9.1 Nitrogen Fixation and Assimilation

Nitrogen fixation and assimilation are crucial processes in the nitrogen cycle. They convert atmospheric nitrogen into forms that living things can use, replenishing the pool of available nitrogen in ecosystems. Without these processes, life as we know it couldn't exist.

Microorganisms play a starring role in nitrogen fixation, with some forming symbiotic relationships with plants. Plants and animals then assimilate this fixed nitrogen, using enzymes to incorporate it into essential biomolecules like proteins and DNA.

Nitrogen fixation: The process and its importance

Conversion of atmospheric nitrogen

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• Nitrogen fixation transforms atmospheric nitrogen (N₂) into biologically available forms (ammonia (NH₃) or nitrates (NO₃⁻))
• Process requires breaking the triple bond in N₂ demands high energy and specialized enzymes
• Occurs through biological, industrial, or atmospheric processes
  • Biological fixation most significant in natural ecosystems
• Fixed nitrogen enables synthesis of amino acids, nucleic acids, and other nitrogen-containing biomolecules in living organisms

Role in the nitrogen cycle

• Nitrogen cycle describes movement of nitrogen through biosphere, atmosphere, and geosphere
• Nitrogen fixation replenishes pool of biologically available nitrogen in ecosystems
• Without fixation, nitrogen cycle would deplete available nitrogen in biosphere
  • Limits ecosystem productivity and function

Microorganisms: Key players in nitrogen conversion

Prokaryotic nitrogen fixers

• Biological nitrogen fixation primarily carried out by prokaryotic microorganisms (bacteria and archaea)
• Diazotrophs fix nitrogen as free-living organisms or in symbiotic relationships with plants
• Rhizobia form symbiotic relationships with leguminous plants
  • Fix nitrogen within specialized root nodules
• Frankia (actinobacteria) form symbiotic relationships with non-leguminous plants (alder trees)
• Cyanobacteria fix nitrogen in aquatic ecosystems and some terrestrial environments
• Free-living nitrogen-fixing bacteria (Azotobacter and Clostridium) contribute to fixation in soil environments

Diverse nitrogen-fixing organisms

• Some archaea capable of nitrogen fixation in extreme environments (hot springs)
• Symbiotic relationships between microorganisms and plants enhance nitrogen fixation efficiency
  • Legume-rhizobia symbiosis
  • Actinorhizal symbiosis (Frankia with non-legumes)

Nitrogen assimilation: From fixed to usable

Plant nitrogen assimilation

• Plants directly assimilate ammonium (NH₄⁺) from nitrogen fixation or organic matter breakdown in soil
• Nitrate (NO₃⁻) assimilation involves reduction to nitrite then ammonium before incorporation into organic compounds
• Glutamine synthetase-glutamate synthase (GS-GOGAT) cycle primary pathway for nitrogen assimilation in plants and microorganisms
• Transamination reactions distribute nitrogen from glutamate to other amino acids
  • Catalyzed by aminotransferases

Animal nitrogen assimilation and excretion

• Animals obtain nitrogen by consuming plants or other animals
  • Assimilate nitrogen-containing compounds through digestion and metabolism
• Excess nitrogen in animals excreted as urea or uric acid
  • Further processed by soil microorganisms
  • Reenters nitrogen cycle

Enzymes: Catalysts for nitrogen fixation and assimilation

Nitrogen fixation enzymes

• Nitrogenase primary enzyme complex for biological nitrogen fixation
  • Consists of dinitrogenase reductase and dinitrogenase components
• Nitrogenase highly sensitive to oxygen
  • Requires anaerobic conditions or specialized protective mechanisms
• Leghemoglobin in legume root nodules maintains low oxygen environment for nitrogenase
  • Facilitates oxygen transport to bacteria

Nitrogen assimilation enzymes

• Nitrate reductase catalyzes reduction of nitrate to nitrite
  • First step in nitrate assimilation by plants and some microorganisms
• Nitrite reductase reduces nitrite to ammonium
  • Completes conversion of nitrate to form directly incorporated into organic compounds
• Glutamine synthetase (GS) catalyzes initial incorporation of ammonium into glutamine
  • Key step in nitrogen assimilation
• Glutamate synthase (GOGAT) transfers amide group from glutamine to α-ketoglutarate
  • Produces two glutamate molecules
  • Completes GS-GOGAT cycle