The submarine emissions of CO2 of the island of Panarea make it an open laboratory for the study of the effects of ocean acidification on calcifying organisms, such as corals. Let’s find out more about this natural study area.
Corals and coral reefs
Corals are a group of invertebrates that belong to the group of Cnidaria (a family that includes organisms like jellyfish and anemones). Each coral is constituted by thousands of tiny organisms genetically identical, called polyps. These organisms produce calcium carbonate used to create an external protective structure (exoskeleton) that will form the coral reef. We will have zooxanthellate corals if the polyp is in a symbiotic relationship with small unicellular algae (zooxanthellae). If the symbiosis is absent, the corals will be called azooxanthellate corals.
Coral reefs are extremely important ecosystems. Firstly, they protect the coastlines from coastal erosion because the waves are dissipated on the reef and do not strike directly on the coast. They also provide suitable habitats for the reproduction and growth of marine organisms (Barbier et al., 2011) and, thanks to their structural complexity, they also have a significant aesthetic, cultural and economic value. In fact, they host a wide range of fish and other marine organisms, representing an essential resource for the economy of coastal communities that mainly depend on fishing (Moberg et al., 1999).
An increasingly endangered ecosystem
Currently, coral reefs are one of the most endangered ecosystems. The continuous exploitation of this habitat through fishing and water pollution caused by agricultural chemicals that end up in the sea, have been, until now, among the causes of the decrease of coral reefs. Moreover, in recent decades, the increase in global temperatures and CO2 concentrations in the atmosphere have weakened the coral exoskeleton (Hughes et al., 2012). In fact, atmospheric carbon dioxide passes by diffusion through the air-ocean interface, resulting in consequent acidification of the oceans, that is, a decrease in the pH of seawater, making the life of many marine organisms almost impossible (Fantazzini et al., 2015).
According to IPCC (Intergovernmental Panel on Climate Change), since 1800, the pH of the ocean has decreased from 8.2 to 8.1 and, if CO2 emissions do not decrease, the sea will reach a value of 7.8, with enormous consequences for calcifying organisms, but in general for all marine species (Fantazzini et al., 2015).
As the Mediterranean Sea is semi-enclosed, it has already been extensively subjected to a decrease in the pH, with values even lower than the other ocean basins, making it an ideal site for studying the impacts of acidification on coral populations (Goffredo et al., 2014).
A natural laboratory in Panarea
The island of Panarea, located along the southwestern Italian coast, is an ideal site for studying the effects of acidification on calcifying organisms. In fact, as a volcanic island, Panarea hides submarine emissions of CO2, creating a natural acidification gradient that mimics the acidification of the oceans (Goffredo et al., 2014).
The species studied in this area are three endemic corals of the Mediterranean Sea: Balanophyllia europaea, Leptopsammia pruvoti and Astroides calycularis. The first one is a zooxanthellate coral, while the other two species are azooxanthellate corals.
The study area consisted of 4 sites, based on the proximity and distance from the crater from which the CO2 concentrations are emitted. These sites show different levels of pH: ranging from the normal pH of the oceans (8.1 – the farther site from the crater) to more acidic values (7.4 – closest area to the crater, with a pH lower than the worst-case scenario depicted in the IPCC projections).
The corals studied were collected in the 4 sites during three different seasons of the year (winter, spring and summer), to compare the calcification of the coral skeleton subjected to both ocean acidification and the effect of temperature.
A different response of corals to stress
The results of the experiment showed two different behaviours between the zooxanthellate coral (B. european) and the two azooxanthellate species (L. Pruvoti and A. calycularis). In fact, in B. European the coral growth was unaffected by the combined action of pH and temperature (except for site 4, where the pH value was not included by the IPCC in the projections for the next century). Conversely, the two azooxanthellate corals showed a decrease in growth as the pH decreased, and its effect was amplified by the increase in temperature. In both cases, however, as the pH decreased, an increase in the coral porosity was observed, that is a more fragile skeleton that results more sensitive to mechanical stress, such as storms or predation.
Although the combined effects of calcification and increased temperatures are undoubtedly harmful, not all corals react similarly. This different behaviour could result from the symbiosis between algae and coral. In fact, the increase in CO2 could stimulate the efficiency of algal photosynthesis, which would provide the energy needed for coral to produce the calcium carbonate necessary for calcification, thus balancing the adverse effects of acidification.
A positive resilience signal
In conclusion, although acidification threatens the survival of calcifying organisms, and in general marine ecosystems, a positive signal is found from the populations observed. Indeed, increased porosity at acidic pH is associated with constant growth, which is necessary to meet reproductive needs, such as the ability to reach a critical size during sexual maturity (Goffredo et al., 2014). This behaviour, moreover, has been observed mainly for extremely acidic pH values that do not even fit in the IPCC projections for the end of the century, showing that we are still in time to reverse the course and help mitigate the effects of climate change.
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