Effects of variations in carbonate chemistry on the calcification rates of Madracis auretenra (= Madracis mirabilis sensu Wells, 1973): bicarbonate concentrations best predict calcification rates

Physiological data and models of coral calcification indicate that corals utilize a combination of seawater bicarbonate and (mainly) respiratory CO₂ for calcification, not seawater carbonate. However, a number of investigators are attributing observed negative effects of experimental seawater acidification by CO₂ or hydrochloric acid additions to a reduction in seawater carbonate ion concentration and thus aragonite saturation state. Thus, there is a discrepancy between the physiological and geochemical views of coral biomineralization. Furthermore, not all calcifying organisms respond negatively to decreased pH or saturation state. Together, these discrepancies suggest that other physiological mechanisms, such as a direct effect of reduced pH on calcium or bicarbonate ion transport and/or variable ability to regulate internal pH, are responsible for the variability in reported experimental effects of acidification on calcification. To distinguish the effects of pH, carbonate concentration and bicarbonate concentration on coral calcification, incubations were performed with the coral Madracis auretenra (= Madracis mirabilis sensu Wells, 1973) in modified seawater chemistries. Carbonate parameters were manipulated to isolate the effects of each parameter more effectively than in previous studies, with a total of six different chemistries. Among treatment differences were highly significant. The corals responded strongly to variation in bicarbonate concentration, but not consistently to carbonate concentration, aragonite saturation state or pH. Corals calcified at normal or elevated rates under low pH (7.6–7.8) when the seawater bicarbonate concentrations were above 1800 μm . Conversely, corals incubated at normal pH had low calcification rates if the bicarbonate concentration was lowered. These results demonstrate that coral responses to ocean acidification are more diverse than currently thought, and question the reliability of using carbonate concentration or aragonite saturation state as the sole predictor of the effects of ocean acidification on coral calcification.     

Differential modification of seawater carbonate chemistry by major coral reef benthic communities

Ocean acidification (OA) resulting from uptake of anthropogenic CO₂ may negatively affect coral reefs by causing decreased rates of biogenic calcification and increased rates of CaCO₃ dissolution and bioerosion. However, in addition to the gradual decrease in seawater pH and Ω _a resulting from anthropogenic activities, seawater carbonate chemistry in these coastal ecosystems is also strongly influenced by the benthic metabolism which can either exacerbate or alleviate OA through net community calcification (NCC = calcification – CaCO₃ dissolution) and net community organic carbon production (NCP = primary production ? respiration). Therefore, to project OA on coral reefs, it is necessary to understand how different benthic communities modify the reef seawater carbonate chemistry. In this study, we used flow-through mesocosms to investigate the modification of seawater carbonate chemistry by benthic metabolism of five distinct reef communities [carbonate sand, crustose coralline algae (CCA), corals, fleshy algae, and a mixed community] under ambient and acidified conditions during summer and winter. The results showed that different communities had distinct influences on carbonate chemistry related to the relative importance of NCC and NCP. Sand, CCA, and corals exerted relatively small influences on seawater pH and Ω _a over diel cycles due to closely balanced NCC and NCP rates, whereas fleshy algae and mixed communities strongly elevated daytime pH and Ω _a due to high NCP rates. Interestingly, the influence on seawater pH at night was relatively small and quite similar across communities. NCC and NCP rates were not significantly affected by short-term acidification, but larger diel variability in pH was observed due to decreased seawater buffering capacity. Except for corals, increased net dissolution was observed at night for all communities under OA, partially buffering against nighttime acidification. Thus, algal-dominated areas of coral reefs and increased net CaCO₃ dissolution may partially counteract reductions in seawater pH associated with anthropogenic OA at the local scale.     

