Hunting for future-proof marine plants in the acidic waters bathing a volcano

A stone causeway connects an islet, the famed Aragonese Castle perched atop it, to the island of Ischia off the coast of Naples in southern Italy. Below water, along the islet’s northern and southern contours, CO₂ bubbles from volcanic rocks. Across these effervescent strips of seabed, pH ranges from an extreme low of 6.6 to a normal 8.1. Between 7.8 and 7.7, values that according to worst-case climate projections will be common in ocean waters globally by the end of the century, seagrass and macroalgae, or seaweed, dominate the seascape.

Since 2008, scientists from all over the world have been using this area as a natural biogeochemistry lab and a window on the future. Researchers working in these waters, naturally acidified due to the presence of underwater volcanoes, are studying animal and seagrass populations that have inhabited these rocks for generations, among other aspects of local ecology.

“We have photosynthetic organisms like [the seagrass] Posidonia and other types of macroalgae, which are already somehow adapted to marine acidification,” Marco Munari, a marine ecologist and ecotoxicologist at Zoological Station Anton Dohrn (SZN) in Naples told Mongabay in an interview. Until earlier this year, Munari was coordinator of SZN’s Ischia Marine Centre; he has since moved to a different SZN branch in Fano. The local populations of these organisms are already prepared for the stressors that populations in other areas might experience in a not-so-distant future.

At the pH levels projected by 2100 under the most pessimistic emission scenarios, seagrass and seaweed will grow lusher. At that level of CO₂ concentration they can maximize their photosynthesis abilities, said Marco Milazzo, an ecology professor at the University of Palermo who works at volcanic seeps in Sicily, about 300 kilometers (186 miles) south of Ischia, and off of Japan.

The problem is that such seagrasses and seaweeds are nearly the only marine organisms that will do better, Munari said.

Since the industrial revolution, the ocean has absorbed about 30% of carbon dioxide emissions from human activities like burning fossil fuels, cement production and changes in land use. This uptake has affected ocean chemistry, decreasing average seawater pH from 8.2 to 8.1. This change might appear minor. But the pH scale is logarithmic, so even variations of 0.1 are significant, and they can unleash a cascade of other changes in seawater composition. In future scenarios of rising CO₂ emissions, declining seawater pH levels are expected to have dire consequences for shell-building organisms such as mussels, clams, sea urchins and corals, including countless tiny organisms that support marine food webs.

Within this context, coastal habitats, such as seagrass meadows, salt marshes and tropical mangroves, have assumed even greater importance. Since 2009, they have been recognized as key ecosystems for pumping carbon dioxide and other greenhouse gases out of the ocean. Like trees and forests on land, these so-called blue carbon ecosystems absorb CO₂ from the environment through photosynthesis, release oxygen and store organic carbon in the sediments. These habitats cover less than 0.5% of the ocean but store half of the carbon that’s buried under the seafloor.

However, much of these ecosystems’ surface area has been lost to coastal development, water quality issues and other anthropogenic pressures. In the Mediterranean region, the iconic seagrass Posidonia oceanica has shrunk by an estimated 34% of its historic area over the last 50 years, according to a 2015 paper mentioned also in a recent review.

“Human activities, such as trawling or the abandonment of fishing nets that with wave motion pull them out … can be seen as the equivalent of the company deforesting the Amazon to make fine wood,” Munari said. By 2030, both the UN and the EU aim to expand conservation and restoration of these CO₂-storing ecosystems.

At Ischia, researchers have been testing whether Posidonia populations adapted to the naturally acidic waters could be used for restoration purposes in other places. Previous experiments near volcanic seeps offered evidence that underwater vegetation, through photosynthesis, helps mitigate acidification and buffer its effects on other species. The photosynthetic uptake of dissolved CO₂ and release of oxygen also seems to help make habitats more resilient to heat waves and possibly other stressors, like pollution, recent studies suggest. Currently SZN researchers are conducting lab experiments to see how seagrasses and seaweeds respond to both heat waves and acidification.

They are also planning field experiments with Cystoseira, a Mediterranean seaweed that does not live near Ischia’s CO₂ vents. They’ll install Cystoseira from different populations along the vent gradient at Ischia to see how they respond to various levels of acidity, with the aim of identifying populations that might potentially be used as donors in restoration projects.

“By testing the response of various populations to warming, acidification, and pollution, it is possible, for example, to identify populations that are inherently more resilient and consequently more suitable for restoration purposes,” Munari said in an email.

In the U.S., scientists have proposed taking seeds of the common seagrass Zostera marina from Virginia and planting them farther north in New York waters. The inspiration came from observing how various species are already migrating northward in response to warming. The idea is to help Z. marina populations that should be well adapted to Virginia’s higher temperatures settle farther north.

Simonetta Fraschetti, an ecology professor at University of Naples Federico II, and Erika Fabbrizzi, a researcher there who just finished her PhD project on macroalgal forest restoration, said they think it’s important to identify populations that can be more resilient than others, especially to temperature anomalies. But Fraschetti’s mantra is: Let’s restore, restore; but first of all, let’s mitigate, mitigate and conserve. Bringing a degraded habitat back to its original condition is extremely costly, she points out, so it’s better to prevent degradation in the first place.

As part of her PhD research, Fabbrizzi worked on identifying criteria that may help prioritize sites with better chances of making a successful comeback. Mapping marine habitats and their environmental conditions is a fundamental first step, she said. Moreover, understanding the causes of a habitat’s disappearance is key to determining its chances of restoration success.

For instance, in Long Island, New York, where Alyson Lowell is carrying out most of her research on seagrass metabolism and its influences on ocean biochemistry as a PhD candidate at Stony Brook University, light is the factor keeping Zostera from performing greater ecological services. Nutrient pollution from a densely populated area like New York can trigger algal blooms that make less light available for seagrass photosynthesis. “In Long Island, for the success of the restoration we have to clean up our water column,” she said.

Tackling the major drivers of habitat degradation sounds more urgent than addressing acidification, according to Fraschetti. “In the Mediterranean, Posidonia disappears for reasons other than acidification,” she said. “We need to pay attention to the causes of its disappearance first.”

The sixth IPCC assessment report stated that Posidonia might go functionally extinct by 2100, mainly as a result of warming waters.

“Natural systems … are characterized by very strong resilience,” Fraschetti said, in a glimpse of optimism. “Let’s build on that; [doing so] is critical to avoiding having an overall desert by the end of the century.”

Banner image: Macroalgae and sea anemones thrive at high CO₂ levels at Vulcano Island, Sicily, Italy. Image by Marco Milazzo.

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