Thermal stress reduces pocilloporid coral resilience to ocean acidification by impairing control over calcifying fluid chemistry

pH regulation in coral photosymbiosis and calcification: a compartmental perspective

The coral-dinoflagellate photosymbiosis and coral calcification underpin shallow water, coral reef ecosystems. This review examines the pivotal role of pH regulation in the cell physiology of these processes. Despite simple tissue organization, photosymbiotic corals maintain a complex internal microenvironment, with distinct compartments exhibiting contrasting pH levels. For example, the acidic 'symbiosome' surrounds the algal symbionts, while the alkaline 'extracellular calcifying medium' occurs at the growing front of the skeleton. We discuss how pH regulation of these compartments is crucial to the functioning of coral photosymbiosis and calcification, as well as mitigating the internal acid-base imbalances that these processes create. The role of pH regulation in the interplay between photosymbiosis and calcification is also discussed, focusing on the influence of symbiont photosynthesis on transepithelial gradients and the distribution of energy sources in the coral colony. Throughout this review, insights into pH regulation derived from previous research on ocean acidification are integrated to deepen understanding. Finally, we propose research priorities to advance knowledge of coral resilience under changing ocean conditions, such as investigating inorganic carbon concentration within coral compartments, species-specific differences and the impacts of thermal stress on pH regulation.

Trade-offs in a reef-building coral after six years of thermal acclimation

There is growing evidence that reef-building corals can acclimate to novel and challenging thermal conditions. However, potential trade-offs that accompany acclimation remain largely unexplored. We investigated physiological trade-offs in colonies of a globally abundant coral species (Pocillopora acuta) that were acclimated ex situ to an elevated temperature of 31 °C (i.e., 1 °C above their bleaching threshold) for six years. By comparing them to conspecifics maintained at a cooler temperature, we found that the energy storage of corals was prioritized over skeletal growth at the elevated temperature. This was associated with the formation of higher density skeletons, lower calcification rates and consequently lower skeletal extension rates, which entails ramifications for future reef-building processes, structural complexity and reef community composition. Furthermore, symbionts were physiologically compromised at 31 °C and had overall lower energy reserves, likely due to increased exploitation by their host, resulting in an overall lower stress resilience of the holobiont. Our study shows how biological trade-offs of thermal acclimation unfold, helping to refine our picture of future coral reef trajectories. Importantly, our observations in this six-year study do not align with observations of short-term studies, where elevated temperatures were often associated with the depletion of energy reserves, highlighting the importance of studying acclimation of organisms at relevant biological scales.

Gene networks governing the response of a calcareous sponge to future ocean conditions reveal lineage-specific XBP1 regulation of the unfolded protein response

Marine sponges are predicted to be winners in the future ocean due to their exemplary adaptive capacity. However, while many sponge groups exhibit tolerance to a wide range of environmental insults, calcifying sponges may be more susceptible to thermo-acidic stress. To describe the gene regulatory networks that govern the stress response of the calcareous sponge, Leucetta chagosensis (class Calcarea, order Clathrinida), individuals were subjected to warming and acidification conditions based on the climate models for 2100. Transcriptome analysis and gene co-expression network reconstruction revealed that the unfolded protein response (UPR) was activated under thermo-acidic stress. Among the upregulated genes were two lineage-specific homologs of X-box binding protein 1 (XBP1), a transcription factor that activates the UPR. Alternative dimerization between these XBP1 gene products suggests a clathrinid-specific mechanism to reversibly sequester the transcription factor into an inactive form, enabling the rapid regulation of pathways linked to the UPR in clathrinid calcareous sponges. Our findings support the idea that transcription factor duplication events may refine evolutionarily conserved molecular pathways and contribute to ecological success.

