Deep-sea hydrothermal vents: Undersea clues to the origin of life

The word “serendipity” is often used to describe an important discovery born out of random chance rather than deliberate action. Such “happy accidents” pepper the annals of scientific history, from Archimedes’ original Eureka!” moment to Alexander Fleming’s discovery of antibiotics, to the invention of everyday items like Velcro, Teflon™, and microwave ovens. Some of these discoveries have been paradigm-shifting or dogma-breaking, sometimes upsetting decades or even centuries of prevailing scientific thought.

In the late 1970s, one such serendipitous finding on the floor of the Pacific Ocean challenged the very foundations of what we believed to be the fundamental requirements of life on earth.

The sun, energy, and carbon

While scientists have long debated a precise definition of “life”, it is widely accepted that all living organisms use energy to maintain their internal environments and carry out life processes, primary among which are the drives to survive and reproduce. The source of this energy is usually sunlight, converted into chemical energy and consumed as food. Living organisms on earth are also carbon-based – our bodies are built out of materials containing carbon. The source of this carbon can be organic – chemicals containing bonds between carbon and hydrogen, such as proteins, sugars, or fats. Or the source can be inorganic – chemicals where carbon is not bound to hydrogen, e.g., carbon dioxide, the gas we all release when we breathe out.

Plants are photoautotrophs that use the sun’s energy and carbon dioxide to produce carbohydrates (Credit: At09kg, Wattcle, Nefronus, CC BY-SA 4.0 , via Wikimedia Commons)

These two factors – source of energy and source of carbon – allow us to neatly categorize almost every organism on earth into one of two main classes. Plants, algae, and many bacteria draw their energy directly from the sun and obtain their carbon from carbon dioxide in the air, using a process called photosynthesis. These organisms are known as “photoautotrophs”. The name comes from the Greek words for light (phôs), self (autós), and nourishment (trophḗ), i.e., “those who nourish themselves using light”.

Photoautotrophs store the sun’s energy in complex organic molecules, which are then used as food by other lifeforms such as ourselves. This second group is called “chemoheterotrophs” (chemo-, “related to chemicals”, héteros, “other, different”) and includes almost all animals (including humans), fungi (e.g., mushrooms and yeast), and many microbes.

Yet, for both these groups, life ultimately depends on sunlight. And until a little more than fifty years ago, we believed that this was true for the food produced and consumed by nearly every organism on earth.

1977 – An accidental discovery

The twist in the tale comes not from the depths of a hi-tech laboratory, but from a group of intrepid ocean-farers acting on a hunch. In the 1960s and 70s, ocean scientists and geologists were rapidly becoming convinced of the existence of deep-sea hydrothermal vents – locations deep under the ocean where volcanic activity leads to the formation of mountain-like ridges. In these places, cracks in the ground can allow cold seawater to mix with lava from beneath the seafloor, throwing up magnificent jets of warm water. While their existence had been hypothesized for a while, no one had yet seen or photographed a deep-sea vent directly. The Galápagos Hydrothermal Expedition of 1977 hoped to change that.

The team of scientists that sailed out of the Panama Canal on Feb 8, 1977, had many things going for them. They had ANGUS – a massive steel cage fitted with temperature sensors, bright strobe lights, and powerful automated cameras.

DSS Alvin (Public domain, via Wikimedia Commons)

They had Alvin, the very first deep-submergence vehicle ever built – a manned submarine capable of diving to astonishing depths. [1] Alvin would later gain international fame due to its use in the exploration of the wreckage of the Titanic.

They had clues from previous expeditions about the possible location of the vent – a spot a few hundred kilometers off the western coast of South America.

And they had an illustrious mix of geologists, geophysicists, and geochemists on board, along with graduate students, the ship’s crew, and a single reporter – David Perlman from the San Francisco Chronicle, who was soon to make a ‘scoop’ most science journalists can only dream of in their careers.

On Feb 12, the first research ship reached the target location, a spot about 330 km from the Galápagos Islands (the same islands where Darwin made his famous observations more than a century ago, eventually giving birth to his theory of natural selection). Three days later, the scientists sent down ANGUS, with the temperature monitoring system turned on and cameras clicking every 10 seconds. All through the day, ANGUS was dragged behind the ship, hovering a slight distance above the seafloor, while researchers up on board continuously monitored the data it sent. Just around midnight, they finally had their reward – a single spike of high-temperature readings lasting about three minutes.

It was the next day before the films could be retrieved and developed (this was 1977), and the researchers were in for a shock. The thirteen photographs taken around the time when the temperature readings spiked all showed the same thing. Hundreds of live shellfish, primarily clams and mussels, coating the seabed where the temperature spike was detected. All around them was a watery desert, but this one spot was teeming with life.

