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Life in the Abyss

If there is a harsher place to live than a hydrothermal vent, it hasn't been found yet. Pitch darkness, poison gas, heavy metals, extreme acidity, enormous pressure, water at turns frigid and searing—this seafloor environment seems more like something from deep space than from our own deep sea.

ByPeter TysonNova


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A fish hovers by a forest of tubeworms at a hydrothermal vent on the ocean floor.
© WGBH Educational Foundation

Yet amazing communities of life exist at hydrothermal vents and the so-called "black smoker" chimneys that, given the right conditions, rise above them like erupting stalagmites. Blind shrimp, giant white crabs, and a variety of tubeworms are just some of the more than 300 species of vent life that biologists have identified since scientists first blundered upon this otherworldly community two decades ago. More than 95 percent of these species are new to science.

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It's hard to say which is more remarkable to scientists studying this bizarre world thousands of feet beneath the sea: what these animals have to cope with, or what they have come up with to do that coping.

black smoker chimney
A black smoker chimney belches scalding, mineral-laden water from the subsurface.
© WGBH Educational Foundation

Dark as night

For starters, it's pitch black at such depths. Sunlight penetrates no farther than a few hundred feet down, leaving the deep-sea floor as dark as the deepest cave. With no sunlight, there are no plants; all vent life belongs to the animal kingdom. And with no plants, there is no photosynthesis. Biologists were flabbergasted when they first learned that creatures lived in total darkness at the seafloor. All other life ever identified, on land or in the sea, derives its energy either directly or indirectly from the sun. How, they wondered, did these animals manage without?

Surprisingly enough considering the vastness of the ocean floor, space is extremely limited.

Through chemosynthesis, it turns out. Vent species rely not on photons from the sun but on chemicals from the Earth's interior. Tiny microbes oxidize the hydrogen sulfide that diffuses out of the vents, providing nutrients for animals higher up the food chain. Some creatures, such as the mollusks known as gastropod snails, feast on the bacteria directly; others, including predatory fish, dine on animals that have eaten or otherwise made use of the microbes; still others, like tubeworms, host the microorganisms in their tissues in exchange for organic compounds that the bacteria fashion from the vent chemicals and seawater. (The only element from above that these microbes require for their artistry is oxygen, which is abundant in seawater and was originally produced, of course, by plants. So when it comes right down to it, even these life forms ultimately rely on sunlight. Which may have made those flabbergasted biologists breathe a little easier.)

smoker & crab
A crab within steps of a toxicant-spewing hydrothermal vent
© WGBH Educational Foundation

Acid rock and heavy metal

As if utter darkness were not enough, vent animals must contend with a witch's cauldron of deadly toxicants. Foremost among them is hydrogen sulfide, one of the principal ingredients of the broiling water spewing from vents and black smokers. While vent microbes thrive on the stuff, this gas is lethal to most other organisms, including the creatures that live within wafting distance. Yet not only do those animals survive it, they depend on it as intrinsically as they do on the microbes. Hydrogen sulfide reacts spontaneously with oxygen, so as soon as vent fluids come into contact with seawater, a swift reaction occurs, releasing energy. All that energy would go to waste if it were it not for the microbes. They harness that reaction and use carbon dioxide to make organic compounds that tubeworms, for example, need to live.

Vents and smokers also release a bevy of heavy metals. Besides being toxic substances, these particles can clog mouthparts and gills. Biologists are still trying to figure out exactly how vent animals cope with these. Several animals have metal-binding proteins in their systems, while others, like some polychaete tubeworms, appear to expel these toxics in mucus. Beyond the toxic gas and particles, vent water can also be extremely acidic. The pH of waters coming out of black smokers can be as low as 2.8, making it more acidic than vinegar. Biologists have seen "naked" snails around hydrothermal vents that could not form their calcium carbonate shells because the water was too acidic.

The pressure on every part of this octopus's bone-white body is over 3,000 pounds per square inch.
© WGBH Educational Foundation

Pressure's on

Another factor these creatures have evolved to live with is the pressure. With every 32.8 feet of descent, the weight of the water above increases by 14.7 pounds per square inch. At 7,500 feet, the pressure animals feel over every square inch of their bodies is over 3,350 pounds. At such pressures, any air pockets, such as lungs, would be crushed flat as a deflated balloon. Vent animals have evolved bodies with no such air spaces.

Vent life appears to be more closely related to ancient animals than anything alive today.

Perhaps the most startling condition these animals cope with is unusual temperatures. For they must deal with both extremes—icy and scalding, often simultaneously. Water at the bottom of the ocean is about 35°F, while vent fluids released from chimneys can reach 750°F. Tubeworms and other vent creatures often live right on the flanks of black smokers, within mere inches of the scorching brew, which only the pressure keeps from boiling. Currents constantly stir up the hot and cold, meaning tubeworms and the like have to deal with ever changing temperatures. Even without currents, the extremes are sobering. Biologists have determined that the difference in temperature between a tubeworm's plumed tip and its base anchored in the side of a vent can be more than 50°F. Vent microbes themselves can take temperatures up to 230°F.

crab clinging to tubeworm
Space is dear—grab what you can.
© WGBH Educational Foundation

Location, location, location

To compound problems, the physical environment of the vent itself has limitations. Surprisingly enough considering the vastness of the ocean floor, space is extremely limited. Talk about the value of location: A tubeworm, for one, must live close enough to a vent to get hydrogen sulfide but not close enough to get burned. To make matters worse, due to geophysical changes taking place beneath them, hydrothermal vents and black smokers can turn off suddenly, choking off the life that depended on them. Even healthy black smokers, though they're made of stone, are fragile structures that eventually crumble beneath their own weight. So vent creatures have to have a means for detecting, traveling to, and colonizing new habitat. Yet vents are spread far and wide throughout the world's seas. How newborn vents acquire new residents is a mystery that continues to keep scientists up at night.

But clearly vent creatures manage to do it. Biologists have discovered these animals at sites right around the mid-ocean ridge that circles the globe. Indeed, they have found that vent animals more closely resemble vent creatures on the other side of the planet than they do animals living even a few feet away from them on the ocean floor.

fish and tubeworms
Did life on Earth get its start at a hydrothermal vent on the seafloor?
© WGBH Educational Foundation

First life?

The irony of vent communities is that, despite the harshness of their home, they appear to have survived for many millions of years, having apparently changed little in that time. Vent life, for one thing, appears to be more closely related to ancient animals than anything alive today. What's more, even during times when all hell was breaking loose on the surface, such as during the periodic mass extinctions that have swept the Earth, vent creatures have calmly gone about their lives, probably little affected. This tenacity, evinced albeit through the most exceptional isolation, bodes well for them in the current mass extinction event.

Some biologists have gone so far as to suggest that a vent-like environment was the place where life on Earth likely got its start. And if such a miracle could have occurred here on Earth, why not on other planets that have the necessary ingredients, including heat, water, and the right mix of chemicals? In the end, there may indeed be a harsher place to live than hydrothermal vents. But again, it hasn't been found. Yet.

Editor's Notes

This feature originally appeared on the site for the NOVA program Volcanoes of the Deep.

National corporate funding for NOVA is provided by Draper. Major funding for NOVA is provided by the David H. Koch Fund for Science, the NOVA Science Trust, the Corporation for Public Broadcasting, and PBS viewers.