Live tissue imaging shows reef corals elevate pH under their calcifying tissue relative to seawater

The threat posed to coral reefs by changes in seawater pH and carbonate chemistry (ocean acidification) raises the need for a better mechanistic understanding of physiological processes linked to coral calcification. Current models of coral calcification argue that corals elevate extracellular pH under their calcifying tissue relative to seawater to promote skeleton formation, but pH measurements taken from the calcifying tissue of living, intact corals have not been achieved to date. We performed live tissue imaging of the reef coral Stylophora pistillata to determine extracellular pH under the calcifying tissue and intracellular pH in calicoblastic cells. We worked with actively calcifying corals under flowing seawater and show that extracellular pH (pHe) under the calicoblastic epithelium is elevated by ～0.5 and ～0.2 pH units relative to the surrounding seawater in light and dark conditions respectively. By contrast, the intracellular pH (pHi) of the calicoblastic epithelium remains stable in the light and dark. Estimates of aragonite saturation states derived from our data indicate the elevation in subcalicoblastic pHe favour calcification and may thus be a critical step in the calcification process. However, the observed close association of the calicoblastic epithelium with the underlying crystals suggests that the calicoblastic cells influence the growth of the coral skeleton by other processes in addition to pHe modification. The procedure used in the current study provides a novel, tangible approach for future investigations into these processes and the impact of environmental change on the cellular mechanisms underpinning coral calcification.     

海洋酸化生态学研究进展

工业革命以来,人类排放的大量二氧化碳引起温室效应的同时,也被海洋吸收使得全球海洋出现了严重的酸化。海洋酸化及伴随的海水碳酸盐化学体系的变化对海洋生物产生深远的影响。以海洋酸化对钙化作用和光合作用的影响为重点,总结了近年来关于海洋酸化的研究,介绍了海洋中不同生态系统对海洋酸化的响应。一方面,海水中CO23-浓度和碳酸钙饱和度的降低对海洋钙化生物造成严重损害,生活在高纬的冷水珊瑚和翼足目等文石生产者是最早的受害者;贝类和棘皮动物在钙化早期对海洋酸化尤其敏感,其幼体存活率受到海洋酸化的严重制约。另一方面,CO2浓度的增加能促进海洋植物的光合作用和生长,增加初级生产力,改变浮游植物的群落组成。此外,海洋酸化可以促进固氮和脱氮作用同时削弱硝化作用,改变溶氧浓度分布和金属的生物可利用性,从而对海洋生物产生间接影响。海洋酸化对海洋生态系统的影响机制复杂,影响程度深远。为了能准确的评估海洋酸化的生态学效应,需要更全面深入的研究。     

A coral polyp model of photosynthesis, respiration and calcification incorporating a transcellular ion transport mechanism

A numerical simulation model of coral polyp photosynthesis, respiration and calcification was developed. The model is constructed with three components (ambient seawater, coelenteron and calcifying fluid), and incorporates photosynthesis, respiration and calcification processes with transcellular ion transport by Ca-ATPase activity and passive transmembrane CO₂ transport and diffusion. The model calculates dissolved inorganic carbon and total alkalinity in the ambient seawater, coelenteron and calcifying fluid, dissolved oxygen (DO) in the seawater and coelenteron and stored organic carbon (CH₂O). To reconstruct the drastic variation between light and dark respiration, respiration rate dependency on DO in the coelenteron is incorporated. The calcification rate depends on the aragonite saturation state in the calcifying fluid (Ωa _cal). Our simulation result was a good approximation of “light-enhanced calcification.” In our model, the mechanism is expressed as follows: (1) DO in the coelenteron is increased by photosynthesis, (2) respiration is stimulated by increased DO in the light (or respiration is limited by DO depletion in the dark), then (3) calcification increases due to Ca-ATPase, which is driven by the energy generated by respiration. The model simulation results were effective in reproducing the basic responses of the internal CO₂ system and DO. The daily calcification rate, the gross photosynthetic rate and the respiration rate under a high-flow condition increased compared to those under the zero-flow condition, but the net photosynthetic rate decreased. The calculated calcification rate responses to variations in the ambient aragonite saturation state (Ωa _amb) were nonlinear, and the responses agreed with experimental results of previous studies. Our model predicted that in response to ocean acidification (1) coral calcification will decrease, but will remain at a higher value until Ωa _amb decreases to 1, by maintaining a higher Ωa _cal due to the transcellular ion transport mechanism and (2) the net photosynthetic rate will increase.     

