And the sponges are forced to follow a somewhat sinister diet in this unhospitable environment.

At first sight, the bottom of the Arctic Ocean is therefore an unattractive place for life. Large parts of the ocean are permanently covered with ice and in such dark waters – due to the absence of light-loving algae – very little food is available. The surprise was therefore great when researchers on the peaks of extinct submarine volcanoes in this Arctic Ocean stumbled upon a surprisingly rich and densely populated ecosystem dominated mainly by sponges. These sponges are not only present in large numbers, but they also manage to reach an impressive size at the bottom of the cold, dark ocean.

Symbiotes

It naturally raises the question of how the sponges manage to maintain themselves in this hospitable environment. Shortly after finding the sponges, the scientists delved into this extensively, says researcher Antje Boetius. “At the top of extinct volcanic mountain ranges, we’ve found huge, thriving sponge gardens that we didn’t know how to get food from before. Our analysis now reveals that these sponges possess microbial symbionts that can utilize ancient organic matter. This enables them to feed on the remains of former, now extinct, inhabitants of the volcanoes, such as the tubes of worms – made up of proteins and chitin – and other waste.”

Food

The sponges therefore work closely with bacteria. We also know such a symbiosis from sponges in other – more hospitable – environments: the bacteria provide the sponges with food and the sponges provide the bacteria with a safe place to stay. There is a similar collaboration on the extinct volcanoes in the Arctic Ocean. It is easy to explain that this cooperation arose precisely here. Because research shows that thousands of years ago these volcanoes harbored a very different, but also rich ecosystem. That ecosystem—which existed by the grace of substances leaking from the ocean floor—disappeared, but its remnants remained. And the sponges – with the help of their microbial collaboration partners – are now taking advantage of that.

In the trap

Incidentally, these microbial collaboration partners appear to be perfect for their role; research shows that they have the genes necessary to extract the necessary substances from the remnants of the decaying ecosystem. But the sponges are not passive profiteers, the researchers emphasize. They seem to contribute by producing skeletal needles. These skeletal needles form a kind of mat on the bottom over which the sponges can move and under and between organic residues, as it were, become trapped. In this way, the sponges create a kind of trap and ensure that there are food sources in the vicinity from which they can draw – in close cooperation with the bacteria involved.

The sponges come in different sizes: some are only an inch in size. Others measure half a meter! Image: Alfred-Wegener-Institut / PS101 AWI OFOS system.

Unique

The result of all this is a well-functioning and simply thriving ecosystem. This was proven when researchers compared this ecosystem with ecosystems at a shallower depth and with more food available to them and concluded that the biomass on the extinct volcanoes in the Arctic Ocean could compete with the biomass in those much more hospitable and nutrient-rich environments. “This is a unique ecosystem,” says Boetius. “We’ve never seen anything like it in the upper Central Arctic.”

It is easy to explain why the ‘sponge gardens’ have only just been discovered. It is not so easy to study and sample the seabed, which is not only hidden under water, but also under sea ice. The discovery now made on the extinct volcanoes suggests that many more interesting ecosystems are waiting to be discovered. Enough reason to keep looking for it, say the researchers. But time is running out. Because the sea ice in the Arctic Ocean is melting rapidly and with it the underlying waters and the ecosystems that host them are also changing.