Spatial variability of and effect of light on the cœlenteron pH of a reef coral

Coral reefs, the largest bioconstruction on Earth, are formed by calcium carbonate skeletons of corals. Coral skeleton formation commonly referred to as calcification occurs in a specific compartment, the extracellular calcifying medium (ECM), located between the aboral ectoderm and the skeleton. Calcification models often assume a direct link between the surrounding seawater and the ECM. However, the ECM is separated from the seawater by several tissue layers and the cœlenteron, which contains the cœlenteric fluid found in both polyps and cœnosarc (tissue connecting the polyps). Symbiotic dinoflagellate-containing cells line the cœlenteron and their photosynthetic activity contributes to changes in the chemistry of the cœlenteric fluid, particularly with respect to pH. The aim of our study is to compare cœlenteron pH between the cœnosarc and polyps and to compare areas of high or low dinoflagellate density based on tissue coloration. To achieve this, we use liquid ion exchange (LIX) pH microsensors to profile pH in the cœlenteron of polyps and the cœnosarc in different regions of the coral colony in light and darkness. We interpret our results in terms of what light and dark exposure means for proton gradients between the ECM and the coelenteron, and how this could affect calcification.

Deciphering temporal gene expression dynamics in multiple coral species exposed to heat stress: Implications for predicting resilience

Coral reefs are facing unprecedented threats due to global climate change, particularly elevated sea surface temperatures causing coral bleaching. Understanding coral responses at the molecular level is crucial for predicting their resilience and developing effective conservation strategies. In this study, we conducted a comprehensive gene expression analysis of four coral species to investigate their long-term molecular response to heat stress. We identified distinct gene expression patterns among the coral species, with laminar corals exhibiting a stronger response compared to branching corals. Heat shock proteins (HSPs) showed an overall decreasing expression trend, indicating the high energy cost associated with sustaining elevated HSP levels during prolonged heat stress. Peroxidases and oxidoreductases involved in oxidative stress response demonstrated significant upregulation, highlighting their role in maintaining cellular redox balance. Differential expression of genes related to calcium homeostasis and bioluminescence suggested distinct mechanisms for coping with heat stress among the coral species. Furthermore, the impact of heat stress on coral biomineralization varied, with downregulation of carbonic anhydrase and skeletal organic matrix proteins indicating reduced capacity for biomineralization in the later stages of heat stress. Our findings provide insights into the molecular mechanisms underlying coral responses to heat stress and highlight the importance of considering species-specific responses in assessing coral resilience. The identified biomarkers may serve as indicators of heat stress and contribute to early detection of coral bleaching events. These findings contribute to our understanding of coral resilience and provide a basis for future research aimed at enhancing coral survival in the face of climate change.

Porites' coral calcifying fluid chemistry regulation under normal- and low-pH seawater conditions in Palau Archipelago: Impacts on growth properties

Ongoing ocean acidification is known to be a major threat to tropical coral reefs. To date, only few studies have evaluated the impacts of natural long-term exposure to low-pH seawater on the chemical regulation and growth of reef-building corals. This work investigated the different responses of the massive Porites coral living at normal (pHsw ~ 8.03) and naturally low-pH (pHsw ~ 7.85) seawater conditions at Palau over the last decades. Our results show that both Porites colonies maintained similar carbonate properties (pHcf, [CO32-]cf, DICcf, and Ωcf) within their calcifying fluid since 1972. However, the Porites skeleton of the more acidified conditions revealed a significantly lower density (~ 1.21 ± 0.09 g·cm-3) than the skeleton from the open-ocean site (~ 1.41 ± 0.07 g·cm-3). Overall, both Porites colonies exerted a strong biological control to maintain stable calcifying fluid carbonate chemistry that favored the calcification process, especially under low-pH conditions. However, the decline in skeletal density observed at low pH provides critical insights into Porites vulnerability to future global change.

Differences in carbonate chemistry up-regulation of long-lived reef-building corals

With climate projections questioning the future survival of stony corals and their dominance as tropical reef builders, it is critical to understand the adaptive capacity of corals to ongoing climate change. Biological mediation of the carbonate chemistry of the coral calcifying fluid is a fundamental component for assessing the response of corals to global threats. The Tara Pacific expedition (2016-2018) provided an opportunity to investigate calcification patterns in extant corals throughout the Pacific Ocean. Cores from colonies of the massive Porites and Diploastrea genera were collected from different environments to assess calcification parameters of long-lived reef-building corals. At the basin scale of the Pacific Ocean, we show that both genera systematically up-regulate their calcifying fluid pH and dissolved inorganic carbon to achieve efficient skeletal precipitation. However, while Porites corals increase the aragonite saturation state of the calcifying fluid (Ω_cf) at higher temperatures to enhance their calcification capacity, Diploastrea show a steady homeostatic Ω_cf across the Pacific temperature gradient. Thus, the extent to which Diploastrea responds to ocean warming and/or acidification is unclear, and it deserves further attention whether this is beneficial or detrimental to future survival of this coral genus.