Coral reefs modify their seawater carbon chemistry – implications for impacts of ocean acidification

Reviews suggest that that the biogeochemical threshold for sustained coral reef growth will be reached during this century due to ocean acidification caused by increased uptake of atmospheric CO₂. Projections of ocean acidification, however, are based on air‐sea fluxes in the open ocean, and not for shallow‐water systems such as coral reefs. Like the open ocean, reef waters are subject to the chemical forcing of increasing atmospheric pCO₂. However, for reefs with long water residence times, we illustrate that benthic carbon fluxes can drive spatial variation in pH, pCO₂ and aragonite saturation state (Ω_a) that can mask the effects of ocean acidification in some downstream habitats. We use a carbon flux model for photosynthesis, respiration, calcification and dissolution coupled with Lagrangian transport to examine how key groups of calcifiers (zooxanthellate corals) and primary producers (macroalgae) on coral reefs contribute to changes in the seawater carbonate system as a function of water residence time. Analyses based on flume data showed that the carbon fluxes of corals and macroalgae drive Ω_ain opposing directions. Areas dominated by corals elevate pCO₂ and reduce Ω_a, thereby compounding ocean acidification effects in downstream habitats, whereas algal beds draw CO₂ down and elevate Ω_a, potentially offsetting ocean acidification impacts at the local scale. Simulations for two CO₂ scenarios (600 and 900 ppm CO₂) suggested that a potential shift from coral to algal abundance under ocean acidification can lead to improved conditions for calcification in downstream habitats, depending on reef size, water residence time and circulation patterns. Although the carbon fluxes of benthic reef communities cannot significantly counter changes in carbon chemistry at the scale of oceans, they provide a significant mechanism of buffering ocean acidification impacts at the scale of habitat to reef.     

BEFORE OCEAN ACIDIFICATION: CALCIFIER CHEMISTRY LESSONS1

Ocean Acidification (OA) has been an important research topic for a decade. Scientists have focused on how the predicted 56% decline in the seawater carbonate ion () concentration will dramatically impair the ability of calcifiers, ranging from coccolithophores to shellfish, to form calcium carbonate (CaCO₃) structures, and the implications of the reduced carbonate saturation state (Ω) for increased dissolution of such structures. However, many published OA studies have overlooked a fundamental issue: most calcifying organisms do not rely on carbonate from seawater to calcify; they use either bicarbonate () or metabolically‐produced CO₂. The ability of important primary (corals, coralline seaweeds, and coccolithophores) and secondary (mollusks) producers to modify their local carbonate chemistry suggests that the primary threat to them from OA is by dissolution rather than impaired calcification. Here, we draw on calcification research from an era before OA and combine it with recent studies that question the source of the carbonate ion, to provide new insights into how OA might affect calcifying organisms. Organismal modification of local carbonate chemistry may enable some calcifiers to successfully form calcareous structures despite OA.     

Slow‐flow habitats as refugia for coastal calcifiers from ocean acidification

The pH of the oceans’ surface water is dropping, termed ocean acidification (OA), and the 0.4 unit reduction in pH by 2100 is projected to negatively impact benthic coastal organisms that produce calcium carbonate “skeletons.” Research has focussed on identifying species that are susceptible to OA, but there is an urgent need to discover refuge habitats that will afford protection to vulnerable species. The susceptibility of calcium carbonate skeletons to dissolution by OA depends on the pH at their surface, and this is controlled by the interaction between seawater velocity and organismal metabolism. This perspective considers how seawater velocity modifies the responses of calcifying organisms (seaweed, shellfish, and tropical corals) to OA through its action on controlling diffusion boundary layer thickness and thereby the pH and calcium carbonate saturation state (Ω) at the organisms’ surface. Evidence is presented to support the idea that slow‐flow habitats, such as wave‐sheltered bays or the within canopies of seaweed/seagrass beds, might provide inexpensive refugia from OA for vulnerable coastal calcifiers.     

