Discovery

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Figure 1. A black smoker emitting plumes of mineral-rich hydrothermal fluid (source)

Hydrothermal vents are found in one of the most extreme environments on earth, they experience extreme cold and hot, intense pressure and zero light, so how do organisms thrive there? Before the mid-19th century the deep ocean was thought to be completely barren until the expedition of the HMS Challenger  in 1875. Led by scientist Dr. Charles Wyville Thompson and only 216 other crew they covered 127,653km of open ocean and discovered 4,500 species completely new to science. Since then, with the development of new technology and equipment the deep ocean has been explored significantly more. Hydrothermal vents are a relatively new finding, having only been discovered in 1977 by accident. They were first explored by geologists hoping to exploit the ocean bed for minerals but instead found an abundance of life where they expected none. This then caught the attention of biologists who have intensely studied and discovered many more areas of hydrothermal activity since.

 

Where can you find hydrothermal vents?

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Figure 2. A world map showing the tectonic plate boundaries and locations of all known hydrothermal vent sites (source)

In 1977 the first discovered hydrothermal vent site along the Galapagos Rift was explored using the deep sea research vessel “Alvin” and it was determined that the cause of this activity was due to magma under the earth’s crust. Hydrothermal vents occur along the boundaries of tectonic plates, mainly in rift valleys, ridges and spreading centres. Some of the main sites are the Galapagos Rift, the East Pacific Rise, Juan de Fuca, the Mid-Atlantic Ridge and in the western Pacific the Mariana Back-Arc.

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Figure 3. A cross section of the earth’s crust showing the flow of seawater and dynamics of a hydrothermal vent below the crust surface (source)

As these plates spread apart, magma is forced upwards and cools when it meets the cold ocean water, forming a new crust. Sea water circulates underneath the crust, accumulating dissolved minerals and heating to 400°C. This water then exits through the crust and cools on contact with cold sea water to form precipitates as minerals solidify in the form of tall chimney-like structures, known as vents. There are a number of different types of vents determined by their different characteristics, compositions, flow rates and locations. Black smokers are one type of vent, that are seen to be one of the most hostile settings imaginable. They are defined by their characteristics of very tall ‘chimneys’ caused by a high rate of water flow that produces large black plumes, dark black in colour, high temperatures and high sulphur concentration. A similar type of structure is a white smoker, with smaller chimneys, a slower flow rate, and cooler temperatures. This type of vent emits high concentrations of barium, calcium and silicon, giving it it’s characteristic pale colour and pale ‘smoke’.

Life in the deep

You may wonder how an ecosystem has been able to thrive in such extreme conditions. In the ocean it is generally known that microscopic photosynthetic organisms called phytoplankton are the key species of every food chain by transforming light into energy. So in the depths where no light can penetrate how is this possible? There is an abundance of chemosynthetic bacteria which are primary producers and provide the foundation for all life at the vents by transforming chemicals dissolved in the water into energy which is vital for survival. Instead of using a photon from sunlight, this bacteria uses electrons from the minerals to start the chain reaction of chemosynthesis, taking carbon dioxide, hydrogen sulphide, and water and producing sulphuric acid (and other sulphur compounds) and sugar. This process produces energy in the form of sugar for many organisms however lowers the pH of the surrounding water significantly which they have evolved to adapt to.

The benthos around hydrothermal vents are dominated mainly by tube worms, clams and mussels which display patterns of diffuse zonation around the vent openings due to the complex topography of the substratum. This untidy distribution and patchiness is difficult to explain as there are so many factors that drive the distribution of organisms including, temperature, pH, flow rate and ocean currents. Nearest to the vent openings is where you will find the most tolerant species like the tube worm Riftia pachyptila which favours the warm waters of between 15 – 17°C, along with other worm species such as the scale worm Branchiplynoe and palm worm, paravinella grasslei found in vent depressions. Tube worms in particular have a symbiotic relationship with the chemosynthetic bacteria that live in their gut and produce energy for them directly into their bodies. Further away from the vent opening in crevices around the peripheral edges of the community is where you will find gastropods such as the giant clam Calyptogena magnifica and mussel Bathymodiolus thermophilus which prefer a more mild environment.

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Figure 4. The recently discovered Hoff crab displaying the many setae on its body for which it got its name (source)

Recently discovered inhabiting a hydrothermal vent near Antarctica is the Hoff crab Kiwa tyleri, a squat lobster, given its name after David Hasselhoff for its hairy chest. It has many setae on its appendages and ventral side of its body that contain filamentous, sulphur-oxidising bacteria. The crabs can then harvest this energy produced by the bacteria that has been retained in the dense setae as it is thought to be a significant source of its nutrition as well as other small molluscs.

 

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Figure 5. The scaly-footed gastropod showing its armour-like iron sulfide plated denticles (source)

Another interesting discovery was that of the scaly-footed gastropod Crysomallon squamiferum on a vent in the Indian ocean. This creature makes use of the high level of dissolved minerals in its environment and has evolved to use it as an advantage by creating a body ‘armour’. It has iron sulphide dermal denticles unlike any natural or synthetically engineered armour made in a lab. Studying this mollusc and other organisms like it can be useful in bio-engineering where inspiration is taken from nature to combat issues in everyday life, for example, these gastropods may be studied by scientists working with the U.S military to improve their armour using this technology. This morphology is akin to that of prehistoric metazoans from the Cambrian era 500 million years ago.

Studying these interesting deep sea creatures that have adapted to live in an environment so extreme could give us the key to uncovering the mystery of the origin of life. If hydrothermal vents are where it all began does that give us an inclination to believe there is a possibility of life on other planets with similar conditions to this too?

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