theme 7(5)

• free living organism (diazotrophs)need protection from oxygen.
• Protection against O2 for nitrogenase of free-living:(strategies)

  1. simple solution: grow anoxically, grow with limited O2.
  2. fix N2 only when oxygen is absent or low concentrationh
• examples of 2:

  • facultative aerobes fix when in anaerobic condition(klebsiella)
  • microaerophiles use less 2% of oxygen
• 3. Compartmentalise nitrogen fixation in area of low oxygen Example Cyanobacteria fix nitrogen within heterocysts.

  4. Protect with capsule or slime layer

  Example: Azotobacter produce extensive capsules or slime layers and respire fast to help protect nitrogenase.
• 5. Oxygen binding proteins

  Example: Rhizobia produce Leghemoglobins to bind oxygen

  6. Use alternative nitrogenases

  Examples Azotobacter chroococcum was the first nitrogen-fixer shown to grow on N, without molybdenum

  forms alternative, less efficient nitrogenases containing either vanadium (V) or Fe in place of Mo

  Example Cyanobacteria and Archaea produce backup nitrogenases