(Lab 3) Isolation of Nitrogen-Fixing Bacteria

Question 1

What bacteria share a mutualistic relationship with legumes?

Answer 1

Rhizobium
- infects root nodules and feeds on sugars
- legumes get usable form of nitrogen (NO3-)

Question 2

Identify the organisms in the images. What are their features?

Answer 2

Azotobacter and Azomonas
- Gram-negative diploovoid
- weakly motile
- obligate aerobes
- can ferment with mannitol as sole carbon source
- facultative N2 fixers

Question 3

Why are Azotobacter and Azomonas facultative N2 fixers?

Answer 3

It is an energy-expensive process

Question 4

Why is enrichment important before isolation?

Answer 4

N2 fixers are a small amount of the microbiome in the soil, and creating an environment where only they can thrive makes it easier to isolate them

Question 5

Why do we incubate the N2-free media with the bottle on its side with the cap slightly loose?

Answer 5

• obligate aerobes
• Need N2 to survive
• increase surface area of bacteria to air

Question 6

Identify the bacteria shown

Answer 6

Azotobacter vinelandii

Question 7

Identify the bacteria shown

Answer 7

Azotobacter chroococcum

Question 8

What N2 fixer has no pigment, is gram variable, and ovoid-rod-shaped?

Answer 8

Azomonas macrocytogenes