Last year, scientists announced the discovery of giant sponges deep below the permanently ice-covered Arctic Ocean – growing on the peaks of extinct underwater volcanoes. Now, they’ve found out that these Arctic sponges (of the order Geodia) survive the nutrient-poor conditions by feeding on the fossilised remains of extinct animals.
“This is a unique ecosystem,” says Antje Boetius, a marine biologist at the Max Planck Institute for Marine Microbiology and the Alfred Wegener Institute, in Germany. “We have never seen anything like it before in the high Central Arctic.”
In 2016, Boetius was the chief scientist aboard the icebreaker research vessel Polarstern (which, incidentally, also recently discovered millions of icefish nests in Antarctica).
On the Arctic voyage, they towed camera systems to examine the seafloor at Langseth Ridge, an underwater mountain range not far from the North Pole, ranging in depth between 700 and 1000 metres. Their images revealed astonishingly rich and densely populated underwater ecosystems atop extinct volcanoes.
This was an intriguing discovery. Scientists previously knew that sponges are abundant and successful in oceans all around the world, from bone-chilling Antarctic waters to shallow tropical reefs. They’re usually filter feeders, able to gobble up nutritious particles from the surrounding seawater.
But life in the Arctic Ocean is difficult. These Geodia sponges live in part of the ocean that has been covered in ice year-found for decades – which means that not much food drifts down from the surface to the depths.
So how, the researchers wondered, do the sponges survive?
Now, as reported in a paper in Nature Communications, they have an answer.
Teresa Morganti, a sponge expert from the Max Planck Institute for Marine Microbiology and a co-author of the paper, analysed images and sponge tissue samples to figure out how old the sponges are and how they eat.
Turns out, the sponges are on average 300 years old and are home to rich bacterial communities that help them feed.
“Our analysis revealed that the sponges have microbial symbionts that are able to use old organic matter,” Morganti explains. “This allows them to feed on the remnants of former, now extinct inhabitants of the seamounts, such as the tubes of worms composed of protein and chitin and other trapped detritus.”
Many sponge species are host to a community of microorganisms, able to transfer nutrients to the sponges from the surrounding water, as well as dispose of excretions and produce antibiotics to protect the sponges’ health.
Morganti and colleagues discovered that these Arctic sponges don’t get their food from the seawater, but rather their resident microorganisms can take in organic matter trapped in the “mat” on which they live.
This mat 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 kinds of animals. Much of that has since died out, leaving these unexpected sponge gardens.
According to microbial analysis, the microorganisms on the sponges can make good use of the fossilised remnants of the extinct ecosystem, using enzymes to break down the remains and pass on nutrients to their sponge host.
“The microbes have just the right toolbox for this habitat,” says Ute Hentschel from the GEOMAR Helmholtz Centre for Ocean Research in Kiel, Germany, who carried out the microbiological analyses.
“The microbes have the genes to digest refractory particulate and dissolved organic matter, and use it as a carbon and nitrogen source, as well as a number of chemical energy sources available there.”
As the climate changes and Arctic sea ice rapidly declines, the researchers say that this kind of research is vital.
“A better knowledge of hotspot ecosystems is essential for protecting and managing the unique diversity of these Arctic seas under pressure,” concludes Boetius.