Ocean acidification does not affect the physiology of the tropical coral Acropora digitifera during a 5-week experiment

The increase in atmospheric CO₂ concentration, which has resulted from the burning of fossil fuels, is being absorbed by the oceans and is causing ocean acidification. Ocean acidification involves the decrease of both the pH and the calcium carbonate saturation state. Ocean acidification is predicted to impact the physiology of marine organisms and reduce the calcification rates of corals. In the present study, we measured the rates of calcification, respiration, photosynthesis, and zooxanthellae density of the tropical coral Acropora digitifera under near-natural summertime temperature and sunlight for a 5-week period. We found that these key physiological parameters were not affected by both mid-CO₂ (pCO₂ = 744 ± 38, pH = 7.97 ± 0.02, Ω_arag = 2.6 ± 0.1) and high-CO₂ conditions (pCO₂ = 2,142 ± 205, pH = 7.56 ± 0.04, Ω_arag = 1.1 ± 0.2) throughout the 35 days experimental period. Additionally, there was no significant correlation between calcification rate and seawater aragonite saturation (Ω_arag). These results suggest that the impacts of ocean acidification on corals physiology may be more complex than have been previously proposed.     

Coralline algae elevate pH at the site of calcification under ocean acidification

Coralline algae provide important ecosystem services but are susceptible to the impacts of ocean acidification. However, the mechanisms are uncertain, and the magnitude is species specific. Here, we assess whether species‐specific responses to ocean acidification of coralline algae are related to differences in pH at the site of calcification within the calcifying fluid/medium (pH_cf) using δ¹¹B as a proxy. Declines in δ¹¹B for all three species are consistent with shifts in δ¹¹B expected if B(OH)₄^? was incorporated during precipitation. In particular, the δ¹¹B ratio in Amphiroa anceps was too low to allow for reasonable pH_cf values if B(OH)₃ rather than B(OH)₄^? was directly incorporated from the calcifying fluid. This points towards δ¹¹B being a reliable proxy for pH_cf for coralline algal calcite and that if B(OH)₃ is present in detectable proportions, it can be attributed to secondary postincorporation transformation of B(OH)₄^?. We thus show that pH_cf is elevated during calcification and that the extent is species specific. The net calcification of two species of coralline algae (Sporolithon durum, and Amphiroa anceps) declined under elevated CO₂, as did their pH_cf. Neogoniolithon sp. had the highest pH_cf, and most constant calcification rates, with the decrease in pH_cf being ¼ that of seawater pH in the treatments, demonstrating a control of coralline algae on carbonate chemistry at their site of calcification. The discovery that coralline algae upregulate pH_cf under ocean acidification is physiologically important and should be included in future models involving calcification.     

Ocean acidification accelerates net calcium carbonate loss in a coral rubble community

Coral rubble communities are an important yet often overlooked component of a healthy reef ecosystem. The organisms inhabiting reef rubble are primarily bioeroders that contribute to the breakdown and dissolution of carbonate material. While the effects of ocean acidification on calcifying communities have been well studied, there are few studies investigating the response of bioeroding communities to future changes in pH and calcium carbonate saturation state. Using a flow-through pH-stat system, coral rubble pieces with a naturally occurring suite of organisms, along with bleached control rubble pieces, were subjected to three different levels of acidification over an 8-week period. Rates of net carbonate loss in bleached control rubble doubled in the acidification treatments (0.02 vs. 0.04% CaCO₃ d^?1 in ambient vs. moderate and high acidification), and living rubble communities experienced significantly increased rates of net carbonate loss from ambient to high acidification conditions (0.06 vs. 0.10% CaCO₃ d^?1, respectively). Although more experimentation is necessary to understand the long-term response and succession of coral rubble communities under projected conditions, these results suggest that rates of carbonate loss will increase in coral rubble as pH and calcium carbonate saturation states are reduced. This study demonstrates a need to thoroughly investigate the contribution of coral rubble to the overall carbonate budget, reef resilience, recovery, and function under future conditions.     

Environmental and physiochemical controls on coral calcification along a latitudinal temperature gradient in Western Australia

The processes that occur at the micro‐scale site of calcification are fundamental to understanding the response of coral growth in a changing world. However, our mechanistic understanding of chemical processes driving calcification is still evolving. Here, we report the results of a long‐term in situ study of coral calcification rates, photo‐physiology, and calcifying fluid (cf) carbonate chemistry (using boron isotopes, elemental systematics, and Raman spectroscopy) for seven species (four genera) of symbiotic corals growing in their natural environments at tropical, subtropical, and temperate locations in Western Australia (latitudinal range of ~11°). We find that changes in net coral calcification rates are primarily driven by pH_cf and carbonate ion concentration []_cf in conjunction with temperature and DIC_cf. Coral pH_cf varies with latitudinal and seasonal changes in temperature and works together with the seasonally varying DIC_cf to optimize []_cf at species‐dependent levels. Our results indicate that corals shift their pH_cf to adapt and/or acclimatize to their localized thermal regimes. This biological response is likely to have critical implications for predicting the future of coral reefs under CO₂‐driven warming and acidification.     

