End of the Century pCO2 Levels Do Not Impact Calcification in Mediterranean Cold-Water Corals

Ocean acidification caused by anthropogenic uptake of CO₂ is perceived to be a major threat to calcifying organisms. Cold-water corals were thought to be strongly affected by a decrease in ocean pH due to their abundance in deep and cold waters which, in contrast to tropical coral reef waters, will soon become corrosive to calcium carbonate. Calcification rates of two Mediterranean cold-water coral species, Lophelia pertusa and Madrepora oculata, were measured under variable partial pressure of CO₂ (pCO₂) that ranged between 380 µatm for present-day conditions and 930 µatm for the end of the century. The present study addressed both short- and long-term responses by repeatedly determining calcification rates on the same specimens over a period of 9 months. Besides studying the direct, short-term response to elevated pCO₂ levels, the study aimed to elucidate the potential for acclimation of calcification of cold-water corals to ocean acidification. Net calcification of both species was unaffected by the levels of pCO₂ investigated and revealed no short-term shock and, therefore, no long-term acclimation in calcification to changes in the carbonate chemistry. There was an effect of time during repeated experiments with increasing net calcification rates for both species, however, as this pattern was found in all treatments, there is no indication that acclimation of calcification to ocean acidification occurred. The use of controls (initial and ambient net calcification rates) indicated that this increase was not caused by acclimation in calcification response to higher pCO₂. An extrapolation of these data suggests that calcification of these two cold-water corals will not be affected by the pCO₂ level projected at the end of the century.     

Oxygen and Heterotrophy Affect Calcification of the Scleractinian Coral Galaxea fascicularis

Heterotrophy is known to stimulate calcification of scleractinian corals, possibly through enhanced organic matrix synthesis and photosynthesis, and increased supply of metabolic DIC. In contrast to the positive long-term effects of heterotrophy, inhibition of calcification has been observed during feeding, which may be explained by a temporal oxygen limitation in coral tissue. To test this hypothesis, we measured the short-term effects of zooplankton feeding on light and dark calcification rates of the scleractinian coral Galaxea fascicularis (n = 4) at oxygen saturation levels ranging from 13 to 280%. Significant main and interactive effects of oxygen, heterotrophy and light on calcification rates were found (three-way factorial repeated measures ANOVA, p<0.05). Light and dark calcification rates of unfed corals were severely affected by hypoxia and hyperoxia, with optimal rates at 110% saturation. Light calcification rates of fed corals exhibited a similar trend, with highest rates at 150% saturation. In contrast, dark calcification rates of fed corals were close to zero under all oxygen saturations. We conclude that oxygen exerts a strong control over light and dark calcification rates of corals, and propose that in situ calcification rates are highly dynamic. Nevertheless, the inhibitory effect of heterotrophy on dark calcification appears to be oxygen-independent. We hypothesize that dark calcification is impaired during zooplankton feeding by a temporal decrease of the pH and aragonite saturation state of the calcifying medium, caused by increased respiration rates. This may invoke a transient reallocation of metabolic energy to soft tissue growth and organic matrix synthesis. These insights enhance our understanding of how oxygen and heterotrophy affect coral calcification, both in situ as well as in aquaculture.     

The skeletal organic matrix from Mediterranean coral Balanophyllia europaea influences calcium carbonate precipitation

