Chemo-autotrophic bacteria are organisms that synthesize their protoplasmic constituents from inorganic substances such as carbon(IV) oxide, water, ammonia or nitrates and obtain the initial energy for the syntheses from the oxidation of inorganic substrates. The substrates which they use are specific to the organisms concerned and can be used in their identification.
Among the best known and important examples of chemo-autotrophic bacteria are the nitrifying bacteria of the soil such as species of Nitrosomonas, Nitrosococcus, Nitrosospira, Nitrosocystis and Nitrosogloea which can oxidize ammonia to nitrite, and species of Nitrobacter and Nitrocystis which can oxidize nitrate to nitrate.
2NH3 + 3O2 → 2HNO2 + 2H2O + 3.31 x 103kJ
While the nitrite can be oxidized to nitrates in the following equations:
HNO2 + O → HNO2 + 9.5 x 103kJ
Both these groups of organisms are obviously strictly aerobic and play very crucial roles in the cycle of nitrogen in nature. Besides nitrogen compounds, a wide variety of substances can be used by other organisms as oxidizable substrates. Among the more interesting chemo-autotrophic organisms besides those mentioned above are the following:
- The colorless sulphur bacteria: These bacteria oxidize hydrogen sulphide according to the following equation
2H2S + O2 → 2S + 2H2O + 5.28 x 103kJ
The sulphur is stored in the cells as granules and the energy released is used to assimilate carbon(IV) oxide. The sulphur can later be used as a reserve oxidizable substrate when H2S is not available. Examples of organisms able to perform these reactions are the genera Beggiatoa, Thiothrix and Thiospirillopsis of the order Chlamydobacteriales and the genera Thiospira and Thiobacillus of the true bacteria. The species Thiobacillus thio-oxidans is a well-known oxidizer of sulphur to sulphates.
- The hydrogen bacteria: These organisms use the energy derived from the oxidation of molecular hydrogen using nitrates, sulphates or even carbon(IV) oxide as oxidizing agents. A typical reaction may be written as
4H2 + H2SO4 → 4H2O + H2S + 4.09 x 103kJ
4H2 + CO2 → 2H2O + CH4 + 2.60 x 103kJ
Organisms of the genus Desulphovibrio perform the first of the these reactions whilst the a mixed collection of species designated the ‘methane bacteria’ perform the second.
Most of these organisms are not obligatory chemo-autotrophs since they can be grown in the absence of hydrogen on media containing the correct organic substances. This means that they can substitute a suitable organic compound for inorganic hydrogen if the need arises, and carry on a heterotrophic existence.
- The iron bacteria: These are bacteria that were once regarded as chemo-autotrophes which obtained energy by the oxidation of ferrous compounds to ferric hydroxide. Members of the genus Leptothrix were believed to perform this reaction as their sole means of obtaining energy, but it has recently been proved that some at least, can live heterotrophically in organic media alone and are quite unable to grow in purely mineral media containing reduced iron. The presence of ferric hydroxide(iron(III) hydroxide, deposited around the bacterial cells in nature is now believed to be the result of the spontaneous conversion of ferrous iron to the ferric condition on exposure to air, if the water, pH, and other conditions are correct. The bacteria possibly play some part in producing the correct conditions for the spontaneous reaction to occur.