Coral calcification responds to seawater acidification: a working hypothesis towards a physiological mechanism

The decrease in the saturation state of seawater, Ω, following seawater acidification, is believed to be the main factor leading to a decrease in the calcification of marine organisms. To provide a physiological explanation for this phenomenon, the effect of seawater acidification was studied on the calcification and photosynthesis of the scleractinian tropical coral Stylophora pistillata. Coral nubbins were incubated for 8 days at three different pH (7.6, 8.0, and 8.2). To differentiate between the effects of the various components of the carbonate chemistry (pH, CO₃²⁻, HCO₃⁻, CO₂, Ω), tanks were also maintained under similar pH, but with 2-mM HCO₃⁻ added to the seawater. The addition of 2-mM bicarbonate significantly increased the photosynthesis in S. pistillata, suggesting carbon-limited conditions. Conversely, photosynthesis was insensitive to changes in pH and pCO₂. Seawater acidification decreased coral calcification by ca. 0.1-mg CaCO₃ g⁻¹ d⁻¹ for a decrease of 0.1 pH units. This correlation suggested that seawater acidification affected coral calcification by decreasing the availability of the CO₃²⁻ substrate for calcification. However, the decrease in coral calcification could also be attributed either to a decrease in extra- or intracellular pH or to a change in the buffering capacity of the medium, impairing supply of CO₃²⁻ from HCO₃⁻.     

Sponge erosion under acidification and warming scenarios: differential impacts on living and dead coral

Ocean acidification will disproportionately impact the growth of calcifying organisms in coral reef ecosystems. Simultaneously, sponge bioerosion rates have been shown to increase as seawater pH decreases. We conducted a 20‐week experiment that included a 4‐week acclimation period with a high number of replicate tanks and a fully orthogonal design with two levels of temperature (ambient and +1 °C), three levels of pH (8.1, 7.8, and 7.6), and two levels of boring sponge (Cliona varians, present and absent) to account for differences in sponge attachment and carbonate change for both living and dead coral substrate (Porites furcata). Net coral calcification, net dissolution/bioerosion, coral and sponge survival, sponge attachment, and sponge symbiont health were evaluated. Additionally, we used the empirical data from the experiment to develop a stochastic simulation of carbonate change for small coral clusters (i.e., simulated reefs). Our findings suggest differential impacts of temperature, pH and sponge presence for living and dead corals. Net coral calcification (mg CaCO₃ cm^?2 day^?1) was significantly reduced in treatments with increased temperature (+1 °C) and when sponges were present; acidification had no significant effect on coral calcification. Net dissolution of dead coral was primarily driven by pH, regardless of sponge presence or seawater temperature. A reevaluation of the current paradigm of coral carbonate change under future acidification and warming scenarios should include ecologically relevant timescales, species interactions, and community organization to more accurately predict ecosystem‐level response to future conditions.     

TESTING THE EFFECTS OF OCEAN ACIDIFICATION ON ALGAL METABOLISM: CONSIDERATIONS FOR EXPERIMENTAL DESIGNS1

Ocean acidification describes changes in the carbonate chemistry of the ocean due to the increased absorption of anthropogenically released CO₂. Experiments to elucidate the biological effects of ocean acidification on algae are not straightforward because when pH is altered, the carbon speciation in seawater is altered, which has implications for photosynthesis and, for calcifying algae, calcification. Furthermore, photosynthesis, respiration, and calcification will themselves alter the pH of the seawater medium. In this review, algal physiologists and seawater carbonate chemists combine their knowledge to provide the fundamental information on carbon physiology and seawater carbonate chemistry required to comprehend the complexities of how ocean acidification might affect algae metabolism. A wide range in responses of algae to ocean acidification has been observed, which may be explained by differences in algal physiology, timescales of the responses measured, study duration, and the method employed to alter pH. Two methods have been widely used in a range of experimental systems: CO₂ bubbling and HCl/NaOH additions. These methods affect the speciation of carbonate ions in the culture medium differently; we discuss how this could influence the biological responses of algae and suggest a third method based on HCl/NaHCO₃ additions. We then discuss eight key points that should be considered prior to setting up experiments, including which method of manipulating pH to choose, monitoring during experiments, techniques for adding acidified seawater, biological side effects, and other environmental factors. Finally, we consider incubation timescales and prior conditioning of algae in terms of regulation, acclimation, and adaptation to ocean acidification.     

