The deep-sea floor under the ice of the Arctic Ocean is extremely poor in nutrients – nevertheless, researchers have discovered astonishingly lush sponge gardens there on the Untersee mountains. As their investigations show, the animals there live on the remains of an ecosystem that has now disappeared: the mountains are extinct underwater volcanoes that, when they were active, allowed tube worms, among other things, to live. The scientists report that the sponges can apparently exploit their millennia-old remains for themselves with the help of microbial symbionts.
What lives at the bottom of the central Arctic Ocean, which is covered with ice all year round? A research team headed by Antje Boetius from the Max Planck Institute for Marine Microbiology in Bremen addressed this question as part of an expedition on the famous research icebreaker Polarstern. Among other things, a towed camera system was used, which provided images from the depths. As the scientists report, they encountered an astonishingly lush habitat in the area of the approximately 1000 meter deep peaks of underwater mountains of the Langseth Ridge. It is dominated by numerous sponges, some of which are over half a meter in diameter.
What are they living on?
As the researchers explain, this is surprising, because hardly any food can trickle down to the bottom from the higher water layers, since the constant sea ice cover in the region means that there is little light for the growth of algae. Nevertheless, the biomass in the sponge gardens is comparable to that in habitats where these simple animal creatures are supplied by a much higher nutrient input. “What the algae produce in the upper water layers in the examined area covers less than one percent of the estimated carbon requirements of the sponges,” explains Boetius. “This is a unique ecosystem. We’ve never seen anything like it in the high Central Arctic.” That’s why the team decided to research the sponges and their habitat more closely. For their study, they evaluated recordings and examined tissue samples from some specimens using different analysis methods.
As the researchers report, these are mainly sponges from the Geodia genus. Traces of their growth rates show that they have an average age of 300 years. Laboratory analysis of the tissue samples showed that they harbor a community of specific bacteria typical of sponges. These microbes live in symbiosis with you – they provide them with certain nutrients and the sponges offer them a safe home in return. The more detailed investigations of the microbes of the Geodia sponges from the Langseth Ridge finally provided indications that they supply the sponges there with nutrients from a special source: they tap into the remains of a biotic community that has now disappeared.
A big leftovers feast
As the researchers explain, it is known from previous studies that the seamounts are extinct volcanoes. In active times, undersea volcanism can, as is well known, provide organisms with a basis for life through certain outgassing. The team also found corresponding remains of such communities, which appeared to be heavily influenced by tubeworms. Radiocarbon dating of these traces indicates that these ancient ecosystems may have existed as late as 2,000 to 3,000 years ago at the then active underwater vents.
“Our results now show that the sponges have microbial symbionts that can utilize old organic material. This allows them to feed on the remains of the former, now extinct inhabitants of the seamounts – for example the burrows of worms, which are made of protein and chitin, and other organic remains stuck there,” says lead author Teresa Morganti. Co-author Ute Hentschel from the GEOMAR Helmholtz Center for Ocean Research Kiel continues: “These microbes have exactly the right toolbox for this habitat. They have genes to break down resilient particulate and dissolved organic matter and use them as a source of carbon and nitrogen, in addition to a range of chemical energy sources available there,” says the scientist.
The research team was also able to show that the sponges act as designers of the bizarre ecosystem: they produce needles that form a mat on which the sponges also slowly crawl. The scientists explain that this rough surface also facilitates the deposition of particles and biogenic materials from the water. Other organisms in the gardens can obviously also benefit from this: “The habitat is therefore rich in other species and even houses soft corals,” says Boetius.
Finally, with a view to the clear consequences of climate change in the far north, the researcher emphasizes: “In view of the rapid retreat of sea ice cover and the changing marine environment, it is essential to better understand such hotspot ecosystems in order to understand the unique diversity of the Arctic seas under pressure protect and manage,” says Boetius.
Source: Max Planck Institute for Marine Microbiology, Article: Nature Communications, doi: 10.1038/s41467-022-28129-7