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Assessing the sources and accumulation patterns of polycyclic aromatic hydrocarbons (PAHs) in corals is critical, as they threaten coral ecosystem resilience in addition to other anthropogenic pressures. We determined acenaphthene, fluorene, fluoranthene, and pyrene concentration in the skeleton and soft tissue of 7 adult and 29 old specimens of the non-zooxanthellate coral Leptopsammia pruvoti from the Mediterranean Sea. Leptopsammia pruvoti accumulated 2–72 times higher PAH concentrations than the previously investigated zooxanthellate Balanophyllia europaea living at the same site at shallower depth, likely in relation to the different trophic strategy. Low molecular weight PAHs were preferentially accumulated compared to high molecular weight PAHs. Detected PAHs were mainly petrogenic, consistently with local pollution sources. Populations of L. pruvoti immobilized PAHs in the skeleton 3–4 orders of magnitude more efficiently than B. europaea. This highlights the need to investigate other non-zooxanthellate species, which represent the majority of Mediterranean scleractinians, but are widely overlooked with respect to the few zooxanthellate species.

Corals exert a strong biological control over their calcification processes, but there is a lack of knowledge on their capability of long-term acclimatization to ocean acidification (OA). We used a dual geochemical proxy approach to estimate the calcifying fluid pH (pH(cf)) and carbonate chemistry of a Mediterranean coral (Balanophyllia europaea) naturally growing along a pH gradient (range: pH(TS) 8.07-7.74). The pH(cf) derived from skeletal boron isotopic composition (delta B-11) was 0.3-0.6 units above seawater values and homogeneous along the gradient (mean +/- SEM: Site 1 = 8.39 +/- 0.03, Site 2 = 8.34 +/- 0.03, Site 3 = 8.34 +/- 0.02). Also carbonate ion concentration derived from B/Ca was homogeneous [mean +/- SEM (mu mol kg(-1)): Site 1 = 579 +/- 34, Site 2 = 541 +/- 27, Site 3 = 568 +/- 30] regardless of seawater pH. Furthermore, gross calcification rate (GCR, mass of CaCO 3 deposited on the skeletal unit area per unit of time), estimated by a "bio-inorganic model" (IpHRAC), was homogeneous with decreasing pH. The homogeneous GCR, internal pH and carbonate chemistry confirm that the features of the "building blocks" - the fundamental structural components - produced by the biomineralization process were substantially unaffected by increased acidification. Furthermore, the pH up-regulation observed in this study could potentially explain the previous hypothesis that less "building blocks" are produced with increasing acidification ultimately leading to increased skeletal porosity and to reduced net calcification rate computed by including the total volume of the pore space. In fact, assuming that the available energy at the three sites is the same, this energy at the low pH sites could be partitioned among fewer calicoblastic cells that consume more energy given the larger difference between external and internal pH compared to the control, leading to the production of less building blocks (i.e., formation of pores skeleton structure, determining increased porosity). However, we cannot exclude that also dissolution may play a role in increasing porosity. Thus, the ability of scleractinian corals to maintain elevated pH(cf) relative to ambient seawater might not always be sufficient to counteract declines in net calcification under OA scenarios.

OCEAN WARMING AND ACIDIFICATION SYNERGISTICALLY INCREASE CORAL MORTALITY CO2-driven ocean warming (OW) and acidification (OA) has prompted urgent research to assess their potential effects on the marine environment, with strong emphasis on calcifying organisms. To correctly predict how these stressors will affect marine organisms, their synergistic effects must be assessed. Here we investigate the interactive effects of OW and OA on the mortality and growth of three Mediterranean coral species: the solitary zooxanthellate Balanophyllia europaea, the solitary non-zooxanthellate Leptopsammia pruvoti and the colonial non-zooxanthellate Astroides calycularis. Corals were transplanted, in different seasons, in proximity to a volcanic CO2 vent where water is naturally acidified to levels matching future IPCC scenarios. We show a strong synergistic adverse effect on mortality rates of the three species. Net calcification rates of the zooxanthellate species were not affected by reduced pH, while those of the two non-zooxanthellate species were negatively affected, indicating that different levels of resilience/resistance to OW and OA among species in symbiosis with photosynthetic algae or asymbiotic ones may be related to different modes of nutrition and/or biomineralization processes. Natural CO2-leaking marine sites can provide essential information on the combined effects of OA and OW on Mediterranean scleractinian corals, which may help understand to which extent different corals will be threatened by increasing atmospheric CO2 pollution.

