What is Azotobacter?

Discovered by Dutch microbiologist Martinus Beijerinck, the genus Azotobacter belongs to the family Pseudomonadaceae.

• Azotobacter spp. are the first identified aerobic, free-living, nitrogen fixers. They are commonly found in soil, water, and sediments but rarely form an association with plant roots. Most of them tolerate alkaline pH, with a few exceptions.
• Azotobacter chroococcum and Azotobacter vinelandii are abundant in tropical alkaline soils (exception- Azotobacter beijerinckii dwells in acidic soil). Azotobacter paspali is unique in that they remain in symbiotic association with the roots of Bahiagrass (Paspalum notatum cv Batatais).
• Azotobacter are relatively large gram negative bacteria, usually having oval or spherical shapes. The cells can be dispersed, form irregular clusters or occasionally arrange into chains.
• They produce convex, circular, slimy colonies which have dark brown colour and wavy edges. They have a unique ability to transform into thick-walled cysts, which is a dormant form to survive tough environmental conditions like extreme temperatures, drought or low nutrient levels.
• Some Azotobacter species are flagellated and motile, while others are not. They serve as model organisms for the study of diazotrophs.
• These bacteria can utilise atmospheric nitrogen for their cellular protein synthesis, later released into the soil. They help fix a significant portion of the gaseous nitrogen for crop plants and improve soil fertility.

The main difference between Azotobacter and Azospirillum lies in the way these nitrogen fixers are found in nature. The Azotobacter species are non-symbiotic nitrogen-fixing bacteria, whereas the Azospirillum species are associative rhizobacteria that fix nitrogen. Azotobacter are aerobic, free-living species. They are widely distributed in soils and waters. On the contrary, Azospirillum spp. are microaerophilic and often found in a beneficial association with plants.

They are present either in the rhizospheres or the roots of important crop plants. Both of them are diazotrophs, meaning they are capable of biological nitrogen fixation (BNF). In this article, we will look into the differences between Azotobacter and Azospirillum in detail.