Aquatics - The Nitrogen Cycle

The nitrogen cycle refers to the cycle of nitrogen atoms through the living and non-living systems on earth. Atmospheric nitrogen is converted to a form which plants can incorporate into new proteins. Nitrogen was originally formed in the hearts of stars through nuclear fusion and when the stars exploded this spread nitrogen all over the universe. When the Earth was first formed nitrogen was the main gas in its atmosphere. Today, the Earth's atmosphere is roughly 78% nitrogen, 21% oxygen, and 1% other gases. Too much oxygen in the atmosphere can be toxic to cells, as well as being flammable. Nitrogen, however, is inert and harmless in its gas form. On the other hand, nitrogen is not accessible to plants and animals to be used in their cells.

• Nitrogen fixation
  
  • bacteria turn nitrogen gas into ammonia
  • nitrogen-fixing bacteria called 'diazotrophs', and have an enzyme called 'nitrogenase' 
  • 'nitrogenase' combines hydrogen and nitrogen atoms
  • ammonia is a nitrogen compound that can dissolve in water, and is easier for other organisms' enzymes to interact with
    
  • 'nitrogenase' can only function when oxygen is not present
  • nitrogen-free compartments for nitrogen fixation
  • Rhizobium nodules of legume roots

• Nitrification
  • Soil bacteria turn ammonia into nitrate
  • Nitrate is the form of nitrogen that can be used by plants and animals
  • Soil bacteria such as Nitrosomonas and Nitrococcus convert ammonia into nitrogen dioxide
  • Nitrobacter then adds a third oxygen atom to form a nitrate
  • These bacteria are 'chemotrophs' that obtain energy from volatile chemicals
    
    • by metabolising nitrogen and oxygen
• Nitrates can be made artificially
  • main ingredient in soil fertilisers 
• If the soil is full of nitrates plants grow quicker without being dependent on the nitrogen fixing bacteria
• High-energy environments can convert nitrogen gas directly into nitrates
  • e.g., lightning strikes and volcanic eruptions
• Assimilation
  • plants consume the nitrates made by soil bacteria and use them to make nucleotides, amino acids and other vital chemicals
  • animals that eat plants are able to use the nucleic and amino acids in their own cell

• Ammonification
  • performed by soil bacteria that decompose plant and animal material
  • ammonia may be taken up again by plants and nitrifying bacteria
  • may be converted back into atmospheric nitrogen through the process of denitrification
• Denitrification
  • final step of the nitrogen cycle
  • anaerobic bacteria can turn nitrates back into nitrogen gas
  • must happen in the absence of oxygen
  • occurs deep in the soil or in wet environments where mud keeps oxygen at bay
  • in some ecosystems this prevents dangerous amounts of nitrogen building up in the soil
• Nitrogen has unique chemical bonding properties that allow it to create DNA and RNA, as well as amino acids
• first nucleotides and amino acids formed naturally under the volatile conditions of early Earth
  • energy sources such as lightning strikes
• too many nitrates can increase cancer risk and interfere with blood chemistry in humans
• too much nitrogen, especially when added through artificial fertilisers, is thought to disrupt ecosystems
  • when artificial nitrate fertilisers get into rivers, lakes and even the ocean, it can cause runaway plant life to grow there
  • can increase algae growth in aquatic systems, which blocks the sun and oxygen
    • can produce toxins that make humans and animals sick