Scleractinian coral skeletons are made mainly of calcium carbonate in the form of aragonite. The mineral deposition occurs in a biological confined environment, but it is still a theme of discussion to what extent the calcification occurs under biological or environmental control. Hence, the shape, size and organization of skeletal crystals from the cellular level through the colony architecture, were attributed to factors as diverse as mineral supersaturation levels and organic mediation of crystal growth. The skeleton contains an intra-skeletal organic matrix (OM) of which only the water soluble component was chemically and physically characterized. In this work that OM from the skeleton of the Balanophyllia europaea, a solitary scleractinian coral endemic to the Mediterranean Sea, is studied in vitro with the aim of understanding its role in the mineralization of calcium carbonate. Mineralization of calcium carbonate was conducted by overgrowth experiments on coral skeleton and in calcium chloride solutions containing different ratios of water soluble and/or insoluble OM and of magnesium ions. The precipitates were characterized by diffractometric, spectroscopic and microscopic techniques. The results showed that both soluble and insoluble OM components influence calcium carbonate precipitation and that the effect is enhanced by their co-presence. The role of magnesium ions is also affected by the presence of the OM components. Thus, in vitro, OM influences calcium carbonate crystal morphology, aggregation and polymorphism as a function of its composition and of the content of magnesium ions in the precipitation media. This research, although does not resolve the controversy between environmental or biological control on the deposition of calcium carbonate in corals, sheds a light on the role of OM, which appears mediated by the presence of magnesium ions.     

Sea surface temperature and the growth of the West Atlantic reef-building coral Montastraea annularis

Relationships were analyzed between sea surface temperature (SST) and annual growth characteristics (density, extension rate and calcification rate) of the Caribbean reef-building coral Montastraea annularis. Colonies were collected from 12 localities in the Gulf of Mexico and the Caribbean Sea. Two well-separated relationships were found, one for the Gulf of Mexico and the other for the Caribbean Sea. Calcification rate and skeletal density increased with increasing SST in both regions, while extension rate tended to decrease. Calcification rate increased ∼0.57 g cm⁻² year⁻¹ for each 1 °C increase in SST. Zero calcification was projected to occur at 23.7 °C in corals from the Gulf of Mexico and at 25.5 °C in corals from the Caribbean Sea. The 24 °C annual average SST isotherm marks the northern limit of distribution of M. annularis. Montastraea annularis populations of the Gulf of Mexico are isolated from those of the Caribbean Sea, and results indicate that corals from the Gulf of Mexico are adapted to growth at lower minimum and average annual SST. Corals from both the Gulf of Mexico and the Caribbean Sea, growing at lower SSTs and having lower calcification rates, extend their skeletons the same or more than those growing at higher SSTs. They achieve this by putting more of their calcification resources into extension and less into thickening, i.e., by sacrificing density.     

Calcification rate and the effect of temperature in a zooxanthellate and an azooxanthellate scleractinian reef coral

Calcification rates, normalized to skeletal mass, in the zooxanthellate Galaxea fascicularis and the azooxanthellate Dendrophyllia sp. were similar over the whole temperature range of 18–29 °C. Calcification was measured by Ca⁴⁵ incorporation in corals that were naturally acclimated to the prevailing seawater temperature. In both species maximum calcification rate occurred at about 25 °C and calcification rates can be fitted to a Gaussian distribution with respect to temperature. The similarity in temperature dependence of the zooxanthellate and azooxanthellate coral suggests that temperature affects some fundamental process of calcification that is independent of light effects. It is shown that two different populations of Galaxea fascicularis have distinctly different ratios of tissue protein to skeletal mass per polyp. This indicates that tissue protein may not be suitable for normalizing calcification rates in individual coral polyps, both within and between species. Intra- and interspecific comparisons of calcification rates may be better made on the basis of skeletal mass when polyps are similar in size and shape.Communicated by Topic Editor C. Barnes     

Calcification process dynamics in coral primary polyps as observed using a calcein incubation method

Calcification processes are largely unknown in scleractinian corals. In this study, live confocal imaging was used to elucidate the spatiotemporal dynamics of the calcification process in aposymbiotic primary polyps of the coral species Acropora digitifera. The fluorophore calcein was used as a calcium deposition marker and a visible indicator of extracellular fluid distribution at the tissue-skeleton interface (subcalicoblastic medium, SCM) in primary polyp tissues. Under continuous incubation in calcein-containing seawater, initial crystallization and skeletal growth were visualized among the calicoblastic cells in live primary polyp tissues. Additionally, the distribution of calcein-stained SCM and contraction movements of the pockets of SCM were captured at intervals of a few minutes. Our experimental system provided several new insights into coral calcification, particularly as a first step in monitoring the relationship between cellular dynamics and calcification in vivo. Our study suggests that coral calcification initiates at intercellular spaces, a finding that may contribute to the general understanding of coral calcification processes.     

