How Can Animals Live on a Deep Seafloor Where Sunlight Never Reaches?

In 1977, a Team Looking for Warm Water Found a Crowd of Animals

In February 1977, the research ship Knorr left the Panama Canal for a spreading ridge on the Pacific seafloor near the Galapagos Islands. A ridge like this is a long underwater crack where the plates that make up Earth’s crust are slowly pulling apart. The scientists on board were geologists, chemists and geophysicists, and there was no biologist among them. The deep seafloor was thought to be too dark and too short of food for crowded life, and the team hoped to find warm water seeping out of the crack [1], [2].

On February 15, a camera and instrument sled towed near the bottom, at about 2,500 meters (8,200 feet) down, picked up a brief temperature spike. Two days later, on February 17, the submersible Alvin made dive 713 to the same area. Its crew found warm water measured at 8 °C (46 °F), which is warm only compared with the near-freezing water around it. Nearby were white clams up to 30 centimeters (about a foot) long, mussels, crabs and a purple octopus. At another site stood tube worms about half a meter (a foot and a half) tall with bright red tops, in water up to 17 °C (63 °F) [1].

The ship was not ready to preserve all this. Its supply of preserving chemicals ran short, and the team used strong vodka bought in Panama as well [2]. The animals were found, but nobody on board yet knew what they lived on.

Water From the Vents Smelled of Rotten Eggs, and That Smell Was a Clue

The answer began with a smell. When chemists on the ship opened water samples from the vent area, the lab filled with the smell of rotten eggs, and people rushed to open portholes [1], [2]. The cause was hydrogen sulfide, a gas that is toxic to us. The scientists saw it as a powerful clue to what the animals might be using for energy in place of sunlight [2].

Sunlight is the starting point of almost every food chain we meet on land and near the sea surface. Plants and algae use its energy to make food, and animals eat them. On a seafloor 2,500 meters deep that starting point is missing, so the question became where the energy came from.

Hot Rock Under the Seafloor Heats Seawater and Loads It With Chemicals

The vents themselves are the source. Cold seawater seeps down through cracks in the crust near the edges of plates. Deep below, it is heated by hot magma and then comes back out onto the seafloor [4]. On its way, it carries chemicals out of the rock, hydrogen sulfide among them.

Vent water can be far hotter than at the 1977 sites. NOAA says it may reach more than 700 °F (about 370 °C), and that it does not boil at the seafloor because the pressure of the deep ocean is so high [4]. Where the hottest fluid meets cold seawater, minerals settle out and build chimneys. “Black smokers” get their color from iron sulfide, and “white smokers” from minerals such as barium, calcium and silicon [4].

Bacteria at the Vents Make Food From Chemicals Instead of Sunlight

The tiny living things that start the food chain here are bacteria. They get energy from chemicals in the vent water, such as hydrogen sulfide, and use it to make food. This is called chemosynthesis [4], [3]. Photosynthesis in a plant starts with light, while chemosynthesis in these bacteria starts with chemical reactions. In the food chain, the bacteria play the part that green plants play in sunlit places.

This overturned an assumption many people had held, that all life ultimately depends on the Sun. The vent animals depend on chemicals that come up from inside the Earth, not on sunlight[4].

A Tube Worm With No Mouth Keeps Its Bacteria Inside a Special Food-Making Organ

That brings us back to the animal with no mouth. It has no mouth, no gut and no apparent way to eat [3]. Instead, much of its body is a spongy organ called the trophosome, which holds its bacteria and makes up over half the worm’s weight [3].

The worm still has to deliver ingredients to its bacterial tenants. The red feathery plume at the top contains a special form of hemoglobin, the same kind of protein that carries oxygen in our blood. In the worm it is modified to carry both oxygen and hydrogen sulfide to the bacteria [3]. So the worm and its bacteria are partners: the worm collects the chemicals from the vent water, and the bacteria turn them into food for the worm.

Vent Communities Are Not One Lucky Spot, Because Hundreds of Active Vent Fields Are Known

The 1977 find turned out to be one of many. The InterRidge Vents Database, a scientists’ catalog, listed 721 known vent fields in its March 2020 version. Of these, 666 were confirmed or inferred to be active and 55 inactive, and 134 active fields had been added in the ten years before [5]. Vents are mostly found along ridges where plates spread apart, though also near some undersea volcanoes [4]. Large parts of the deep seafloor have never been explored, so the catalog is a count of what has been found so far.

Draw Two Food Chains, One That Starts With the Sun and One That Starts at a Vent

You can test your understanding with paper and pencil. Draw a chain of arrows for a shallow sea or a pond: sunlight, tiny algae, a small animal, a fish. Now draw a vent chain: hot rock, vent water full of hydrogen sulfide, bacteria, a tube worm. In each chain, which step builds food from simple ingredients? What does the first step in each chain supply?

If you would like to see the real animals, the Woods Hole page on the 1977 discovery (the first source below) tells the story and has pictures. Ask a grown-up to look at it with you.

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