Ocean acidification and seaweed reproduction: increased CO2 ameliorates the negative effect of lowered pH on meiospore germination in the giant kelp Macrocystis pyrifera (Laminariales,Phaeophyceae)

The worldwide effects of ocean acidification (OA) on marine species are a growing concern. In temperate coastal seas, seaweeds are dominant primary producers that create complex habitats and supply energy to higher trophic levels. Studies on OA and macroalgae have focused on calcifying species and adult stages, but critically, they have overlooked the microscopic stages of the reproductive life cycle, which, for other anthropogenic stressors, e.g., UV‐B radiation, are the most susceptible life‐history phase. Also, environmental cues and stressors can cause changes in the sex ratio, which has implications for the mating system and recruitment success. Here, we report the effects of pH (7.59–8.50) on meiospore germination and sex determination for the giant kelp, Macrocystis pyrifera (Laminariales), in the presence and absence of additional dissolved inorganic carbon (DIC). Lowered pH (7.59–7.60, using HCl‐only) caused a significant reduction in germination, whereas added DIC had the opposite effect, indicating that increased CO₂ at lower pH ameliorates physiological stress. This finding also highlights the importance of appropriate manipulation of seawater carbonate chemistry when testing the effects of OA on photosynthetic organisms. The proportion of male to female gametophytes did not vary significantly between treatments, suggesting that pH was not a primary environmental modulator of sex. Relative to the baseline (pH 8.19), gametophytes were 32% larger under moderate OA (pH 7.86) and 10% larger under extreme OA (pH 7.61). We suggest that metabolically active cells can compensate for the acidification of seawater. This homeostatic function minimizes the negative effects of lower pH (high H⁺ ions) on cellular activity. The 6–9% reduction in germination success under extreme OA suggests that meiospores of M. pyrifera may be resistant to future OA.     

Coral reefs modify their seawater carbon chemistry – case study from a barrier reef (Moorea,French Polynesia)

Changes in the carbonate chemistry of coral reef waters are driven by carbon fluxes from two sources: concentrations of CO₂ in the atmospheric and source water, and the primary production/respiration and calcification/dissolution of the benthic community. Recent model analyses have shown that, depending on the composition of the reef community, the air‐sea flux of CO₂ driven by benthic community processes can exceed that due to increases in atmospheric CO₂ (ocean acidification). We field test this model and examine the role of three key members of benthic reef communities in modifying the chemistry of the ocean source water: corals, macroalgae, and sand. Building on data from previous carbon flux studies along a reef‐flat transect in Moorea (French Polynesia), we illustrate that the drawdown of total dissolved inorganic carbon (C_T) due to photosynthesis and calcification of reef communities can exceed the draw down of total alkalinity (A_T) due to calcification of corals and calcifying algae, leading to a net increase in aragonite saturation state (Ω_a). We use the model to test how changes in atmospheric CO₂ forcing and benthic community structure affect the overall calcification rates on the reef flat. Results show that between the preindustrial period and 1992, ocean acidification caused reef flat calcification rates to decline by an estimated 15%, but loss of coral cover caused calcification rates to decline by at least three times that amount. The results also show that the upstream–downstream patterns of carbonate chemistry were affected by the spatial patterns of benthic community structure. Changes in the ratio of photosynthesis to calcification can thus partially compensate for ocean acidification, at least on shallow reef flats. With no change in benthic community structure, however, ocean acidification depressed net calcification of the reef flat consistent with findings of previous studies.     