The physiological response of reef corals to diel fluctuations in seawater temperature

An opportunity to explore the effects of fluctuating temperatures on tropical scleractinian corals arose when diurnal warming (as large as 4.7 °C) was detected over the rich coral communities found within the back reef of Moorea, French Polynesia. In April and May 2007, experiments were completed to determine the effects of fluctuating temperature on Pocillopora meandrina and Porites rus, and consecutive trials were used to expose them for 13 days to 26 °C, 28 °C (ambient conditions), 30 °C, or a fluctuating treatment ranging from 26 to 30 °C over 24 h. The multivariate response was assessed using maximum dark-adapted quantum yield of PSII (F_V/F_M), Symbiodinium density, chlorophyll-a content, and calcification. In trial 1, multivariate physiology of both species was significantly affected by treatments, with the fluctuating temperature resulting in a 17-45% decline in Symbiodinium density (relative to the ambient) matching that occurring at a constant 30 °C; F_V/F_M, chlorophyll-a content, and calcification, did not differ between the fluctuating and the steady treatments. In contrast, in trial 2 that utilized corals collected two weeks after those used in trial 1, the corals were unaffected by the treatments, likely due to an environment × trial interaction caused by seasonal declines in Symbiodinium density. Together, these results demonstrate that short transgressions to ecologically relevant high and low temperatures can elicit a potentially detrimental response equivalent to that occurring upon exposure to a constant high temperature. The dissimilar responses among dependent variables and consecutive trials underscore the importance of temporal replication and multivariate approaches in coral ecophysiology. It is likely that recent history has a stronger effect on the response of corals to treatments than is currently recognized.     

Effect of zooplankton availability on the rates of photosynthesis, and tissue and skeletal growth in the scleractinian coral Stylophora pistillata

This work investigated the effect of light and feeding on tissue composition as well as on rates of photosynthesis and calcification in the zooxanthellae (zoox) scleractinian coral, Stylophora pistillata. Microcolonies were maintained at three different light levels (80, 200, 300 μmol m⁻² s⁻¹) and subjected to two feeding regimes (starved and fed) over 9 weeks. Corals were fed both natural plankton and Artemia salina nauplii four times a weeks and samplings were made after 2, 5, and 9 weeks. Results confirmed that feeding enhances coral growth rate and increases both the dark and light calcification rates. These rates were 50-75% higher in fed corals (FC; 60±20 and 200±40 nmol Ca²⁺ cm⁻² h⁻¹ for dark and light calcification, respectively) compared to control corals (CC; 30±9 and 124±23 nmol Ca²⁺ cm⁻² h⁻¹). The dark calcification rates, however, were four times lower than the rates of light calcification (independent of trophic status). After 5 weeks, chlorophyll a (chl-a) concentrations were four to seven times higher in fed corals (7-21 μg cm⁻²) than in control corals (2-5 μg cm⁻²). The amount of protein was also significantly higher in fed corals (2.11-2.50 mg cm⁻²) than in control corals (1.08-1.52 mg cm⁻²). Rates of photosynthesis in fed corals were 2-10 times higher (1.24±0.75 μmol O₂ h⁻¹ cm⁻²) than those measured in control corals (0.20±0.08 μmol O₂ h⁻¹ cm⁻²).     

The rôle of zooxanthellae in the hermatypic coral Plesiastrea urvillei (Milne Edwards and Haime) From cold waters

The presence of zooxanthellae in tissues of the cold-temperate water coral Plesiastrea urvillei (Milne Edwards and Haime) has been confirmed histologically. Numbers of zooxanthellae per unit surface area and increases in submerged wet weight as a measure of calcification have been followed for 150 days under four different conditions: light-fed, light-starved, dark-fed, and dark-starved. No significant difference was found in density of zooxanthellae or calcification rates between light-fed and light-starved, and between dark-fed and dark-starved. After Day 48 the calcification rate in the dark dropped, however, by a factor of ≈4 to a constant lower rate and was correlated with a decrease in density of zooxanthellae. Zooxanthellae thus enhance calcification about 4 times during photosynthesis. Measurements of oxygen consumption and production indicated that even at the low light intensities experienced on a cloudy winter day by the coral in its natural environment more energy was fixed during photosynthesis than was required by the host. The retention of zooxanthellae and continued calcification in the dark for upwards of 48 days is considered to be an adaptation to the lower light levels experienced by P. urvillei compared with tropical corals.     

