Chemoautotrophic and chemolithotrophic bacteria as weathering agents

The inorganic processes associated with chemoautotrophic and chemolithotrophic bacteria may make these bacteria one of the most important sources of weathering and erosion of rocks on Earth.

Autotrophic bacteria obtain the carbon that they need to sustain survival and growth from carbon dioxide (CO₂). To process this carbon source, the bacteria require energy. Chemoautotrophic bacteria and chemolithotrophic bacteria obtain their energy from the oxidation of inorganic (non-carbon) compounds. That is, they derive their energy from the energy already stored in chemical compounds. By oxidizing the compounds, the energy stored in chemical bonds can be utilized in cellular processes. Examples of inorganic compounds that are used by these types of bacteria are sulfur, ammonium ion (NH⁴⁺), and ferrous iron (Fe²⁺).

The designation autotroph means "self nourishing." Indeed, both chemoautotrophs and chemolithotrophs are able to grow on medium that is free of carbon. The designation lithotrophic means "rock eating," further attesting to the ability of these bacteria to grow in seemingly inhospitable environments.

Most bacteria are chemotrophic. If the energy source consists of large chemicals that are complex in structure, as is the case when the chemicals are derived from once-living organisms, then it is the chemoautotrophic bacteria that utilize the source. If the molecules are small, as with the elements listed above, they can be utilized by chemolithotrophs.

Only bacteria are chemolithotrophs. Chemoautotrophs include bacteria, fungi, animals, and protozoa.

There are several common groups of chemoautotrophic bacteria. The first group is the colorless sulfur bacteria. These bacteria are distinct from the sulfur bacteria that utilize sunlight. The latter contain the compound chlorophyll, and so appear colored. Colorless sulfur bacteria oxidize hydrogen sulfide (H₂S) by accepting an electron from the compound. The acceptance of an electron by an oxygen atom creates water and sulfur. The energy from this reaction is then used to reduce carbon dioxide to create carbohydrates. An example of a colorless sulfur bacteria is the genus Thiothrix.

Another type of chemoautotroph is the "iron" bacteria. These bacteria are most commonly encountered as the rusty colored and slimy layer that builds up on the inside of toilet tanks. In a series of chemical reactions that is similar to those of the sulfur bacteria, iron bacteria oxidize iron compounds and use the energy gained from this reaction to drive the formation of carbohydrates. Examples of iron bacteria are Thiobacillus ferrooxidans and Thiobacillus thiooxidans. These bacteria are common in the runoff from coal mines. The water is very acidic and contains ferrous iron. Chemoautotrophs thrive in such an environment.

A third type of chemoautotrophic bacteria includes the nitrifying bacteria. These chemoautotrophs oxidize ammonia (NH₃) to nitrate (NO₃⁻). Plants can use the nitrate as a nutrient source. These nitrifying bacteria are important in the operation of the global nitrogen cycle. Examples of chemoautotrophic nitrifying bacteria include Nitrosomonas and Nitrobacter.

The evolution of bacteria to exist as chemoautotrophs or chemolithotrophs has allowed them to occupy niches that would otherwise be devoid of bacterial life. For example, in recent years scientists have studied a cave near Lovell, Wyoming. The groundwater running through the cave contains a strong sulfuric acid. Moreover, there is no sunlight. The only source of life for the thriving bacterial populations that adhere to the rocks are the rocks and the chemistry of the groundwater.

The energy yield from the use of inorganic compounds is not nearly as great as the energy that can be obtained by other types of bacteria. Regardless, chemoautotrophs and chemolithotrophs do not usually face competition from other microorganisms, so the energy they are able to obtain is sufficient to sustain their existence.

The ability of chemoautotrophic and chemolithotrophic bacteria to thrive through the energy gained by inorganic processes is the basis for the metabolic activities of the socalled extremophiles. These are bacteria that live in extremes of pH, temperature , or pressure, as three examples. Moreover, it has been suggested that the metabolic capabilities of extremophiles could be duplicated on extraterrestrial planetary bodies.

See also Erosion; Geochemistry; Weathering and weathering series