New discovery in the deep sea: worms growing in the stones!

Produced by: Science Popularization China

Author: Zhang Zhao

Producer: China Science Expo

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The “unknown realm” of the deep sea

Are there any unknown corners on Earth?

Although humans today have explored the highest peaks on land, the deepest parts of the ocean, and even the moon and Mars, there are still many unknowns about the vast ocean on our planet. According to estimates by the U.S. Atmospheric and Oceanic Administration, we have only explored about 5% of the ocean area, which means that 95% of the area is still unknown to humans.

Deep in the vast Pacific Ocean, there is a mysterious submarine mountain range - the East Pacific Ridge. This is where two major tectonic plates meet, where new crust is generated and moves to both sides at a rate of 70 mm per year. This has also caused frequent volcanic activity and spawned many hydrothermal vents. Hydrothermal vents are like "hot springs" on the seabed, where seawater penetrates into the crust, meets magma, and then gushes out with a large amount of minerals. Some hot springs are black because they are rich in sulfides, so they are figuratively named "black chimneys."

Deep-sea hydrothermal vents "black chimneys" on the Atlantic Ocean

(Image credit: National Oceanic and Atmospheric Administration)

What is even more striking is the unique ecological environment here. The place that was originally thought to be a desert of life is unexpectedly "lively": giant tube worms as long as 2 meters, snails with "iron feet", Pompeii worms that can withstand temperatures of 80°C... Their existence has broken the traditional biological cognition and become a hot spot for studying the way life survives in extreme environments.

Giant tube worms living in hydrothermal vents on the East Pacific Ridge

(Image credit: National Oceanic and Atmospheric Administration)

But since the first discovery of the submarine hydrothermal vent ecosystem in 1977, scientists have generally believed that only microorganisms and viruses can survive in the seafloor crust beneath these hydrothermal vents. Whether there is other life hidden under the hard rock has always been unknown.

This mystery was recently solved. A research team from the University of Vienna operated an underwater robot, SuB-astian, to explore the hydrothermal vent area at a depth of 2,515 meters, uncovering a whole new world of life and completely refreshing our understanding of the deep-sea ecosystem.

2,500 meters deep into the ocean

The team's initial goal was to drill small holes in the seafloor to study how giant tubeworm larvae settle in hydrothermal areas. However, as the underwater robot carefully chiseled a hole about 2 centimeters wide, scientists captured some unexpected scenes through real-time video transmission.

This unexpected discovery prompted them to take a bolder approach. They controlled the robot remotely and carefully lifted a 10-15 cm thick volcanic rock plate. When the rock plate was turned over, everyone was shocked by the scene before their eyes: Under the hard rock, there was a cavity about 10 cm high, and the cavity was full of vibrant deep-sea creatures! Under the rock was like a hidden "underground city".

Undersea "Dungeon"

The architectural structure of this deep-sea "underground city" is very complex. You can imagine a multi-story parking building: each floor is separated by a thick concrete floor, and the floors are connected by ramps. The underwater "underground city" is also a similar structure: it is made up of multiple layers of volcanic rock plates stacked on top of each other, forming cavities about 10 cm high between each layer. These cavities are interconnected through cracks in the rock, and warm seawater flows in them.

Multi-layered cavity structure beneath the rock

(Image source: Literature)

In this unique underground world, giant tube worms (Riftia pachyptila) are still the protagonists. They can reach a length of 50 cm, and their huge bodies grow wantonly, full of cavities, just like the roots of trees underground. These tube worms have no mouth or digestive system. They coexist with bacteria in their bodies and rely on bacteria to decompose sulfide for energy. Scientists have found that these tube worms not only survive underground, but can even reproduce here.

Giant tube worms growing upward from beneath a rock

(Image source: Literature)

In addition to the giant tube worms, there are also some smaller tube worms (Oasisia alvinae), which particularly like to grow on the "rock stalactites" protruding from the top of the cavity. Some individuals are up to 20 cm long, which is longer than any of their kind found on the seabed surface before.

In addition to the tubeworms, some mobile residents also live in the underground city, including polychaete worms (Paralvinella), which feed on the white microbial mat at the bottom of a cavity.

