These sponges survive the deep sea by feeding on remains of long-dead animals | Science

It can be hard to find food in the central Arctic Ocean. The water is frigid and the surface is blanketed in ice, making it nearly impossible for the tiny organisms that power many marine food chains to photosynthesize. Now, researchers have unraveled how a gigantic, newly discovered sponge garden gets around the lack of nutrients: by feeding on the fossilized remains of other underwater invertebrates that lived thousands of years ago. Some of the sponges have apparently survived on such a diet for more than 300 years

“It’s a really cool” finding, says Stephanie Archer, a marine ecologist at the Louisiana Universities Marine Consortium who wasn’t involved in the study. The work, she says, reveals how rarely studied deep Arctic ecosystems continue to function, even as melting sea ice threatens to disrupt them.

Sponges have been around for at least 600 million years and were likely the first multicellular organisms on Earth. They filter water through their pores, digesting microscopic photosynthesizing organisms called phytoplankton and other food particles to help cycle nutrients such as carbon, nitrogen, and phosphorus through the underwater ecosystem. “They’re very opportunistic and can tap into food sources that others cannot,” says Jasper de Goeij, a marine ecologist at the University of Amsterdam, also not involved with the study.

But sponges weren’t necessarily top of mind when Antje Boetius, a marine biologist at the Max Planck Institute for Marine Microbiology and one of the study’s co-authors, set out on a research mission to the central Arctic Ocean in 2016. Among other projects, she planned to survey and map the Langseth Ridge, a V-shaped, 125-kilometer-long underwater mountain range located north of Svalbard at the top of the globe. “We thought we might see some rocks and maybe one or two” deep-sea sponges, Boetius says of the effort.

She and her colleagues devised an underwater sledge packed with equipment to measure and sample the ocean floor. The rig included cameras, lights, sensors, and other devices encased in a steel frame about as large as a Volkswagen Beetle. While traversing the underwater peaks and valleys of the Langseth Ridge, the rig came across a dense patch of sponges stretching for at least 15 square kilometers, nearly the size of 3000 U.S. football fields—a total shock, Boetius says.

“Imagine yourself going into the desert, and you find the most spectacular oasis where everyone has told you there is no life,” she explains. The sponge ecosystem sat as far as 1000 meters below a thick sheet of ice through which no sunlight could penetrate. Boetius and her colleagues wondered how the animals could survive in such an inhospitable home.

In many parts of the deepest ocean, life congregates around seeps—vents in the sea floor where gasses leak out from Earth’s innards and fuel microbial growth, attracting communities of deep-sea invertebrates. But there was no such gas source along this ridge and no currents or upwelling that could be carrying nutrients or particles to the sponges, explains study co-author Teresa Morganti, who studies sponges at Max Planck. The water was still and stripped of food, but the sponges were thriving.

So the researchers extracted samples of the underwater sponge “oasis” to figure out how the animals were surviving—some were several centuries old, carbon dating revealed. The sponges, some as wide as dinner plates, were growing on a curious substrate: a blackened tangle of fossilized siboglinid tube worms, which are deep-sea worms that live in clusters of tubes stuck to the ocean floor. The team measured the carbon and nitrogen isotopes of the samples and sequenced the DNA of microbes that colonize the sponges and help them process their food.

The sponges were replete with microbes that digest organic matter, the team reports today in Nature Communications. This suggests the animals were pulling nutrients from the fossilized layer below them, essentially eating the 1000-year-old dead invertebrates with help from their symbiotic bacteria, the team says.

The former tube worms likely sprung up around gaseous vents that were active thousands of years ago but then closed, leaving the fossilized husks ready for the taking by the hungry sponges. It’s the first time such an eating strategy has been observed for sponges, Morganti believes.

Goeij is cautious, however, noting the analyses come from just a few chunks of the ocean floor. Still, he says, it’s complicated to obtain these samples, and “this is a really good basis” for the hypothesis that sponges are capable of this strategy.

Both he and Archer say the finding makes clear how important the often-underestimated animals are in building biological hot spots. Because sponges help cycle nutrients through their environment, everything they eat and do has “consequences for the rest of the ocean,” Archer says. “Every time we think we have sponges figured out,” she says, “a paper like this comes along and there’s something new they can do.”