Separate and combined effects of elevated pCO2 and temperature on the branching reef corals Acropora digitifera and Montipora digitata

Ocean acidification (OA) and warming (OW) are major global threats to coral reef ecosystems; however, studies on their combined effects (OA + OW) are scarce. Therefore, we evaluated the effects of OA, OW, and OA + OW in the branching reef corals Acropora digitifera and Montipora digitata, which have been found to respond differently to environmental changes. Our results indicate that OW has a greater impact on A. digitifera and M. digitata than OA and that the former species is more vulnerable to OW than the latter. OW was the main stressor for increased mortality and decreased calcification in the OA + OW group, and the effect of OA + OW was additive in both species. Our findings suggest that the relative abundance and cover of M. digitata are expected to increase whereas those of A. digitifera may decrease in the near future in Okinawa.

Corals adapted to extreme and fluctuating seawater pH increase calcification rates and have unique symbiont communities

Ocean acidification (OA) is a severe threat to coral reefs mainly by reducing their calcification rate. Identifying the resilience factors of corals to decreasing seawater pH is of paramount importance to predict the survivability of coral reefs in the future. This study compared corals adapted to variable pHT (i.e., 7.23-8.06) from the semi-enclosed lagoon of Bouraké, New Caledonia, to corals adapted to more stable seawater pHT (i.e., 7.90-8.18). In a 100-day aquarium experiment, we examined the physiological response and genetic diversity of Symbiodiniaceae from three coral species (Acropora tenuis, Montipora digitata, and Porites sp.) from both sites under three stable pHNBS conditions (8.11, 7.76, 7.54) and one fluctuating pHNBS regime (between 7.56 and 8.07). Bouraké corals consistently exhibited higher growth rates than corals from the stable pH environment. Interestingly, A. tenuis from Bouraké showed the highest growth rate under the 7.76 pHNBS condition, whereas for M. digitata, and Porites sp. from Bouraké, growth was highest under the fluctuating regime and the 8.11 pHNBS conditions, respectively. While OA generally decreased coral calcification by ca. 16%, Bouraké corals showed higher growth rates than corals from the stable pH environment (21% increase for A. tenuis to 93% for M. digitata, with all pH conditions pooled). This superior performance coincided with divergent symbiont communities that were more homogenous for Bouraké corals. Corals adapted to variable pH conditions appear to have a better capacity to calcify under reduced pH compared to corals native to more stable pH condition. This response was not gained by corals from the more stable environment exposed to variable pH during the 100-day experiment, suggesting that long-term exposure to pH fluctuations and/or differences in symbiont communities benefit calcification under OA.

Deoxygenation lowers the thermal threshold of coral bleaching

Exposure to deoxygenation from climate warming and pollution is emerging as a contributing factor of coral bleaching and mortality. However, the combined effects of heating and deoxygenation on bleaching susceptibility remain unknown. Here, we employed short-term thermal stress assays to show that deoxygenated seawater can lower the thermal limit of an Acropora coral by as much as 1 °C or 0.4 °C based on bleaching index scores or dark-acclimated photosynthetic efficiencies, respectively. Using RNA-Seq, we show similar stress responses to heat with and without deoxygenated seawater, both activating putative key genes of the hypoxia-inducible factor response system indicative of cellular hypoxia. We also detect distinct deoxygenation responses, including a disruption of O2-dependent photo-reception/-protection, redox status, and activation of an immune response prior to the onset of bleaching. Thus, corals are even more vulnerable when faced with heat stress in deoxygenated waters. This highlights the need to integrate dissolved O2 measurements into global monitoring programs of coral reefs.

