In the cold, dark depths of the Arctic Ocean, a feast of the dead is under way.
A vast community of sponges, the densest group of these animals found in the Arctic, is consuming the remains of an ancient ecosystem to survive, researchers report February 8 in Nature Communications.
The study highlights just how opportunistic sponges are, says Jasper de Goeij, a deep-sea ecologist at the University of Amsterdam not involved with this work. Evolutionarily speaking, sponges “are more than 600 million years old, and they inhabit all parts of our globe,” he says. Scientists might not know about all of them because many places that sponges inhabit are really difficult to get to, he adds.
Sponges are predominantly filter feeders, and are crucial to nutrient recycling throughout the oceans. The existence of this colony, discovered by a research ship in 2016, however, has been an enigma.
The sponges, which include the species Geodia parva, G. hentscheli and Stelletta rhaphidiophora, live between 700 and 1,000 meters down in the central Arctic Ocean, where there are virtually no currents to provide food, and sea ice covers the water year-round. What’s more, sponges are largely immobile, yet in 2021 researchers, including Teresa Morganti, a marine biologist at the Max Planck Institute for Marine Microbiology in Bremen, Germany, reported that these ones slowly move, using their spicules — microscopic skeletal structures — and leaving them as thick brown trails in their wake.
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In the new study, Morganti and colleagues turned their attention to the matted layer underneath the sponge colony, a smorgasbord of discarded spicules and blackened fossilized life, including empty worm tubes and mollusk shells. To see if this thick mat was a food source, the team analyzed samples of the sponges, the mat material and the surrounding water. The researchers also investigated the genetic makeup of the microbes that live within the sponge tissues, and those in the sediment.
Carbon and nitrogen isotopes — atoms with different numbers of neutrons — in the sponge tissues closely matched those of the dead matter below, suggesting the animals were consuming it. The genetic signature of the microbes showed they had enzymes capable of breaking down the material and were likely dissolving the dead organic matter into food for the sponges (SN: 12/27/13).
The matted layer is up to 15 centimeters thick in places, the researchers found. Assuming that the layer is, on average, greater than 4 centimeters thick, it could provide almost five times the carbon that the sponges would need to survive, the team calculates.
The discovery that the sponges are feeding from below means they are likely moving to access more food, Morganti and colleagues suggest. The scientists also found many sponges to be budding, or breaking off parts to form new individuals, showing active reproduction.
Radiocarbon dating showed the adult sponges — spread across more than 15 square kilometers on the peaks of an underwater volcanic mountain range — to be over 300 years old on average, a “truly outstanding” finding, says Paco Cardenas, a sponge expert at Uppsala University in Sweden who was not involved with the new study. “We expected sponges to grow very slowly, but this had never been measured in the deep sea,” he says.
The dead ecosystem below the sponges is around 2,000 to 3,000 years older, a once-thriving community of animals that lived in the nutrient-rich conditions created when the volcanoes were last active, the researchers suggest.
Sponges often appear to take advantage of the most abundant carbon sources, which may change as global warming alters the composition of the oceans, says ecologist Stephanie Archer of the Louisiana Universities Marine Consortium in Chauvin, who was not involved in the work. “One big question will be how flexible sponge-microbe associations are, and how quickly they change to take advantage of shifting carbon sources,” she says. More