How do deep-sea hydrothermal vent creatures survive without sunlight?
― The chemistry of turning hydrogen sulfide into energy
Past 200m depth, sunlight barely reaches. Past 1000m, it's total darkness. And yet, around deep-sea "hydrothermal vents" past 2000m, crowds of distinctive creatures like tube worms and clams cluster together. Where photosynthesis should be impossible, what are these creatures living on?
Trace the food chain on land or in shallow water, and you almost always reach plants or phytoplankton. Herbivores eat plants; carnivores eat herbivores — trace it back far enough, and "photosynthesis," which uses sunlight to make food, is the starting point for everything.
So on the deep-sea floor, where sunlight never reaches, how do creatures create that "starting point"?
From cracks in the seafloor, hot water loaded with chemicals like hydrogen sulfide is thought to gush out.
Certain bacteria are thought to work by using the energy released when hydrogen sulfide is oxidized to build nutrients (organic matter) — a process called chemosynthesis.
Let's look, step by step, at the chemicals in the hydrothermal water and the chemosynthesis that uses them.
Making food with chemosynthesis instead of photosynthesis
Photosynthesis, carried out by plants and phytoplankton, is a reaction that uses the sun's light energy to build nutrients (organic matter) from carbon dioxide and water. Around deep-sea hydrothermal vents, bacteria are thought to exist that build the same kind of nutrients using the energy held in a chemical, hydrogen sulfide, instead of light energy. This process is called "chemosynthesis." When hydrogen sulfide combines with oxygen (is oxidized), energy is released, and bacteria use that energy to assemble organic matter from carbon dioxide.
Tube worms have no mouth and no stomach
Tube worms (relatives of the beard worm), commonly seen at hydrothermal vents, are thought to lose their mouth and digestive tract entirely as they mature. So how do they get nutrients? It's thought they pack a special internal organ densely with chemosynthesizing bacteria, and receive the nutrients the bacteria produce directly. In turn, the bacteria are thought to be in a mutually beneficial relationship (symbiosis), receiving a steady supply of the hydrogen sulfide and oxygen they need for chemosynthesis via the tube worm's body.
The deep-sea floor a little way from a hydrothermal vent is thought to be a nutrient-poor, sparsely populated "desert." Within that desert, only the area around a vent becomes an "oasis," a densely packed ecosystem made possible by chemosynthesis.
They've found a different energy source, one the Earth itself provides.
Something you can check for yourself
- Write "Photosynthesis" on paper, then connect a sun icon and the words carbon dioxide, water, food, oxygen with arrows
- Next to it, write "Chemosynthesis," swap the sun icon for the words "hydrogen sulfide," and connect it the same way
- Compare how only the input (energy source) differs, while the output (making food) is the same
This gives a feel for how the deep-sea ecosystem stands independent of the land ecosystem, which needs sunlight.
Summary
Deep-sea hydrothermal vent creatures are thought to survive without sunlight because of chemosynthesizing bacteria that make food using the energy in a chemical, hydrogen sulfide. Some creatures, like tube worms, house these bacteria inside their bodies in symbiosis, using that energy indirectly. Just as land ecosystems depend on the sun, deep-sea ecosystems depend on chemical energy from inside the Earth.
If photosynthesis is a "child of the sun," chemosynthesis is a "child of the Earth itself."
For how creatures withstand the extreme conditions of the deep sea, see this article.
Want to know more? ― Terms, numbers, and textbook connectionsWe mark clearly which level each part belongs to, from middle-school science to active research
- Middle schoolCovered in middle-school science
- High schoolCovered in high-school "Basic Biology"
- High school+Treated as advanced/column content in high-school textbooks
- UniversityNot taught in high school — university-level specialist content (geochemistry, deep-sea biology)
- ResearchNot yet settled even at university level — a topic researchers are actively investigating
Middle schoolTerms: words used around hydrothermal vents
- Hydrothermal vent: A place where hot water gushes from a crack in the seafloor.
- Chemosynthesis: The process of making nutrients using chemical energy instead of light.
- Symbiosis: Different species of organisms living together, helping each other.
High schoolChecking with a formula: how much water pressure is there at hydrothermal vent depth?
