Chemosynthesis is the process by which certain bacteria and archaea produce food using energy from chemical reactions rather than sunlight. Where photosynthesis powers nearly all life on Earth’s surface, chemosynthesis sustains entire ecosystems in places sunlight never reaches, from the deep ocean floor to sealed underground caves. It was first observed in 1977 at hydrothermal vents along the Galápagos Rift, and it fundamentally changed how scientists understand where life can exist.
How Chemosynthesis Works
In photosynthesis, plants capture solar energy and use it to convert carbon dioxide and water into sugar and oxygen. Chemosynthesis follows a similar logic but swaps out sunlight for chemical energy. Bacteria harvest energy by breaking down inorganic compounds, things like hydrogen sulfide, methane, ammonia, or dissolved iron, and use that energy to build organic molecules from carbon dioxide.
The most common version at deep-sea vents uses hydrogen sulfide, the compound responsible for the rotten-egg smell. In the presence of carbon dioxide and oxygen, bacteria oxidize hydrogen sulfide to produce simple carbohydrates, elemental sulfur, and water. Other bacteria run similar reactions using methane or ammonia as their fuel source. In each case, the inorganic compound acts as the electron donor, the same role that water plays in photosynthesis.
Not all of these reactions require oxygen. Some chemosynthetic microbes operate in completely oxygen-free environments, pairing methane oxidation with sulfate reduction. This anaerobic version of chemosynthesis is widespread on the ocean floor, particularly at cold seeps where methane bubbles up through sediment. Recent research in the South China Sea has identified additional pathways where microbes couple methane oxidation with nitrate or nitrite reduction, expanding the known ways organisms can extract energy from methane in the deep sea.
Where Chemosynthetic Ecosystems Exist
The most famous chemosynthetic habitats are hydrothermal vents, cracks in the ocean floor where superheated, mineral-rich water gushes into the cold deep sea. These vents occur along mid-ocean ridges and other tectonically active zones. The 1977 Galápagos expedition, led by scientists Jack Corliss, Jack Dymond, and John Edmond using the submersible Alvin, found thriving animal communities 2,500 meters below the surface, in total darkness, clustered around these vents. It was the first direct evidence that complex life could be sustained entirely without photosynthesis.
Cold seeps are another major habitat. Unlike vents, which blast out hot fluid, cold seeps release methane and hydrogen sulfide slowly through the seafloor. They support dense communities of mussels, clams, and microbial mats, all ultimately fueled by chemosynthetic bacteria.
Chemosynthesis also occurs on land. Movile Cave in southeastern Romania has been sealed off from the surface for roughly 6 million years. Its waters are rich in hydrogen sulfide, methane, and ammonia, and the entire food web runs on chemosynthetic bacteria rather than anything derived from sunlight. Sulfur-oxidizing and ammonium-metabolizing microbes form the base of a community that includes more than 30 species of cave-dwelling invertebrates found nowhere else on Earth. Amphipods in the cave carry sulfur-oxidizing bacteria on their bodies, a direct partnership between animal and chemosynthetic microbe.
The Organisms Behind It
Chemosynthesis is carried out exclusively by bacteria and archaea. At hydrothermal vents alone, researchers have cultured dozens of genera spanning multiple major groups. These include sulfur-oxidizing bacteria, hydrogen-oxidizing archaea, and methane-producing archaea called methanogens. The diversity is enormous: a single vent field can host organisms from at least nine different bacterial phyla and two major branches of archaea.
Iron-oxidizing bacteria represent a particularly energy-constrained form of chemosynthesis. Oxidizing dissolved iron yields less energy per reaction than oxidizing hydrogen sulfide, as little as 29 kilojoules per mole under standard conditions. That is a tiny energy budget for building a cell. These bacteria compensate by living at the boundary between iron-rich, oxygen-poor water and oxygenated water, where lower oxygen concentrations can triple their energy yield. The first iron-oxidizing bacterium ever described, back in 1837, was a stalk-forming microbe found in iron-rich freshwater. Similar organisms turn up today in groundwater seeps, wetlands, and even the soil around plant roots near acid mine drainage.
Giant Tube Worms and Symbiosis
Many of the large animals at hydrothermal vents don’t eat chemosynthetic bacteria in the traditional sense. Instead, they host them internally. The giant tube worm is the most striking example. These worms, which can grow over a meter long, have no mouth, no gut, and no anus. They survive entirely through a symbiotic relationship with sulfur-oxidizing bacteria housed in a specialized organ called the trophosome.
The worm’s feathery red plume, which extends from the top of its tube into the vent water, is packed with hemoglobin proteins that make up roughly one-fifth of the plume’s total protein content. These hemoglobins are unusual because they bind both oxygen and hydrogen sulfide simultaneously, something that would be toxic to most animals. The blood carries both compounds through the worm’s circulatory system directly to the bacteria in the trophosome, which use them to produce the organic carbon the worm needs to survive. It is a complete outsourcing of nutrition to an internal microbial partner.
How Much Energy Chemosynthesis Produces
Globally, chemosynthesis is a minor player compared to photosynthesis. The total carbon fixed by chemosynthetic bacteria beneath the seafloor at deep-sea hot springs amounts to roughly 1.4 teragrams of carbon per year at most, representing about 0.43% of the photosynthetic material that sinks to ocean depths below 2,000 meters.
Locally, though, the picture is very different. Carbon fixation rates measured at individual vent sites range from around 380 to 9,300 grams of carbon per square meter per year. Those numbers are comparable to rates measured in the sunlit coastal ocean and are two to four orders of magnitude higher than the amount of photosynthetic material that drifts down to the same depth. In other words, a hydrothermal vent field is a biological hotspot surrounded by what would otherwise be a food desert.
Chemosynthesis and the Search for Alien Life
Chemosynthesis has become central to astrobiology because it removes sunlight from the equation for life. Two moons in our solar system, Jupiter’s Europa and Saturn’s Enceladus, are thought to have liquid water oceans beneath their icy surfaces, kept warm by tidal heating from their parent planets. If life exists in those oceans, it would almost certainly depend on chemical energy rather than light.
On Enceladus, the Cassini spacecraft detected molecular hydrogen in plumes of water vapor shooting from the moon’s south pole. Hydrogen is exactly the fuel that methane-producing archaea use on Earth, and methanogenesis is now the most commonly proposed metabolism for potential life on Enceladus. Scientists have modeled scenarios where tidal heating drives hydrothermal activity on Enceladus’s ocean floor, producing hydrogen through water-rock reactions called serpentinization, the same process that powers Earth’s Lost City hydrothermal field in the Atlantic.
Europa presents a different set of possibilities. Radiation from Jupiter constantly bombards Europa’s icy surface, splitting water molecules and generating oxygen and other oxidants. If those compounds can migrate downward through the ice into the ocean below, they could pair with chemical reductants from the seafloor to power aerobic or anaerobic metabolisms. The most commonly proposed pathways for Europa include methane oxidation coupled with sulfate reduction, a pairing found widely on Earth’s ocean floor. Because there is no direct detection of specific chemicals in Europa’s ocean yet, the range of hypothesized metabolisms is broader than for Enceladus, but they all trace back to the same principle: life running on chemistry, not light.