Hydrothermal Vents

 by Ruthie Gawley

Imagine grotesque towers looming in perpetual blackness. Twisted rockforms littering a cratered floor. Jets of superheated fluid belching into crushingly pressurised water.

No, you aren’t visualising Mordor, or a hellish apocalypse. In fact, you are picturing a hydrothermal vent - one of the most productive, diverse ecosystems on Earth. Scientists believe vents might even hold the millenia-old mystery of the origin of life. 

At first glance though, you would write them off. Found on the seafloor, they are sometimes three miles underwater. For context, this is the height of six Burj Khalifas stacked on top of each other. At these depths, the pressure is 500 atmospheres and the temperature is a frigid 0-3 degrees celsius. But life always finds a way; and in this case the solution lies in the Earth’s molten-hot innards, bubbling away below the ocean floor. At hydrothermal vents, these two violently different worlds collide - and the results are beyond astounding.

We are going to explore the most dramatic type of vent; black smokers. They form in places where tectonic plates are moving apart underwater, and new seafloor is being formed. As a result, cracks and fissures in the floor lead down to the boiling hot magma destined to become new seabed. Seawater trickles down through these. On the water’s journey, metals and minerals in the forming rock dissolve into it. But, finally, the magma heats it up so much that it jets back upwards. At this point, it can be 400 degrees! But, when it once again reaches the freezing deep sea, it quickly cools. All of the minerals and metals it was carrying almost instantly precipitate - they are shocked out of dissolved solution and emerge, fully solid, in the seawater. Because this happens so much, the precipitating materials start to build up. Sulphides of various metals begin to stack, and eventually form huge chimneys around the fissures that water is spewing out of. These are our Black Smokers, which can be an incredible 18 stories high! Imagining the scale of these extreme ecosystems gives me a headache. 

Due to the valuable chemicals and the extra heat offered by hydrothermal vents, life is able to thrive. Animals such as yeti crabs, eyeless shrimp, and dumbo octopus flourish. Incredible bacteria that proliferate in the vents play their part in the explosion of life as well. This is demonstrated by the remarkable Riftia pachyptila. The giant tube worm - as it is more affectionately known - has a cylindrical white body which can grow to three metres tall. It’s topped with a single, blood-red structure that looks like a feather, but is really a lung. Incredibly, it doesn’t possess a mouth, stomach, anus, eyes, or ears. But Rifta still manages to feed itself - with the help of the extremely chemically fertile vents and the handy bacteria

To produce food, Riftia first needs energy. Forming the sugars that it needs to sustain itself requires an input of the stuff. The worm passes this energy-production job to a colony of bacteria living inside of it. These bacteria have adapted so that they can carry out respiration using the chemically rich minerals found in the vents. Their form of respiration, a chemical reaction that releases energy, combines hydrogen sulphide with oxygen. Seawater contains elements found in the air, so oxygen is abundant, and hydrogen sulphide spews out of the vents from deep in the crust. Using its lung, the Riftia takes in both of these and supplies them to the bacteria. The tiny organisms do their respiration, release energy, and the worm can take a tax of this - a fair deal, given the bacteria now have a stable supply of ingredients for respiration, and a safe home within the worm.

Having obtained the energy it needs from the bacteria, Riftia can then craft itself some food. Glucose is the type of sugar that fuels the worm, formed from carbon, oxygen, and hydrogen. To form this delicious food, the worm uses the extra energy to kick start a reaction between carbon dioxide and hydrogen ions- the former occurring in seawater, the latter a byproduct of bacteria respiration. The whole process is called chemosynthesis; using chemical energy (produced by the bacteria, with chemicals alone) to produce food! On land, we are used to photosynthesis, where energy from the sun is required. Chemosynthesis on the seafloor, powered by bacteria, is pretty alien to us. It’s a process which demonstrates the amazing adaptations of life in the vents. 

And it goes beyond just a fascinating quirk of evolution; Riftia’s story is significant on a larger scale. There are many similar tales in hydrothermal vents, and they demonstrate the chemical potential of the ecosystem; minerals and metals from under the Earth mixing with the seawater - which, remember, contains elements from our atmosphere like carbon dioxide and oxygen. A very potent combination; this clash spurs reactions which release energy and produce organic matter, the substances that make up living things. This is exactly what happened in Riftia’s case, with hydrogen sulphide and oxygen releasing energy and carbon dioxide converted into organic glucose. The realisation of this chemical fertility - the ability to easily produce organic matter without the need for sun energy, like is often needed on land - is huge.

So huge that it means that hydrothermal vents are the prime candidate for origin of life. Earth was once just a hunk of rock, nothing but a geological process. At some point 3.7 billion years ago, the first living organism sprung into being. Our planet gained real meaning for the first time. 

The reality is, we don’t know exactly how this happened - how something exploded out of nothing. But our best guesses centre around hydrothermal vents. I’ll say it one more time: the hydrothermal vent’s unique chemistry gift it (a) the chemicals and situations needed to release energy, and (b) the chemicals and situations needed to form organic matter with that energy. That's what the start of life looked like, because you can’t have organisms without organic matter.

The theory we will focus on is scientific revolutionary Gunter Wachtershauser’s. One of the first to pioneer the field, he said that the formation of substances in the black smokers themselves could release the energy we need. His theory centred around iron pyrite, a mineral found in the tower. The pyrite is formed when our old friend hydrogen sulphide comes into contact with iron, after they both explode out of the vents. They meld together, sped up with the help of catalysts: other metals which aren’t affected by the reaction, but increase the rate. Like Riftia’s bacterial respiration, the end products of the reaction have less energy than the starting ones, and the surplus, leftover energy is released.

Unlike Riftia, the amount of energy is much lower, and so not enough to use its trick of converting carbon dioxide into glucose. Instead, Wachtershauser hit on an even better solution - the water also contains the more reactive carbon monoxide. The pyrite’s energy can be harnessed, and, in theory, the carbon monoxide morphs into organic matter. 

Organic matter is the basis of life. Within the productive and energetic waters of the hydrothermal vents, Wachtershauser believed that all of the effort thrown into getting our lovely life-building blocks became worthwhile. In other words, we have black smokers to thank for our very existence today. 

Whether Wachtershauser was right or wrong, it doesn't change the fact- hydrothermal vents are, truely, remarkable. The fascinating mixture of chemistry, geology and biology that takes place in these seafloor fissures is like nothing else. The bizarre creatures; the mind-boggling reactions; the ethereal structures; No, it can’t be argued. There is no place quite like it.