Future reef decalcification under a business-as-usual CO2 emission scenario

Long-term coral microbial community acclimatization is associated with coral survival in a changing climate

The plasticity of some coral-associated microbial communities under stressors like warming and ocean acidification suggests the microbiome has a role in the acclimatization of corals to future ocean conditions. Here, we evaluated the acclimatization potential of coral-associated microbial communities of four Hawaiian coral species (Porites compressa, Porites lobata, Montipora capitata, and Pocillopora acuta) over 22-month mesocosm experiment. The corals were exposed to one of four treatments: control, ocean acidification, ocean warming, or combined future ocean conditions. Over the 22-month study, 33-67% of corals died or experienced a loss of most live tissue coverage in the ocean warming and future ocean treatments while only 0-10% died in the ocean acidification and control. Among the survivors, coral-associated microbial communities responded to the chronic future ocean treatment in one of two ways: (1) microbial communities differed between the control and future ocean treatment, suggesting the potential capacity for acclimatization, or (2) microbial communities did not significantly differ between the control and future ocean treatment. The first strategy was observed in both Porites species and was associated with higher survivorship compared to M. capitata and P. acuta which exhibited the second strategy. Interestingly, the microbial community responses to chronic stressors were independent of coral physiology. These findings indicate acclimatization of microbial communities may confer resilience in some species of corals to chronic warming associated with climate change. However, M. capitata genets that survived the future ocean treatment hosted significantly different microbial communities from those that died, suggesting the microbial communities of the survivors conferred some resilience. Thus, even among coral species with inflexible microbial communities, some individuals may already be tolerant to future ocean conditions. These findings suggest that coral-associated microbial communities could play an important role in the persistence of some corals and underlie climate change-driven shifts in coral community composition.

Evaluation of different 16S rRNA gene hypervariable regions and reference databases for profiling engineered microbiota structure and functional guilds in a swine wastewater treatment plant

High-throughput 16S rRNA gene amplicon sequencing technology is widely applied for environmental microbiota structure analysis to derive knowledge that informs microbiome-based surveillance and oriented bioengineering. However, it remains elusive how the selection of 16S rRNA gene hypervariable regions and reference databases affects microbiota diversity and structure profiling. This study systematically evaluated the fitness of different frequently used reference databases (i.e. SILVA 138 SSU, GTDB bact120_r207, Greengenes 13_5 and MiDAS 4.8) and primers of 16S rRNA gene in microbiota profiling of anaerobic digestion and activated sludge collected from a full-scale swine wastewater treatment plant (WWTP). The comparative results showed that MiDAS 4.8 achieved the highest levels of taxonomic diversity and species-level assignment rate. For whichever sample groups, microbiota richness captured by different primers decreased in the following order: V4 > V4-V5 > V3-V4 > V6-V8/V1-V3. Using primer-bias-free metagenomic data results as the judging standard, V4 region also best characterized microbiota structure and well represented typical functional guilds (e.g. methanogens, ammonium oxidizers and denitrifiers), while V6-V8 regions largely overestimated the archaeal methanogens (mainly Methanosarcina) by over 30 times. Therefore, MiDAS 4.8 database and V4 region are recommended for best simultaneous analysis of bacterial and archaeal community diversity and structure of the examined swine WWTP.

Microbial Diversity, Community Turnover, and Putative Functions in Submarine Canyon Sediments under the Action of Sedimentary Geology

Sampling challenges in deep-sea ecosystems lead to a lack of knowledge about the distribution of microbes in different submarine canyons. To study microbial diversity and community turnover under different ecological processes, we performed 16S/18S rRNA gene amplicon sequencing for sediment samples from a submarine canyon in the South China Sea. Bacteria, archaea, and eukaryotes made up 57.94% (62 phyla), 41.04% (12 phyla), and 1.02% (4 phyla) of the sequences, respectively. Thaumarchaeota, Planctomycetota, Proteobacteria, Nanoarchaeota, and Patescibacteria are the five most abundant phyla. Heterogeneous community composition was mainly observed in vertical profiles rather than horizontal geographic locations, and microbial diversity in the surface layer was much lower than that in deep layers. According to the null model tests, homogeneous selection dominated community assembly within each sediment layer, whereas heterogeneous selection and dispersal limitation dominated community assembly between distant layers. Different sedimentation processes of sediments, i.e., rapid deposition caused by turbidity currents or slow sedimentation, seem to be primarily responsible for these vertical variations. Finally, functional annotation through shotgun-metagenomic sequencing found that glycosyl transferases and glycoside hydrolases are the most abundant carbohydrate-active enzyme categories. The most likely expressed sulfur cycling pathways include assimilatory sulfate reduction, the link between inorganic and organic sulfur transformation, and organic sulfur transformation, while the potentially activated methane cycling pathways include aceticlastic methanogenesis and aerobic and anaerobic oxidation of methane. Overall, our study revealed high levels of microbial diversity and putative functions in canyon sediments and the important influence of sedimentary geology on microbial community turnover between vertical sediment layers. IMPORTANCE Deep-sea microbes have received growing attention due to their contribution to biogeochemical cycles and climate change. However, related research lags due to the difficulty of collecting samples. Based on our previous study, which revealed the formation of sediments under the dual action of turbidity currents and seafloor obstacles in a submarine canyon in the South China Sea, this interdisciplinary research provides new insights into how sedimentary geology influences microbial community assembly in sediments. We proposed some uncommon or new findings, including the following: (i) microbial diversity was much lower on the surface than in deeper layers (ii) archaea and bacteria dominated the surface and deep layers, respectively; (iii) sedimentary geology played key roles in vertical community turnover; and (iv) the microbes have great potential to catalyze sulfur, carbon, and methane cycling. This study may lead to extensive discussion of the assembly and function of deep-sea microbial communities in the context of geology.

Metabolic and metatranscriptional characteristics of corals bleaching induced by the most severe marine heatwaves in the South China Sea

Coral bleaching significantly affects the function and health of coral reef ecosystems; however, the mechanisms underlying metabolism and transcription in corals remain unclear. In this study, untargeted metabolomics and metatranscriptomic analyses were performed to analyze the differences between unbleached and bleached Pocillopora corals during the most severe marine heatwaves. Difference analysis showed that bleached corals had significant metabolomic characteristics compared with those in unbleached corals. These differences were significant (p < 0.05) according to partial least squares discriminant analysis (PLS-DA). Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analysis revealed that the metabolites were significantly enriched in numerous pathways in bleached or unbleached corals, such as steroid hormone biosynthesis, biosynthesis of unsaturated fatty acids, and pyrimidine metabolism. Bleaching greatly affects coral reproduction as well as the tolerance of coral symbionts to heat stress. In metatranscriptomic analysis, we observed large gene expression differences between unbleached and bleached corals. Three Gene Ontology directed acyclic graphs (DAGs) were constructed to show the significantly differentially expressed genes (DEGs). Many biological and molecular processes were significantly enriched between bleached corals to unbleached corals, such as metabolic processes, lipid metabolic processes, oxidation-reduction processes, single-organism metabolic processes, and protein metabolic processes. Metabolome and metatranscriptome analyses showed that bleaching caused substantial physiological damage to corals. This study provides insight into the metabolic and transcriptional changes that occur in corals during bleaching.