Global change differentially modulates Caribbean coral physiology

Global change driven by anthropogenic carbon emissions is altering ecosystems at unprecedented rates, especially coral reefs, whose symbiosis with algal symbionts is particularly vulnerable to increasing ocean temperatures and altered carbonate chemistry. Here, we assess the physiological responses of three Caribbean coral (animal host + algal symbiont) species from an inshore and offshore reef environment after exposure to simulated ocean warming (28, 31°C), acidification (300–3290 μatm), and the combination of stressors for 93 days. We used multidimensional analyses to assess how a variety of coral physiological parameters respond to ocean acidification and warming. Our results demonstrate reductions in coral health in Siderastrea siderea and Porites astreoides in response to projected ocean acidification, while future warming elicited severe declines in Pseudodiploria strigosa. Offshore S. siderea fragments exhibited higher physiological plasticity than inshore counterparts, suggesting that this offshore population was more susceptible to changing conditions. There were no plasticity differences in P. strigosa and P. astreoides between natal reef environments, however, temperature evoked stronger responses in both species. Interestingly, while each species exhibited unique physiological responses to ocean acidification and warming, when data from all three species are modelled together, convergent stress responses to these conditions are observed, highlighting the overall sensitivities of tropical corals to these stressors. Our results demonstrate that while ocean warming is a severe acute stressor that will have dire consequences for coral reefs globally, chronic exposure to acidification may also impact coral physiology to a greater extent in some species than previously assumed. Further, our study identifies S. siderea and P. astreoides as potential ‘winners’ on future Caribbean coral reefs due to their resilience under projected global change stressors, while P. strigosa will likely be a ‘loser’ due to their sensitivity to thermal stress events. Together, these species-specific responses to global change we observe will likely manifest in altered Caribbean reef assemblages in the future.

Is Ocean Acidification Really a Threat to Marine Calcifiers? A Systematic Review and Meta-Analysis of 980+ Studies Spanning Two Decades

Ocean acidification is considered detrimental to marine calcifiers, but mounting contradictory evidence suggests a need to revisit this concept. This systematic review and meta-analysis aim to critically re-evaluate the prevailing paradigm of negative effects of ocean acidification on calcifiers. Based on 5153 observations from 985 studies, many calcifiers (e.g., echinoderms, crustaceans, and cephalopods) are found to be tolerant to near-future ocean acidification (pH ≈ 7.8 by the year 2100), but coccolithophores, calcifying algae, and corals appear to be sensitive. Calcifiers are generally more sensitive at the larval stage than adult stage. Over 70% of the observations in growth and calcification are non-negative, implying the acclimation capacity of many calcifiers to ocean acidification. This capacity can be mediated by phenotypic plasticity (e.g., physiological, mineralogical, structural, and molecular adjustments), transgenerational plasticity, increased food availability, or species interactions. The results suggest that the impacts of ocean acidification on calcifiers are less deleterious than initially thought as their adaptability has been underestimated. Therefore, in the forthcoming era of ocean acidification research, it is advocated that studying how marine organisms persist is as important as studying how they perish, and that future hypotheses and experimental designs are not constrained within the paradigm of negative effects.

Adaptive Responses of the Sea Anemone Heteractis crispa to the Interaction of Acidification and Global Warming

Ocean acidification and warming are two of the most important threats to the existence of marine organisms and are predicted to co-occur in oceans. The present work evaluated the effects of acidification (AC: 24 ± 0.1 °C and 900 μatm CO2), warming (WC: 30 ± 0.1 °C and 450 μatm CO2), and their combination (CC: 30 ± 0.1 °C and 900 μatm CO2) on the sea anemone, Heteractis crispa, from the aspects of photosynthetic apparatus (maximum quantum yield of photosystem II (PS II), chlorophyll level, and Symbiodiniaceae density) and sterol metabolism (cholesterol content and total sterol content). In a 15-day experiment, acidification alone had no apparent effect on the photosynthetic apparatus, but did affect sterol levels. Upregulation of their chlorophyll level is an important strategy for symbionts to adapt to high partial pressure of CO2 (pCO2). However, after warming stress, the benefits of high pCO2 had little effect on stress tolerance in H. crispa. Indeed, thermal stress was the dominant driver of the deteriorating health of H. crispa. Cholesterol and total sterol contents were significantly affected by all three stress conditions, although there was no significant change in the AC group on day 3. Thus, cholesterol or sterol levels could be used as important indicators to evaluate the impact of climate change on cnidarians. Our findings suggest that H. crispa might be relatively insensitive to the impact of ocean acidification, whereas increased temperature in the future ocean might impair viability of H. crispa.

