Experiment 14 - Nonsymbiotic Nitrogen Fixation

What is nitrogen fixation?

The conversion of nitrogen gas (of air) into a fixed form

How does nitrogen fixation occur?

Only a few bacteria and cyanobacteria can transform molecular nitrogen into cellular (protein) nitrogen which becomes available to organisms.

What is the most studied species of nonsymbiotic nitrogen fixers?

Azotobacter

What are Azotobacter

A group of free-living, aerobic soil organisms

Why is Azotobacter studied so much?

They’re capable of utilizing atmospheric nitrogen as their only source of nitrogen

What anaerobic microorganism is considered an important nonsymbiotic nitrogen fixer?

Clostridium pasteurianum

What are some photosynthetic bacteria?

Rhodospirillum rubrum

What is the mechanism of nitrogen fixation?

Reducing free nitrogen to ammonia

What can the ammonia be coupled with?

alpha-ketoglutaric acid in the cell to form a versatile amino acid (glutamic acid)

What do Azotobacter species produce

Capsular slime

What are the characteristics of Azotobacter?

• Gram-negative

• Thick-walled cysts

What do some species of Azotobacter differ in?

Presence/absence of:

• Water-insoluble melanin pigment

• Water-soluble pigment

• Fluorescent pigment

What are the characteristics of Azotobacter chroococcum?

• Large ovoid rod

• Found in pairs

• NO water-soluble pigment

• NO fluorescence under UV

• Growth on agar = water-insoluble pigment

  • Strains later become black

What are the characteristics of Azotobacter beijerinckii?

• Similar in size + shape to A. chroococcum

• Single, pairs or chains of several cells w large capsules

• NO fluoresce under UV

What are the characteristics of older Azotobacter beijerinckii cultures?

• Coccoid

• Form cysts

• Turn yellow/cinnamon w water-insoluble pigment

What are the characteristics of Azotobacter vinelandii?

• Large ovoid rod

• Found in pairs

• NO water-insoluble pigment

• Water-soluble fluorescent pigment appears green under UV

What are the characteristics of Azotobacter paspali?

• Large ovoid OR pleomorphic rod

• Variations from coccoid to elongated filaments

• Water-soluble fluorescent pigment appears green under UV

• NO water-insoluble pigment

What are the characteristics of other soil microorganisms?

They resemble Azotobacter in colony morphology and characteristics

What are characteristics of Azomonas?

• Large ovoid cells

• In pairs OR clumps

• Capable of nitrogen fixation

• No cysts are produces

• Some strains has a water-soluble fluorescent pigment appearing white under UV

What genera are closely allied with Azotobacter?

• Beijerinckia

• Derxia