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Mediterranean corals are a natural model for studying global warming, as the Mediterranean basin is expected to be one of the most affected regions and the increase in temperature is one of the greatest threats for coral survival. We have analyzed for the first time with time-domain nuclear magnetic resonance (TD-NMR) the porosity and pore-space structure, important aspects of coral skeletons, of two scleractinian corals, Balanophyllia europaea (zooxanthel- late) and Leptopsammia pruvoti (nonzooxanthellate), taken from three different sites on the western Italian coast along a temperature gradient. Comparisons have been made with mercury intrusion porosimetry and scanning electron micros- copy images. TD-NMR parameters are sensitive to changes in the pore structure of the two coral species. A parameter, related to the porosity, is larger for L. pruvoti than for B. europaea, confirming previous non-NMR results. Another parameter representing the fraction of the pore volume with pore sizes of less than 10−20 μm is inversely related, with a high degree of statistical significance, to the mass of the specimen and, for B. europaea, to the temperature of the growing site. This effect in the zooxanthellate species, which could reduce its resistance to mechanical stresses, may depend on an inhibition of the photosynthetic process at elevated temperatures and could have particular consequences in determining the effects of global warming on these species.

Scleractinian corals are a major source of biogenic calcium carbonate, yet the relationship between their skeletal microstructure and mechanical properties has been scarcely studied. In this work, the skeletons of two coral species: solitary Balanophyllia europaea and colonial Stylophora pistillata , were investigated by nanoindentation. The hardness H IT and Young's modulus E IT were determined from the analysis of several load–depth data on two perpendicular sections of the skeletons: longitudinal (parallel to the main growth axis) and transverse. Within the experimental and statistical uncertainty, the average values of the mechanical parameters are independent on the section's orientation. The hydration state of the skeletons did not affect the mechanical properties. The measured values, E IT in the 76–77 GPa range, and H IT in the 4.9–5.1 GPa range, are close to the ones expected for polycrystalline pure aragonite. Notably, a small difference in H IT is observed between the species. Different from corals, single-crystal aragonite and the nacreous layer of the seashell Atrina rigida exhibit clearly orientation-dependent mechanical properties. The homogeneous and isotropic mechanical behaviour of the coral skeletons at the microscale is correlated with the microstructure, observed by electron microscopy and atomic force microscopy, and with the X-ray diffraction patterns of the longitudinal and transverse sections.

Ocean acidification is predicted to impact ecosystems reliant on calcifying organisms, potentially reducing the socioeconomic benefits these habitats provide. Here we investigate the acclimation potential of stony corals living along a pH gradient caused by a Mediterranean CO2 vent that serves as a natural long-term experimental setting. We show that in response to reduced skeletal mineralization at lower pH, corals increase their skeletal macroporosity (features >10 μm) in order to maintain constant linear extension rate, an important criterion for reproductive output. At the nanoscale, the coral skeleton's structural features are not altered. However, higher skeletal porosity, and reduced bulk density and stiffness may contribute to reduce population density and increase damage susceptibility under low pH conditions. Based on these observations, the almost universally employed measure of coral biomineralization, the rate of linear extension, might not be a reliable metric for assessing coral health and resilience in a warming and acidifying ocean., Global warming and ocean acidification impact coral ecosystems. Here, the authors show higher skeletal porosity and reduced bulk density at lower pH in corals living along a natural pH gradient in the Mediterranean, which may contribute to reduce population density and increase damage susceptibility.

This is the first study exploring skeletal mechanical properties in temperate corals. The variation of skeletal mechanical properties with distance from the aboral pole, coral age, and population was investigated in the corals Balanophyllia europaea (zooxanthellate) and Leptopsammia pruvoti (non-zooxanthellate), collected at three sites along a wide latitudinal gradient. Mechanical properties were measured by nanoindentation, a technique applied here in detail for the first time to a scleractinian. In both species, a reduction of Young’s modulus was observed toward the oral pole, which is the youngest part of the skeleton. Skeletons of B. europaea increased their Young’s modulus with age, unlike L. pruvoti. Only the zooxanthellate species showed reduced Young’s modulus in southern populations, coherently with the observed reduced skeletal bulk density and porosity with SST (decreasing latitude) in B. europaea, and with the lack of correlations with SST and latitude for skeletal bulk density and porosity in L. pruvoti. These results support previous hypotheses on differences in the skeletal mechanical properties of these two coral species in relation to the observed variations of skeletal bulk density and porosity with temperature/latitude, which may have consequences related to the envisaged seawater warming of next decades.

