Final Report Summary - CORALWARM (Corals and global warming: The Mediterranean versus the Red Sea)
CoralWarm aimed at generating for the first time projections of temperate and subtropical coral survival by integrating warming and acidification effects on metabolic and skeletal processes of Mediterranean and Red Sea coral key species. High-temperature and CO2 tolerant species were revealed, allowing future bioremediation actions and establishment of coral refuges, saving corals for future generations. Biologists, chemists, physicists, physiologists, and molecular ecologists worked together, producing new interdisciplinary approaches and scientific ventures. A new underwater natural laboratory on ocean acidification was discovered at Panarea Island (Mediterranean Sea). At this site, the natural CO2 vents from a volcanic crater at shallow depth acidify seawater to levels predicted by the IPCC for the end of this century and beyond. Long-term effect of pCO2 exposure was tested on 4 calcifying organisms, naturally living around the natural laboratory of Panarea. The tolerance of these species to ocean acidification seems inversely related to their degree of control on biomineralization. By modifying macro-scale (while preserving nano-scale) skeletal properties, zooxanthellate corals reduce skeletal mineralization with increasing acidification, but keep a constant linear extension rate to reach sexual maturity size, suggesting a mechanism of phenotypical plasticity. The project shaded a light on the mechanism of calcium carbonate deposition in coral skeletal growth. The skeleton of stony corals is commonly believed to be composed entirely of aragonite due to the current Mg/Ca molar ratio of seawater, which favors the deposition of this form of calcium carbonate (CaCO3). CoralWarm team showed that a significant amount of calcite (up to 23%) can be found in Mediterranean coral skeletons, in addition to aragonite. The amount of calcite inside coral skeletons can potentially affect the analysis of coral archives in paleoclimatic studies, which requires pure aragonitic substrates, introducing errors up to 4-11 °C. Thus, CoralWarm team strongly recommends to scientists working on paleoclimatology an accurate mineralogical preliminary inspection before performing the paleoclimatic analyses. Scientific community also debated on what extent the calcification occurs under biological or environmental control, even if the mineral deposition occurs in a biological confined environment. Coral skeleton, other than CaCO3, also contains an intra-skeletal organic matrix (OM), produced by the coral at the time of mineral deposition. CoralWarm showed how OM influences CaCO3 crystal morphology, aggregation and polymorphism as a function of its composition and the crystallization environment. In particular, in-vitro experiments on different coral species showed that only the OM of Balanophyllia europaea is able to precipitate aragonite even in an environment completely depleted of magnesium, one of the main ions enhancing the precipitation of aragonite. This in-vitro result may indicate a stronger biological control, and a higher independence from the environment of the biomineralization processes in this species, compared to those so far tested from the Mediterranean and tropical seas. The sensitivity to temperature variation was tested in the field, along an 8° latitudinal gradient in the Mediterranean. Along this transect B. europaea increases its skeletal porosity with increasing temperature, leading to more fragile skeletons in warmer populations, showing a less efficient reproductive strategy. This trend was not apparent in the non-photosynthetic Leptopsammia pruvoti and Caryophyllia inornata, suggesting a higher tolerance of this species to temperature increase. CoralWarm found evidence that the increase in porosity with increasing temperatures observed in B. europaea could depend on an attenuation of calcification due to an inhibition of its photosynthetic process at elevated temperatures. Caryophyllia inornata displayed an unusual pattern of embryogenesis, with embryos found in male and sexually inactive polyps, suggesting the possibility of an asexual origin of larvae, coupled with a canonic sexual reproduction, a reproductive strategy that may increase survivorship of populations in a rapidly changing environment. These results highlight differences in the sensitivity/tolerance of coral species to temperature changes in face of global climate change, possibly related to trophic strategies (i.e. presence or absence of photosynthesis). Non-photosynthetic corals could be less sensitive to global warming. CoralWarm research efforts on determining the effects of warming and acidification on the poorly studied non-photosynthetic corals 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.