The Effect of Ocean Acidification on Skeletal Structures

It is well known that the increasing partial pressure of atmospheric CO2 (pCO2) is reducing surface ocean pH, a process known as ocean acidification (OA) [...]

Stylophora under stress: A review of research trends and impacts of stressors on a model coral species

Sometimes called the "lab rat" of coral research, Stylophora pistillata (Esper, 1797) has been extensively used in coral biology in studies ranging from reef ecology to coral metabolic processes, and has been used as a model for investigations into molecular and cellular biology. Previously thought to be a common species spanning a wide distribution through the Indo-Pacific region, "S. pistillata" is in fact four genetically distinct lineages (clades) with different evolutionary histories and geographical distributions. Here, we review the studies of stress responses of S. pistillatasensulato (clades 1-4) and highlight research trends and knowledge gaps. We identify 126 studies on stress responses including effects of temperature, acidification, eutrophication, pollutants and other local impacts. We find that most studies have focused on the effect of single stressors, especially increased temperature, and have neglected the combined effects of multiple stressors. Roughly 61% of studies on S. pistillata come from the northern Red Sea (clade 4), at the extreme limit of its current distribution; clades 2 and 3 are virtually unstudied. The overwhelming majority of studies were conducted in laboratory or mesocosm conditions, with field experiments constituting only 2% of studies. We also note that a variety of experimental designs and treatment conditions makes it difficult to draw general conclusions about the effects of particular stressors on S. pistillata. Given those knowledge gaps and limitations in the published research, we suggest a more standardized approach to compare responses across geographically disparate populations and more accurately anticipate responses to predicted future climate conditions.

Changes in physiological performance and protein expression in the larvae of the coral Pocillopora damicornis and their symbionts in response to elevated temperature and acidification

Climate change causes ocean warming and acidification, which threaten coral reef ecosystems. Ocean warming and acidification cause bleaching and mortality, and decrease calcification in adult corals, leading to changes in the composition of coral communities; however, their interactive effects on coral larvae are not comprehensively understood. To examine the underlying molecular mechanisms of larval responses to elevated temperature and pCO₂, we examined the physiological performance and protein expression profiles of Pocillopora damicornis at two temperatures (29 and 33 °C) and pCO₂ levels (500 and 1000 μatm) for 5 d. Extensive physiological and proteomic changes were observed in coral larvae. The results indicated a significant decrease in net photosynthesis (P_NET) and autotrophic capability (P_NET/R_D) of larvae exposed to elevated temperature but a marked increase in P_NET and P_NET/R_D of larvae exposed to high pCO₂ levels. Elevated temperature significantly reduced endosymbiont densities by 70% and photochemical efficiency, indicating that warming impaired host-symbiont symbiosis. Expression of photosynthesis-related proteins, the photosystem (PS) I reaction center subunits IV and XI as well as oxygen-evolving enhancer 1, was downregulated at higher temperatures in symbionts, whereas expression of the PS I iron‑sulfur center protein was increased under high pCO₂ conditions. Furthermore, expression of phosphoribulokinase (involved in the Calvin cycle) and phosphoenolpyruvate carboxylase (related to the C₄ pathway) was downregulated in symbionts under thermal stress; this finding suggests reduced carbon fixation at high temperatures. The abundance of carbonic anhydrase-associated proteins, which are predicted to exert biochemical roles in dissolved inorganic carbon transport in larvae, was reduced in coral host and symbionts at high temperatures. These results elucidate potential mechanisms underlying the responses of coral larvae exposed to elevated temperature and acidification and suggest an important role of symbionts in the response to warming and acidification.