Balanophyllia europaea is a shallow water solitary zooxanthellate coral, endemic to the Mediterranean Sea. Extensive field studies across a latitudinal temperature gradient highlight detrimental effects of rising temperatures on its growth, demography and skeletal characteristics, suggesting that depression of photosynthesis at high temperatures might cause these negative effects. Here we test this hypothesis by analyzing, by means of pulse amplitude modulation fluorometry, the photosynthetic efficiency of B. europaea specimens exposed in aquaria to the annual range of temperatures experienced in the field (13°C, 18°C, and 28°C), and two extreme temperatures expected for 2100 as a consequence of global warming (29°C and 32°C). The indicators of photosynthetic performance analyzed (maximum and effective quantum yield) showed that maximum efficiency was reached at 20.0-21.6°C, slightly higher than the annual mean temperature in the field (18°C). Photosynthetic efficiency decreased from 20.0°C to 13°C and even more strongly from 21.6°C to 32°C. An unusual form of bleaching was observed, with a maximum zooxanthellae density at 18°C that strongly decreased from 18°C to 32°C. Chlorophyll a concentration per zooxanthellae cell showed an opposite trend as it was minimal at 18°C and increased from 18°C to 32°C. Since the areal chlorophyll concentration is the product of the zooxanthellae density and its cellular content, these trends resulted in a homogeneous chlorophyll concentration per coral surface across temperature treatments. This confirms that B. europaea photosynthesis is progressively depressed at temperatures >21.6°C, supporting previous hypotheses raised by the studies on growth and demography of this species. This study also confirms the threats posed to this species by the ongoing seawater warming.

In contrast with the number of studies on tropical species, analyses of the variation of growth parameters with environmental variables in temperate areas are very scarce. The growth of only four species of scleractinians has been analyzed in natural Mediterranean populations: Cladocora caespitosa (Family: Faviidae), Balanophyllia europaea and Leptopsammia pruvoti (Family: Dendrophylliidae), and Caryophyllia inornata (Family: Caryophylliidae). Notwithstanding the importance of obtaining information on coral population dynamics, the first study on a Mediterranean scleractinian dates back less than 10 years. To date, field analyses of the relationships between environmental parameters and growth and population dynamics of Mediterranean scleractinians are available only for two solitary dendrophyliid species: B. europaea and L. pruvoti. Available literature on growth and population dynamics of natural scleractinian populations of the Mediterranean Sea is reviewed in the present work. As general trends, it seems that: (1) solitary species have a definite growth pattern, in contrast with colonial species; and (2) symbiotic species are more sensitive to increasing temperatures and more vulnerable to global warming. Knowledge on the growth and population dynamics of Mediterranean species has significantly increased in the last 20 years, and more effort is still required to gather the necessary information for protecting a hotspot of biodiversity like the Mediterranean Sea from the threats of global environmental change.

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.

The demographic traits of the solitary azooxanthellate scleractinian Leptopsammia pruvoti were determined in six populations on a sea surface temperature (SST) gradient along the western Italian coasts. This is the first investigation of the growth and demography characteristics of an azooxanthellate scleractinian along a natural SST gradient. Growth rate was homogeneous across all populations, which spanned 7 degrees of latitude. Population age structures differed between populations, but none of the considered demographic parameters correlated with SST, indicating possible effects of local environmental conditions. Compared to another Mediterranean solitary scleractinian, Balanophyllia europaea, zooxanthellate and whose growth, demography and calcification have been studied in the same sites, L. pruvoti seems more tolerant to temperature increase. The higher tolerance of L. pruvoti, relative to B. europaea, may rely on the absence of symbionts, and thus the lack of an inhibition of host physiological processes by the heat-stressed zooxanthellae. However, the comparison between the two species must be taken cautiously, due to the likely temperature differences between the two sampling depths. Increasing research effort on determining the effects of temperature on the poorly studied azooxanthellate scleractinians may shed light on the possible species assemblage shifts that are likely to occur during the current century as a consequence of global climatic change.

