BIOL/CSES 4684

AZOTOBACTER

SOIL MICROBIOLOGY

This webpage was created by Amy Crum

1. IDENTIFYING CHARACTERISTICS

Aerobic but can grow under reduced oxygen conditions.
Large rod of varying length down to coccoid morphology.
2 um or more in diameter; may be almost the size of yeasts.
Can occur singly, in chains, or in clumps.
No endospores are formed, but do form thick-walled cysts.
Gram negative.
May be mobile by peritrichous flagella or non-mobile.
Non-symbiotic nitrogen fixers.
Copious amounts of capsular slime may be formed.
Catalase positive.
Can produce a water soluble pigment, either yellow-green, fluorescent or red-violet/brownish-black.
The percent G+C = 63 - 66 %.
Optimum temperature range between 20 and 30°C.
Growth range is between pH 5.5 - 8.5, but optimum pH is 7.0 - 7.5.

The above picture shows an electron micrograph of a dividing vegetative A. vinelandii cell.

The pictures at the top of the page show vegetative cells (on the left) and cysts (on the right) of A. Vinelandii by phase contrast microscopy.

2. TAXONOMIC DESCRIPTION
The genus Azotobacter has six species that are rather large atmospheric nitrogen fixers, but can grow on alternate sources of nitrogen such as ammonia, urea, and nitrate. They are able to grow on various carbohydrates, alcohols, and organic acids. Carbon metabolism is strictly oxidative and rarely produces acids or any other fermentation products. Azotobacter is aerobic but can grow in reduced oxygen conditions. Like other nitrogenases, the nitrogenase produced by Azotobacter is oxygen-sensitive. It is believed that because of its high respiration rate (highest of any living organism) and ability to perform conformational protection, Azotobacter is able to protect its nitrogenase from oxygen. Molybdenum is required for nitrogen fixation, but can be replaced by vanadium. Iron can also be substituted if molybdenum and vanadium are not present.

Azotobacter does not produce endospores, but does form thick-walled cysts as part of its life cycle. These cysts are resistant to desiccation and to some deleterious chemical and physic agents. They, however, cannot withstand extreme temperatures. While in the cyst stage of their life cycle, they do not fix nitrogen and are optically refractile.

Mobility is seen in A. chroococcum, , A. armeniacus, and A. paspali by means of peritrichous flagella. A. beijerickii and A. nigricans are non-motile. A water-soluble, yellow-green, fluorescent pigment is excreted by A. vinelandii and A. paspali; a red-violet or brownish-black pigment is seen in A. nigricans, A. armeniacus, and under certain conditions A. paspali.

Azotobacter is categorized along with the gamma group of purple Bacteria.

The picture above shows an electron micrograph of a thin slice of a cyst of A. vinelandii gram stain.

3. ISOLATION AND ECOLOGY
Azotobacter was first discovered using a culture that was devoid of a combined nitrogen source. It is capable of growing on media that has atmospheric nitrogen but grows most rapidly on ammonia. Molybdenum needs to be present in order for optimal substrate reduction. If grown on carbohydrate-containing media, capsules or slime layers will be produced. Different methods have been used for isolation of Azotobacter, such as Soil-Paste, Nutrient Solution and Agar Medium.

Azotobacter is found on neutral to alkaline soils, in aquatic environments, in the plant rhizosphere and phyllosphere. A.chroococcum is the most common species of Azotobacter present in the soil. Most of the studies on Azotobacter have been to compare its role as a nitrogen fixer to that of C. pasteurianum and Rhizobium. There is also interest in Azotobacter because of it has the highest metabolic rate of any living organism and because of its cyst formation.

The picture above shows a typical Azotobacter gram stain.

4. REFERENCES
Balows, Albert, Hans G. Truper, Martin Dworkin, Wim Harderk, Karl-Heinz Schleifer. 1992. The Prokaryotes. Second edition. Volume 1. Springer-Verlag. pg. 212-213.

Balows, Albert, Hans G. Truper, Martin Dworkin, Wim Harderk, Karl-Heinz Schleifer. 1992. The Prokaryotes. Second edition. Volume 4. Chapter 165. Jan Hendrick Becking. Springer-Verlag. pg. 3144-3154.

Buchanan, R.E. and N.E. Gibbons. 1974. Bergey's Manual of Determinative Bacteriology. Eighth edition. The Williams & Wilkins Company. Baltimore, Maryland. pg. 254-255.

Madigan, Michael T., John M. Martinko, Jack Parker. 1997. Brock Biology of Microorganisms. Eighth edition. Prentice Hall. Upper Saddle River, New Jersey. pg.701-703.

5. LINKS TO SITES RELATED TO AZOTOBACTER
Nitrogen Cycle

Azotobacter