Hydrothermal vents lie in the deep sea, past 2000m depth. Let's estimate the water pressure at that depth using the relationship between depth and pressure.
| In symbols | P ＝ P0 ＋ ρ × g × h |
| In words | Water pressure ＝ atmospheric pressure ＋ seawater density × gravitational acceleration × depth |
| Where it comes from | This is the hydrostatic pressure formula: the weight of the seawater directly overhead becomes the pressure. |
| What the symbols mean | P is water pressure, P0 is atmospheric pressure at the surface, ρ is seawater density (about 1025 kilograms per cubic meter), g is gravitational acceleration (9.8 meters per second per second), h is depth (in meters) |
Water pressure (atm) ≒ 1 ＋ depth (m) ÷ 10
| Baseline 1 atm | Atmospheric pressure at the surface (depth 0m) |
| Increase per 10m of depth | Working out ρ × g × 10 in the formula above gives roughly 1 atm |
| Depth ÷ 10 | 2500 ÷ 10 ＝ 250 |
| Water pressure (atm) | 1 ＋ 250 ＝ 251 |
| Result | About 251 atmospheres |
251 atmospheres is close to having about 251kg bearing down on every square centimetre. Chemosynthesizing bacteria and the creatures that live with them are thought to manage just fine even under this much pressure. In other words, they're adapted to a doubly harsh environment: extreme darkness and extreme pressure at once.
High school+The reaction that oxidizes hydrogen sulfide
Most chemosynthesizing bacteria are thought to extract energy by reacting hydrogen sulfide (H₂S) with oxygen to form sulfur or sulfate ions. The energy released in this step drives a reaction that builds organic matter, such as sugars, from carbon dioxide. While photosynthesis, which uses light energy, is called "photoautotrophy," this process, which uses chemical energy, is called "chemoautotrophy."
UniversityThe detailed mechanisms of symbiosis and metabolism studied in deep-sea biology
In deep-sea biology and geochemistry, researchers study in detail the metabolic pathways of the bacteria inside the tube worm's internal organ (the trophosome), and the exchange of substances with the host. Hydrogen sulfide is normally highly toxic to organisms, but the host is also thought to have a mechanism for transporting it safely using specialized proteins.
ResearchWhat's still not settled
- The hypothesis that life itself may have originated in an environment like a hydrothermal vent (the hydrothermal-vent origin-of-life hypothesis) is one of the major themes still under active research in the life sciences.
- Astronomers and astrobiologists are discussing the possibility that similar hydrothermal environments — and life — may exist in the oceans inside icy extraterrestrial moons, such as Jupiter's moon Europa and Saturn's moon Enceladus.
- Research is also ongoing into at what evolutionary stage, and how, the symbiotic relationship between tube worms and bacteria came about.
Even a single small ecosystem in the darkness of the deep sea is packed with a rich, still-active theme where geochemistry and the life sciences meet.
Connections to textbooks (by level)
| Level | Subject/Unit | Where in this article |
|---|---|---|
| Middle school | Science, connections between organisms | Basic terms: hydrothermal vents, chemosynthesis, symbiosis |
| High school | Basic Biology, water pressure (advanced) | Calculating depth and water pressure |
| High school+ | Biology, autotrophy (advanced) | Oxidation of hydrogen sulfide and energy |
| University | Deep-sea biology, geochemistry | Trophosome metabolism, hydrogen sulfide transport |
| Research | Life sciences, astrobiology (ongoing) | Hydrothermal-vent origin-of-life hypothesis, search for life in extraterrestrial oceans |
- Explanations of hydrothermal vent ecosystems and chemosynthesis in deep-sea biology textbooks.
- Explanations of hydrogen sulfide oxidation and energy metabolism in geochemistry reference materials.
- Explanations of the possibility of life on extraterrestrial ocean worlds in astrobiology reference materials.
※ The depth and pressure figures are rough values meant to aid understanding of the mechanism. Actual hydrothermal vent depths and pressures are thought to vary by location.
※This article is a general-audience science explainer. The figures given are approximations to aid understanding of the mechanism. Actual values are thought to vary by location and conditions.