Fossil-eating sponges discovered in the Arctic

Sponges are tucking into a buffet of leftovers deep beneath the Arctic ice.

The fossilised remains of an ancient ecosystem have provided an unlikely food source for the marine life, providing sustenance in an area of the ocean where vital nutrients are hard to come by.

The co-author of the first study on this unusual ecosystem, Professor Antje Boetius, says, 'This is a unique ecosystem, and we have never seen anything like it before in the high central Arctic.

'Primary productivity in the overlying water provides less than one percent of the sponges' carbon demand, with fossil matter contributing a significant part of the remaining amount. Thus, this sponge garden may be a transient ecosystem, but it is rich in species, including soft corals.'

While scientists now have a better understanding of this ecosystem, there are likely to be many others which haven't been discovered and would need protection from future climate change.

Dr Ana Riesgo Gil, a specialist in sponges at the Museum and who was not involved in this study, says, 'There could be more sites like this to be found. The oldest sponges that are discussed in the paper are from the order Tetractinellida, which are the most abundant in the deep sea of the north and southern hemispheres.

'These sponges are pretty well adapted to the deep sea, so I don't think this feeding is necessarily limited to these individuals and could be extrapolated to other areas where there were formerly seeps.'

What is a cold seep?

While food chains on land often start with photosynthesis, those underwater can begin at a variety of other points. Without light, other processes, such as chemosynthesis, can provide the energy to support unique biological communities.

Some of the most unique communities surround hydrothermal vents, where water that infiltrates into the Earth's crust emerges superheated and full of minerals. Reactions making use of these compounds allow life to survive thousands of metres below the ocean's surface, with some suggestions life may have originated in this environment.

Aside from hydrothermal vents, other underwater communities can form around dead whales, sunken wood and cold seeps. The latter are where hydrocarbons, primarily methane, seep out of the Earth's crust as a result of tectonic activity. Microorganisms can take advantage of this to release energy through reactions involving these compounds, producing carbonates as a byproduct.

Over time, this allows organisms which feed on these microbes to arrive and survive, followed by other animals including predators. This creates areas that are significantly more diverse than the surrounding ocean, with one study describing them as 'oases' of the Arctic.

However, the seeps are dependent on the continued release of hydrocarbons. As the underground reservoirs that feed them run dry, organisms with the highest energy demands begin to die out, until eventually nothing is left.

The carbonates released by seep microbes over their lifetime precipitate as rock, which is left behind when the seep disappears. This can provide a surface for animals like corals and sponges to grow on following the end of the seep.

However, it now appears that the remains of seep ecosystems can provide food for the communities which replace them.

How do the Arctic sponges survive?

Sitting high in the Arctic circle, the Langseth Ridge varies from around 500 metres to over five kilometres deep. During the time that Rome was becoming one of the world's first superpowers over 2,000 years ago, a seep community was thriving on the ridge.

As the western Roman Empire wound down a thousand years later, its fortunes were mirrored by the seep community, which died out as its methane supply dried up. Following a period of low diversity for the ridge, which may have lasted for at least a millennium, sponges subsequently began to establish themselves on the same site.

Over time, a range of species, including one previously unknown to science, established the densest community of sponges found anywhere in the Arctic. However, when the site was discovered by researchers, they were unsure as to how such a community could survive in nutrient-poor waters.

Analysis of samples taken from the site subsequently revealed that the sponges have a mixed diet to obtain the carbon they need to grow, including the fossils of both polychaete worm tubes and bivalves.

'The main limit to how far and deep the sponges can spread is their supply of food,' Ana explains. 'Sponges primarily live by filtering food, such as plankton or bacteria, from the water, but they also make use of other sources. Some sponges are carnivorous and have adapted to trap and consume prey.

'While the species composition and size of the sponges at the Langseth Ridge site are not particularly unusual, what they feed on is.  This paper is very interesting because it shows that sponges can feed off refractory dissolved organic matter, which as far as I'm aware isn't something that we previously knew they could do.'

The sponges' ability to digest the remains is attributed to the bacteria that live in a symbiotic relationship with them. While the sponges may have some ability to break down the fossils themselves, the microbes contain a variety of enzymes that release additional nutrients locked within it.

'There is no sponge without a symbiont,' Ana explains. 'Which symbiont they have mostly depends on what source of energy the sponges use. Sponges in shallow waters have bacteria that can use light, for instance, whereas these sponges have bacteria able to consume the refractory dissolved organic matter.'

This has allowed the sponges to grow substantially compared with others in nearby waters, with those at the centre of the fossil community growing largest. The organisms also show signs of reproducing, suggesting an abundance of nutrients and energy being available.

As more of the fossil matter is eaten away, the abundance of food at the site will decrease. Eventually, this means that the unique community will die out, just as the cold seep organisms did before it.

The scientists behind the discovery of this site hope that the knowledge gained will enable similar 'astonishingly rich ecosystems' to be found before they exhaust their food resources. They also hope to find how to better protect these communities in the face of climate change.

'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,' Antje says.