Is Rhizobium a nitrifying bacterium

Bacteria are unicellular organisms without a real nucleus (prokaryotes). They can be spherical (cocci), rod-shaped or helical (spirils) and have an average size of 0.1-20 µm. They colonize the soil with great biodiversity and a high density of individuals.

Bacteria can multiply within a few hours and often form large cell chains and colonies. Over 100 million bacteria can live in one gram of garden soil. Bacteria preferentially live in the thin film of water that surrounds the soil particles, on root surfaces and in the root space, the rhizosphere. They can move actively with flagella or passively with the ground water and are sensitive to dehydration. Bacteria have developed various adaptations and prefer a weakly acidic to weakly alkaline environment.

Soil bacteria are active in a variety of ways. Most species are heterotrophic and feed on dead organic matter and excretions from the organisms (e.g. feces). They prefer easily degradable carbon and nitrogen compounds (e.g. sugar, starch, cellulose, proteins, peptides, amino acids) and decompose the substrate by excreting enzymes. Due to their large spectrum of enzymes, they play a major role in the decomposition and are the most important decomposers (= reducers or destructors). There are no naturally occurring organic compounds and only a few artificial substrates that cannot decompose them.

The end products of bacterial degradation and the prerequisites for plant life are: carbon dioxide, water and mineral salts. The decomposition processes usually take place in an oxygen-containing environment, i.e. under aerobic conditions. But there are also anaerobic types of bacteria that decompose in the absence of oxygen. In this case, it usually involves fermentation and putrefaction processes.

Bacteria can be differentiated according to different criteria, e.g. with regard to energy supply, diet, metabolic types or environmental conditions.

With regard to the diet, two groups can be distinguished:

1.autotrophic bacteria:They can live without organic foreign substances and get their energy from sunlight (= photoautotrophic), e.g. those containing chlorophyll Blue-green bacteria (Cyanobacteria; formerly: blue-green algae), or inorganic compounds (= chemoautotrophic), e.g. nitrifying bacteria (see below). Both groups usually use carbon dioxide (CO2).
2. heterotrophic bacteria: They feed on organic matter. Lots of these
Species can also exist without oxygen (= anaerobic

Bacteria that are relevant for the soil can also be classified in a simplified manner with regard to their function and energy generation method, e.g .:

Carbohydrate-breaking bacteria: they break down substances containing carbohydrates (e.g. cellulose, hemicellulose, sugar, starch) - aerobic: e.g. Cellulomonas, anaerobic: e.g. Clostridium.
Protein-decomposing and ammonifying bacteria: they build proteins into amino acids, ammonia (NH3) and ammonium (NH4) from; aerobic and anaerobic types of bacteria, e.g. Bacillus mycoides.
Nitrifying agents: they oxidize ammonium (NH4) to nitrite (NO2) or nitrate (NO3), e.g. the species Nitrosomonas and Nitrobacter. The process is known as nitrification
(NH4+ -> NO2- -> NO3- ).
Denitrificants: they reduce nitrogen oxides in the anaerobic environment down to elemental nitrogen (N2), e.g. the species Pseudomonas and Achromobacter. The process is known as denitrification (NO3--> NO2- -> N2O -> N2).
Nitrogen-binding bacteria: they fix elementary atmospheric nitrogen (N.2) and convert it into organic N-compounds, e.g. the species Azotobacter, Amylobacter, Clostridium, Rhizobium, partly in symbiosis with plant roots.
Sulfuricants (sulfur bacteria): they build hydrogen sulfide (H.2S) to sulfur (S) and sulfates (SO4), e.g. Art Thiobacillus.
Desulfuricants (sulfur bacteria): they reduce sulfate (SO4) to hydrogen sulfide (H2S), e.g. Art Desulfovibrio.

The soil bacteria can be systematically divided into four groups
(according to GISI et al. 1997, p. 52 ff.):

Slime bacteria (myxobacteria): mostly rod-shaped, heterotrophic organisms that primarily feed on other bacteria.
Blue-green bacteria (cyanobacteria): autotrophic bacteria that photosynthesize and produce oxygen. They live freely or in symbiosis with fungi (see lichens) and green plants and are partly capable of fixing atmospheric nitrogen, e.g. the species Anabaena.
Eubacteria: different families. Most species are (chemo-) heterotrophic, i.e. they decompose organic compounds through respiration (aerobic) and fermentation (anaerobic) and derive energy and carbon from them. Some specialists are chemoautotrophic, i.e. they draw energy from the oxidation of inorganic compounds. Important families are: the aerobes Pseudomonadaceae, the free-living, nitrogen-fixing ones Azotobacteriaceae and the symbiotic, nitrogen-fixing ones Rhizobiaceae (e.g. the bacterium Rhizobiumthat lives in symbiosis with the roots of butterfly flowers and induces the formation of root nodules; see interactions).

Radiation fungi (actinomycetes): unicellular organisms that form a mycelium-like, thread-like network and represent a transitional form between bacteria and fungi, are often directly assigned to bacteria (e.g. by GISI et al. 1997 and BRAUNS 1968; see radiation fungi).

Additional Information:

  • Diet types and diets
  • decomposition
  • Material cycle


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