There are also predators, such as a sandworm from the family Nereis sandersi. Even mollusks have found their place here: the researchers found some gastropods and semi-sessile sea snails (Neomphalus fretterae).

Other creatures that live under rocks

(Image source: Literature)

These animals form a complete ecosystem, with corresponding species from the bottom to the top of the food chain. They either live in a fixed way or swim freely, and they may have lived and multiplied for tens of thousands or even hundreds of millions of years in this unknown place.

The Secret of the "Dungeon"

After the initial surprise, scientists began to think further: How do these creatures survive in the "underground city"?

Scientists have found through measurements that the environment of these underground cavities is actually very similar to the hydrothermal areas on the seafloor: the temperature is maintained at about 18 degrees Celsius, the pH value is about 6.1, it contains rich sulfides, and sufficient oxygen. Such an environment provides a natural condition for organisms on the seabed to enter the underground development.

In fact, the ecosystems on the seabed and under the rocks are likely to be connected and unified, which is well reflected in the giant tube worms. In the underground space, the tube worms either grow upward from the cracks in the rocks to the seabed, or hang down from the top of the cavity to obtain nutrients from the surrounding water. Their lifestyle is not different from that of the tube worms on the seabed, that is, they use symbiotic bacteria to decompose sulfides in seawater to provide themselves with energy.

What is interesting is how they reproduce. Scientists have discovered that the larvae of tube worms may not drift in the seawater as previously thought, but instead penetrate the crust with cold seawater, migrate in underground spaces through hydrothermal systems, and then erupt from other cracks. These cavities and cracks under the rock are like a well-connected "subway system" that allows the larvae to take any route and reach any suitable place for settlement.

Scientists speculate on the connection between the surface ecosystem of the seabed and the ecosystem in the rock cavity. The larvae of giant tube worms may migrate through an underground "transportation system"

(Image source: Literature)

Life is everywhere

If we use one sentence to summarize the shock this discovery has caused to mankind, it must be "life is everywhere." In the past, we thought that the deep sea was a "desert of life", but the discovery of the submarine hydrothermal ecosystem broke this view.

After that, people thought it was impossible for large life to exist under the rocks, but this research has once again shattered people's assumptions. The significance of this discovery is: if life can adapt to such extreme environments, then is it possible to nurture life in similar environments?

There are similar environments in the solar system, one of which is Jupiter's satellite, Europa. This satellite is slightly smaller than the moon and may have an iron-nickel core and silicate shell similar to the Earth. Most importantly, it has a water-ice crust: under its ice layer hundreds of kilometers thick, there is probably a liquid ocean hidden.

Scientists speculate that in the ocean of Europa, there is an environment similar to the deep-sea hydrothermal vents on Earth, and today's research gives us a glimpse into the possibility of discovering life on other planets. If there is extraterrestrial life in the solar system, it is very likely that they do not rely on sunlight to survive.

An image of Europa, its interior structure, and an artist's conception of possible hydrothermal vents in Europa's ocean.

(Image credit: NASA, Kelvinsong)

In addition, this study has also triggered new thinking about the origin of life. Hydrothermal vents have long been considered one of the possible birthplaces of early life on Earth. Now, we have discovered that there are not only simple microorganisms, but also large organisms with more complex cells. Perhaps the "cradle period" of life on Earth was carried out "secretly" underground.

Now, perhaps we can answer the question at the beginning of the article. Humans have learned arrogance from repeated "conquests", but nature is tirelessly teaching us humility. Life under the rocks of the deep sea is telling us: Never think that you know enough about this planet and the life on it.

(Note: Latin parts in the text should be italicized)

References:

[1]Bright, M., Gollner, S., de Oliveira, AL et al. Animal life in the shallow subseafloor crust at deep-sea hydrothermal vents. Nat Commun 15, 8466 (2024).

[2]García, María Colín et al. “Hydrothermal vents and prebiotic chemistry: a review.” (2016).

[3]http://www.nasa.gov/mission_pages/cassini/

[4] Peter Lonsdale, Clustering of suspension-feeding macrobenthos near abyssal hydrothermal vents at oceanic spreading centers, Deep Sea Research, Volume 24, Issue 9, (1977).