Researchers have discovered the secret to survival of "massive" sponge gardens thriving on top of a ridge of extinct volcanoes beneath the ocean, 350 kilometres from the North Pole.
It's a region permanently covered in sea ice in one of the most oligotrophic seas on the planet. An oligotrophic sea is one generally devoid of plant life and very high in dissolved oxygen.
Though the scientists found the sponges a few years ago, they weren't able to figure out how they were living in such a hostile environment.
But in a paper published today in Nature Communications, they reveal that the sponges survive by getting nutrients from dead sea life that once inhabited the seamounts, according to lead author Teresa Morganti from the Max Planck Institute for Marine Microbiology.
"The extinct community was mostly composed of tubeworms and other polychaetes [bristle worms]," Dr Morganti said.
So how was there a community of animals living there before, and what happened to them?
The researchers concluded that there was an underwater seep of gases like sulfide and methane from the volcanoes, until around 2,000 to 3,000 years ago.
"The previous community … relied on the chemical compounds from the seepage as energy and food sources," Dr Morganti said.
But the sponges inhabiting the area today have a microbial symbiont — a micro-organism that can extract carbon and nitrogen from the bodies of the animals that once lived there.
In doing so, these microbial symbionts provide nutrients to the sponges, and the sponges in turn provide structure, and also further nutrients to their symbionts as they filter the passing water.
"Both benefit from each other," Dr Morganti said.
The find is the most northerly sponge garden ever discovered, the scientists said.
It's also the "densest sponge community found in the Boreal-Arctic regions", according to Dr Morganti.
The seamounts consist of three extinct volcanic peaks, known collectively as the Langseth Ridge.
The ridge is about 125 kilometres long with the largest peak rising 2,500 metres off the seafloor to about 570 metres below the ocean surface.
The first sponge from the area was "casually" collected back in 2011, and a proper expedition of the seamount was launched in 2016.
That expedition discovered sponge gardens covering an area more than 35 kilometres long and nearly 10 kilometres across at their widest point.
Solving the food supply mystery
But scientists then had to answer the question of how an area that should be virtually devoid of life was supporting such a rich diversity.
"Our study here was focused on solving the mystery around the food source," Dr Morganti said.
"It took quite a while because we tested so many different hypotheses and used very novel methods in dating and identifying the food web structure, and the symbionts and their functions."
Along with the sponges, the researchers identified shrimp, starfish, and soft corals. They estimated around 60 species are inhabiting the seamounts in all.
But the physical conditions that have led to the formation of this unique environment are going to change quite dramatically.
The IPCC predicts that at 2 degrees Celsius of warming, the Arctic will experience ice-free summers around once every 10 years.
More sunlight penetrating the ocean could lead to an increase in the amount of food reaching the sponge gardens, Dr Morganti said.
"Such change might not harm these sponges themselves but rather favour the settlement of other species," she said.
The researchers are calling for increased marine protection zones for the Arctic to be put in place before receding sea ice opens it up for further exploitation.
"But when it recedes, it's important to know which biodiversity hotspots are to be protected, especially from [bottom] trawling that is destructive for such [immobile] forms of life," Dr Morganti said.