Environmental memory gained from exposure to extreme pCO2 variability promotes coral cellular acid-base homeostasis

Ocean acidification is a growing threat to coral growth and the accretion of coral reef ecosystems. Corals inhabiting environments that already endure extreme diel pCO₂ fluctuations, however, may represent acidification-resilient populations capable of persisting on future reefs. Here, we examined the impact of pCO₂ variability on the reef-building coral Pocillopora damicornis originating from reefs with contrasting environmental histories (variable reef flat versus stable reef slope) following reciprocal exposure to stable (218 ± 9) or variable (911 ± 31) diel pCO₂ amplitude (μtam) in aquaria over eight weeks. Endosymbiont density, photosynthesis and net calcification rates differed between origins but not treatment, whereas primary calcification (extension) was affected by both origin and acclimatization to novel pCO₂ conditions. At the cellular level, corals from the variable reef flat exhibited less intracellular pH (pHi) acidosis and faster pHi recovery rates in response to experimental acidification stress (pH 7.40) than corals originating from the stable reef slope, suggesting environmental memory gained from lifelong exposure to pCO₂ variability led to an improved ability to regulate acid–base homeostasis. These results highlight the role of cellular processes in maintaining acidification resilience and suggest that prior exposure to pCO₂ variability may promote more acidification-resilient coral populations in a changing climate.

Potential Resilience to Ocean Acidification of Benthic Foraminifers Living in Posidonia oceanica Meadows: The Case of the Shallow Venting Site of Panarea

This research shows the results regarding the response to acidic condition of the sediment and Posidonia foraminiferal assemblages collected around the Panarea Island. The Aeolian Archipelago represents a natural laboratory and a much-promising study site for multidisciplinary marine research (carbon capture and storage, geochemistry of hydrothermal fluids and ocean acidification vs. benthic and pelagic organisms). The variability and the complexity of the interaction of the ecological factors characterizing extreme environments such as shallow hydrothermal vents did not allow us to carry out a real pattern of biota responses in situ, differently from those observed under controlled laboratory conditions. However, the study provides new insights into foraminiferal response to increasing ocean acidification (OA) in terms of biodiversity, faunal density, specific composition of the assemblages and morphological variations of the shells. The study highlights how the foraminiferal response to different pH conditions can change depending on different environmental conditions and microhabitats (sediments, Posidonia leaves and rhizomes). Indeed, mineral sediments were more impacted by acidification, whereas Posidonia microhabitats, thanks to their buffer effect, can offer “refugia” and more mitigated acidic environment. At species level, rosalinids and agglutinated group represent the most abundant taxa showing the most specific resilience and capability to face acidic conditions.

Integrating environmental variability to broaden the research on coral responses to future ocean conditions

Our understanding of the response of reef-building corals to changes in their physical environment is largely based on laboratory experiments, analysis of long-term field data, and model projections. Experimental data provide unique insights into how organisms respond to variation of environmental drivers. However, an assessment of how well experimental conditions cover the breadth of environmental conditions and variability where corals live successfully is missing. Here, we compiled and analyzed a globally distributed dataset of in-situ seasonal and diurnal variability of key environmental drivers (temperature, pCO₂ , and O₂ ) critical for the growth and livelihood of reef-building corals. Using a meta-analysis approach, we compared the variability of environmental conditions assayed in coral experimental studies to current and projected conditions in their natural habitats. We found that annual temperature profiles projected for the end of the 21st century were characterized by distributional shifts in temperatures with warmer winters and longer warm periods in the summer, not just peak temperatures. Furthermore, short-term hourly fluctuations of temperature and pCO₂ may regularly expose corals to conditions beyond the projected average increases for the end of the 21st century. Coral reef sites varied in the degree of coupling between temperature, pCO₂ , and dissolved O₂ , which warrants site-specific, differentiated experimental approaches depending on the local hydrography and influence of biological processes on the carbonate system and O₂ availability. Our analysis highlights that a large portion of the natural environmental variability at short and long timescales is underexplored in experimental designs, which may provide a path to extend our understanding on the response of corals to global climate change.

Decrease in volume and density of foraminiferal shells with progressing ocean acidification

Rapid increases in anthropogenic atmospheric CO₂ partial pressure have led to a decrease in the pH of seawater. Calcifying organisms generally respond negatively to ocean acidification. Foraminifera are one of the major carbonate producers in the ocean; however, whether calcification reduction by ocean acidification affects either foraminiferal shell volume or density, or both, has yet to be investigated. In this study, we cultured asexually reproducing specimens of Amphisorus kudakajimensis, a dinoflagellate endosymbiont-bearing large benthic foraminifera (LBF), under different pH conditions (pH 7.7–8.3, NBS scale). The results suggest that changes in seawater pH would affect not only the quantity (i.e., shell volume) but also the quality (i.e., shell density) of foraminiferal calcification. We proposed that pH and temperature affect these growth parameters differently because (1) they have differences in the contribution to the calcification process (e.g., Ca²⁺-ATPase and Ω) and (2) pH mainly affects calcification and temperature mainly affects photosynthesis. Our findings also suggest that, under the IPCC RCP8.5 scenario, both ocean acidification and warming will have a significant impact on reef foraminiferal carbonate production by the end of this century, even in the tropics.

Coral micro- and macro-morphological skeletal properties in response to life-long acclimatization at CO2 vents in Papua New Guinea

This study investigates the effects of long-term exposure to OA on skeletal parameters of four tropical zooxanthellate corals naturally living at CO₂ seeps and adjacent control sites from two locations (Dobu and Upa Upasina) in the Papua New Guinea underwater volcanic vent system. The seeps are characterized by seawater pH values ranging from 8.0 to about 7.7. The skeletal porosity of Galaxea fascicularis, Acropora millepora, massive Porites, and Pocillopora damicornis was higher (up to ~ 40%, depending on the species) at the seep sites compared to the control sites. Pocillopora damicornis also showed a decrease of micro-density (up to ~ 7%). Thus, further investigations conducted on this species showed an increase of the volume fraction of the larger pores (up to ~ 7%), a decrease of the intraskeletal organic matrix content (up to ~ 15%), and an increase of the intraskeletal water content (up to ~ 59%) at the seep sites. The organic matrix related strain and crystallite size did not vary between seep and control sites. This multi-species study showed a common phenotypic response among different zooxanthellate corals subjected to the same environmental pressures, leading to the development of a more porous skeletal phenotype under OA.

Identifying Metocean Drivers of Turbidity Using 18 Years of MODIS Satellite Data: Implications for Marine Ecosystems under Climate Change

Turbidity impacts the growth and productivity of marine benthic habitats due to light limitation. Daily/monthly synoptic and tidal influences often drive turbidity fluctuations, however, our understanding of what drives turbidity across seasonal/interannual timescales is often limited, thus impeding our ability to forecast climate change impacts to ecologically significant habitats. Here, we analysed long term (18-year) MODIS-aqua data to derive turbidity and the associated meteorological and oceanographic (metocean) processes in an arid tropical embayment (Exmouth Gulf in Western Australia) within the eastern Indian Ocean. We found turbidity was associated with El Niño Southern Oscillation (ENSO) cycles as well as Indian Ocean Dipole (IOD) events. Winds from the adjacent terrestrial region were also associated with turbidity and an upward trend in turbidity was evident in the body of the gulf over the 18 years. Our results identify hydrological processes that could be affected by global climate cycles undergoing change and reveal opportunities for managers to reduce impacts to ecologically important ecosystems.

