Recycling within ecosystems

• Nitrogen fixation

  Nitrogen-fixing bacteria such as Azotobacter and Rhizobium contain the enzyme nitrogenase, which combines atmospheric nitrogen (N2) with hydrogen (H2) to produce ammonia (NH3)

  A form of nitrogen that can be absorbed and used by plants.
• Nitrogen- fixing bacteria

  Azotobacter is an example of a free-living soil bacterium.

  Rhizobium live inside root nodules= these are growths on the roots of leguminous plants such as peas, beans.

  The bacteria have a mutualistic relationship with the plant:

  • Plant gains amino acids from Rhizobium, which are produced by fixing nitrogen gas in the air into ammonia in the bacteria.
  • The bacteria gain carbohydrates produced by the plant during photosynthesis, which they use as an energy source.
• Nitrification

  = Process by which ammonium compounds in the soil are converted into nitrogen- containing molecules that can be used by plants.

  1. Nitrifying bacteria (Nitrosomonas) oxidise ammonium compounds into nitrites. (NO2-)
  2. Nitrobacter oxidise nitrites into nitrates (NO3-) - Nitrate ions are highly soluble and are therefore the form in which most nitrogen enters a plant.
• Denitrification

  In the absence of oxygen eg: waterlogged soils, denitrifying bacteria convert nitrates in the soil back to nitrogen gas.

  • Only happens under anaerobic conditions.
  • The bacteria use the nitrates as a source of energy for respiration and nitrogen gas is released.
• Ammonification

  = Decomposers convert nitrogen- containing molecules in dead organisms and urine into ammonium compounds.
• Decomposition

  = Chemical process in which a compound is broken down into smaller molecules.

  • Often an essential element such as nitrogen or carbon, can't be used directly by an organism in the organic form it is in (dead or waste matter)
  • This organic material must be processed into inorganic elements and compounds, which are more usable and returned to the environment.
• Decomposer

  = An organism that feeds on and breaks down dead plant or animal matter, turning organic compounds into inorganic ones (nutrients) available to photosynthetic producers in the ecosystems.

  • Saprotrophs because they obtain their energy from dead or waste organic material.
  • They digest their food externally by secreting enzymes onto dead organisms. The enzymes break down complex organic molecules into simpler soluble molecules- the decomposers then absorb these molecules.
  • Decomposers release stored inorganic compounds and elements back into the environment.
• Detritivores

  = Help speed up the decay process by feeding on detritus- dead and decaying material.

  • They break it down into smaller pieces of organic material, which increases the surface area for the decomposers to work on.
  • Perform internal digestion.
• Atmospheric carbon dioxide levels 


  = Samples are taken from deep within a glacier.

  • When the ice was formed air bubbles were trapped within the ice- these bubbles reflected the composition of the atmosphere at this time.
  • Analysis of gases present within these bubbles revels the composition of the atmosphere.
• Increased levels of atmospheric carbon dioxide trap more thermal energy in the atmosphere.

  • The amount of carbon dioxide dissolved in seas and oceans is affected by the temperature (the higher the temp the less gas is dissolved