Our oceans are still considered less well explored than the surface of the moon. But marine researchers are constantly working to close the gaps in our knowledge about the cradle of all life. A large international team of scientists from 13 institutions, including the University of Geneva (Switzerland), the Norwegian Research Center – NORCE (Norway), IFREMER (France), the Center for Marine Environmental Sciences – MARUM in Bremen, and the Senckenberg Research Institute and Natural History Museum Frankfurt (both Germany), conducted a comprehensive study to map seafloor biodiversity in all major ocean regions and link it to existing data from the water column. The result is a unified overview that for the first time maps the entire biodiversity of multicellular organisms in the world’s oceans from the surface to the deep sea. In line with this, scientists from the Alfred Wegener Institute for Polar and Marine Research yesterday published their fascinating discovery in the Arctic deep sea: giant sponge gardens grow on extinct underwater volcanoes not far from the North Pole!
The large-scale study to determine ocean biodiversity, published in the journal Science Advances, examined sediments from all major ocean regions from the Arctic Ocean to the Mediterranean Sea and the Atlantic and Pacific oceans to the Southern Ocean as part of 15 international deep-sea expeditions. Combining the results with existing data from both the light and completely dark water layers provided, for the first time, a complete picture of the marine biodiversity of multicellular organisms.
The species that live in deep-sea sediments and their ecology are still largely unknown. Here, seafloor organisms – from animals to microbes – play a crucial role in the ocean’s nutrient cycle. They help bind or metabolise sinking organic and inorganic matter, much of which comes from microscopic plankton in upper water layers, and return it to the cycle. Thus, benthic (seafloor-dwelling) organisms are the basis for the functioning of marine food webs and the sequestration of carbon, both of which have a crucial influence on global climate.
“It is important to understand this and then also to be able to take appropriate protective measures. Because the deep-sea ecosystem is under enormous pressure caused by humans: climate change, deep-sea mining, oil and gas exploration, trawling and pollution threaten life in the depths of the sea,” says Prof. Dr. Angelika Brandt, of the Senckenberg Research Institute and Natural History Museum Frankfurt, and a co-author of the study.
All samples collected by the scientists during the various expeditions were genetically analyzed. Sequencing the DNA of the organisms the researchers found in the deep-sea sediment samples allowed them to differentiate between benthic organisms and sinking plankton that had reached the seafloor from the overlying water column. According to Brandt, the data revealed that only about one-third of the organisms found are already known.
“With nearly 1,700 samples and two billion DNA sequences from the surface to the deep-ocean floor worldwide, high-throughput environmental genomics vastly expands our capacity to study and understand deep-sea biodiversity, its connection to the water masses above and to the global carbon cycle,” said Tristan Cordier, a researcher at NORCE and Bjerknes Centre for Climate Research and the lead author of the study.
The study also found that polar regions act as hotspots of carbon sequestration, confirmed by the abundance and composition of plankton DNA in deep-sea sediments. For the first time, scientists were also able to determine which organisms in the plankton community contribute most to the sequestration of atmospheric carbon and thus help regulate Earth’s climate.
Arctic underwater volcanoes are hotspot for sponges
No less exciting are the discoveries made by researchers at the Alfred Wegener Institute for Polar and Marine Research in the Arctic deep sea, which have not yet been included in the biodiversity study. They came across giant sponge gardens on Langseth Ridge, just a few hundred kilometers from the North Pole, spread across the tops of extinct underwater volcanoes.
Initially, it was a mystery to the AWI scientists what the sponges feed on in one of the most nutrient-poor ocean regions permanently covered by sea ice. Analysis by sponge expert Teresa Morganti, a researcher at the Max Planck Institute for Marine Microbiology in Bremen, Germany, and lead author of the study, found that the sponges have microbial symbionts that can recycle old organic material. Morganti explains that this allows them to feed on the remains of former, now extinct, inhabitants of the seamounts – for example, the tubes of worms composed of protein and chitin and other trapped detritus.
Together with Anna de Kluijver, of Utrecht University, and Gesine Mollenhauer’s lab at the AWI, Morganti found that thousands of years ago, substances seeping from inside the seafloor supported a lush ecosystem with numerous animals. The remains left behind after their extinction now form the basis for these rich sponge gardens.
“Prior to our study, no similar sponge ground has been identified in the high Central Arctic, an area of the ice-covered ocean which remains understudied given the difficulties associated with observing and sampling such ice-covered deep-sea ecosystems,” Morganti emphasises.
The biomass of sponges on Langseth Ridge is comparable to that in nutrient-rich, shallower sponge beds. “This is a unique ecosystem. We have never seen anything like it before in the high Central Arctic. In the study area, primary productivity in the overlying water provides less than one percent of the sponges’ carbon demand. Thus, this sponge garden may be a transient ecosystem, but it is rich in species, including soft corals,” says Antje Boetius, the head of the Deep-Sea Ecology and Technology Research Group at the Max Planck Institute for Marine Microbiology, and the director of the AWI.
“With sea-ice cover rapidly declining and the ocean environment changing, a better knowledge of hotspot ecosystems is essential for protecting and managing the unique diversity of these Arctic seas under pressure,” Boetius concludes.
Julia Hager, PolarJournal
Link to the study Biological diversity in the world’s oceans: Tristan Cordier, Inès Barrenechea Angeles, Nicolas Henry, Franck Lejzerowicz, Cédric Berney, Raphaël Morard, Angelika Brandt, Marie-Anne Cambon-Bonavita, Lionel Guidi, Fabien Lombard, Pedro Martinez Arbizu, Ramon Massana, Covadonga Orejas, Julie Poulain, Craig R. Smith, Patrick Wincker, Sophie Arnaud-Haond, Andrew J. Gooday, Colomban de Vargas, Jan Pawlowski. Patterns of eukaryotic diversity from the surface to the deep-ocean sediment. Science Advances, 2022; 8 (5) DOI: 10.1126/sciadv.abj9309.
Link to the study Sponge gardens in the Arctic deep sea: Morganti, T.M., Slaby, B.M., de Kluijver, A. et al. Giant sponge grounds of Central Arctic seamounts are associated with extinct seep life. Nat Commun 13, 638 (2022). https://doi.org/10.1038/s41467-022-28129-7