International audience; The skeleton of scleractinian corals is commonly believed to be composed entirely of aragonite due to the current Mg/Ca molar ratio of seawater, which thermodynamically favours the deposition of this polymorph of calcium carbonate (CaCO3). However, some studies have shown that other forms of CaCO3 such as calcite can be present in significant amount (1-20%) inside tropical coral skeletons, significantly impacting paleo-reconstructions of SST or other environmental parameters based on geochemical proxies. This study aims at investigating for the first time, (1) the skeletal composition of two Mediterranean solitary corals, the azooxanthellate Leptopsammia pruvoti and the zooxanthellate Balanophyllia europaea, across their life cycle, (2) the distribution of the different CaCO3 forms inside skeletons, and (3) their implications in paleoclimatology. The origin of the different forms of CaCO3 observed inside studied coral skeletons and their relationships with the species' habitat and ecological strategies are also discussed. CaCO3 composition of L. pruvoti and B. europaea was investigated at six sites located along the Italian coasts. Skeleton composition was studied by means of X-ray powder diffraction and Fourier transform infrared spectroscopy. A significant amount of calcite (1-23%) was found in more than 90% of the studied coral skeletons, in addition to aragonite. This calcite was preferentially located in the basal and intermediate areas than at the oral pole of coral skeletons. Calcite was also mainly located in the epitheca that covered the exposed parts of the coral in its aboral region. Interestingly in B. europaea, the calcite content was negatively correlated with skeleton size (age). The presence of calcite in scleractinian corals may result from different mechanisms: (1) corals may biologically precipitate calcite crystals at their early stages in order to insure their settlement on the substrate of fixation, especially in surgy environments; (2) calcite presence may result from skeletons of other calcifying organisms such as crustose coralline algae; and/or (3) calcite may result from the infilling of galleries of boring microorganisms which are known to colonize coral skeletons. We suggest that more than one of the above mentioned processes are involved.

Coral polyps inside a colony may differ in reproductive activity and ecological function even while sharing the same genetic identity. Although polyps are the basic units of coral colonies, their size, biometry and size structure have rarely been studied. This study investigated, for the first time, colony and polyp biometric relationships and intra-colony polyp population size structure in the Mediterranean endemic Astroides calycularis (Pallas, 1766). Biometric parameters for 160 colonies and 4162 polyps were measured with consideration of polyp position inside the colony (central or peripheral). The positive allometric relationship between polyp width and length, resulting in a progressively circular oral disc as polyp size increases, may relate to the low-sedimentation characteristics of the habitat of this species. The smaller size of peripheral polyps compared to central ones suggests that polyp budding occurs preferentially at the outskirts of the colonies, possibly increasing the competitive advantage for space utilization. Larger colonies had polyps with smaller size than small and medium colonies, due to an over-representation of the size class containing polyp size at sexual maturity. It is proposed that large colonies may invest energy in increasing polyp size up to the size at sexual maturity, rather than increasing the size of already mature polyps.

The correlation between two environmental factors (solar radiation and sea surface temperature), biometry, and population density was assessed along a latitudinal gradient in the zooxanthellate coral Balanophyllia europaea and in the azooxanthellate coral Leptopsammia pruvoti. With increasing polyp size, the oral disc of B. europaea assumed an oval shape, while that of L. pruvoti retained a circular shape. In both species, biometric parameters varied more with temperature than with solar radiation. In the zooxanthellate species, temperature explained a higher percentage of biometric parameter variance than in the azooxanthellate species. While environmental factors did not co-vary with demographic characteristics in L. pruvoti, temperature was negatively related to the population density of B. europaea. It is hypothesized that the negative effect of temperature on biometry and population density of B. europaea depends on photosynthesis inhibition of symbiotic zooxanthellae at high temperatures, which would lower the calcification rate and availability of energetic resources.

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.