Large-scale interventions may delay decline of the Great Barrier Reef

On the iconic Great Barrier Reef (GBR), the cumulative impacts of tropical cyclones, marine heatwaves and regular outbreaks of coral-eating crown-of-thorns starfish (CoTS) have severely depleted coral cover. Climate change will further exacerbate this situation over the coming decades unless effective interventions are implemented. Evaluating the efficacy of alternative interventions in a complex system experiencing major cumulative impacts can only be achieved through a systems modelling approach. We have evaluated combinations of interventions using a coral reef meta-community model. The model consisted of a dynamic network of 3753 reefs supporting communities of corals and CoTS connected through ocean larval dispersal, and exposed to changing regimes of tropical cyclones, flood plumes, marine heatwaves and ocean acidification. Interventions included reducing flood plume impacts, expanding control of CoTS populations, stabilizing coral rubble, managing solar radiation and introducing heat-tolerant coral strains. Without intervention, all climate scenarios resulted in precipitous declines in GBR coral cover over the next 50 years. The most effective strategies in delaying decline were combinations that protected coral from both predation (CoTS control) and thermal stress (solar radiation management) deployed at large scale. Successful implementation could expand opportunities for climate action, natural adaptation and socioeconomic adjustment by at least one to two decades.

Impacts of hypoxic events surpass those of future ocean warming and acidification

Over the past decades, three major challenges to marine life have emerged as a consequence of anthropogenic emissions: ocean warming, acidification and oxygen loss. While most experimental research has targeted the first two stressors, the last remains comparatively neglected. Here, we implemented sequential hierarchical mixed-model meta-analyses (721 control-treatment comparisons) to compare the impacts of oxygen conditions associated with the current and continuously intensifying hypoxic events (1-3.5 O₂ mg l^-1) with those experimentally yielded by ocean warming (+4 °C) and acidification (-0.4 units) conditions on the basis of IPCC projections (RCP 8.5) for 2100. In contrast to warming and acidification, hypoxic events elicited consistent negative effects relative to control biological performance-survival (-33%), abundance (-65%), development (-51%), metabolism (-33%), growth (-24%) and reproduction (-39%)-across the taxonomic groups (mollusks, crustaceans and fish), ontogenetic stages and climate regions studied. Our findings call for a refocus of global change experimental studies, integrating oxygen concentration drivers as a key factor of ocean change. Given potential combined effects, multistressor designs including gradual and extreme changes are further warranted to fully disclose the future impacts of ocean oxygen loss, warming and acidification.

Responses of branching reef corals Acropora digitifera and Montipora digitata to elevated temperature and pCO2

Anthropogenic emission of CO₂ into the atmosphere has been increasing exponentially, causing ocean acidification (OA) and ocean warming (OW). The “business-as-usual” scenario predicts that the atmospheric concentration of CO₂ may exceed 1,000 µatm and seawater temperature may increase by up to 3 °C by the end of the 21^st century. Increases in OA and OW may negatively affect the growth and survival of reef corals. In the present study, we separately examined the effects of OW and OA on the corals Acropora digitifera and Montipora digitata, which are dominant coral species occurring along the Ryukyu Archipelago, Japan, at three temperatures (28 °C, 30 °C, and 32 °C) and following four pCO₂ treatments (400, 600, 800, and 1,000 µatm) in aquarium experiments. In the OW experiment, the calcification rate (p = 0.02), endosymbiont density, and maximum photosynthetic efficiency (Fv/Fm) (both p < 0.0001) decreased significantly at the highest temperature (32 °C) compared to those at the lower temperatures (28 °C and 30 °C) in both species. In the OA experiment, the calcification rate decreased significantly as pCO₂ increased (p < 0.0001), whereas endosymbiont density, chlorophyll content, and Fv/Fm were not affected. The calcification rate of A. digitifera showed greater decreases from 30 °C to 32 °C than that of M. digitata. The calcification of the two species responded differently to OW and OA. These results suggest that A. digitifera is more sensitive to OW than M. digitata, whereas M. digitata is more sensitive to OA. Thus, differences in the sensitivity of the two coral species to OW and OA might be attributed to differences in the endosymbiont species and high calcification rates, respectively.

Metabolic performance and thermal and salinity tolerance of the coral Platygyra carnosa in Hong Kong waters

Stress-tolerant coral species, such as Platygyra spp., are considered to be well adapted to survive in marginal reefs, but their physiological response to short term exposure to abnormally high temperature and lowered salinity remains poorly understood. Using non-invasive techniques to quantitatively assess the health of Platygyra carnosa (e.g. respiration, photosynthesis, biocalcification and whiteness), we identified the plasticity of its energetics and physiological limits. Although these indicators suggest that it can survive to increasing temperature (25-32 °C), its overall energetics were seriously diminished at temperatures >30 °C. In contrast, it was well adapted to hyposaline waters (31-21 psu) but with reduced biocalcification, indicating short term adaptation for expected future changes in salinity driven by increased amounts and intensities of precipitation. Our findings provide useful insights to the effect of these climate drivers on P. carnosa metabolism and thus better forecast changes in their health status under future climate change scenarios.

Paradise lost: End-of-century warming and acidification under business-as-usual emissions have severe consequences for symbiotic corals

Despite recent efforts to curtail greenhouse gas emissions, current global emission trajectories are still following the business-as-usual representative concentration pathway (RCP) 8.5 emission pathway. The resulting ocean warming and acidification have transformative impacts on coral reef ecosystems, detrimentally affecting coral physiology and health, and these impacts are predicted to worsen in the near future. In this study, we kept fragments of the symbiotic corals Acropora intermedia (thermally sensitive) and Porites lobata (thermally tolerant) for 7 weeks under an orthogonal design of predicted end-of-century RCP8.5 conditions for temperature and pCO2 (3.5°C and 570 ppm above present-day, respectively) to unravel how temperature and acidification, individually or interactively, influence metabolic and physiological performance. Our results pinpoint thermal stress as the dominant driver of deteriorating health in both species because of its propensity to destabilize coral-dinoflagellate symbiosis (bleaching). Acidification had no influence on metabolism but had a significant negative effect on skeleton growth, particularly when photosynthesis was absent such as in bleached corals or under dark conditions. Total loss of photosynthesis after bleaching caused an exhaustion of protein and lipid stores and collapse of calcification that ultimately led to A. intermedia mortality. Despite complete loss of symbionts from its tissue, P. lobata maintained small amounts of photosynthesis and experienced a weaker decline in lipid and protein reserves that presumably contributed to higher survival of this species. Our results indicate that ocean warming and acidification under business-as-usual CO2 emission scenarios will likely extirpate thermally sensitive coral species before the end of the century, while slowing the recovery of more thermally tolerant species from increasingly severe mass coral bleaching and mortality. This could ultimately lead to the gradual disappearance of tropical coral reefs globally, and a shift on surviving reefs to only the most resilient coral species.

