Difference between Azotobacter and Azospirillum

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.

Difference Between Azotobacter and Azospirillum

The Azotobacter vs Azospirillum spp. are discussed below:

Criteria	Azotobacter	Azospirillum
Definition	Azotobacter spp. are aerobic, non-symbiotic diazotrophs	Azospirillum spp. are microaerophilic, associative diazotrophs
Habitat	Free living, widely found in soil, water and sediments	Surface colonising rhizobacteria, commonly found in the rhizosphere or roots of crops like rice, wheat, corn etc.
Biological interaction	Non-symbiotic	Symbiotic or associative
Oxygen tolerance	Aerobic	Microaerophilic, facultative anaerobe
pH tolerance	Neutral to alkaline, optimal growth & N2 fixation at pH- 7-7.5	Acidic to alkaline, optimal growth & N2 fixation at pH- 7
Growth temperature	Mesophilic bacteria, grows between 25 °C to 30 °C, optimal growth temperature- 30 °C	Grows between 5 °C to 42 °C, optimal growth temperature- 37 °C
Shape	Oval or spherical shape	Curved rod shape
Colony characteristics	Convex, circular, mucoid colonies with dark brown colour	Flat, circular colonies with red colour
Family	Pseudomonadaceae	Azospirillaceae
Functions	Mainly function as free living nitrogen fixer Carry out phosphorus solubilisation in soil Improve soil fertility Used in biofertilizers and Used for production of biopolymers Can induce root growth by secreting certain plant metabolites and hormones	Function as associative nitrogen fixer Promote plant growth by producing phytohormones and plant metabolites- polyamine, and trehalose Produce antioxidants to relieve oxidative stress of plants Carry out degradation of soil pollutant Used in biofertilizers for non-legume crops
Examples	Azotobacter chroococcum, Azotobacter vinelandii, Azotobacter beijerinckii	Azospirillum lipoferum, Azospirillum brasilense, Azospirillum amazonense

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.

What is Azospirillum?

The genus Azospirillum was also initially described by M.W. Beijerinck. It belongs to the class of alpha-Proteobacteria and the family Azospirillaceae.

• The genus mainly includes nitrogen-fixing rhizobacteria. They can be found in soil and freshwater but are most abundant in the rhizospheres and roots of crop plants like corn, rice, wheat, sugarcane etc.
• These bacteria enhance plant growth by forming beneficial associations with them. Hence they are called plant growth-promoting rhizobacteria (PGPR).
• Azospirillum lipoferum, Azospirillum brasilense and Azospirillum amazonense efficiently fix nitrogen in the rice rhizosphere. They also stimulate plant growth by producing and releasing various plant hormones.
• Azospirillum species are gram-negative and microaerophilic. Some are facultative anaerobes, capable of surviving without oxygen.
• They can tolerate a wide range of soil pH, from 5 (acidic) to 9 (alkaline). They are curved rods and motile. The cells usually remain dispersed.
• They can produce spherical cysts in response to nutrient deprivation or desiccation. Their colonies have a circular, flat morphology with prominent red colouring and wavy edges.

Similarities Between Azotobacter and Azospirillum

Apart from differences, there are some similarities between these two species of bacteria. These similarities are summarised below:

• Both of these genera are diazotrophs.
• Both are gram-negative bacteria.
• They both can form resistant cysts under extreme environmental conditions.
• They improve soil fertility.
• They both are used in biofertilizers.
• Both were first described by Dutch microbiologist Martinus Beijerinck.

Usage of Azotobacter and Azospirillum

They are used as biofertilizers and seed inoculants for non-leguminous plants for the following reasons:

• They can convert atmospheric nitrogen into ammonia.
• They have a crucial role in maintaining the usable form of N₂ , a vital macronutrient in soil.
• They can help to mineralise other nutrients like phosphorus, carbon, sulphur, overall increasing soil fertility.
• They increase uptake of macronutrients like N, P and some micronutrients like Zn, Fe.
• They release vital phytohormones and other biologically active substances to promote growth of crop plants.
• They also stimulate flowering, rooting, foliage formation, seed vigour.
• These microorganisms help in biological remediation of heavy metals and other soil pollutants.
• Azospirillum spp. can produce antioxidants to mitigate oxidative stress of plants.
• These bacteria induce plant defence mechanisms to fight various biotic and abiotic stresses.
• Azotobacter spp. are also used in industrial production of important biopolymers like- alginate (have biomedical applications), poly 3-hydroxybutyrate (used in biodegradable plastics) etc.

Conclusion – Difference between Azotobacter and Azospirillum

Azotobacter and Azospirillum are two distinct genera of diazotrophs found in different ecological niches. Azotobacter spp. function as free-living nitrogen fixers. Azospirillum spp., on the other hand, act as associative nitrogen fixers promoting plant growth through hormone secretion. Both of them are valuable for soil fertility and biofertilizer production in the agriculture industry.

FAQs on Difference between Azotobacter and Azospirillum

What is the Difference between Rhizobium and Azotobacter?

Rhizobium species form an endosymbiotic relationship with leguminous plants during nitrogen fixation. On the other hand, Azotobacter species are free-living bacteria that fix nitrogen in soil.

How do you Identify Azotobacter?

Azotobacter spp. exhibit gram-negative character. They form slimy, smooth, dark colonies. Biochemical tests, as well as 16S rRNA sequencing methods can be used to identify them.

Is Azospirillum Symbiotic or Non Symbiotic?

Azospirillum species are primarily found in an associative relationship with many crop plants such as rice, wheat, sorghum, maize.

Is Azospirillum a Cyanobacteria?

No, Azospirillum spp. are non-photosynthetic rhizobacteria that promote plant growth by producing plant hormones and other metabolites.

Is Azospirillum Gram Positive or Gram Negative?

All members of the genus Azospirillum are Gram-negative and appear pink or red under the microscope after Gram staining.

Is Azospirillum a Nitrogen Fixer?

Yes, Azospirillum is a nitrogen-fixing bacterium that forms a symbiotic relationship with plants to fix atmospheric nitrogen for their growth.

What are the Advantages of Using Azospirillum over Azotobacter?

Azospirillum offer better root surface colonization, wider plant compatibility, and additional growth-promoting benefits as compared to Azotobacter.

What are the Similarities Between Azotobacter and Azospirillum?

Azotobacter and Azospirillum are both nitrogen-fixing bacteria that form symbiotic relationships with plants, helping in their growth. They share similarities in increasing soil fertility and reducing the need for synthetic fertilizers.