An undersea oasis

To understand why this finding was so extraordinary, consider that this was an area 2500 meters (8200 feet) beneath the sea’s surface. If the Burj Khalifa, the Shanghai Tower, and the Abraj-Al-Bait Clock Tower – the three tallest buildings in the world – were stacked on top of each other and placed on the ocean floor here, the topmost point of the resulting structure still wouldn’t break the ocean’s surface.

The sun’s rays cannot penetrate much beyond 200 meters down, which means that this is a zone of absolute darkness and biting cold. Add to that the weight of 2.5 kilometers of seawater, which ensures that an animal living here would have to face almost 250 times the pressure one would experience on the earth’s surface.

If a human were to be exposed to such conditions, their lungs would collapse and they would suffocate to death within minutes, if they did not die of hypothermia first. There are no nutrients, no light for photosynthesis, not even enough debris floating down from above to sustain scavengers. By all rights, the place should have been a desert.

The next day, two of the scientists – Jack Corliss of Oregon State University and Tjeerd van Andel from Stanford University – decided to take a look for themselves. Accompanied by a pilot, they rode down in Alvin to the area photographed by ANGUS. This would be Alvin’s 713th dive.  

At first, there was nothing. But as soon as they reached the area photographed by ANGUS, the scenery transformed. Warm shimmering water was flowing upwards around cracks in the ocean floor, colored softly blue by minerals dissolved in the water – the researchers had finally found their hydrothermal vent. And all around it, there was life. Foot-long clams covered the floor. “There are crabs, clams – both live and dead shells, and some puffy-looking things,” said an amazed Corliss into the radio.

Over the next few days, the scientists made several more dives, discovering four more vent sites, each with a unique ecosystem of living creatures. As Perlman, the expedition’s sole journalist later wrote, everywhere the researchers went, they found “rich clusters of living organisms basking in the warmth of the geysers.”.

Giant tubeworm colony in the Galapagos rift (Credit: NOAA Okeanos Explorer Program, Galapagos Rift Expedition 2011, Public domain, via Wikimedia Commons)

The fifth and final site was perhaps the most incredible of all. The vent was surrounded by swaying fields of gigantic snow-white tubeworms, bright red plumes covering their heads. The eerily beautiful worms had no discernible mouths or guts, many reaching over 2 meters (6.6 feet) in length. The researchers named this spot the “Garden of Eden”. In doing so, they unknowingly ended up hinting at a critical inference supported by life in these depths, one that would not become apparent until much later.

So unexpected was this discovery, that the whole contingent of scientists on the expedition did not include a single biologist. When the need to collect and preserve living samples and specimens became obvious, intense scavenging turned up half a flask of formaldehyde that one of the students had happened to carry. The bulk of the samples had to be preserved in the next best alternative – bottles of strong Russian Vodka.

Symbiosis under the ocean

How? This was the question that puzzled scientists worldwide. How could life thrive in such mindboggling abundance in places so far removed from sunlight, so devoid of organic nutrients, and at temperatures and pressures scarcely imaginable for us soft, land-living creatures?

The answers lay in the rotten-egg smell of the water and in the biological samples the expedition had collected. Fiercely sought after in the initial years, these samples eventually revealed a way of life as perfectly logical as it was unexpected. At its heart, it involved a symbiosis – biological partnership – that made these unlikely undersea communities possible.

The giant red-tipped tubeworms that the researchers saw were later named Riftia pachyptila. Riftia can inhabit these nutrient-deprived waters thanks to a specialized organ called the trophosome. The trophosome is stuffed with billions of bacteria that serve as the worms’ home-grown food factory. These bacteria have become specialized in harnessing the energy stored in hydrogen sulfide – a highly toxic gas that smells like rotten eggs and is found dissolved in the water around the vents.

The blood-red plumes of Riftia contain hemoglobin, that carry both oxygen and hydrogen sulfide (Credit: Kbseah, CC BY-SA 4.0 , via Wikimedia Commons)

The bright red color of the Riftia’s head region comes from hemoglobin, the same molecule that gives our blood its characteristic red hue. The hemoglobin binds hydrogen sulfide from the seawater and carries it to the bacteria living in the trophosome, who then break it down to release energy and create organic compounds. The worms do not have mouths for a simple reason – all the food they need is produced within their bodies, delivered up by helpful little tenants in return for a bit of safety and shelter.

Hydrothermal vents are sites of great mixing – the cold water meeting the heat from deep within the earth churns up vast quantities of minerals, including compounds containing sulfur, ammonia, and metals such as iron and manganese. Many species of bacteria and archae (another type of single-celled microorganism) can utilize these as energy sources to carry out all the processes required for life. Instead of photosynthesis, they carry out chemosynthesis. Instead of releasing oxygen as a byproduct, the sulfide-metabolizing bacteria excrete crystals of pure sulfur. In place of photoautotrophs, these microorganisms are “chemoautotrophs” and the organic compounds they produce are enough to sustain entire ecosystems.