Living coral tissue slows skeletal dissolution related to ocean acidification

Climate change is causing major changes to marine ecosystems globally, with ocean acidification of particular concern for coral reefs. Using a 200 d in situ carbon dioxide enrichment study on Heron Island, Australia, we simulated future ocean acidification conditions, and found reduced pH led to a drastic decline in net calcification of living corals to no net growth, and accelerated disintegration of dead corals. Net calcification declined more severely than in previous studies due to exposure to the natural community of bioeroding organisms in this in situ study and to a longer experimental duration. Our data suggest that reef flat corals reach net dissolution at an aragonite saturation state (Ω_AR) of 2.3 (95% confidence interval: 1.8-2.8) with 100% living coral cover and at Ω_AR > 3.5 with 30% living coral cover. This model suggests that areas of the reef with relatively low coral mortality, where living coral cover is high, are likely to be resistant to carbon dioxide-induced reef dissolution.

The human imperative of stabilizing global climate change at 1.5°C

Increased concentrations of atmospheric greenhouse gases have led to a global mean surface temperature 1.0°C higher than during the pre-industrial period. We expand on the recent IPCC Special Report on global warming of 1.5°C and review the additional risks associated with higher levels of warming, each having major implications for multiple geographies, climates, and ecosystems. Limiting warming to 1.5°C rather than 2.0°C would be required to maintain substantial proportions of ecosystems and would have clear benefits for human health and economies. These conclusions are relevant for people everywhere, particularly in low- and middle-income countries, where the escalation of climate-related risks may prevent the achievement of the United Nations Sustainable Development Goals.

Using B isotopes and B/Ca in corals from low saturation springs to constrain calcification mechanisms

Ocean acidification is expected to negatively impact calcifying organisms, yet we lack understanding of their acclimation potential in the natural environment. Here we measured geochemical proxies (δ¹¹B and B/Ca) in Porites astreoides corals that have been growing for their entire life under low aragonite saturation (Ω_sw: 0.77–1.85). This allowed us to assess the ability of these corals to manipulate the chemical conditions at the site of calcification (Ω_cf), and hence their potential to acclimate to changing Ω_sw. We show that lifelong exposure to low Ω_sw did not enable the corals to acclimate and reach similar Ω_cf as corals grown under ambient conditions. The lower Ω_cf at the site of calcification can explain a large proportion of the decreasing P. astreoides calcification rates at low Ω_sw. The naturally elevated seawater dissolved inorganic carbon concentration at this study site shed light on how different carbonate chemistry parameters affect calcification conditions in corals.

Ocean acidification is expected to have a negative impact on calcifying organisms, however, our understanding of the acclimation potential of corals in their natural habit is currently limited. Here, the authors find that scleractinian corals living in high pCO₂ conditions cannot fully adapt the chemistry of their internal calcifying fluid compared to corals growing in ambient conditions.

Living in future ocean acidification, physiological adaptive responses of the immune system of sea urchins resident at a CO2 vent system

The effects of ocean acidification, a major anthropogenic impact on marine life, have been mainly investigated in laboratory/mesocosm experiments. We used the CO₂ vents at Ischia as a natural laboratory to study the long-term effects of ocean acidification on the sea urchin Paracentrotus lividus population resident in low-pH (7.8 ± 0.2) compared to that at two control sites (pH 8.02 ± 0.00; 8.02 ± 0.01). The novelty of the present study is the analysis of the sea urchin immune cells, the sentinels of environmental stress responses, by a wide-ranging approach, including cell morphology, biochemistry and proteomics. Immune cell proteomics showed that 311 proteins were differentially expressed in urchins across sites with a general shift towards antioxidant processes in the vent urchins. The vent urchin immune cells showed higher levels of total antioxidant capacity, up-regulation of phagosome and microsomal proteins, enzymes of ammonium metabolism, amino-acid degradation, and modulation of carbon metabolism proteins. Lipid-hydroperoxides and nitric oxide levels were not different in urchins from the different sites. No differences in the coelomic fluid pH, immune cell composition, animal respiration, nitrogen excretion and skeletal mineralogy were observed. Our results reveal the phenotypic plasticity of the immune system of sea urchins adapted to life at vent site, under conditions commensurate with near-future ocean acidification projections.

Climate change drives trait-shifts in coral reef communities

Climate change is expected to have profound, partly unforeseeable effects on the composition of functional traits of complex ecosystems, such as coral reefs, and some ecosystem properties are at risk of disappearing. This study applies a novel spatially explicit, individual-based model to explore three critical life history traits of corals: heat tolerance, competitiveness and growth performance under various environmental settings. Building upon these findings, we test the adaptation potential required by a coral community in order to not only survive but also retain its diversity by the end of this century under different IPCC climate scenarios. Even under the most favourable IPCC scenario (Representative Concentration Pathway, RCP 2.6), model results indicate that shifts in the trait space are likely and coral communities will mainly consist of small numbers of temperature-tolerant and fast-growing species. Species composition of coral communities is likely to be determined by heat tolerance, with competitiveness most likely playing a subordinate role. To sustain ~15% of current coral cover under a 2 °C temperature increase by the end of the century (RCP 4.5), coral systems would have to accommodate temperature increases of 0.1-0.15 °C per decade, assuming that periodic extreme thermal events occurred every 8 years. These required adaptation rates are unprecedented and unlikely, given corals' life-history characteristics.

Thresholds and drivers of coral calcification responses to climate change

Increased temperature and CO₂ levels are considered key drivers of coral reef degradation. However, individual assessments of ecological responses (calcification) to these stressors are often contradicting. To detect underlying drivers of heterogeneity in coral calcification responses, we developed a procedure for the inclusion of stress-effect relationships in ecological meta-analyses. We applied this technique to a dataset of 294 empirical observations from 62 peer-reviewed publications testing individual and combined effects of elevated temperature and pCO₂ on coral calcification. Our results show an additive interaction between warming and acidification, which reduces coral calcification by 20% when pCO₂ levels exceed 700 ppm and temperature increases by 3°C. However, stress levels varied among studies and significantly affected outcomes, with unaffected calcification rates under moderate stresses (pCO₂  ≤ 700 ppm, ΔT < 3°C). Future coral reef carbon budgets will therefore depend on the magnitude of pCO₂ and temperature elevations and, thus, anthropogenic CO₂ emissions. Accounting for stress-effect relationships enabled us to identify additional drivers of heterogeneity including coral taxa, life stage, habitat, food availability, climate, and season. These differences can aid reef management identifying refuges and conservation priorities, but without a global effort to reduce CO₂ emissions, coral capacity to build reefs will be at risk.

Eutrophication may compromise the resilience of the Red Sea coral Stylophora pistillata to global change

Environmental stressors are adversely affecting coral reef ecosystems. There is ample evidence that scleractinian coral growth and physiology may be compromised by reduced pH, and elevated temperature, and that this is exacerbated by local environmental stressors. The Gulf of Aqaba is considered a coral reef refuge from acidification and warming but coastal development and nutrient effluent may pose a local threat. This study examined the effects of select forecasted environmental changes (acidification, warming, and increased nutrients) individually and in combination on the coral holobiont Stylophora pistillata from the Gulf of Aqaba to understand how corals in a potential global climate change refugia may fare in the face of local eutrophication. The results indicate interactions between all stressors, with elevated nutrient concentrations having the broadest individual and additive impacts upon the performance of S. pistillata. These findings highlight the importance of maintaining oligotrophic conditions to secure these reefs as potential refugia.