Ocean acidification is expected to significantly affect calcifying marine organisms, such as corals. While ocean acidification effects on coral calcification have been widely investigated, the effects on coral gametogenesis are almost unknown. A volcanic vent at Panarea Island (Mediterranean Sea, Italy) characterized by continuous and localized CO2 emission, generates a stable pCO2 gradient from the centre to its periphery, providing a unique natural laboratory for ocean acidification studies. Here we assessed the effects of ocean acidification on gametogenesis and embryogenesis of the Mediterranean non-zooxanthellate coral Leptopsammia pruvoti transplanted along this natural pCO2 gradient. After 3 months under experimental conditions, increasing pCO2 seemed to negatively influence spermary production and development. In the control Site, all spermaries showed the latest (most advanced) maturation stage while in the most acidic Sites they also displayed earlier stages, indicating a delay in spermatogenesis and consequently of fertilization and planulation processes. Considering the negative response showed by this species after a short-term exposure, an even stronger negative effect on sexual reproduction is hypothesized under a long-term exposure.

Ocean Acidification (OA) is predicted to have profound impacts on marine ecosystems because carbonate ions are an essential substrate for the biomineralization of shells and skeletons of calcifying marine organisms, from phytoplankton and corals to fishes1,2,3. Volcanic CO2 vent systems, where seawater is naturally acidified, offer a unique opportunity to investigate the response of benthic organisms and habitats to OA. The Ischia Island (Tyrrhenian Sea, Italy) offers a natural laboratory for OA studies, allowing us to investigate how a suite of habitats and species responds to acidification. A. calycularis is a Mediterranean endemic azooxanthellate coral. It is long-lived species and commonly found in dim light shallow rocky habitats of the south-western Mediterranean Sea4. It broods its larvae5 and thus has relatively low dispersal capacities and high potential for local adaptation. This coral is reported as vulnerable in the IUCN red list6. There is one population of A. calycularis that naturally occurs in a semi-submersed cave (Grotta del Mago) affected by CO2 venting, where is highly abundant (70% cover at 1 m depth). Here, we assess population structure and the skeletal characteristics of A. calycularis originating from different sites (naturally acidified and ambient pH sites) along the coast of Ischia Island . We hypothesize that the population thriving under naturally acidified conditions shows higher population dynamics and differences in biomineralization process than the populations studied from other reference sites with ambient pH.Colonies in the Grotta del Mago have encrusting morphology, with smaller size and consequent, lower weight and lower number of polyps compared to conspecifics from sites at normal pH conditions. With increasing acidification (lower pH), the skeletal porosity decreased while the bulk and micro- density increased. Given the reduced calcification rate that may be expected in acidified waters, the observed increase in skeletal density may be counterbalanced by a strong decrease in linear extension rate.

Increasing research effort on marine calcifying organisms is important and urgent, given the projected CO2-driven ocean acidification for the coming century. Here we assess the growth and population dynamics of the Mediterranean endemic zooxanthellate solitary coral B. europaea at three sites along a natural pCO2 gradient of the volcanic vents off Panarea Island (Italy), in the Mediterranean Sea. B. europaea was the only scleractinian found along the gradient and its abundance decreased with increasing pCO2. The age-length relationships, obtained for each site by sclerochronology through computerized tomography, resulted homogeneous and then data were pooled to obtain a general age-length relationship. Skeletal linear extension rate and bulk skeletal density were measured and net calcification rate was calculated for each sample at each site. While linear extension rate did not vary with pCO2, bulk skeletal density and net calcification rate decreased with increasing pCO2. The population age structure was obtained by analyzing photoquadrats and several observed and theoretical demographic parameters were derived (age structure stability, instantaneous rate of mortality, average age, % of immature individuals, age at maximum biomass, average age of biomass) and compared among sites. Populations showed a deficiency of young individuals with increasing pCO2. The species seems sensitive to ocean acidification even if the photosynthetic efficiency of its symbionts increases with pCO2, suggesting an even higher sensitivity for shallow water non-zooxanthellate scleractinians.