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7.3.3. Chemosynthesis


The chemosynthesis is the synthesis of ATP from the energy released in reactions of small inorganic compounds. The organisms that perform chemosynthesis are called  chemoautotrophs,  chemolyttrophs or chemosynthetics, all are bacteria that use carbon dioxide as a carbon source in a process similar to the Calvin cycle of plants.

The global equation of chemosynthesis, using hydrogen sulfide as an oxidizable molecule is the following:

CO2  + O2  + 4H2S → CH2O + 4S + 3H2O

Many bacteria that live at the bottom of the oceans use chemosynthesis as a way to produce energy without the need for sunlight, where photosynthetic organisms cannot live. Many of these bacteria are the basic food source for the rest of the organisms on the oceanic floor, with very common symbiotic behavior.

Many of the reduced compounds used by bacteria, such as NH3 or H2S, are substances from the decomposition of organic matter. When oxidized, they transform them into mineral substances, NO3- and SO42-, respectively, which can be absorbed by plants. Therefore, these bacteria close the biogeochemical cycles, making life possible on the planet.

Phases of chemosynthesis

  • First phase. Oxidative phase: oxidation of inorganic compounds.

This phase is similar to the photochemical or light phase of photosynthesis.

In this phase, the energy released in the oxidation reactions of inorganic substances (ammonia, nitrite, sulfur, iron, etc.) is used to synthesize ATP  (Oxidative phosphorylation of ADP) and reducing power (NADH in bacteria instead of NADPH as in plants). Part of the ATP is used to cause a reverse transport of electrons in the respiratory chain to obtain NADH.

  • Second stage. Biosynthetic phase: biosynthesis of organic compounds.

This phase is similar to the biosynthetic or dark phase of photosynthesis .

It uses the ATP and NADH obtained in the previous phase to reduce inorganic compounds (CO2 , NO-3, SO2-4 ) and obtain organic compounds.

Types of chemosynthetic bacteria

Depending on the substrate used, bacteria are classified into the following groups:

  • Colorless sulfur bacteria: These bacteria use H2S from the decomposition of organic matter, which is abundant in wastewater, hydrothermal vents and environments rich in sulfur or H2S.
  • Nitrogen bacteria: They live in soil and water. They oxidize reduced nitrogen compounds, specifically ammonia (NH3), which they transform into nitrates (NO3-), which can be used by plants. This oxidation of ammonia from the decomposition of organic matter takes place in two phases:
    • Nitrosifying bacteria: They oxidize ammonia to nitrites.
    • Nitrifying bacteria: They oxidize nitrites to nitrates.
  • Iron bacteria: They oxidize ferrous to ferric compounds.
  • Hydrogen bacteria  Facultative chemoautotrophs, which can use molecular hydrogen.