Chemosynthesis

Historical background and core concepts

Chemosynthesis is the unique ability of chemosynthetic organisms to use the energy released during the oxidation of inorganic substances. This energy is expended on the synthesis of complex organic compounds necessary to build the cell's own structures.

The main difference between chemosynthesizers and photoautotrophs lies in their energy source: while plants and cyanobacteria require sunlight, chemosynthesizing bacteria rely entirely on chemical reactions (exothermic processes).

The phenomenon of chemosynthesis was discovered in 1889 by the prominent Russian microbiologist S.N. Winogradsky. He was the first to prove the existence of autotrophic non-green organisms in nature by studying the life activity of nitrified bacteria in detail. This discovery revolutionized scientific views on how living creatures obtain nutrition.

Nitrifying bacteria

Nitrifying bacteria inhabit soil and various water bodies predominantly. They play a colossal role in nature, serving as a critical link in the global nitrogen cycle.

Their specific mechanism involves secreting enzymes directly into the external environment. The oxidation of nitrogen-containing compounds occurs in two consecutive stages:

1. Oxidation of ammonia to nitrous acid (Stage I). At this stage, ammonia reacts with oxygen:

$2NH_3 + 3O_2 \rightarrow 2HNO_2 + 2H_2O + E$

1. Oxidation of nitrous acid to nitric acid (Stage II). The resulting nitrous acid undergoes further oxidation:

$2HNO_2 + O_2 \leftrightarrow 2HNO_3 + E$

The energy ($E$) released at both stages is captured by the bacteria and expended on their internal plastic processes.

Colorless sulfur bacteria and iron bacteria

Other important representatives of chemosynthesizers include organisms that utilize sulfur and iron compounds.

Colorless sulfur bacteria inhabit water bodies rich in hydrogen sulfide ($H_2S$). They synthesize organic molecules from carbon dioxide ($CO_2$), obtaining energy through the oxidation of hydrogen sulfide.

• In the first stage, $H_2S$ is oxidized with the release of free sulfur and energy:

$2H_2S + O_2 \rightarrow 2H_2O + 2S + E$

• If the environment experiences a shortage of hydrogen sulfide, the bacteria are capable of oxidizing the accumulated free sulfur into sulfuric acid to obtain additional energy:

$2S + 3O_2 + 2H_2O \rightarrow 2H_2SO_4 + E$

Iron bacteria specialize in altering the valence of iron. Their mechanism is based on the oxidation of ferrous iron to ferric iron: $4Fe^{2+} + O_2 + 4H^+ \rightarrow 4Fe^{3+} + 2H_2O + E$. As a result of these microorganisms' activity, bog iron ore deposits are formed in nature.

Hydrogen bacteria

Another unique group is hydrogen bacteria, which inhabit the soil. They use molecular hydrogen as their energy substrate. This gas does not come from nowhere: it is formed as a result of anaerobic (oxygen-free) decomposition of various organic residues by other microorganisms.

The chemical reaction providing hydrogen bacteria with energy is as follows: $2H_2 + O_2 \rightarrow 2H_2O + E$

As in other cases of chemosynthesis, the energy released during water formation goes toward autotrophic assimilation.

Class and exam questions

What else to study on this topic

• Mechanisms of enzyme secretion into the external environment by nitrifying bacteria
• The connection between hydrogen bacteria and the anaerobic decomposition of organics
• The two-stage oxidation mechanism in colorless sulfur bacteria (to sulfur and to sulfuric acid)
• The role of nitrifying bacteria in the global nitrogen cycle

Other topics in the "Metabolism and Energy Conversion" section

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.