Hydrothermal vents: survival at the ocean's hot springs

Hydrothermal vents, located on the seafloor, release a fluid that has been superheated within the Earth's crust. But despite this heat, the surrounding environment is a perfect habitat for a range of organisms.

Dr Maggie Georgieva, a vent researcher at the Museum, explains how the vent sites help some animal communities survive and thrive in the cold, dark depths of the deep sea.

What is a hydrothermal vent?

Hydrothermal vents are naturally forming structures found in the ocean. They usually occur on divergent plate boundaries, where tectonic plates are moving apart. The vents expel a fluid that was heated to extreme temperatures when seeping through the Earth's crust from the ocean.

Maggie explains, 'Hydrothermal vents are like hot springs on the seafloor. As tectonic plates of oceanic crust move apart, the crust is stretched, and in places it breaks, forming cracks and fissures within it.

'Seawater then percolates into these cracks and seeps deep into the crust, where it comes into close contact with the underlying mantle and is heated.'

As the heated seawater moves through the crust, it picks up dissolved gases and minerals. The high temperature of the fluid makes it buoyant, and the superheated water eventually begins to move back up to the seabed where it is expelled through a vent.

'When the hot fluid erupts from the seafloor,' says Maggie, 'it starts to mix with the colder surrounding seawater. The minerals that are dissolved in the hot fluid then start to come out of solution.'

As the minerals precipitate, they form a solid structure onto the seabed around the venting fluid known as a vent chimney.

Hydrothermal vents are often divided into two types: 'black smokers' and 'white smokers'.

Maggie says, 'There is normally a temperature difference associated with the two types of vent chimney. Depending on how hot the fluid is, it can carry different minerals in solution, and different minerals will precipitate out at different temperatures.

White smokers typically occur at lower temperatures. The light appearance is due to the minerals carried, which can include silica and barite. When these precipitate, they appear white.

Black smokers are hotter and spew out a fluid that carries mostly iron sulphides, which make them look darker.

Life at hydrothermal vents

Initially the temperature of the fluid released from hydrothermal vents is extreme - it can reach over 400°C. But despite the scalding heat, the environment around the vents is habitable for a range of animals.

'While these fluids are hot, they tend to cool very quickly as they mix with seawater,' explains Maggie. 'The vent might be very hot, but when you move away from it a little, you can have a temperature of 20°C or so, which is quite nice for lots of animals.'

The food chain at these ocean oases relies on a core process called chemosynthesis, which is carried out by bacteria. This is similar to photosynthesis used by plants on land, but instead of using light energy from the Sun, the bacteria use chemicals drawn from the vent fluid.

'Chemosynthesis is a process in which chemical energy is used to convert carbon dioxide into sugar - essentially into food - in the complete absence of sunlight.

'The vents influence the evolution of animals in the deep sea. There are lots of somewhat strange animals that have adapted to what may initially seem like fairly inhospitable conditions.'

Some species appear to have become fully reliant on the thermal sites. Animals such as scaly-foot gastropods (Chrysomallon squamiferum) and yeti crabs (Kiwa species) have only been recorded at hydrothermal vents. Large colonies of vent mussels and tube worms can also be found living there.  

In 1980, the Pompeii worm (Alvinella pompejana) was identified living on the sides of vent chimneys. It is one of the most heat-resistant multicellular animals on the planet, able to withstand temperature spikes of over 80°C.

'Most animals can't cope with anything over 40°C. Very close to the hot fluid, there are typically only microorganisms. These can survive in temperatures up to around 120°C,' explains Maggie.

Hydrothermal vents and the origin of life

The study of hydrothermal vents is challenging, not least because their deep-sea locations are difficult to access. They were first discovered in 1977, when a black smoker was located during an expedition around the Galápagos Islands.

'Exploring hydrothermal vents requires fairly specialist equipment. For other deep-sea habitats you can generally use simpler sampling devices such as grabs and trawls, but you can't really do that at vents because of the rocky, uneven terrain,' says Maggie.

'Studying the vents involves using ships with equipment such as remotely operated vehicles (ROVs) or manned submersibles. They're equipped with lights, cameras and manipulator arms to collect samples. It can be quite an expensive process to explore a vent.

'Hydrothermal vents are fascinating environments. There have been hydrothermal vents since the very early history of the planet - and they could be where life on Earth first started.'

Records of life on Earth stretch back a long way. Fossils of multicellular animals have been found dating back to over 500 million years ago, while the oldest animal fossils from hydrothermal vents are approximately 440 million years old.

Some of the earliest evidence for microbial life on Earth comes from rocks located in Canada that formed within hydrothermal vent environments around 4 billion years ago.

The hostility of the planet's surface at that time suggests that life is more likely to have begun within the Earth's crust or in the deep sea. Research also indicates that early life relied on chemosynthetic processes, like those seen in the ocean today.

This makes hydrothermal vents a likely candidate for the origin of life on Earth.