Inferred calcification rate of a Mediterranean azooxanthellate coral is uncoupled with sea surface temperature along an 8° latitudinal gradient

Introduction

Correlations between sea surface temperature (SST) and growth parameters of the solitary azooxanthellate Dendrophylliid Leptopsammia pruvoti were assessed along an 8° latitudinal gradient on western Italian coasts (Mediterranean Sea), to check for possible negative effects of increasing temperature as the ones reported for a closely related, sympatric but zooxanthellate species.

Results

Calcification rate was correlated with skeletal density but not with linear extension rate, indicating that calcium carbonate deposition was preferentially allocated to keep a constant skeletal density. Unlike most studies on both temperate and tropical zooxanthellate corals, where calcification rate is strongly related to environmental parameters such as SST, in the present study calcification rate was not correlated with SST.

Conclusions

The lower sensitivity of L. pruvoti to SST with respect to other sympatric zooxanthellate corals, such as Balanophyllia europaea, may rely on the absence of a temperature induced inhibition of photosynthesis, and thus the absence of an inhibition of the calcification process. This study is the first field investigation of the relationship between SST and the three growth parameters of an azooxanthellate coral. Increasing research effort on determining the effects of temperature on biological traits of the poorly studied azooxanthellate scleractinians may help to predict the possible species assemblage shifts that are likely to occur in the immediate future as a consequence of global climatic change.

     

Calcification by juvenile corals under heterotrophy and elevated CO2

Ocean acidification (OA) threatens the existence of coral reefs by slowing the rate of calcium carbonate (CaCO₃) production of framework-building corals thus reducing the amount of CaCO₃ the reef can produce to counteract natural dissolution. Some evidence exists to suggest that elevated levels of dissolved inorganic nutrients can reduce the impact of OA on coral calcification. Here, we investigated the potential for enhanced energetic status of juvenile corals, achieved via heterotrophic feeding, to modulate the negative impact of OA on calcification. Larvae of the common Atlantic golf ball coral, Favia fragum, were collected and reared for 3 weeks under ambient (421 μatm) or significantly elevated (1,311 μatm) CO₂ conditions. The metamorphosed, zooxanthellate spat were either fed brine shrimp (i.e., received nutrition from photosynthesis plus heterotrophy) or not fed (i.e., primarily autotrophic). Regardless of CO₂ condition, the skeletons of fed corals exhibited accelerated development of septal cycles and were larger than those of unfed corals. At each CO₂ level, fed corals accreted more CaCO₃ than unfed corals, and fed corals reared under 1,311 μatm CO₂ accreted as much CaCO₃ as unfed corals reared under ambient CO₂. However, feeding did not alter the sensitivity of calcification to increased CO₂; ? calcification/?Ω was comparable for fed and unfed corals. Our results suggest that calcification rates of nutritionally replete juvenile corals will decline as OA intensifies over the course of this century. Critically, however, such corals could maintain higher rates of skeletal growth and CaCO₃ production under OA than those in nutritionally limited environments.     