Ocean acidification alters early successional coral reef communities and their rates of community metabolism

Ocean acidification is expected to alter community composition on coral reefs, but its effects on reef community metabolism are poorly understood. Here we document how early successional benthic coral reef communities change in situ along gradients of carbon dioxide (CO₂), and the consequences of these changes on rates of community photosynthesis, respiration, and light and dark calcification. Ninety standardised benthic communities were grown on PVC tiles deployed at two shallow-water volcanic CO₂ seeps and two adjacent control sites in Papua New Guinea. Along the CO₂ gradient, both the upward facing phototrophic and the downward facing cryptic communities changed in their composition. Under ambient CO₂, both communities were dominated by calcifying algae, but with increasing CO₂ they were gradually replaced by non-calcifying algae (predominantly green filamentous algae, cyanobacteria and macroalgae, which increased from ~30% to ~80% cover). Responses were weaker in the invertebrate communities, however ascidians and tube-forming polychaetes declined with increasing CO₂. Differences in the carbonate chemistry explained a far greater amount of change in communities than differences between the two reefs and successional changes from five to 13 months, suggesting community successions are established early and are under strong chemical control. As pH declined from 8.0 to 7.8, rates of gross photosynthesis and dark respiration of the 13-month old reef communities (upper and cryptic surfaces combined) significantly increased by 10% and 20%, respectively, in response to altered community composition. As a consequence, net production remained constant. Light and dark calcification rates both gradually declined by 20%, and low or negative daily net calcification rates were observed at an aragonite saturation state of <2.3. The study demonstrates that ocean acidification as predicted for the end of this century will strongly alter reef communities, and will significantly change rates of community metabolism.

Seagrass habitat metabolism increases short-term extremes and long-term offset of CO2 under future ocean acidification

The role of rising atmospheric CO₂ in modulating estuarine carbonate system dynamics remains poorly characterized, likely due to myriad processes driving the complex chemistry in these habitats. We reconstructed the full carbonate system of an estuarine seagrass habitat for a summer period of 2.5 months utilizing a combination of time-series observations and mechanistic modeling, and quantified the roles of aerobic metabolism, mixing, and gas exchange in the observed dynamics. The anthropogenic CO₂ burden in the habitat was estimated for the years 1765-2100 to quantify changes in observed high-frequency carbonate chemistry dynamics. The addition of anthropogenic CO₂ alters the thermodynamic buffer factors (e.g., the Revelle factor) of the carbonate system, decreasing the seagrass habitat's ability to buffer natural carbonate system fluctuations. As a result, the most harmful carbonate system indices for many estuarine organisms [minimum pH_T, minimum Ω_arag, and maximum pCO_2(s.w.)] change up to 1.8×, 2.3×, and 1.5× more rapidly than the medians for each parameter, respectively. In this system, the relative benefits of the seagrass habitat in locally mitigating ocean acidification increase with the higher atmospheric CO₂ levels predicted toward 2100. Presently, however, these mitigating effects are mixed due to intense diel cycling of CO₂ driven by aerobic metabolism. This study provides estimates of how high-frequency pH_T, Ω_arag, and pCO_2(s.w.) dynamics are altered by rising atmospheric CO₂ in an estuarine habitat, and highlights nonlinear responses of coastal carbonate parameters to ocean acidification relevant for water quality management.

Bleaching and mortality of a photosymbiotic bioeroding sponge under future carbon dioxide emission scenarios

The bioeroding sponge Cliona orientalis is photosymbiotic with dinoflagellates of the genus Symbiodinium and is pervasive on the Great Barrier Reef. We investigated how C. orientalis responded to past and future ocean conditions in a simulated community setting. The experiment lasted over an Austral summer under four carbon dioxide emission scenarios: a pre-industrial scenario (PI), a present-day scenario (PD; control), and two future scenarios of combined ocean acidification and ocean warming, i.e., B1 (intermediate) and A1FI (extreme). The four scenarios also simulated natural variability of carbon dioxide partial pressure and temperature in seawater. Responses of C. orientalis generally remained similar between the PI and PD treatments. C. orientalis under B1 displayed a dramatic increase in lateral tissue extension, but bleached and displayed reduced rates of respiration and photosynthesis. Some B1 sponge replicates died by the end of the experiment. Under A1FI, strong bleaching and subsequent mortality of all C. orientalis replicates occurred at an early stage of the experiment. Mortality arrested bioerosion by C. orientalis under B1 and A1FI. Overall, the absolute amount of calcium carbonate eroded by C. orientalis under B1 or A1FI was similar to that under PI or PD at the end of the experiment. Although bioerosion rates were raised by short-term experimental acidification in previous studies, our findings from the photosymbiotic C. orientalis imply that the effects of bioerosion on reef carbonate budgets may only be temporary if the bioeroders cannot survive long-term in the future oceans.

Calcification in Caribbean reef-building corals at high pCO2 levels in a recirculating ocean acidification exposure system

Projected increases in ocean pCO₂ levels are anticipated to affect calcifying organisms more rapidly and to a greater extent than other marine organisms. The effects of ocean acidification (OA) have been documented in numerous species of corals in laboratory studies, largely tested using flow-through exposure systems. We developed a recirculating ocean acidification exposure system that allows precise pCO₂ control using a combination of off-gassing measures including aeration, water retention devices, venturi injectors, and CO₂ scrubbing. We evaluated the recirculating system performance in off-gassing effectiveness and maintenance of target pCO₂ levels over an 84-day experiment. The system was used to identify changes in calcification and tissue growth in response to elevated pCO₂ (1000 μatm) in three reef-building corals of the Caribbean: Pseudodiploria clivosa, Montastraea cavernosa, and Orbicella faveolata. All three species displayed an overall increase in net calcification over the 84-day exposure period regardless of pCO₂ level (control +0.28- 1.12 g, elevated pCO₂ +0.18- 1.16 g), and the system was effective at both off-gassing acidified water to ambient pCO₂ levels, and maintaining target elevated pCO₂ levels over the 3-month experiment.

Coral physiology and microbiome dynamics under combined warming and ocean acidification

Rising seawater temperature and ocean acidification threaten the survival of coral reefs. The relationship between coral physiology and its microbiome may reveal why some corals are more resilient to these global change conditions. Here, we conducted the first experiment to simultaneously investigate changes in the coral microbiome and coral physiology in response to the dual stress of elevated seawater temperature and ocean acidification expected by the end of this century. Two species of corals, Acropora millepora containing the thermally sensitive endosymbiont C21a and Turbinaria reniformis containing the thermally tolerant endosymbiont Symbiodinium trenchi, were exposed to control (26.5°C and pCO₂ of 364 μatm) and treatment (29.0°C and pCO₂ of 750 μatm) conditions for 24 days, after which we measured the microbial community composition. These microbial findings were interpreted within the context of previously published physiological measurements from the exact same corals in this study (calcification, organic carbon flux, ratio of photosynthesis to respiration, photosystem II maximal efficiency, total lipids, soluble animal protein, soluble animal carbohydrates, soluble algal protein, soluble algal carbohydrate, biomass, endosymbiotic algal density, and chlorophyll a). Overall, dually stressed A. millepora had reduced microbial diversity, experienced large changes in microbial community composition, and experienced dramatic physiological declines in calcification, photosystem II maximal efficiency, and algal carbohydrates. In contrast, the dually stressed coral T. reniformis experienced a stable and more diverse microbiome community with minimal physiological decline, coupled with very high total energy reserves and particulate organic carbon release rates. Thus, the microbiome changed and microbial diversity decreased in the physiologically sensitive coral with the thermally sensitive endosymbiotic algae but not in the physiologically tolerant coral with the thermally tolerant endosymbiont. Our results confirm recent findings that temperature-stress tolerant corals have a more stable microbiome, and demonstrate for the first time that this is also the case under the dual stresses of ocean warming and acidification. We propose that coral with a stable microbiome are also more physiologically resilient and thus more likely to persist in the future, and shape the coral species diversity of future reef ecosystems.

