Hydrothermal Vent fauna living among the mussels include shrimp, limpets, and Galatheid crabs. Mariana Arc region, Western Pacific Ocean.
Hydrothermal Vent fauna living among the mussels include shrimp, limpets, and Galatheid crabs. Mariana Arc region, Western Pacific Ocean. Credit: Pacific Ring of Fire 2004 Expedition.

In the year 1977, one of the most extreme habitats on Earth was discovered in the deep sea, Hydrothermal Vents. This extreme environment is found at depths of up to >2km , with high pressure constraints that cause a Styrofoam cup to shrink to the size of a 50p, along with water filled with chemicals such as Hydrogen Sulphide being pumped out the seafloor up to 400oC and no source of sunlight. Located at various rifts and ridges in the deep sea, Hydrothermal Vents are created due to the movement of tectonic plates.

These harsh physical conditions makes it more interesting when life was found at the vent sites by ALVIN, a deep sea research vessel (DSRV). From bacteria to invertebrates, hydrothermal vents are packed with extremophiles, which thrive in the extreme conditions. With the success of this environment, it is important to look at how life came to be in such extreme conditions. Research into life at vent sites has led to links that life on Earth potentially started at hydrothermal sites. Could this be the link when searching for life, in the extremes of other planets and moons in the universe?

How does life survive at hydrothermal vent sites?

Without a source of sunlight to drive the ecosystem of hydrothermal vents, energy is needed from another source. The superheated fluids coming out of the vents provides the heat and chemicals for life to exist. The bacteria that inhabit the vents are the key to the success of fauna located near the vents.

Similar to how plants use photosynthesis for energy, bacteria use a chemical process called ‘Chemosynthesis’. This reaction is perfect for the environment as the bacteria have the ability to use the chemicals pumped out of vents to provide energy. Several fauna use the bacteria in a form of symbiotic relationship with the bacteria.

Image showing the biology of Riftia pachyptila, tubeworm species found at hydrothermal vent sites. Credit: David Fischer
Image showing the biology of Riftia pachyptila, tubeworm species found at hydrothermal vent sites. Credit: David Fischer

Vestimentiferan tube worms (Riftia pachyptila) have evolved to not have a digestive system. This is due to their relationship with the bacteria located at vents. Instead, an organ known as a trophosome is used. Taking up 60% of the tube worm, the trophosome acts a ‘home’ for the bacteria, which in turn produce organic compounds for energy, from the Hydrogen Sulphide for the tube worm to survive using chemosynthesis. Species of mollusc like the Giant Vent Clam (Calyptogena magnifica) also use bacteria for energy, with a reduced sized gut due to being more reliant on the bacteria.

How life on other planets link to vents on Earth?

Origin of life on Earth has often been thought to be a ‘primordial soup’, where a mix of chemical reactions and an energy source such as UV light from the sun caused life to begin. With the discovery of hydrothermal vents and the use of chemosynthesis as an energy source, it is possible that life on Earth began in the depth of the ocean, without sunlight.

In the earliest history of Earth, the sun was more luminous, producing more harmful UV levels than the modern day. The radiation from UV-B rays would be able to damage the deoxyribonucleic acid (DNA) obscuring the development of organisms. Hydrothermal Vents are around 2km from the reach of the harmful sunlight above, life would be able to survive until UV radiation decrease as the life of the sun increased, eventually transcending to using the UV rays as a energy source via photosynthesis.

Search for life on other worlds has always been an important subject. Is Earth the only planet to hold life in the universe? Looking at how life on Earth could have potentially began at hydrothermal vents, it gives hope that other planets and even moons could harbour life, even if it is only at microscopic levels such as bacteria.

Illustration of Enceladus showing a global liquid water ocean between its rocky core and icy crust.
Illustration of Enceladus showing a global liquid water ocean between its rocky core and icy crust. Credits: NASA/JPL-Caltech

For life to exist, there needs to be four aspects present: The presence of water, a heat source, mixture of chemicals including carbon, nitrogen, oxygen, hydrogen, phosphorus and sulphur and time for life to develop. The planet or moon should also be in an area known as the ‘Habitable Zone‘ also known as the Goldilock zone. The Earth receives enough energy from the sun for life to exist.

Earth has all these aspects, which has led to life developing. While Earth is the only Planet in the Habitatable zone in our solar system, it is now the view of many that one moon in our solar system is thought to be life supporting, Enceladus.

Enceladus is the sixth largest moon orbiting Saturn. The Cassini mission has shown that Enceladus has geysers blasting water-rich gas into space. The plumes from the geysers have traces of organic compounds, suggesting it can harbour life. Enceladus is known to have an ocean, which is heated between the icy crust surface and rocky core.

As there would not be a suitable UV level to sustain photosynthesis, the link to hydrothermal vents on Earth has been made. Life could develop in the oceans on Enceladus in the same way life possibly began on Earth, with organisms using chemosynthesis to drive an ecosystem. This has also led to tidal habitable zones being look at, where planets with tidal heating can survive without high level of energy from the sun found in the habitable zone.

More to come?

There is still much to learn about hydrothermal vents, the origin of life on Earth and the universe. The more research into these topics, links can continue to develop. Humans have only explored around 5% of the ocean on Earth, there is still a lot to learn and discover. This is similar case with life on other planets. While we know that there are planets and moons that have water present such Europa, more exploration is needed to enhance our knowledge and help to establish how life came to be. The search for life still has hope thanks to this extreme environment.

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