Searching for future-proof marine plants in the acidic waters that bathe a volcano
Low pH levels in naturally acidic waters near an underwater volcano in Italy, according to worst-case climate predictions, will become a common phenomenon by the end of the century and beyond.
Scientists are studying the reactions of local seagrasses and algae to acidic conditions.
One of the questions scientists are asking is whether seagrasses could be used for environmental restoration in other areas that may become more acidic in the near future.
However, some researchers report that these marine plants capable of sequestering carbon must deal with more imminent dangers such as pollution, the degradation of their habitat and warming waters: problems that must be addressed to make their restoration efforts effective.
A raised cobblestone road connects an islet, on which the famous Aragonese castle perches, to the island of Ischia, off the coast of the city of Naples. Along the northern and southern edges of the islet, carbon dioxide bubbles from the volcanic rocks underwater. Around these effervescent strips of sea floor, the pH of the water varies from a low of 6.6 to the normal 8.1. Between 7.8 and 7.7, values that will be common to all marine waters on the planet by the end of the century, according to the worst climate forecasts, phanerogams and macroalgae dominate the landscape.
Since 2008 this area has been used as a natural biogeochemistry laboratory and as a window into the future by scientists from all over the world. Researchers working in these waters, acidic by nature due to the presence of underwater volcanoes, among the various aspects of the ecology of the place, are studying animals and populations of seagrasses that have populated these rocks for generations.
“There are photosynthetic organisms such as Posidonia and other types of macroalgae that, in some way, have already adapted to the acidification of sea water,” said Marco Munari, researcher in Marine Ecology and Ecotoxicology at the Anton Dohrn Zoological Station (SZN ) of Naples, in an interview with Mongabay. Until the beginning of this year, Munari was the coordinator of the Ischia Marine Center of the SZN, then he moved to a branch of the SZN in Fano.
Local populations of these organisms are already prepared for the stressors that populations in other areas may experience in the near future.
With pH at the levels predicted by 2100 under the most pessimistic of emissions scenarios, seagrasses and algae will thrive. In fact, at the expected CO2 concentration levels, these plants can make the most of their photosynthesis capabilities, claims Marco Milazzo, professor of Ecology at the University of Palermo who is studying underwater volcanic vents in Sicily and off the coast of Japan.
The problem is that these seagrasses and macroalgae are almost the only marine organisms that are better off, he added.
Since the industrial revolution, the oceans have absorbed about 30% of carbon dioxide emissions from human activities such as the burning of fossil fuels, cement production and changes in land use. This absorption affected marine chemistry, decreasing the average pH level of the water from 8.2 to 8.1 . Such a change might seem insignificant, but the pH scale is logarithmic and even changes of 0.1 are important and can trigger a series of cascading changes in the composition of marine waters. In future scenarios, with CO2 emissions increasing, the decrease in the pH level of marine waters is expected to have catastrophic consequences for shell-building organisms such as mussels, clams, sea urchins and corals, including countless microorganisms that they support the marine food web.
In a similar context, coastal habitats, made up of seagrass beds, salt marshes and tropical mangroves, have taken on even greater importance. Since 2009, the key role that these systems play in absorbing carbon dioxide and other greenhouse gases from the seas has been recognized. Like trees and forests on earth, these so-called “blue” ecosystems absorb carbon dioxide from the environment through photosynthesis, releasing oxygen and storing organic carbon in sediments. These habitats cover less than 0.5% of the seas but store half of the carbon, which is buried carbon, which is buried.
The largest area of underwater volcanic vents on the island of Vulcano. Photo by Dimitri Kleitou.
Unfortunately, much of the surface area of these ecosystems has been lost due to coastal development, water quality, and other anthropogenic pressures. In the Mediterranean region, according to a 2015 article also cited in a more recent review, the iconic seagrass Posidonia oceanica , over the last 50 years, has retreated by 34% compared to its historical area.
“Human activities, such as trawling or the abandonment of fishing nets which, with the movement of the waves, uproot these marine plants, correspond to industries that are deforesting the Amazon to produce valuable wood,” he argues Munari. By 2030, both the UN and the European Community aim to expand the conservation and restoration of these carbon dioxide-storing ecosystems.
Experiments already carried out near underwater volcanic vents, in fact, have demonstrated that underwater vegetation, through photosynthesis, contributes to mitigating acidification and attenuates its effects on other species. The uptake of dissolved CO2 and the release of oxygen due to photosynthesis, according to some recent studies, appear to make habitats more resilient to heat waves and, presumably, other stressors such as pollution . Currently, SZN researchers are conducting laboratory experiments to test the response of seagrasses and macroalgae to both heat waves and water acidification.
Additionally, they are planning field experiments with Cystoseira, a Mediterranean macroalgae not found around Ischia's volcanic vents. In fact, they will place different populations of Cystoseira along the slopes of the vents to test their responses to various levels of acidity with the aim of identifying populations for use in other environmental restoration projects.
A marine biologist places sensors along the slope of a volcanic vent in Shikinejima, Japan, to continuously measure the concentration of dissolved CO2 in seawater, pH, temperature and salinity.
In the United States, scientists have proposed taking seeds of the common seagrass Zostera marina from Virginia and planting them far north, in New York waters. The inspiration came from the observation of the northward migration of some species following warming waters. The idea is to help Zostera marina , which should be well accustomed to Virginia's higher temperatures, become established much further north as well.
Simonetta Fraschetti, professor of Ecology at the Federico II University of Naples, and Erika Fabbrizzi, a researcher who has just completed a doctoral project on the forest restoration of macroalgae, believe it is important to identify populations that adapt better than others, especially to temperature anomalies. Fraschetti's mantra, however, is: we restore, we restore but, first of all, we mitigate, we mitigate and we conserve. Restoring a degraded habitat to its original condition is extremely expensive: so it is best to prevent degradation in the first place.
During her PhD, Erika Frabbrizzi worked on identifying priority criteria for those sites most likely to recover. Mapping marine habitats and their environmental conditions is a critical first step, she said. Furthermore, understanding the causes of the disappearance of a habitat is the key to the success of its restoration.
For example, on Long Island, New York, where Alyson Lowell is conducting much of her research on seagrass metabolism and its effects on marine water biochemistry as a graduate student at Stony Brook University, light is the factor preventing Zostera from providing better ecological services. Nutrient pollution in a densely populated area like New York stimulates algae blooms, which steal light from seagrass photosynthesis.
Carbon dioxide bubbles coming from volcanic rocks off the island of Ischia. Photo by Pasquale Vassallo/SZN.
According to Fraschetti, the factors that contribute to the degradation of a habitat must be combated more urgently than the acidification of water. “In the Mediterranean, Posidonia is disappearing for reasons other than acidification,” he said. “We must first focus on the causes of this disappearance.”
The IPCC Sixth Assessment Report claims that Posidonia could reach functional extinction by 2100, mainly due to warming marine waters.
“Natural systems… are characterized by a very strong resilience,” says Fraschetti in a burst of optimism. “Let's start from this to avoid having global desertification by the end of the century.”
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