Altered sediment biota and lagoon habitat carbonate dynamics due to sea cucumber bioturbation in a high-pCO2 environment

The effects of global change on biological systems and functioning are already measurable, but how ecological interactions are being altered is poorly understood. Ecosystem resilience is strengthened by ecological functionality, which depends on trophic interactions between key species and resilience generated through biogenic buffering. Climate-driven alterations to coral reef metabolism, structural complexity and biodiversity are well documented, but the feedbacks between ocean change and trophic interactions of non-coral invertebrates are understudied. Sea cucumbers, some of the largest benthic inhabitants of tropical lagoon systems, can influence diel changes in reef carbonate dynamics. Whether they have the potential to exacerbate or buffer ocean acidification over diel cycles depends on their relative production of total alkalinity (A_T ) through the dissolution of ingested calcium carbonate (CaCO₃ ) sediments and release of dissolved inorganic carbon (C_T ) through respiration and trophic interactions. In this study, the potential for the sea cucumber, Stichopus herrmanni, a bêche-de-mer (fished) species listed as vulnerable to extinction, to buffer the impacts of ocean acidification on reef carbonate chemistry was investigated in lagoon sediment mesocosms across diel cycles. Stichopus herrmanni directly reduced the abundance of meiofauna and benthic primary producers through its deposit-feeding activity under present-day and near-future pCO₂ . These changes in benthic community structure, as well as A_T (sediment dissolution) and C_T (respiration) production by S. herrmanni, played a significant role in modifying seawater carbonate dynamics night and day. This previously unappreciated role of tropical sea cucumbers, in support of ecosystem resilience in the face of global change, is an important consideration with respect to the bêche-de-mer trade to ensure sea cucumber populations are sustained in a future ocean.

Simultaneous use of caustic and oxygen for efficient sulfide control in sewers

Periodic caustic shock-loading is a commonly used method for sulfide control in sewers. Caustic shock-loading relies on the elevation of the sewage pH to ≥10.5 for several hours, thereby removing sewer pipe biofilms as well as deactivating SRB activity in the remaining biofilm. Although a widely used method, SRB activity is often not completely inhibited, and as such sulfide is still being generated. Here, we propose and experimentally demonstrate an innovative approach which combines caustic with oxygen, another commonly used method, as a dosing strategy for overcoming the drawbacks of caustic shock-loading. Six laboratory-scale rising main reactors were subjected to three dosing schemes over a period of three months, namely (i) simultaneous caustic and oxygen addition, (ii) caustic addition and (iii) no chemical addition. Our results showed that the combination of caustic and oxygen achieved efficient sulfide control, leading to a prolonged biofilm recovery period in between caustic shocks. In addition, methane emissions were reduced to a negligible level compared to caustic treatment only. To translate the findings to real-life application, the key parameters obtained during the long-term lab-scale experiments were subjected to extensive simulation studies using the SeweX model under a wide range of conditions commonly found in sewers. Overall, this study highlights the potential of periodic shock-loading and intermittent oxygen injection as combined dosing strategy for efficient sulfide control in sewers.

Trends and frontiers for the science and management of the oceans

People have an enduring fascination with the biology of the oceans. When the BBC's 'Blue Planet' series first aired on British television almost a quarter of the nation tuned in. As the diversity of science in this special issue of Current Biology attests, the ocean presents a challenging environment for study while also exhibiting some of the most profound and disruptive symptoms of global change. Marine science has made major advances in the past few decades, which were primarily made possible through important technological innovations. This progress notwithstanding, there are persistent challenges in achieving an understanding of marine processes at appropriate scales and delivering meaningful insights to guide ocean policy and management. Naturally, the examples chosen below betray my ecological leanings, but I hope that many of the issues raised resonate with readers in many different disciplines.

Linking fishes to multiple metrics of coral reef structural complexity using three-dimensional technology

Structural complexity strongly influences biodiversity and ecosystem productivity. On coral reefs, structural complexity is typically measured using a single and small-scale metric (‘rugosity’) that represents multiple spatial attributes differentially exploited by species, thus limiting a complete understanding of how fish associate with reef structure. We used a novel approach to compare relationships between fishes and previously unavailable components of reef complexity, and contrasted the results against the traditional rugosity index. This study focused on damselfish to explore relationships between fishes and reef structure. Three territorial species, with contrasting trophic habits and expected use of the reef structure, were examined to infer the potential species-specific mechanisms associated with how complexity influences habitat selection. Three-dimensional reef reconstructions from photogrammetry quantified the following metrics of habitat quality: 1) visual exposure to predators and competitors, 2) density of predation refuges and 3) substrate-related food availability. These metrics explained the species distribution better than the traditional measure of rugosity, and each species responded to different complexity components. Given that a critical effect of reef degradation is loss of structure, adopting three-dimensional technologies potentially offers a new tool to both understand species-habitat association and help forecast how fishes will be affected by the flattening of reefs.

Sponge bioerosion on changing reefs: ocean warming poses physiological constraints to the success of a photosymbiotic excavating sponge

Excavating sponges are prominent bioeroders on coral reefs that in comparison to other benthic organisms may suffer less or may even benefit from warmer, more acidic and more eutrophic waters. Here, the photosymbiotic excavating sponge Cliona orientalis from the Great Barrier Reef was subjected to a prolonged simulation of both global and local environmental change: future seawater temperature, partial pressure of carbon dioxide (as for 2100 summer conditions under "business-as-usual" emissions), and diet supplementation with particulate organics. The individual and combined effects of the three factors on the bioerosion rates, metabolic oxygen and carbon flux, biomass change and survival of the sponge were monitored over the height of summer. Diet supplementation accelerated bioerosion rates. Acidification alone did not have a strong effect on total bioerosion or survival rates, yet it co-occurred with reduced heterotrophy. Warming above 30 °C (+2.7 °C above the local maximum monthly mean) caused extensive bleaching, lower bioerosion, and prevailing mortality, overriding the other factors and suggesting a strong metabolic dependence of the sponge on its resident symbionts. The growth, bioerosion capacity and likelihood of survival of C. orientalis and similar photosymbiotic excavating sponges could be substantially reduced rather than increased on end-of-the-century reefs under "business-as-usual" emission profiles.