CO(2)-Driven Ocean Acidification Alters and Weakens Integrity of the Calcareous Tubes Produced by the Serpulid Tubeworm, Hydroides elegans

As a consequence of anthropogenic CO(2-)driven ocean acidification (OA), coastal waters are becoming increasingly challenging for calcifiers due to reductions in saturation states of calcium carbonate (CaCO(3)) minerals. The response of calcification rate is one of the most frequently investigated symptoms of OA. However, OA may also result in poor quality calcareous products through impaired calcification processes despite there being no observed change in calcification rate. The mineralogy and ultrastructure of the calcareous products under OA conditions may be altered, resulting in changes to the mechanical properties of calcified structures. Here, the warm water biofouling tubeworm, Hydroides elegans, was reared from larva to early juvenile stage at the aragonite saturation state (Ω(A)) for the current pCO(2) level (ambient) and those predicted for the years 2050, 2100 and 2300. Composition, ultrastructure and mechanical strength of the calcareous tubes produced by those early juvenile tubeworms were examined using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM) and nanoindentation. Juvenile tubes were composed primarily of the highly soluble CaCO(3) mineral form, aragonite. Tubes produced in seawater with aragonite saturation states near or below one had significantly higher proportions of the crystalline precursor, amorphous calcium carbonate (ACC) and the calcite/aragonite ratio dramatically increased. These alterations in tube mineralogy resulted in a holistic deterioration of the tube hardness and elasticity. Thus, in conditions where Ω(A) is near or below one, the aragonite-producing juvenile tubeworms may no longer be able to maintain the integrity of their calcification products, and may result in reduced survivorship due to the weakened tube protection.     

Sensitivity of coral calcification to ocean acidification: a meta‐analysis

To date, meta‐analyses of effects of acidification have focused on the overall strength of evidence for statistically significant responses; however, to anticipate likely consequences of ocean acidification, quantitative estimates of the magnitude of likely responses are also needed. Herein, we use random effects meta‐analysis to produce a systematically integrated measure of the distribution of magnitudes of the response of coral calcification to decreasing Ω_Arag. We also tested whether methodological and biological factors that have been hypothesized to drive variation in response magnitude explain a significant proportion of the among‐study variation. We found that the overall mean response of coral calcification is ~15% per unit decrease in Ω_Arag over the range 2 < Ω_Arag < 4. Among‐study variation is large (standard deviation of 8% per unit decrease in Ω_Arag). Neither differences in carbonate chemistry manipulation method, study duration, irradiance level, nor study species growth rate explained a significant proportion of the among‐study variation. However, studies employing buoyant weighting found significantly smaller decreases in calcification per unit Ω_Arag (~10%), compared with studies using the alkalinity anomaly technique (~25%). These differences may be due to the greater tendency for the former to integrate over light and dark calcification. If the existing body of experimental work is indeed representative of likely responses of corals in nature, our results imply that, under business as usual conditions, declines in coral calcification by end‐of‐century will be ~22%, on average, or ~15% if only studies integrating light and dark calcification are considered. These values are near the low end of published projections, but support the emerging view that variability due to local environmental conditions and species composition is likely to be substantial.     

Shifts in coralline algae,macroalgae, and coral juveniles in the Great Barrier Reef associated with present‐day ocean acidification

Seawater acidification from increasing CO₂ is often enhanced in coastal waters due to elevated nutrients and sedimentation. Our understanding of the effects of ocean and coastal acidification on present‐day ecosystems is limited. Here we use data from three independent large‐scale reef monitoring programs to assess coral reef responses associated with changes in mean aragonite saturation state (Ω_ar) in the Great Barrier Reef World Heritage Area (GBR). Spatial declines in mean Ω_ar are associated with monotonic declines in crustose coralline algae (up to 3.1‐fold) and coral juvenile densities (1.3‐fold), while non‐calcifying macroalgae greatly increase (up to 3.2‐fold), additionally to their natural changes across and along the GBR. These three key groups of organisms are important proxies for coral reef health. Our data suggest a tipping point at Ω_ar 3.5–3.6 for these coral reef health indicators. Suspended sediments acted as an additive stressor. The latter suggests that effective water quality management to reduce suspended sediments might locally and temporarily reduce the pressure from ocean acidification on these organisms.