I.

Introduction

Chemosynthesis, process in which some types of microorganisms use energy from chemical reactions to produce organic (carbon-containing) compounds as food for themselves. Chemosynthesis differs from photosynthesis, in which plants and certain microorganisms use light energy from the Sun to produce carbohydrates as food from carbon dioxide (CO₂) and water. Organisms that manufacture their own food are called autotrophs. Microbes that use chemicals in the process are called chemoautotrophs and include members of both the bacteria and the archaea.

Bacteria that use chemosynthesis are important in fixing nitrogen to make the element available in a chemical form that plants can use as part of the nitrogen cycle. Scientists have discovered unusual communities of marine life on the ocean floor that depend on chemosynthetic microorganisms as the first step in a food chain. Other chemosynthetic microbial life has been found in caves and deep inside rocks in Earth’s crust. Different types of chemoautotrophic microbes use hydrogen, hydrogen sulfide (H₂S), or methane (CH₄) as a chemical energy source.

Some chemosynthetic organisms are extremophiles that thrive in environments that have extreme temperatures, pressures, or chemical conditions. However, not all extremophiles are chemosynthetic (some use photosynthesis) and not all chemosynthetic microbes are extremophiles found in extreme conditions.

II.

Oxidizing Microorganisms

Chemosynthetic microorganisms that use oxygen play a major role in the nitrogen cycle and other processes. So-called nitrifying bacteria that live in the soil change ammonia into nitrite, which other bacteria then oxidize to nitrate as part of a process called nitrification. Some oxidizing bacteria that can cause corrosion on metal use the electrochemical potential between iron ions as an energy source.

III.

Chemosynthetic Biological Communities in the Ocean

Scientists have found different types of communities of organisms on the ocean floor that depend on chemosynthetic microbes as the basis of a food chain. In such biological communities, the microbes that produce their own food using chemical energy instead of sunlight are either eaten directly or produce byproducts that other animals use as nutrients, often in symbiotic relationships. Special types of animals such as tube worms, clams, and crustaceans are often associated with such communities. Tube worms and clams can host chemosynthetic microbes on their tissues to extract food.

The best-known of such biological communities are found around hydrothermal vents where volcanically heated water rich in minerals and in chemicals such as hydrogen sulfide rise from the sea floor. Chimneylike structures called “black smokers” are found in geologically active areas of the ocean crust in different parts of the world. Similar communities can also form where water heated by chemical reactions involving minerals in the mantle emerges from the ocean floor.

Another type of chemosynthetic community can occur where chemical-rich fluids ooze out of the ocean floor at cool or lukewarm temperatures. These locations are often called “cold seeps” to distinguish them from volcanically heated vents. The fluids can be produced by the decomposition of organic materials and may be in the form of hydrocarbons such as petroleum or natural gas (methane). Some fluids may come from deep in the ocean floor and be produced by inorganic chemical reactions that originally took place at high temperatures and pressures. Such communities usually occur around the margins of continental plates (see Plate Tectonics).

Cold seeps are associated with a number of geologic features. Undersea mud volcanoes are conelike structures of mud or clay that erupt with methane gas. Brine pools are deep depressions in the ocean floor that contain extremely salty water—about four times saltier than ordinary seawater—and supply methane to bacteria that are food sources for mussels and other animals that live along the upper margins of the brine pools. Frozen methane hydrate deposits are found buried in many places on the sea floor. The cold temperatures and high pressures cause the methane gas to be trapped with water in an icelike state. In some regions, methane hydrates are exposed on the surface of the sea floor and can support chemosynthetic communities. In other locations, bacteria that live beneath the surface of the ocean floor produce hydrogen sulfide. Other types of microorganisms can then use the hydrogen sulfide released as an energy source.

Chemosynthetic microbes also live on the bones of dead whales and on water-soaked logs or tree trunks that sink to the sea floor. The microorganisms in turn become the basis of a food chain. Scientists think such scattered oases of chemosynthetic communities may allow such organisms to spread between distant regions that have hydrothermal vents or cold seeps.

IV.

Underground Chemosynthesis

Chemosynthetic microbes have been found deep in Earth’s crust. In 1995 geologists found bacteria living 1,500 m (5,000 ft) below the surface in Columbia River basalts created by ancient volcanic eruptions. Iron-bearing minerals in the rock reacted with oxygen-poor water to release hydrogen. Chemosynthetic bacteria then used hydrogen and dissolved carbon dioxide to produce methane and other hydrocarbon molecules to use as food (see autotroph). Similar processes involving bacteria may happen in basalt under the sea and in other locations.

Underground microbes that derive energy from hydrogen sulfide are found in caves. The hydrogen sulfide may be produced by other microorganisms feeding on petroleum deposits deep below the caves. The cave organisms create sulfuric acid (H₂SO₄) in combination with oxygen and water. The sulfuric acid in turn dissolves limestone, hollowing out the chambers of the caves and leaving gypsum formations.

V.

Study of Chemosynthesis

Chemosynthetic organisms are of special interest to scientists who study the origin of life and the possibility of life on other bodies in space (see astrobiology). Some chemosynthetic organisms such as archaea can also be extremophiles that exist in extreme heat or chemical conditions similar to those thought to exist on early Earth. Researchers hope to find clues to how life came to exist on Earth or might exist elsewhere in the universe. Within our solar system, the planet Mars and the moons Europa and Titan might have conditions similar to those where chemosynthetic microbes are found on Earth.

Scientists also study chemosynthetic microorganisms to understand chemical processes in living things (see biochemistry). The special chemistry used by chemosynthetic microbes could have application in industry and manufacturing. The microbes themselves or genetically modified versions could also be used to treat pollution from industrial and toxic chemicals.