Acclimatization to high-variance habitats does not enhance physiological tolerance of two key Caribbean corals to future temperature and pH

Corals are acclimatized to populate dynamic habitats that neighbour coral reefs. Habitats such as seagrass beds exhibit broad diel changes in temperature and pH that routinely expose corals to conditions predicted for reefs over the next 50-100 years. However, whether such acclimatization effectively enhances physiological tolerance to, and hence provides refuge against, future climate scenarios remains unknown. Also, whether corals living in low-variance habitats can tolerate present-day high-variance conditions remains untested. We experimentally examined how pH and temperature predicted for the year 2100 affects the growth and physiology of two dominant Caribbean corals (Acropora palmata and Porites astreoides) native to habitats with intrinsically low (outer-reef terrace, LV) and/or high (neighbouring seagrass, HV) environmental variance. Under present-day temperature and pH, growth and metabolic rates (calcification, respiration and photosynthesis) were unchanged for HV versus LV populations. Superimposing future climate scenarios onto the HV and LV conditions did not result in any enhanced tolerance to colonies native to HV. Calcification rates were always lower for elevated temperature and/or reduced pH. Together, these results suggest that seagrass habitats may not serve as refugia against climate change if the magnitude of future temperature and pH changes is equivalent to neighbouring reef habitats.

Effects of elevated nutrients and CO2 emission scenarios on three coral reef macroalgae

Coral reef macroalgae are expected to thrive in the future under conditions that are deleterious to the health of reef-building corals. Here we examined how macroalgae would be affected by exposure to future CO2 emission scenarios (pCO2 and temperature), enriched nutrients and combinations of both. The species tested, Laurencia intricata (Rhodophyta), Turbinaria ornata and Chnoospora implexa (both Phaeophyceae), have active carbon-concentrating mechanisms but responded differently to the treatments. L. intricata showed high mortality under nutrient enriched RCP4.5 ("reduced" CO2 emission) and RCP8.5 ("business-as-usual" CO2 emission) and grew best under pre-industrial (PI) conditions, where it could take up carbon using external carbonic anhydrase combined, potentially, with proton extrusion. T. ornata's growth rate showed a trend for reduction under RCP8.5 but was unaffected by nutrient enrichment. In C. implexa, highest growth was observed under PI conditions, but highest net photosynthesis occurred under RCP8.5, suggesting that under RCP8.5, carbon is stored and respired at greater rates while it is directed to growth under PI conditions. None of the species showed growth enhancement under future scenarios, nutrient enrichment or combinations of both. This leads to the conclusion that under such conditions these species are unlikely to pose an increasing threat to coral reefs.

Reformation of tissue balls from tentacle explants of coral Goniopora lobata: self-organization process and response to environmental stresses

Coral has strong regeneration ability, which has been applied for coral production and biodiversity protection via tissue ball (TB) culture. However, the architecture, morphological processes, and effects of environmental factors on TB formation have not been well investigated. In this study, we first observed TB formation from the cutting tentacle of scleractinia coral Goniopora lobata and uncovered its inner organization and architecture by confocal microscopy. We then found that the cutting tentacle TB could self-organize and reform a solid TB (sTB) in the culture media. Using chemical drug treatment and dissection manipulation approaches, we demonstrated that the mechanical forces for bending and rounding of the cutting fragments came from the epithelial cells, and the cilia of epithelial cell played indispensable roles for the rounding process. Environmental stress experiments showed that high temperature, not CO₂-induced acidification, affected TB and sTB formation. However, the combination of high temperature and acidification caused additional severe effects on sTB reformation. Our studies indicate that coral TB has strong regeneration ability and therefore could serve as a new model to further explore the molecular mechanism of TB formation and the effects of environmental stresses on coral survival and regeneration.

Ocean acidification: Linking science to management solutions using the Great Barrier Reef as a case study

Coral reefs are one of the most vulnerable ecosystems to ocean acidification. While our understanding of the potential impacts of ocean acidification on coral reef ecosystems is growing, gaps remain that limit our ability to translate scientific knowledge into management action. To guide solution-based research, we review the current knowledge of ocean acidification impacts on coral reefs alongside management needs and priorities. We use the world's largest continuous reef system, Australia's Great Barrier Reef (GBR), as a case study. We integrate scientific knowledge gained from a variety of approaches (e.g., laboratory studies, field observations, and ecosystem modelling) and scales (e.g., cell, organism, ecosystem) that underpin a systems-level understanding of how ocean acidification is likely to impact the GBR and associated goods and services. We then discuss local and regional management options that may be effective to help mitigate the effects of ocean acidification on the GBR, with likely application to other coral reef systems. We develop a research framework for linking solution-based ocean acidification research to practical management options. The framework assists in identifying effective and cost-efficient options for supporting ecosystem resilience. The framework enables on-the-ground OA management to be the focus, while not losing sight of CO2 mitigation as the ultimate solution.

Biological responses to environmental heterogeneity under future ocean conditions

Organisms are projected to face unprecedented rates of change in future ocean conditions due to anthropogenic climate-change. At present, marine life encounters a wide range of environmental heterogeneity from natural fluctuations to mean climate change. Manipulation studies suggest that biota from more variable marine environments have more phenotypic plasticity to tolerate environmental heterogeneity. Here, we consider current strategies employed by a range of representative organisms across various habitats - from short-lived phytoplankton to long-lived corals - in response to environmental heterogeneity. We then discuss how, if and when organismal responses (acclimate/migrate/adapt) may be altered by shifts in the magnitude of the mean climate-change signal relative to that for natural fluctuations projected for coming decades. The findings from both novel climate-change modelling simulations and prior biological manipulation studies, in which natural fluctuations are superimposed on those of mean change, provide valuable insights into organismal responses to environmental heterogeneity. Manipulations reveal that different experimental outcomes are evident between climate-change treatments which include natural fluctuations vs. those which do not. Modelling simulations project that the magnitude of climate variability, along with mean climate change, will increase in coming decades, and hence environmental heterogeneity will increase, illustrating the need for more realistic biological manipulation experiments that include natural fluctuations. However, simulations also strongly suggest that the timescales over which the mean climate-change signature will become dominant, relative to natural fluctuations, will vary for individual properties, being most rapid for CO2 (~10 years from present day) to 4 decades for nutrients. We conclude that the strategies used by biota to respond to shifts in environmental heterogeneity may be complex, as they will have to physiologically straddle wide-ranging timescales in the alteration of ocean conditions, including the need to adapt to rapidly rising CO2 and also acclimate to environmental heterogeneity in more slowly changing properties such as warming.

Internal pH regulation facilitates in situ long-term acclimation of massive corals to end-of-century carbon dioxide conditions

The resilience of tropical corals to ocean acidification depends on their ability to regulate the pH within their calcifying fluid (pH_cf). Recent work suggests pH_cf homeostasis under short-term exposure to pCO₂ conditions predicted for 2100, but it is still unclear if pH_cf homeostasis can be maintained throughout a corals lifetime. At CO₂ seeps in Papua New Guinea, massive Porites corals have grown along a natural seawater pH gradient for decades. This natural gradient, ranging from pH 8.1–7.4, provides an ideal platform to determine corals’ pH_cf (using boron isotopes). Porites maintained a similar pH_cf (~8.24) at both a control (pH 8.1) and seep-influenced site (pH 7.9). Internal pH_cf was slightly reduced (8.12) at seawater pH 7.6, and decreased to 7.94 at a site with a seawater pH of 7.4. A growth response model based on pH_cf mirrors the observed distribution patterns of this species in the field. We suggest Porites has the capacity to acclimate after long-time exposure to end-of-century reduced seawater pH conditions and that strong control over pH_cf represents a key mechanism to persist in future oceans. Only beyond end-of-century pCO₂ conditions do they face their current physiological limit of pH homeostasis and pH_cf begins to decrease.