Ocean acidification bends the mermaid's wineglass

Ocean acidification lowers the saturation state of calcium carbonate, decreasing net calcification and compromising the skeletons of organisms such as corals, molluscs and algae. These calcified structures can protect organisms from predation and improve access to light, nutrients and dispersive currents. While some species (such as urchins, corals and mussels) survive with decreased calcification, they can suffer from inferior mechanical performance. Here, we used cantilever beam theory to test the hypothesis that decreased calcification would impair the mechanical performance of the green alga Acetabularia acetabulum along a CO₂ gradient created by volcanic seeps off Vulcano, Italy. Calcification and mechanical properties declined as calcium carbonate saturation fell; algae at 2283 µatm CO₂ were 32% less calcified, 40% less stiff and 40% droopier. Moreover, calcification was not a linear proxy for mechanical performance; stem stiffness decreased exponentially with reduced calcification. Although calcifying organisms can tolerate high CO₂ conditions, even subtle changes in calcification can cause dramatic changes in skeletal performance, which may in turn affect key biotic and abiotic interactions.     

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.     

Ocean Acidification Accelerates Reef Bioerosion

In the recent discussion how biotic systems may react to ocean acidification caused by the rapid rise in carbon dioxide partial pressure (pCO₂) in the marine realm, substantial research is devoted to calcifiers such as stony corals. The antagonistic process – biologically induced carbonate dissolution via bioerosion – has largely been neglected. Unlike skeletal growth, we expect bioerosion by chemical means to be facilitated in a high-CO₂ world. This study focuses on one of the most detrimental bioeroders, the sponge Cliona orientalis, which attacks and kills live corals on Australia’s Great Barrier Reef. Experimental exposure to lowered and elevated levels of pCO₂ confirms a significant enforcement of the sponges’ bioerosion capacity with increasing pCO₂ under more acidic conditions. Considering the substantial contribution of sponges to carbonate bioerosion, this finding implies that tropical reef ecosystems are facing the combined effects of weakened coral calcification and accelerated bioerosion, resulting in critical pressure on the dynamic balance between biogenic carbonate build-up and degradation.     

Cloning and Use of a Coral 36B4 Gene to Study the Differential Expression of Coral Genes Between Light and Dark Conditions

This paper aims to validate reference genes for gene expression studies between light and dark conditions in the scleractinian coral Stylophora pistillata for future gene expression studies of the “light-enhanced calcification” phenomenon. For this purpose, we cloned, sequenced, and characterized a candidate reference gene, the 36B4 gene from the coral S. pistillata, and validated 36B4 and β-actin as reference genes. To illustrate the future applications of these reference genes, we tested the dark and light expression of two photosynthetic genes (Rubisco and D1 protein of the photosystem II) and two genes encoding proteins involved in calcium transport for coral calcification (a calcium ATPase and a calcium channel). Results show that both photosynthetic genes are enhanced during the light when standardized against 36B4 and β-actin, whereas the two genes encoding proteins involved in calcium transport are not differentially expressed between light and dark conditions. The characterization of a coral 36B4 and the establishment of such valid reference genes will be useful for future gene expression studies between diverse conditions (aposymbiotic/symbiotic, stress/control, light/dark conditions) in scleractinian corals. Nucleotide sequence of the coral 36B4 gene cloned in this study is available in the Genbank database under the accession number EU069460.     

Coral calcification: autoradiography of a scleractinian coral Galaxeafascicularis after incubation in 45Ca and 14C

 The uptake of ⁴⁵Ca and/or ¹⁴C by the skeleton of coral colonies has been commonly used to investigate the processes of calcification. This study reports the differential uptake of these tracers within different regions of the skeleton and tissues of individual corallites and polyps of the hermatypic coral Galaxea fascicularis. Incubation in ⁴⁵Ca in the light resulted in 80 percent of the ⁴⁵Ca taken up being deposited in the skeleton. Autoradiography of transverse and longitudinal slices of freeze-substituted polyps and corallites showed that in the light ⁴⁵Ca was incorporated into the exsert septa, the outside of the thecal walls of the corallite and the inner edges of the septa. Incorporation did not occur in the costae. The radioactivity in the skeleton was considerably greater than in the tissues. In the dark, or in the presence of the photosynthetic inhibitor Diuron, ⁴⁵Ca was taken up by the exsert septa and was patchily distributed in the corallite walls which suggests that it was not a result of isotopic exchange. The differential incorporation of ⁴⁵Ca onto the exsert septa was confirmed by scintillation counting. Negligible radioactivity remained in the extrathecal coelenteron after a brief 5 min rinse in non-radioactive seawater. Only 0.1% of ¹⁴C taken up in the light was incorporated into the skeleton and this was confirmed by autoradiography. In the presence of Diuron or in the dark, very little ¹⁴C was incorporated into tissues or skeleton and in autoradiographs was either not evident in the skeleton or the distribution was similar to that seen in autoradiographs of ⁴⁵Ca uptake. These results show that the deposition of ⁴⁵Ca, and therefore calcium carbonate, occurs at specific loci on the skeleton of a corallite. In the dark, deposition occurs specifically at the growing points of the corallite. Differential deposition of calcium carbonate within individual corallites has not been previously reported. Accepted: 27 May 1997     

