observed around Sesoko Island, Okinawa, Japan. The objective of this study was to examine thermal tolerance patterns of Acropora corals, across reefs with low to moderate thermal exposure (degree heating weeks ~2–5°C week). Field surveys on permanent plots were conducted from October 2015 to April 2017 to compare the population dynamics of adult Acropora corals 6 months before and after the bleaching events around Sesoko Island. Variability in thermal stress response was driven primarily by the degree of thermal stress. Wave action and turbidity may have mediated the thermal stress. Tabular and digitate coral morphologies were the most tolerant and susceptible to thermal stress, respectively. Growth inhibition after bleaching was more pronounced in the larger digitate and corymbose coral morphologies. This study indicates that Acropora populations around Sesoko Island can tolerate short-term, moderate thermal challenges.

Diurnally Fluctuating pCO2 Modifies the Physiological Responses of Coral Recruits Under Ocean Acidification.

Diurnal pCO2 fluctuations have the potential to modulate the biological impact of ocean acidification (OA) on reef calcifiers, yet little is known about the physiological and biochemical responses of scleractinian corals to fluctuating carbonate chemistry under OA. Here, we exposed newly settled Pocillopora damicornis for 7 days to ambient pCO2, steady and elevated pCO2 (stable OA) and diurnally fluctuating pCO2 under future OA scenario (fluctuating OA). We measured the photo-physiology, growth (lateral growth, budding and calcification), oxidative stress and activities of carbonic anhydrase (CA), Ca-ATPase and Mg-ATPase. Results showed that while OA enhanced the photochemical performance of in hospite symbionts, it also increased catalase activity and lipid peroxidation. Furthermore, both OA treatments altered the activities of host and symbiont CA, suggesting functional changes in the uptake of dissolved inorganic carbon (DIC) for photosynthesis and calcification. Most importantly, only the fluctuating OA treatment resulted in a slight drop in calcification with concurrent up-regulation of Ca-ATPase and Mg-ATPase, implying increased energy expenditure on calcification. Consequently, asexual budding rates decreased by 50% under fluctuating OA. These results suggest that diel pCO2 oscillations could modify the physiological responses and potentially alter the energy budget of coral recruits under future OA, and that fluctuating OA is more energetically expensive for the maintenance of coral recruits than stable OA.

Molecular Processes and Hub Genes of Acropora Palmata in Response to Thermal Stress And Bleaching

Yuan, J.; Liu, L.; Zhang, Y.; Shen, C., and Lin, S., 2019. Molecular processes and hub genes of Acropora palmata in response to thermal stress and bleaching. Journal of Coastal Research, 35(1), 26–32. Coconut Creek (Florida), ISSN 0749-0208.Among many environmental factors, high temperature is the main factor that causes coral bleaching. Thus, analyses of the bleaching mechanism and important genes involved in coral responses to thermal stress and bleaching are important. This study explored the gene expression profiling data (GSE16151) from the Gene Expression Omnibus database by using bioinformatics analysis to study the molecular processes and hub genes involved in the thermal stress and bleaching responses of the coral Acropora palmata. The results suggest that thermal stress down-regulated the snoRNP genes, which led to a reduction of some ribosomal proteins, and then decreased biosynthesis, resulting in disruption of a series of biological processes such as the glyoxylate cycle and vitamin metabolic processes. Moreover, the coral cytoskeleton showed breakdown because the CCT7 gene was down-regulated and because of insufficient adenosine triphosphate production caused by the thermal stress. All these responses will destroy the P53 pathway and initiate apoptosis and ultimately necrosis. In addition, 107 differentially expressed genes were detected between the normal and bleached corals, indicating that snoRNPs, CCT7, RPL37, and RPS27A may be the hub genes responsible for thermal stress and bleaching responses. These genes can be used as new biomarkers to help solve the problem of coral bleaching.

Linking climate variability and growth in coral skeletal records from the Great Barrier Reef

The remoteness of the northern Great Barrier Reef makes observations of environmental change and coral health sparse, but provides opportunities for paleoclimate and paleoecology proxies to contribute new insights into coral health in a changing climate. These proxies include geochemical measures (δ18O, δ13C, and Sr/Ca) that track sea surface temperature (SST), salinity, and physiological processes; luminescence, which records freshwater input; and annual growth parameters (density, extension, and calcification). Merging these approaches provides insight into the historical role of environmental variability in coral health. This study uses Porites spp. corals from five sites on the northern Great Barrier Reef (12–13.5°S) to produce combined monthly resolved records of geochemistry, growth banding, and luminescence between 1972 and 2008. We demonstrate that SST reconstructed from Sr/Ca accurately captures Indo-Pacific Warm Pool variability, and hydrological proxies (luminescence and seawater δ18O) accurately reconstruct summer rainfall and river discharge in nearshore corals. Concurrent Sr/Ca minima and density and luminescence peaks from two sites demonstrate that high-density bands are generally formed during summer at these sites, which aids the development of future coral paleoclimate and paleoecology chronologies. Regional hydrological proxies showed more consistent responses to El Nino–Southern Oscillation (ENSO) events than SST proxies, in agreement with instrumental data. Because these corals, like instrumental records, show no consistently large ENSO heat extremes, we find no consistent growth anomalies during historical ENSO events. Our results highlight the unusual nature of recent widespread and severe coral bleaching and establish groundwork for exploring the response of northern GBR corals to past climate variations.

Ocean warming drives decline in coral metabolism while acidification highlights species-specific responses

ABSTRACTOcean warming and acidification can have negative implications on coral reefs. This mechanistic study aims to evaluate the proximal causes of the observed negative response of Hawaiian corals to climate change scenarios. Net calcification (Gnet), gross photosynthesis, and dark respiration were measured in three species of Hawaiian corals across a range of temperature and acidification regimes using endpoint incubations. Calcification rates showed a curvilinear response with temperature, with the highest calcification rates observed at 26°C. Coral response to ocean acidification (OA) was species dependent and highly variable. OA enhanced calcification rates by 45% in the perforate coral, Montipora capitata, but had no short-term effect on the calcification or photosynthetic rates of imperforate corals, Pocillopora damicornis or Leptastrea purpurea. Further investigations revealed M. capitata to effectively dissipate protons (H+) while increasing uptake of bicarbonate (), therefore maintaining high rates of Gnet under acute OA stress. This study demonstrates the first experimental evidence of the ability of a coral species to take advantage of increased dissolved inorganic carbon and overcome an increasing proton gradient in the boundary layer under OA conditions. These observed differences in coral metabolism may underlie the species-specific responses to climate change.