Echinometra sea urchins acclimatized to elevated pCO2 at volcanic vents outperform those under present-day pCO2 conditions

Rising atmospheric CO2 concentrations will significantly reduce ocean pH during the 21st century (ocean acidification, OA). This may hamper calcification in marine organisms such as corals and echinoderms, as shown in many laboratory-based experiments. Sea urchins are considered highly vulnerable to OA. We studied an Echinometra species on natural volcanic CO2 vents in Papua New Guinea, where they are CO2 -acclimatized and also subjected to secondary ecological changes from elevated CO2 . Near the vent site, the urchins experienced large daily variations in pH (>1 unit) and pCO2 (>2000 ppm) and average pH values (pHT 7.73) much below those expected under the most pessimistic future emission scenarios. Growth was measured over a 17-month period using tetracycline tagging of the calcareous feeding lanterns. Average-sized urchins grew more than twice as fast at the vent compared with those at an adjacent control site and assumed larger sizes at the vent compared to the control site and two other sites at another reef near-by. A small reduction in gonad weight was detected at the vents, but no differences in mortality, respiration, or degree of test calcification were detected between urchins from vent and control populations. Thus, urchins did not only persist but actually 'thrived' under extreme CO2 conditions. We suggest an ecological basis for this response: Increased algal productivity under increased pCO2 provided more food at the vent, resulting in higher growth rates. The wider implication of our observation is that laboratory studies on non-acclimatized specimens, which typically do not consider ecological changes, can lead to erroneous conclusions on responses to global change.

Effects of High Dissolved Inorganic and Organic Carbon Availability on the Physiology of the Hard Coral Acropora millepora from the Great Barrier Reef

Coral reefs are facing major global and local threats due to climate change-induced increases in dissolved inorganic carbon (DIC) and because of land-derived increases in organic and inorganic nutrients. Recent research revealed that high availability of labile dissolved organic carbon (DOC) negatively affects scleractinian corals. Studies on the interplay of these factors, however, are lacking, but urgently needed to understand coral reef functioning under present and near future conditions. This experimental study investigated the individual and combined effects of ambient and high DIC (pCO2 403 μatm/ pHTotal 8.2 and 996 μatm/pHTotal 7.8) and DOC (added as Glucose 0 and 294 μmol L-1, background DOC concentration of 83 μmol L-1) availability on the physiology (net and gross photosynthesis, respiration, dark and light calcification, and growth) of the scleractinian coral Acropora millepora (Ehrenberg, 1834) from the Great Barrier Reef over a 16 day interval. High DIC availability did not affect photosynthesis, respiration and light calcification, but significantly reduced dark calcification and growth by 50 and 23%, respectively. High DOC availability reduced net and gross photosynthesis by 51% and 39%, respectively, but did not affect respiration. DOC addition did not influence calcification, but significantly increased growth by 42%. Combination of high DIC and high DOC availability did not affect photosynthesis, light calcification, respiration or growth, but significantly decreased dark calcification when compared to both controls and DIC treatments. On the ecosystem level, high DIC concentrations may lead to reduced accretion and growth of reefs dominated by Acropora that under elevated DOC concentrations will likely exhibit reduced primary production rates, ultimately leading to loss of hard substrate and reef erosion. It is therefore important to consider the potential impacts of elevated DOC and DIC simultaneously to assess real world scenarios, as multiple rather than single factors influence key physiological processes in coral reefs.

Diuron tolerance and potential degradation by pelagic microbiomes in the Great Barrier Reef lagoon

Diuron is a herbicide commonly used in agricultural areas where excess application causes it to leach into rivers, reach sensitive marine environments like the Great Barrier Reef (GBR) lagoon and pose risks to marine life. To investigate the impact of diuron on whole prokaryotic communities that underpin the marine food web and are integral to coral reef health, GBR lagoon water was incubated with diuron at environmentally-relevant concentration (8 µg/L), and sequenced at specific time points over the following year. 16S rRNA gene amplicon profiling revealed no significant short- or long-term effect of diuron on microbiome structure. The relative abundance of prokaryotic phototrophs was not significantly altered by diuron, which suggests that they were largely tolerant at this concentration. Assembly of a metagenome derived from waters sampled at a similar location in the GBR lagoon did not reveal the presence of mutations in the cyanobacterial photosystem that could explain diuron tolerance. However, resident phages displayed several variants of this gene and could potentially play a role in tolerance acquisition. Slow biodegradation of diuron was reported in the incubation flasks, but no correlation with the relative abundance of heterotrophs was evident. Analysis of metagenomic reads supports the hypothesis that previously uncharacterized hydrolases carried by low-abundance species may mediate herbicide degradation in the GBR lagoon. Overall, this study offers evidence that pelagic phototrophs of the GBR lagoon may be more tolerant of diuron than other tropical organisms, and that heterotrophs in the microbial seed bank may have the potential to degrade diuron and alleviate local anthropogenic stresses to inshore GBR ecosystems.

Animal behaviour shapes the ecological effects of ocean acidification and warming: moving from individual to community-level responses

Biological communities are shaped by complex interactions between organisms and their environment as well as interactions with other species. Humans are rapidly changing the marine environment through increasing greenhouse gas emissions, resulting in ocean warming and acidification. The first response by animals to environmental change is predominantly through modification of their behaviour, which in turn affects species interactions and ecological processes. Yet, many climate change studies ignore animal behaviour. Furthermore, our current knowledge of how global change alters animal behaviour is mostly restricted to single species, life phases and stressors, leading to an incomplete view of how coinciding climate stressors can affect the ecological interactions that structure biological communities. Here, we first review studies on the effects of warming and acidification on the behaviour of marine animals. We demonstrate how pervasive the effects of global change are on a wide range of critical behaviours that determine the persistence of species and their success in ecological communities. We then evaluate several approaches to studying the ecological effects of warming and acidification, and identify knowledge gaps that need to be filled, to better understand how global change will affect marine populations and communities through altered animal behaviours. Our review provides a synthesis of the far-reaching consequences that behavioural changes could have for marine ecosystems in a rapidly changing environment. Without considering the pervasive effects of climate change on animal behaviour we will limit our ability to forecast the impacts of ocean change and provide insights that can aid management strategies.

Framework of barrier reefs threatened by ocean acidification

To date, studies of ocean acidification (OA) on coral reefs have focused on organisms rather than communities, and the few community effects that have been addressed have focused on shallow back reef habitats. The effects of OA on outer barrier reefs, which are the most striking of coral reef habitats and are functionally and physically different from back reefs, are unknown. Using 5-m long outdoor flumes to create treatment conditions, we constructed coral reef communities comprised of calcified algae, corals, and reef pavement that were assembled to match the community structure at 17 m depth on the outer barrier reef of Moorea, French Polynesia. Communities were maintained under ambient and 1200 μatm pCO2 for 7 weeks, and net calcification rates were measured at different flow speeds. Community net calcification was significantly affected by OA, especially at night when net calcification was depressed ~78% compared to ambient pCO2 . Flow speed (2-14 cm s(-1) ) enhanced net calcification only at night under elevated pCO2 . Reef pavement also was affected by OA, with dissolution ~86% higher under elevated pCO2 compared to ambient pCO2 . These results suggest that net accretion of outer barrier reef communities will decline under OA conditions predicted within the next 100 years, largely because of increased dissolution of reef pavement. Such extensive dissolution poses a threat to the carbonate foundation of barrier reef communities.