The Nitrogen Cycle

• Nitrogen Cycle


  Nutrients (such as nitrogen and phosphorus) are recycled through food webs in natural ecosystems
• Decomposition


  Microorganisms recycle nutrients stored within dead organisms and waste products
• Saprobionts


  True decomposers, mainly from fungi and bacteria kingdoms, that secrete enzymes to digest dead organic matter externally and absorb the products
• Saprobionts


  • Secrete a wide range of digestive enzymes to hydrolyse a large variety of biological molecules, releasing mineral ions like ammonium and phosphate that are important for producers
  • Not all products of extracellular digestion are absorbed, many remain in the environment and are available to other organisms
• Mycorrhizae


  Symbiotic relationships between plant roots and fungi, where the fungi increase the surface area of the roots to absorb more water and mineral ions, and the plant provides organic compounds to the fungi
• Nitrogen Fixation


  1. Nitrogen-fixing bacteria convert atmospheric nitrogen gas into ammonium ions

  2. Nitrifying bacteria convert ammonium ions into nitrites and nitrates that can be absorbed by plants

  3. Denitrifying bacteria use nitrates for respiration and release nitrogen gas back to the atmosphere
• Roles of microorganisms in recycling minerals

  • Saprobionts decompose waste and dead matter via extracellular digestion
  • Nitrogen-fixing bacteria carry out ammonification
  • Nitrifying bacteria convert atmospheric nitrogen to ammonium, nitrites and nitrates
  • Denitrifying bacteria release nitrogen gas
  • Mycorrhizal fungi increase plant root surface area to absorb water and minerals