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A colony of roughly 10 sea sponges, ranging in size, are seen here growing along the undersea Langseth Ridge in the Arctic Ocean. (courtesy of the Alfred Wegener Institute for Polar and Marine Research)

The deep-sea regions of the Arctic are one of the most inhospitable environments for life.

In addition to the frigid temperatures, the area is covered in thick ice throughout the year, which blocks nearly all the sunlight needed for photosynthesis. The distance from land also means that there are no nutrients filtering in.

Yet within this environment, a unique ecosystem has been discovered — one dominated by giant sponges.

With an average age of 300 years and diameters of up to 1 meter, the sponges were found, surprisingly enough, to be feeding on fossil remnants from tubeworms that survived on volcanoes active thousands of years ago.

The Alfred Wegener Institute for Polar and Marine Research’s icebreaker, the Polarstern, can be seen here breaking through icy waters. (courtesy of the Alfred Wegener Institute for Polar and Marine Research)

An international research team on the Polarstern, an icebreaker affiliated with Germany’s Alfred Wegener Institute for Polar and Marine Research, conducted a survey on the Langseth Ridge, a deep-sea mountain range in the Arctic Ocean to the northwest of Greenland. In the latest issue of the scientific journal Nature Communications, they report discovering a large sponge habitat spanning a 15-square kilometer area around seamounts.

In a press release from the institute, the article’s corresponding author Antje Boetius said, “Thriving on top of extinct volcanic seamounts of the Langseth Ridge we found massive sponge gardens, but did not know what they were feeding on.”

Previously, the seamount was home to a diverse range of life, including tubeworms and shellfish, thanks to the warm, sulfur-rich water emitted from hydrothermal vents. But 2,000 years ago, that activity stopped.

The sponges were found to grow as big as 1 meter in diameter and 25 kilograms in weight and functioned as the dominant species in the seamount ecosystem. The sponge’s main habitat was the peak of the seamounts, at depths of 500-700 meters.

The sea sponges act as “ecosystem engineers” in the Arctic Ocean, which severely lacks nutrients for life. (courtesy of the Alfred Wegener Institute for Polar and Marine Research)

An analysis of the collected sponges showed that a symbiotic relationship with microorganisms is what enabled them to thrive in such an extreme environment.

“The microbes have the genes to digest refractory particulate and dissolved organic matter and use it as a carbon and nitrogen source,” said Ute Hentschel, a microbiologist from the GEOMAR Helmholtz Centre for Ocean Research who was on the research team. Hentschel explained that those are the energy sources the sponges need to survive.

The sponges and microbes feed on the fossilized tubeworms around hydrothermal vents that were blocked up thousands of years ago.

“Our analysis revealed that the sponges have microbial symbionts that are able to use old organic matter [. . .] to feed on [. . .] the tubes of worms composed of protein and chitin and other trapped detritus,” said first author Teresa Morganti, a sponge expert at the Max Planck Institute for Marine Microbiology in Bremen, Germany.

This is the first time that scientists have discovered animals that can eat fossils. Another notable fact is that this dietary habit was made possible through symbiosis with microorganisms.

Tubeworms are seen here growing in the vicinity of hydrothermal vents. The sponges and microbes feed on the fossilized tubeworms around hydrothermal vents that were blocked up thousands of years ago. (courtesy of the US’ National Oceanic and Atmospheric Administration)

The researchers said the sponges function as “ecosystem engineers” on the seamount. The sponges weave silica-based spicules into a mat that provides a living space for other organisms and also catches particles that they can eat.

The research explained that the sponges help form a hot spot that is “rich in species, including soft corals,” despite the extremely nutrient-poor environment.

But how long can this sponge garden be maintained in the Arctic Ocean? The researchers predicted that the sponges could continue absorbing the tubeworm detritus for at least several thousand more years because the cold temperature of the water slows their metabolism.

A more immediate threat is climate change.

“With sea-ice cover rapidly declining and the ocean environment changing, [research] is essential for protecting and managing the unique diversity of these Arctic seas,” Boetius said.

Starfish gather to feed on the decomposing sea sponges. (courtesy of the Alfred Wegener Institute for Polar and Marine Research)

This finding shows how much we still don’t know about our planet’s life-forms, and in particular ecosystems under the ice.

“There is so much alien-like life and especially in the ice-covered seas where we barely have the technology to access, to look around and to make a map,” she said in an interview with the BBC.

One of the first multicellular organisms to evolve, there are more than 8,000 species of sponges distributed across the world’s oceans. They have a remarkable knack for adaptation that was fine-tuned by numerous brushes with extinction. They are thus equipped to endure low-oxygen environments, as well as ocean acidification and warming.

The article “Giant sponge grounds of Central Arctic seamounts are associated with extinct seep life” was published in the journal Nature Communications; DOI: 10.1038/s41467-022-28129-7.

By Cho Hong-sup, environment correspondent

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