Scientists have been mystified by how the gardens of massive sponges at the bottom of the Arctic Ocean thrive in the cold, dark environment that is otherwise hostile to life.
In the permanently frozen Arctic Ocean, there is little light and therefore low productivity of algae plants, which form the basis of the ocean food chain. During the voyage of the research vessel Polarstern, German scientists were stunned to discover sponges living on top of volcanic seamounts in the central Arctic Ocean.
As reported in the journal Nature Communications, the Polarstern expedition determined that sponges appear to survive on what remains of extinct animals. In bustling arctic hotspots, despite little food reaching darkness, sponges grow to impressive size in dense populations on Langseth Ridge: a submerged mountain range. No other such sponge garden has been discovered in the High Arctic.
Study co-author Antje Boetius of the Max Planck Institute in Germany said that when his team saw the blooming sponges, they “didn’t know what they were feeding on”. However, team member and co-author Teresa Morganti identified how sponges survive in an impoverished environment.
“Our analysis revealed that the sponges contain microbial symbionts capable of utilizing ancient organic matter. This allows them to feed on the remains of former seamount inhabitants, now extinct, such as worm tubes composed of protein and chitin and other trapped detritus,” Morganti said.
Geodia sponges dominate seamounts. The biomass the researchers found is comparable to places with 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,” Boetius said.
While sponges are among the most rudimentary forms of life, they abound in oceans around the world, including the Arctic. Many sponges exhibit communities of symbiotic microorganisms, including bacteria and algae, which not only provide food, but also transfer nutrients and eject waste products. Microorganisms also contribute to the health of sponges by producing antibiotics. This symbiotic unit is called a spongy holobiont.
The researchers hypothesized that thousands of years ago, substances seeped from the earth’s crust and supported a vibrant ecosystem and a variety of animals. After the animals died, their remains provided a living base for the sponges. An analysis of microbial life on seamounts supported this hypothesis.
“Microbes have the genes to digest refractory particles and dissolved organic matter and use them as a source of carbon and nitrogen, as well as a number of chemical energy sources available there,” said Ute Hentschel of the German GEOMAR Helmholtz Center for Ocean Research.
Another spectacular discovery is that sponges actually create their own ecosystem with their interlocking spicules – tiny structural elements made of silica or calcium carbonate, which form their skeleton. The spicules in turn form a mat on which the sponges can crawl while helping organic particles to settle. The symbiosis of sponges and microorganisms then consumes this detritus in their own food trap.
“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,” Boetius said. .
Edited by Siân Speakman and Kristen Butler
