Giant Arctic sponges feed on extinct ecosystems

Last year, scientists announced the discovery of giant sponges deep beneath the permanently ice-covered Arctic Ocean – growing on the tops of extinct underwater volcanoes. Now they have discovered that these arctic sponges (of the order Geodia) survive nutrient-poor conditions by feeding on the fossilized remains of extinct animals.

“It’s a unique ecosystem,” says Antje Boetius, a marine biologist at the Max Planck Institute for Marine Microbiology and the Alfred Wegener Institute, Germany. “We’ve never seen anything like this before in the central High Arctic.”

In 2016, Boetius was the chief scientist aboard the icebreaker research vessel Polarstern (which, by the way, also recently discovered millions of icefish nests in Antarctica).

On the Arctic voyage, they towed camera systems to examine the seabed at Langseth Ridge, an underwater mountain range not far from the North Pole, which ranges in depth from 700 to 1,000 meters. Their images revealed surprisingly rich and densely populated underwater ecosystems atop extinct volcanoes.

Credit: Alfred-Wegener-Institut / System PS101 AWI OFOS / Antje Boetius

It was an intriguing discovery. Scientists previously knew that sponges are abundant and thriving in oceans around the world, from the freezing waters of Antarctica to shallow tropical reefs. They are generally filter feeders, capable of engulfing nutrient particles from the surrounding seawater.

But life in the Arctic Ocean is tough. Those Geodia sponges live in a part of the ocean that has been covered in ice for decades – meaning little food drifts from the surface to the depths.

So, the researchers ask, how do sponges survive?

Now, as reported in an article by Nature Communicationthey have an answer.

Teresa Morganti, a sponge expert from the Max Planck Institute for Marine Microbiology and co-author of the paper, analyzed images and samples of sponge tissue to determine how old the sponges were and how they were eaten.

It turns out that sponges are on average 300 years old and harbor rich bacterial communities that help them feed.

“Our analysis revealed that sponges have microbial symbionts capable of utilizing old organic matter,” says Morganti. “This allows them to feed on the remains of now-extinct former inhabitants of seamounts, such as worm tubes made up of protein and chitin and other trapped detritus.”

Many species of sponges harbor a community of microorganisms, capable of transferring nutrients to sponges from the surrounding water, as well as removing excretions and producing antibiotics to protect sponge health.

Morganti and his colleagues found that these arctic sponges do not get their food from seawater, but their resident microorganisms can absorb organic matter trapped in the “carpet” on which they live.

This carpet is a remnant of an ancient ecosystem. Thousands of years ago, gases from volcanic systems below the seabed supported a much richer ecosystem, home to many species of animals. Much of that has since disappeared, leaving those unexpected sponge gardens.

According to microbial analysis, microorganisms on sponges can make good use of the fossilized remains of the extinct ecosystem, using enzymes to break down the remains and pass nutrients to their sponge host.

Top down images of a sponge on the bottom of the sea
On average, sponges are 300 years old, many are even older. They host a complex community of microorganisms in a symbiotic relationship, which contributes to the health and nutrition of sponges. Credit: Alfred-Wegener-Institut / System PS101 AWI OFOS / Antje Boetius

“The microbes have exactly the right toolkit for this habitat,” says Ute Hentschel of the GEOMAR Helmholtz Center for Ocean Research in Kiel, Germany, who carried out the microbiological analyses.

“Microbes have the genes to digest refractory particles and dissolved organics, and use them as a source of carbon and nitrogen, as well as a number of chemical energy sources available there.”

As the climate changes and Arctic sea ice shrinks rapidly, researchers say this type of research is vital.

“Better knowledge of hotspot ecosystems is essential to protect and manage the unique diversity of these pressurized Arctic seas,” concludes Boetius.