In addition to the tubeworms, certain clams and mussels from the vent ecosystems have also been shown to harbor bacteria that they depend on for nutrition. Other animals don’t bother with growing the bacteria within their bodies, choosing instead to munch on the thick layers of microorganisms that grow on the rocks surrounding the vents, swallowing them whole for nutrition. Yet others survive by preying on the animals that depend on the bacteria for survival. An intricate food-web, complete with producers, grazers, predators, and prey, that has nothing to do with the sun above.

It should be noted here that this wasn’t humanity’s first encounter with the phenomenon of chemosynthesis. In fact, it had been proposed as far back as the late 19th century, to explain the behavior and growth of certain bacteria. However, it would have been dismissed as an oddity – yet another peculiarity of the microbial world that could be filed away in the backrooms of scientific knowledge – if not for the findings of the Galápagos Hydrothermal Expedition. No one, absolutely no one had imagined that chemoautotrophic microorganisms could sustain the large variety and quantity of life the scientists found near the vents.

Since their original discovery, chemosynthesis-dependent food chains have also been observed in large freshwater lakes and dark caves, while soil-living chemoautotrophs are now known to be critical for maintaining the earth’s nitrogen cycle. And as scientists are now hypothesizing, deep-sea chemoautotrophs may hold the answer to one of the most fundamental questions in biology – how did life on earth originate?

From early earth to space and beyond

Scientists estimate that the earth formed about 4.5 billion years ago. The earliest fossil evidence of lifeforms dates back to about 3.5 billion years ago, suggesting that life arose pretty early on in the earth’s lifespan. Given that every organism living today shares a common ancestor, it also appears that life may have only evolved once on our planet (or as far as we know, anywhere in the universe). The conditions that led to the origin of life, therefore, were likely both rare and relatively short-lived (pun intended).

While we have no way to determine the exact conditions that prevailed on early earth, of one thing we are reasonably certain – it looked nothing like the lush, temperate planet we know today. Soon after its formation, the earth was probably still being regularly barraged by rogue asteroids and meteorite collisions, which resulted in extremely high temperatures and dense clouds blocking out much of the available sunlight. Oxygen in the atmosphere was vanishingly small, and surfaces were continuously bombarded by ultraviolet radiation since the ozone layer was yet to form.

Earlier, scientists believed that the atmosphere was rich in gases like methane and ammonia, which could have reacted in the presence of water to give rise to the organic molecules that are the precursors of life. However, it now appears much more likely that instead of methane and ammonia, the atmosphere was made up of nitrogen and carbon dioxide, gases that are much more inert and unlikely to react spontaneously.

Given this, of all the places on early earth likely to sustain conditions necessary for the origin of life, the hydrothermal vents probably came the closest. Their high temperatures and pressures coupled with their mineral richness would contribute to bringing together many of the key ingredients for the earliest metabolic pathways – dissolved methane, ammonia, hydrogen, metal sulfides that could act as catalysts. And once the building blocks of life were created, over time, chance interactions might result in some of them assembling into proto-cells and later gaining the property of self-replication (the ability to copy themselves) – the hallmark of life. Such proto-lifeforms would ultimately move out from the vent surroundings, spreading through the oceans, making this the actual ‘Garden of Eden’ from which all life on earth would eventually descend. 

In 2017, a team of researchers reported observing fossilized microorganisms in rocks collected near a hydrothermal vent near Canada. They dated these fossils as an astounding 3.77 – 4.28 billion years old. While still controversial, if validated, this finding would make these fossils the oldest evidence of life on earth, appearing only a few hundred million years after its formation. This would lend strong support to the theory of life originating at hydrothermal vents.

Jupiter’s moon Europa may have hydrothermal vents under its surface oceans (Credit: NASA / Jet Propulsion Lab-Caltech / SETI Institute, Public domain, via Wikimedia Commons)

Similarly, the search for extraterrestrial life has received a boost from the observation of deep-sea chemoautotrophs. The vast oceans of Jupiter’s moon Europa, one of the likeliest places in the solar system to harbor life, are believed to contain hydrothermal vents. Deep-sea hydrothermal vents are also proposed to have existed on Mars, back when it still had liquid water, a possible home for ancient life in one of our nearest neighbors.  

And once we step out of the solar system, the possibilities only multiply. The amount of stellar energy received by exoplanets is likely to vary, and the surface of many such planets may be inhospitable due to radiation, meteor strikes, or harsh weather conditions. It is possible that in such cases, life would originate below the ground or deep underneath liquid oceans, where the conditions would be ripe for chemoautotrophism to take center stage.

As American physicist Joseph Henry said in 1877, “The seeds of great discoveries are constantly floating around us, but they only take root in minds well-prepared to receive them.” The discovery of deep-sea ecosystems strengthens this view of scientific inquiry and hints at the many secrets that may still be hiding in the depths of one of the world’s greatest wildernesses – its oceans. 

Further Reading

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