Hydrothermal vents were one of the most significant discoveries of the 20th century. First discovered in 1977 off the coast of the Galapagos Islands, at the bottom of the Pacific Ocean; hydrothermal vents showed for the first time that an organism could survive without sunlight. For the first time, we knew an organism could succeed without direct or indirect influence from photosynthetic processes.

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Figure 1. Hydrothermal vent covered by giant tube worms (Riftia pachyptila) – SOURCE

A hydrothermal vent is a chimney-like structure that can reach heights of up to 60 metres (Figure 1). They are usually found on mid-ocean ridges ranging from depths of 1500-4000m deep, occupying from the bathyal to the abyssal zone. Discovered by the deep-sea research vessel ‘ALVIN’, scientists may have expected to find hot water springs on the seafloor; however, they did not anticipate finding them surrounded by complex ecosystems. Since their discovery in 1977, ALVIN has gone on to locate over 24 hydrothermal vent sites, resulting in the identification of over 300 new species.

Formation
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Figure 2. Graphic demonstrating how cracks caused by spreading and cooling of the sea floor leads to the formation of mineral-rich hydrothermal fluid. – SOURCE

Hydrothermal vents often occur on mid-ocean ridges as a result of plate tectonics. When the mid-ocean ridges form, the sea floor begins to spread away from the ridge. As the surface spreads, it cools and solidifies, causing fissures and cracks to occur. Cold water can seep down these cracks; if the water can percolate far enough down towards the oceanic crust, then it is heated by the molten rock beneath. Upon heating, the water chemically reacts with the rock it passes, dissolving the minerals within. The more the water heats up, the more mineral-rich the sea water becomes (Figure 2). Once the hydrothermal fluid becomes hot enough, it turns buoyant and forces its way back to the surface. Despite entering the rock through numerous cracks, the now hot fluid returns through a reduced number of channels. Once expelled from the chimney, the hot mineral-rich sea water comes in contact with the cold waters of the abyssal. The hydrothermal fluid cools rapidly and the dissolved minerals begin to precipitate out as the temperature decreases, resulting in black smoke. As these minerals solidify and sink, they culminate to form the large ‘chimney’ structures from which the fluid is released.

Chemosynthesis

Chemosynthesis is the process by which chemosynthetic bacteria use inorganic molecules, such as hydrogen sulphide (H2S) as an energy source to produce organic compounds. Despite first being observed over 100 years ago, the significance of Chemosynthesis as a mechanism was not recognised as it does not play a substantial role in photosynthesis. It was not until the discovery of deep-sea hydrothermal vents, where photosynthetic processes are absent, did the biogeochemical importance of this reaction become apparent. The bacteria oxidise the hydrogen sulphide in the presence of carbon dioxide and oxygen, to make sugar, sulphur and water: CO2 + 4H2S + O2 → CH2O + 4S + 3H20. They are therefore primary producers, as they provide their own food.

Giant tube worms

Giant tube worms (Riftia pachyptila) are an example of how vent species rely on specialised adaptations and symbiotic relationships with chemosynthetic bacteria to survive. The tube worm must be able to survive in crushing pressures and a range of temperatures due to the location of its habitat. Not only that, they must be able to adjust to these temperature changes at a moment’s notice. From the deep-sea cold of around 2-3°C to heat produced by hydrothermal vent fluids which at the centre of the cloud can reach 350°C. The worm lives within a chitin tube made from the same composition that forms a crustacean’s shell. This hardened tube provides the soft, colourless body of the worm protection against predators.

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Figure 3. The plume of a giant tube worm (Riftia pachyptila). Formed by densely packed tentacles which select nutrients for the chemosynthetic bacteria. Red colour due to an abundance of blood vessels at the surface to increase diffusion rate. – SOURCE

The large red feature on the end of the worm is called the ‘plume’ (Figure 3). This plume is the worms breathing apparatus and can be retracted back into the tube when under threat. It contains specialised haemoglobin which has a particularly high affinity for oxygen in the seawater. Large quantities of this oxygen-carrying haemoglobin pigment are the reason the plume is bright red in colour. Giant tube worms have neither a mouth nor a stomach. Instead, they possess a large specialised organ called a trophosome. The spongy tissue of the trophosome homes high quantities of this chemosynthetic bacteria estimated to be 285 billion bacteria per ounce of tissue. Large numbers of chemosynthetic bacteria allow for greater production of organic matter in exchange for hydrogen sulphide. The trophosome makes up about 60% of the organism to allow as much bacterial colonisation as possible. The plume is also responsible for the transfer of reactants to the chemosynthetic bacteria in the trophosome; drawing oxygen, hydrogen sulphide and carbon dioxide from the sea water. These circulate the body via the specialised haemoglobin before being transported towards the trophosome. The bonding of hydrogen sulphide to haemoglobin occurs so that the oxygen cannot oxidise the H2S in the blood; this prevents the tube worm against sulphide poisoning. Once at the trophosome, the bacteria convert these compounds into a sugar carbohydrate that can be used by both the bacteria and the host worm.

Where next?

Despite making up 95% of the Earth’s living space, relatively little is known about the deep-sea. The immense pressures and pitch black surroundings make the deep-sea a difficult place to visit on a regular basis, even with ALVIN. Therefore, we don’t fully understand the adverse effects that may be occurring as a result that deep-sea mining and waste disposal. Some reports even suggest that up to 1 in 5 hydrothermal vents could be under threat from deep-sea mining. In addition, this unique, unexplored environment has provided us with evidence that life on other planets may exist. Proof that life can exist absent of sunlight has led researchers to believe life may exist on planets that have oceans with a rocky bottom. One such satellite that may support chemosynthetic bacteria is Saturn’s moon Europa. Furthermore, we have not gained enough knowledge about the deep-sea to begin to exploit it, as it has important influences on life, not just in higher oceanic zones but possibly other planets.

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