Sponges grow in large numbers and impressive size on the tops of extinct underwater volcanoes.
Huge gardens of sponges thrive atop seamounts in the central Arctic Ocean, one of the most oligotrophic seas on earth. They seem to feed on the remains of an extinct fauna. Microorganisms help sponges exploit this fluffy material as a source of food and energy. Scientists from Bremen, Bremerhaven and Kiel and their international partners discovered this unique hotspot of life during a POLARSTERN expedition and are now reporting their findings in the journal Nature Communication. They highlight the need for a better understanding of the diversity and uniqueness of Arctic ecosystems in light of global and local changes.
Little food reaches the depths under the permanently ice-covered Arctic Ocean because light limits the productivity of algae. However, scientists from Bremen, Bremerhaven and Kiel have now discovered a surprisingly rich and densely populated ecosystem on the tops of extinct underwater volcanoes, reporting their findings in the journal Nature Communication. These hotspots of life were dominated by sponges, which grew there in great numbers and impressive size.
“Thriving atop the extinct volcanic seamounts of Langseth Ridge, we found huge gardens of sponges, but we had no idea what they were feeding on,” reports Antje Boetius, chief scientist of the expedition, head of the seabed ecology and technology research group. at the Max Planck Institute for Marine Microbiology and Director of the Alfred Wegener Institute, Helmholtz Center for Polar and Marine Research. Using samples from the mission, first author Teresa Morganti, a sponge expert at the Max Planck Institute for Marine Microbiology in Bremen, was able to identify how sponges adapt to the most nutrient-poor environment. . Morganti explains: “Our analysis revealed that sponges have microbial symbionts capable of utilizing ancient organic matter. This allows them to feed on the remains of now-extinct former inhabitants of seamounts, such as worm tubes composed of protein and chitin and other trapped detritus.
living off leftovers
Sponges are considered one of the most basic forms of animal life. They are nonetheless thriving and abundant in all oceans, from shallow tropical reefs to deep Arctic seabeds. Many sponges harbor a complex community of microorganisms in a symbiotic relationship, which contribute to sponge health and nutrition by producing antibiotics, transferring nutrients, and eliminating excreta. This also applies for Geodia-sponges, which dominated the community on arctic seamounts. The unit of sponge and associated microbes is called a sponge holobiont. Teresa Morganti cooperated with Anna de Kluijver, an expert from the University of Utrecht, and with the laboratory of Gesine Mollenhauer at the Alfred Wegener Institute to identify the food source, growth and age of the sponges. They learned that thousands of years ago, substances seeping from the interior of the seabed supported a rich ecosystem, home to a variety of animals. When they died, their remains remained. Now these form the basis of this unexpected sponge garden.
Microbial analysis of the microorganisms supported the researchers’ hypothesis. “The microbes have exactly the right toolkit for this habitat,” says Ute Hentschel of the GEOMAR Helmholtz Center for Ocean Research in Kiel, who carried out the microbiological analyzes with her team. “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.”
Scientists have also shown that sponges act as ecosystem engineers: they produce spicules that form a carpet on which they crawl. This can further facilitate the local sedimentation of particles and biogenic materials. Sponge holobionts can tap into this detrital material, creating their own food trap.
Protecting requires understanding
Langseth Ridge is an underwater mountain range not far from the North Pole that lies below the surface of water that is permanently covered in ice. There, sponge biomass was comparable to that of shallower sponge soils with a much higher nutrient supply. “It’s a unique ecosystem. We have never seen anything like this before in the central High Arctic. In the study area, the primary productivity in the overlying water provides less than one percent of the carbon demand of the sponges. So this sponge garden may be a transient ecosystem, but it is rich in species, including soft corals,” explains Antje Boetius.
The Arctic is one of the regions most affected by climate change. “Prior to our study, no similar spongy soil had been identified in the central High Arctic, an area of ice-covered ocean that remains understudied given the difficulties associated with observing and sampling such ice-covered deep-sea ecosystems,” Morganti points out. . The close collaboration of scientists from different institutions, including the Max Planck Institute for Marine Microbiology, the Alfred Wegener Institute and GEOMAR, has resulted in a comprehensive understanding of this surprising hotspot of life in the cold depths. “With sea ice cover rapidly diminishing and the ocean environment changing, better knowledge of hotspot ecosystems is essential to protect and manage the unique diversity of these pressurized Arctic seas,” concludes Boetius.
Reference: “Giant sponge bottoms of central Arctic seamounts are associated with extinct seep life” by TM Morganti, BM Slaby, A. de Kluijver, K. Busch, U. Hentschel, JJ Middelburg , H. Grotheer, G. Mollenhauer, J. Dannheim, HT Rapp, A. Purser and A. Boetius, February 8, 2022, Nature Communication.
- Max Planck Institute for Marine Microbiology, Celsiusstr. 1, 28359 Bremen, Germany
- Alfred Wegener Institute Helmholtz Center for Polar and Marine Research, Am Handelshafen 12, 27570 Bremerhaven, Germany
- GEOMAR Helmholtz Center for Ocean Research Kiel, Duesternbrooker Weg 20, 24105 Kiel, Germany
- University of Utrecht, Department of Earth Sciences, Princetonlaan 8a, 3584 CB Utrecht, The Netherlands
- Christian-Albrechts-University Kiel, Christian-Albrechts-Platz 4, 24118 Kiel, Germany
- MARUM and Department of Geosciences, University of Bremen, 28359 Bremen, Germany
- Helmholtz Institute for Functional Marine Biodiversity, Ammerländer Heerstraße 231, 26129 Oldenburg, Germany
- University of Bergen, Department of Biological Sciences and KG Jebsen Center for Deep-Sea Research, PO Box 7803, 5020 Bergen, Norway