The biology and physiology of alga-invertebrate symbioses. III. In situ measurements of photosynthesis and calcification in some hermatypic corals

In situ measurements of the rates of photosynthesis and calcification in three species of hermatypic corals were made at Eilat, in the Gulf of Aqaba, Red Sea. Experiments were made at 5, 20 and 35 m depth under unusually poor conditions of submarine illumination for the region, and at the relatively low water temperature (21°C) for coral growth which prevails there all year. Estimates of photosynthetic rates by both the ¹⁴C and oxygen methods indicated that the ¹⁴C method does measure gross photosynthesis in these organisms even at, and below, light compensation points. Substantial rates of carbon fixation in Acropora and Millepora show that, even under bad conditions, these organisms could survive autotrophically to at least 10 m depth, as also could the massive coral Goniastrea although this had much lower photosynthetic rates under the same conditions, compensated for by a much lower respiratory rate than the other two corals.Calcification rates were variable but showed a considerable increase in light as compared with the dark in all three species, and the rates did not decrease with depth as much as might have been anticipated from the reduction in photosynthesis and ambient light energy. Photosynthetic and calcification rates were similar to those reported for similar organisms both in the Caribbean and on the Great Barrier Reef.     

Coral reef calcifiers buffer their response to ocean acidification using both bicarbonate and carbonate

Central to evaluating the effects of ocean acidification (OA) on coral reefs is understanding how calcification is affected by the dissolution of CO₂ in sea water, which causes declines in carbonate ion concentration [CO₃²⁻] and increases in bicarbonate ion concentration [HCO₃⁻]. To address this topic, we manipulated [CO₃²⁻] and [HCO₃⁻] to test the effects on calcification of the coral Porites rus and the alga Hydrolithon onkodes, measured from the start to the end of a 15-day incubation, as well as in the day and night. [CO₃²⁻] played a significant role in light and dark calcification of P. rus, whereas [HCO₃⁻] mainly affected calcification in the light. Both [CO₃²⁻] and [HCO₃⁻] had a significant effect on the calcification of H. onkodes, but the strongest relationship was found with [CO₃²⁻]. Our results show that the negative effect of declining [CO₃²⁻] on the calcification of corals and algae can be partly mitigated by the use of HCO₃⁻ for calcification and perhaps photosynthesis. These results add empirical support to two conceptual models that can form a template for further research to account for the calcification response of corals and crustose coralline algae to OA.     

Carbonic anhydrases in anthozoan corals—A review

Coral reefs are among the most biologically diverse and economically important ecosystems on the planet. The deposition of massive calcium carbonate skeletons (biomineralization or calcification) by scleractinian corals forms the coral reef framework/architecture that serves as habitat for a large diversity of organisms. This process would not be possible without the intimate symbiosis between corals and photosynthetic dinoflagellates, commonly called zooxanthellae. Carbonic anhydrases play major roles in those two essential processes of coral’s physiology: they are involved in the carbon supply for calcium carbonate precipitation as well as in carbon-concentrating mechanisms for symbiont photosynthesis. Here, we review the current understanding of diversity and function of carbonic anhydrases in corals and discuss the perspective of theses enzymes as a key to understanding impacts of environmental changes on coral reefs.