The metabolic diversity of the prokaryotes makes them key players in the cycles that keep elements moving through ecosystems. Many prokaryotes are decomposers: organisms that metabolize organic compounds in dead organic material and return the products to the environment as inorganic substances. Prokaryotes, along with fungi, return tremendous quantities of carbon to the atmosphere as carbon dioxide, thus carrying out a key step in the carbon cycle.

The key metabolic reactions of many prokaryotes involve nitrogen or sulfur. For example, some bacteria carry out respiratory electron transport without using oxygen as an electron acceptor. These organisms use oxidized inorganic ions such as nitrate, nitrite, or sulfate as electron acceptors. Examples include the denitrifiers, which release nitrogen to the atmosphere as nitrogen gas (N₂). These normally aerobic bacteria, mostly species of the genera Bacillus and Pseudomonas, use nitrate (NO₃^–) as an electron acceptor in place of oxygen if they are kept under anaerobic conditions:

2 NO₃^– + 10 e^– + 12 H⁺ → N₂ + 6 H₂O

Denitrifiers play a key role in the cycling of nitrogen through ecosystems. Without denitrifiers, which convert nitrate ions back into nitrogen gas, all forms of nitrogen would leach from the soil and end up in lakes and oceans, making life on land much more difficult.

Nitrogen fixers convert atmospheric nitrogen gas into a chemical form (ammonia) that is usable by the nitrogen fixers themselves as well as by other organisms:

N₂ + 6 H → 2 NH₃

All organisms require nitrogen in order to build proteins, nucleic acids, and other important compounds. *Nitrogen fixation is thus vital to life as we know it. This all-important biochemical process is carried out by a wide variety of prokaryotic archaea and bacteria (including cyanobacteria) but by no eukaryotes, so we depend on these prokaryotes for our very existence.

Ammonia is oxidized to nitrate in soil and in seawater by chemoautotrophic bacteria called nitrifiers. Bacteria of two genera, Nitrosomonas and Nitrosococcus, convert ammonia (NH₃) to nitrite ions (NO₂^–), and Nitrobacter oxidize nitrite to nitrate (NO₃^–), the form of nitrogen most easily used by many plants. What do the nitrifiers get out of these reactions? Their metabolism is powered by the energy released by the oxidation of ammonia or nitrite. For example, by passing the electrons from nitrite through an electron transport system, Nitrobacter can make ATP and, using some of this ATP, can also make NADH. With this ATP and NADH, the bacterium can convert CO₂ and H₂O into glucose.

546

We have already seen the importance of the cyanobacteria in the cycling of oxygen: in ancient times, the oxygen generated by their photosynthesis converted Earth’s atmosphere from an anaerobic to an aerobic environment. Other prokaryotes—both bacteria and archaea—contribute to the cycling of sulfur. Deep-sea hydrothermal vent ecosystems depend on chemoautotrophic prokaryotes that are incorporated into large communities of crabs, mollusks, and giant worms, all living at a depth of 2,500 m—below any hint of sunlight. These bacteria obtain energy by oxidizing hydrogen sulfide and other substances released in the near-boiling water flowing from volcanic vents in the ocean floor.