Genetic and physiological traits conferring tolerance to ocean acidification in mesophotic corals

The integrity of coral reefs worldwide is jeopardized by ocean acidification (OA). Most studies conducted so far have focused on the vulnerability to OA of corals inhabiting shallow reefs while nothing is currently known about the response of mesophotic scleractinian corals. In this study, we assessed the susceptibility to OA of corals, together with their algal partners, inhabiting a wide depth range. We exposed fragments of the depth generalist coral Stylophora pistillata collected from either 5 or 45 m to simulated future OA conditions, and assessed key molecular, physiological and photosynthetic processes influenced by the lowered pH. Our comparative analysis reveals that mesophotic and shallow S. pistillata corals are genetically distinct and possess different symbiont types. Under the exposure to acidification conditions, we observed a 50% drop of metabolic rate in shallow corals, whereas mesophotic corals were able to maintain unaltered metabolic rates. Overall, our gene expression and physiological analyses show that mesophotic corals possess a greater capacity to cope with the effects of OA compared to their shallow counterparts. Such capability stems from physiological characteristics (i.e., biomass and lipids energetics), a greater capacity to regulate cellular acid-base parameters, and a higher baseline expression of cell adhesion and extracellular matrix genes. Moreover, our gene expression analysis suggests that the enhanced symbiont photochemical efficiency under high pCO₂ levels could prevent acidosis of the host cells and it could support a greater translocation of photosynthates, increasing the energy pool available to the host. With this work, we provide new insights on the response to OA of corals living at mesophotic depths. Our investigation discloses key genetic and physiological traits underlying the potential for corals to cope with future OA conditions.

Combined responses of primary coral polyps and their algal endosymbionts to decreasing seawater pH

With coral reefs declining globally, resilience of these ecosystems hinges on successful coral recruitment. However, knowledge of the acclimatory and/or adaptive potential in response to environmental challenges such as ocean acidification (OA) in earliest life stages is limited. Our combination of physiological measurements, microscopy, computed tomography techniques and gene expression analysis allowed us to thoroughly elucidate the mechanisms underlying the response of early-life stages of corals, together with their algal partners, to the projected decline in oceanic pH. We observed extensive physiological, morphological and transcriptional changes in surviving recruits, and the transition to a less-skeleton/more-tissue phenotype. We found that decreased pH conditions stimulate photosynthesis and endosymbiont growth, and gene expression potentially linked to photosynthates translocation. Our unique holistic study discloses the previously unseen intricate net of interacting mechanisms that regulate the performance of these organisms in response to OA.

Marine heatwaves depress metabolic activity and impair cellular acid-base homeostasis in reef-building corals regardless of bleaching susceptibility

Ocean warming is causing global coral bleaching events to increase in frequency, resulting in widespread coral mortality and disrupting the function of coral reef ecosystems. However, even during mass bleaching events, many corals resist bleaching despite exposure to abnormally high temperatures. While the physiological effects of bleaching have been well documented, the consequences of heat stress for bleaching-resistant individuals are not well understood. In addition, much remains to be learned about how heat stress affects cellular-level processes that may be overlooked at the organismal level, yet are crucial for coral performance in the short term and ecological success over the long term. Here we compared the physiological and cellular responses of bleaching-resistant and bleaching-susceptible corals throughout the 2019 marine heatwave in Hawai'i, a repeat bleaching event that occurred 4 years after the previous regional event. Relative bleaching susceptibility within species was consistent between the two bleaching events, yet corals of both resistant and susceptible phenotypes exhibited pronounced metabolic depression during the heatwave. At the cellular level, bleaching-susceptible corals had lower intracellular pH than bleaching-resistant corals at the peak of bleaching for both symbiont-hosting and symbiont-free cells, indicating greater disruption of acid-base homeostasis in bleaching-susceptible individuals. Notably, cells from both phenotypes were unable to compensate for experimentally induced cellular acidosis, indicating that acid-base regulation was significantly impaired at the cellular level even in bleaching-resistant corals and in cells containing symbionts. Thermal disturbances may thus have substantial ecological consequences, as even small reallocations in energy budgets to maintain homeostasis during stress can negatively affect fitness. These results suggest concern is warranted for corals coping with ocean acidification alongside ocean warming, as the feedback between temperature stress and acid-base regulation may further exacerbate the physiological effects of climate change.