theme 7(4)(1)N fixing

• Diazotrophs – fix N2 gas into NH3.
• free living: Azotobacter, Azospirullum
• fix N nonsymbiotically

  extensive slime layers (protect Nitrogenase from O2)
• symbiotic: Rhizobia (mesorhizobium, Bradyrhizobium)

  • provide fixed N to host
  • w/ leguminous plants
  • shipworms w/ Teredinibacter.
  • termite guts w/ Treponema
  • fungi, algae, and plants w/ Cyanobacteria
• Rhizobia nitrogenases

  • Nitrogen-fixing need O2 to generate energy for N2 fixation but nitrogenases are inactivated by O2
  • In the nodule, O2 levels are controlled by the O2 binding protein = leghemoglobin (Rhizobia specific).