PIRAN, Slovenia — What if you could fight one threat to the world's oceans with another?

That's what scientists in Piran, on the Slovene riviera, are trying to find out. Together with researchers from across Europe and Israel, they're investigating whether jellyfish — which are booming in the Mediterranean and beyond, threatening to destabilize ecosystems as well as local economies — can be used to combat microplastic pollution.

Their EU-funded project, dubbed GoJelly, also aims to make use of the boom by developing sustainable, jellyfish-derived products ranging from fertilizer to human food.

At its heart, however, lies the ambitious effort to create a filter for wastewater treatment plants using the animal's mucus — in short, to reduce microplastics in the world's oceans by trapping them in jellyfish slime.

"As a biomaterial, it's really interesting," said Katja Klun, a marine chemist at Piran's National Institute of Biology. In 2015, scientists found that jellyfish mucus can trap nanoparticles, sparking the GoJelly idea. Tests conducted as part of the project suggest that the goo also has "the potential to absorb microparticles," Klun said.

Thanks to a combination of factors, including overfishing and global warming, jellyfish populations are on the rise, threatening to disrupt the delicate balance of marine ecosystems.

Opening a fridge full of containers and test tubes in one of the institute's labs, she took out a glass beaker filled with murky liquid. "You see?" she said, swirling it until the liquid turned almost clear except for a goopy, greenish spiral at its center. "We put the mucus in some water and then spiked it with microparticles. The slimy part attached to the particles."

The microparticles used to test the mucus were fluorescent microbeads — tiny, non-degradable plastic pieces found in products such as rinse-off cosmetics and toothpaste, which enter sewage systems and eventually oceans, where they can harm marine life.

Unlike plastics, jellyfish are natural inhabitants of the oceans. But thanks to a combination of factors, including overfishing and global warming, jellyfish populations are on the rise, threatening to disrupt the delicate balance of marine ecosystems.

The Piran institute has a treasure trove of historical documents detailing more than two centuries of jellyfish activity in the Gulf of Trieste, the Adriatic bay shared by Italy, Croatia and Slovenia. The records demonstrate that blooms, or population explosions, are increasing in the area.

"We have a time series of observation of different species since the 1790s. We know they formed blooms here in the past," said Tjaša Kogovšek, a jellyfish ecologist at the institute. "But these blooms have become more frequent and last longer in recent decades, particularly since the 1980s."

The problem extends beyond the Mediterranean: In 2012, for example, researchers found jellyfish populations had increased in 62 percent of the marine ecosystems they studied. (Some scientists have cast doubt that a worldwide boom exists; it doesn't help that jellyfish are difficult to count.)

The causes aren't clear-cut, Kogovšek said, but human activity plays a significant role. Overfishing means jellyfish face fewer predators and competitors for food, and many species thrive in warmer waters, handing them an advantage in an era of rising sea temperatures.

Stronger than ever

Jellyfish are also outstanding survivors — they've lived through all of the Earth's mass extinction events — and are able to adjust more quickly than other creatures to changes in the ecosystem. "Because they're very old, evolutionarily speaking, they are more flexible," Kogovšek said, noting that they aren't picky eaters, for example.

Their proliferation can disrupt food chains and throw marine ecosystems off-balance. But they also increasingly interfere with human activity, to the point of causing economic damage: clogging fishing nets, shutting down power stations, decimating salmon farms and forcing beach closures.

Invasive species present another problem. One such unwelcome guest, the comb jelly, popped up on the Slovenian coast a few years ago — likely via the ballast water of passing ships — and has blighted Piran's otherwise crystal-clear waters every summer since.

Researchers in Piran are currently examining the mucus of different species to analyse their absorption properties and potential methods of preservation.

"It's like a soup," said Lena Radevich, a guide on a tourist boat, pointing overboard at countless comb jellies. The ghostly, roughly fist-sized creatures don't sting, she said, but their arrival nevertheless spells trouble: When they appeared in the Black Sea in the 1980s, they devastated local fish stocks.

Radevich works on a glass-bottom boat that takes tourists to see local fish and mussel farms close up. But in late August, there were far more comb jellies surrounding the boat than fish, much to the horror of tourists who had planned on going swimming. "Sometimes," Radevich said, "there's so many you can't even see the fish."

Though they resemble each other, comb jellies aren't actually jellyfish, but they're similar enough for the GoJelly team to look at uses for their mucus and biomass. At the Piran institute, water-filled buckets of comb jellies stood next to larger tubs of yellowish specimens known as fried-egg jellyfish.

The researchers had already extracted their mucus, which involves exposing the animals to air — triggering the production of mucus, a defense mechanism — and placing them on a funnel, letting the slime drip through.

Creating a microplastics filter is a painstaking process and the road to a prototype is long. (GoJelly runs for four years, until December 2021, having received €6 million in EU funding under the bloc's Horizon 2020 program.)

Researchers in Piran are currently examining the mucus of different species to analyse their absorption properties and potential methods of preservation. But finding out how to stabilize and store the material is tricky, said Klun, the marine chemist.

"Bacteria really like this mucus," she said. "It can last months in the freezer, but outside it degrades within a day. Even in the fridge, it degrades really fast."

Klun also acknowledged that a jellyfish-derived filter for plastics might not be sustainable on a large scale. For that reason, and due to the challenge of preservation, she believes that in the long term, it could make sense to attempt making a synthetic version of the mucus — an approach that would also solve the issue of continuous supply, given that jellyfish blooms are seasonal.

Meanwhile, in labs across Europe, the Piran institute's partners are investigating other uses for jellyfish. Since the filter project only requires the mucus, the animals' biomass is shipped to other GoJelly scientists — including Antonella Leone, who is hoping to tempt Europeans into eating jellyfish.

Jellyfish for dinner?

At the Institute of Science of Food Production in Lecce, at the heel of Italy's boot, Leone studies methods of processing jellyfish for food, as well as its nutritional benefits. Unlike in Asia, jellyfish aren't a traditional food in Europe, but Leone thinks that could change — particularly in food-obsessed Italy.

"People are curious," Leone said in Brussels, where GoJelly researchers met for a review of the project in August. "In only 10 years, sushi spread across Italy. I think a similar spread of jellyfish is possible."

As for what jellyfish tastes like? "When uncooked, the taste is similar to oysters," she said. Surprisingly, she described the texture of cooked jellyfish as "crunchy."

Before serving jellyfish to the European public, however, Leone and her colleagues need to demonstrate that it's safe to eat. Venom is obviously a concern — though at least in barrel jellyfish, Leone has found that heat renders the toxin harmless.

Leone is also excited about the potential health benefits of jellyfish, and is looking into potential anti-inflammatory effects. Plus, she added, jellyfish might be 95 percent water but the rest is "very good quality protein," mostly collagen.

GoJelly researchers think that protein could be useful for food supplements, sustainable fish feed and even cosmetics. Other potential uses include fertilizer. They also hope that any products they develop could create a demand for European jellyfish, and therefore a new source of income for fishermen.

Not all the products might become reality, said Ana Rotter, who leads the Piran GoJelly team. But many are making good progress, she added, and if any of them become a success, it "would be a dream come true" — especially if they utilized invasive species like the little comb jellies polluting the waters around Piran.

"Let's say they're useful for fertilizer," Rotter said. "Can you imagine — we have this invasive species, we collect it and protect the environment from getting out of equilibrium and then make something useful out of it. It would be win-win-win."

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