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Thursday, June 20, 2024

Huge Sponges Are Eating an Extinct Arctic Ecosystem

Stretching for nearly 80 miles across the seafloor of the central Arctic, the Langseth Ridge is craggy, barren, and generally inhospitable. And it should be: Unlike more productive oceans, few nutrients swirl here, thanks to the ice above blocking the light. Thousands of years ago, however, the peaks of the ridge hummed with volcanic activity, which produced sulfur that fed tube worms—the ones you may have seen from videos of hydrothermal vents elsewhere in the world. Or more accurately, the sulfur fed the symbiotic bacteria inside the worms, which processed it into energy, keeping the animals alive.

That volcanic activity at Langseth Ridge died long ago, yet life has remained. Today in the journal Nature Communications, scientists describe how a previously unknown kind of ecosystem has been thriving under the ice, along the towering ridge around 2,000 feet deep. “No one knows what is living on these giant mounds,” says Antje Boetius, director of the Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research and coauthor of the paper. “And when I say giant mounds, just imagine we would have on Earth an undiscovered mountain that is 3.8 kilometers—really huge—and no one has walked there. No one has taken a photograph, no one knows what types of plants and animals are living there.”

With the help of a remotely operated vehicle dangling from an icebreaker, Boetius and her colleagues discovered that the ridge is now dominated not by worms, but by enormous sponges, each up to 3 feet wide. They are, on average, 300 years old, but some are much older. Strangely enough, the sponges have evolved a similar microbe-based survival strategy—only they eat the tubes left behind by the worms, which have been dead for 2,000 years. Thus an extinct, fossilized hydrothermal ecosystem fuels an even more bizarre assemblage of life. 

“It's like a forest,” says Teresa Maria Morganti, an ecologist and expert on sponges at the Max Planck Institute for Marine Microbiology, lead author of the new paper. “It's really a hot spot of life in the middle of the desert. It's really fascinating how they could exploit this ancient previous community.”

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The foundation of the ridge ecosystem is a dense mat made of spicules, tiny structures of silica that sponges use to construct their bodies. This mat creates a complex three-dimensional matrix for other animals like shrimp, but it also betrays what the sponges have been up to in the darkness: They’re moving around looking for food, leaving trails in the mat. 

That food is the ancient worms’ fossilized tubes, which are made of protein and chitin (the same stuff crustaceans use to make their shells). The researchers know the sponges are feeding on it because they used the ROV to gather specimens and found the same chemical signatures in both the animals and the worm tubes. Looking inside the sponges, they also discovered symbiotic bacteria that would help the animals process such tough material. “It's organic matter that no one can digest, and these were heavily enriched in the sponge biomass,” says Boetius. “We are probably looking at a recycling system where one community lives using biomass of an ancient community that is no longer alive.”

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And that’s not all they’re eating: The same bacterial community may be helping the sponges pull food from the water itself. “We have found that the symbionts inside the sponges can use the solid organic matter that is in the water column, and at the same time they can use dissolved inorganic carbon in the water column as well,” says Morganti. “So it's really a joint effort between sponges and their symbionts.” This is more in line with how sponges in nutrient-rich seas eat, by sucking in water and filtering out food. But because there’s so little to eat in the water column in the middle of the Arctic, these giant sponges evolved the extra microbial trick needed to eat the fossilized worm tubes.

Life on the ridge moves slowly, which gives the sponges an extra advantage. “Sponge metabolism is very low in general, and here we are talking about very big individuals, so that means that they have even lower metabolism,” says Morganti. And, she points out, water as cold as the Arctic’s slows metabolism further. As a result, the sponges need less food, so the supply of leftover tubes might last them thousands more years. 

“To me this discovery shows that sponges, one of the earliest forms of multicellular life to evolve, are capable of extreme adaptation and that nothing is wasted in the deep sea,” says biologist Huw Griffiths of the British Antarctic Survey, who wasn’t involved in the research. (Almost exactly a year ago, Griffiths reported finding strange creatures under a half mile of ice in Antarctica.) “These sponges and their associated bacterial buddies are recycling nutrients laid down by hydrothermal vent worms thousands of years before the sponges arrived, when the vents were still active.”

Another biological trick helps the sponges thrive where there shouldn’t be life. These animals reproduce by “budding,” or generating little copies of themselves. “With budding, they produce exactly the same individual—so the same genetics and also the same symbionts, which are fundamental for these sponges to survive in such an environment,” says Morganti. That is, the new sponges arrive with the genes needed to survive this harsh existence and the microbes they require to extract energy from the extinct ecosystem. 

Because the sponges provide surfaces for other life to attach to—corals and, in a strange twist of fate, even tube-dwelling worms—they are critical engineers of their whole ecosystem. But for all their toughness, this community is facing massive change, as the Arctic is warming much faster than the rest of the planet. “It is a telling sign that we can only make these kinds of discoveries because the sea ice in the region is reducing,” says Griffiths. “Sea mounts are often the target of the fishing industry, and this amazing habitat is now under a growing threat as the ice melts and it becomes more accessible to ships.” 

With less ice blocking the sun, these waters might also become more biologically productive, meaning there could be more food in the water column and less evolutionary need for a fossil-eating sponge. The race is on, then, to discover what other strangeness might lurk in these frigid waters before they change into something unrecognizable. “Because of their logistical challenges,” Griffiths says, the polar regions “probably have many more surprises waiting in the depths.”


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