Chemosynthesis is the use of energy released by inorganic chemical reactions to produce carbohydrates. It is analogous to the more familiar process of photosynthesis. In photosynthesis, plants grow in sunlight, capturing solar energy to make organic matter. In chemosynthesis, bacteria grow in mineral-rich water, harnessing chemical energy to make organic material.

Although chemosynthesis receives much less fanfare than photosynthesis, its activity is equally as important to the global ecosystem. Chemosynthetic organisms drive several major biogeochemical cycles. Chemosynthesis was crucial to the establishment of life on Earth, and is a likely candidate for powering life on other worlds.

There is no single chemosynthetic equation – different inorganic compounds may be utilized, depending on the circumstances. Methane, sulfides, nitrites, ferrous iron, and ammonia are all candidates for chemosynthesis.

Chemosynthetic Reaction
Chemosynthesis uses the energy generated by redox reactions to fix carbon into carbohydrate. Microbes absorb compounds that can function as electron donors, and oxidize them, beginning an electron flow that eventually produces the energy carrier ATP.. This energy is then used to combine carbon with other molecules into a carbohydrate, usually sugar.

There are numerous chemosynthetic pathways, and the exact compounds involved in these reactions vary by organism and environmental conditions. Most chemosynthesizers are autotrophs – they take up inorganic compounds to produce organic matter, and function as the primary producers in their communities. Others are heterotrophs – they obtain some compounds from organic sources, such as those derived from the decomposition of other organisms, but still use the energy obtained from inorganic chemical reactions to process those compounds.

Sulfur-based Chemosynthesis
The most extensive ecosystem based on chemosynthesis is centered around undersea hot springs called hydrothermal vents, where a chemical-rich soup bubbles out of the earth’s crust and into the sea. Boiling hot, saturated with toxic chemicals and heavy metals, and more acidic than vinegar, vent waters are deadly to most marine animals.

But this noxious brew is paradise to bacteria, which coat the rocks around the vent in thick orange and white mats. The bacteria absorb hydrogen sulfide streaming from the vents, and oxidize it to sulfur. They use the chemical energy released during oxidation to fix carbon, hydrogen, and oxygen into sugar molecules.

This reaction - CO2+ 4H2S + O2→ CH2O + 4S + 3H2O - is one of the most important chemosynthetic pathways, supporting diverse deep-sea communities and helping to regulate the chemistry of ocean water.

Nitrogen-based Chemosynthesis
In soils and in freshwater, chemosynthetic activity by nitrifying bacteria is vital to plant growth. All organisms need nitrogen to make amino acids, but the most abundant species of nitrogen, atmospheric N2, is a form that most organisms cannot take up. Luckily, nitrogen-fixing chemosynthetic bacteria oxidize N2into ammonia to generate energy. Then nitrifying bacteria oxidize ammonium into nitrates and nitrates, forms of nitrogen that plants can utilize.