As humans we thought that we had a fairly good understanding of the limits of the existence of life on this planet. As humans we thought that elements like sulphur are highly toxic to living organisms. That was, however, until the discovery of hydrothermal vents  in 1977. The discovery of these deep sea vents completely rewrote the textbooks on what we, as humans, believed was possible for the survival of life. With water at temperatures as high as 400°C flowing out of black smokers  and toxic elements such as sulphur and heavy metals such as mercury makes these vents a very inhospitable environment and almost uninhabitable…almost.

There are a range of organisms that do inhabit these vent systems, which are found along the Mid-Atlantic ridge, Pacific rim of Fire and the East Pacific Ridge. These organisms range from small chemosynthetic bacteria, to bivalves the size of rugby balls. All of these organisms are adapted to live in certain parts of a vent system, able to withstand the high temperatures and toxic elements. There is one species of worm, however, which is able to use the toxic sulphur in its blood to gain energy. This is the giant tube worm, Riftia pachyptila.

What is the Giant Tube Worm?

A cross-section of the Giant Tube Worm, Riftia pacyptila showing the internal and external anatomy. Image by David Fischer.
A cross-section of the Giant Tube Worm, Riftia pacyptila showing the internal and external anatomy. Image by David Fischer.

The giant tube worm, Riftia pachyptila, is only found at hydrothermal vents systems around the world. The worm can grow up to 8ft in length and live in tubes that they construct around themselves and on the substrate of the vent. These tube worms are reliant on the chemosynthetic bacteria that are present in the trophosome of the worm. The trophosome is the fleshy and spongy tissue of the worm that is protected boxic Bloody the tube that the worm builds. Per ounce of trophosome tissue there is an estimated 285 billion sulphur oxidising bacteria, which makes up around half of the worms weight. The bacteria and the worms have a symbiotic relationship where the worm provides oxygen, carbon dioxide and the sulphide that the bacteria need for their metabolism. for the bacteria in the cells in the trophosome, while the bacteria oxides sulphur dioxide that is produced from the vents. This process releases metabolic energy that the tube worms can use to survive. There is no light in the deep ocean where the vents are located so the worms could not receive the needed energy rom photosynthesis. The tube worms also have no intestinal tract, so cannot gain the energy they require from heterotrophy. The worms, therefore, need the symbiotic relationship with the bacteria in order to survive at the hydrothermal vent sites. This relationship does come with a potentially fatal drawback, however.

Toxic Blood

Since the worm needs to transport the sulphur from the surrounding waters to the bacteria in its cells, it requires a system to do so. The only part of the worm’s body that exists the tube that it builds is the Obturacular or plume. This plume that emerges from the tube has a high number of blood vessels in it which contain haemoglobin that carries the oxygen, carbon dioxide and sulphide to the bacteria inside the trophosome. The plume is also made up of branchial lamellae. These lamellae increase the surface area of the plume, allowing for more sulphide to enter the blood via the plume.

Since it is the haemoglobin in the worm that carries the sulphide to the bacteria in the trophosome, the haemoglobin can contain very high concentrations of the sulphide. These concentrations can be up to 1.1 mM. The concentrations of sulphide that are present in the blood of the tube worm would cause sulphide poisoning in humans. This poisoning would occur through the oxidation of hydrogen sulphide that is inhaled, which leads to a reduction in ATP production and anaerobic metabolism.

Adaptations to sulphide

Tube worms, on the other hand, do not get sulphide poisoning despite transporting hydrogen sulphide in their blood. The worms do this by binding sulphide molecules to the haemoglobin in its blood. This means that the sulphide does not come into contact with the oxygen in the blood of the worm. This allows the worm to carry the sulphide in the blood as no oxidation of the sulphide will occur, meaning that the worm will not suffer from the effects of sulphide poisoning.

By doing this, the tube worms accumulate sulphide in their vascular blood. This accumulation of sulphide is not seen in humans or bovine serum albumin. This accumulation of sulphide in the blood can be reversed by the worm back down to lower concentrations, however. The tube worm does this via the reduction of blood pH. The reduction in blood pH releases the sulphide from the blood.

It has, however, been suggested that Riftia reduce the risk of sulphide poisoning further by taking up HS , rather than hydrogen sulphide. HS is less toxic to the tube worms than hydrogen sulphide and the uptake of hydrogen sulphide is more limited. This is because the hydrogen sulphide molecules can only be bound to haemoglobin, whereas, HS can be transported around the blood by the tube worms.

The tube worm, Riftia pachyptila demonstrated that with the discovery of hydrothermal vent systems, there are is a variety of organisms that can survive in hat we as humans previously considered completely uninhabitable environments. Not only does Riftia demonstrate that it has evolved in order to survive in the toxic waters that erupt from hydrothermal vents, but have evolved methods in which they can use the toxic sulphide chemicals in the water as a viable food source.

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