3.5.4 Nutrient Cycles

• Nutrient cycle - The cycling of elements nitrogen and phosphorus between inorganic molecules in the environment and organic molecules within organisms
• Saprobiotic nutrition:

  1. Secretes extracellular enzymes onto dead organism/urine/faeces
  2. Larger molecules are hydrolysed into smaller molecules
  3. Amino acids etc are taken up and used in cell metabolism
  4. Secretes ammonium ions/ammonia and phosphates into soil for uptake by producers. Producers convert into organic N and P containing molecules
• Mycorrhizae - Facilities the uptake of water and inorganic ions by plants.

  • Increases uptake of water/ions by increasing the surface area for osmosis/active transport
  • Fungus benefits by gaining glucose/amino acids from plant
• Nitrogen cycle:

  1. Ammonification - Saprobionts turn organic nitrogen containing compounds into ammonia/ammonium ions
  2. Nitrification - Nitrifying bacteria causes ammonia/ammonium to turn to nitrites to nitrates (requires aerobic conditions)
  3. Nitrogen fixation - Nitrogen fixing bacteria in soil/root nodules of legumes cause nitrogen gas to form ammonium ions (requires nitrogenase enzyme)
  4. Denitrification - Denitrifying bacteria causes nitrates to form nitrogen gas (requires anaerobic condition e.g waterlogged soil)
• Phosphorus cycle: Cycling of inorganic phosphates into organic molecules e.g DNA, Triose phosphate, phospholipids

  1. Phosphates released into the soil/water from weathering of rocks
  2. Taken up by producers
  3. Passed through consumers
  4. Released into soil from saprobiont metabolism of dead organic matter/faeces/urine
  5. (From aquatic ecosystems - seabirds excrete a lot of phosphate in guans)
• Use of fertilisers:

  • Farming results in the n/p in soil being slowly used up by plants. Fertilisers are used to increase the soils levels
  • Artificial fertilisers - controlled quantities of n/p in powders/pellets, are quickly absorbed by plants
  • Natural fertilisers - Animal waste still contains biological molecules so need to undergo ammonification then nitrification before nitrates are available to plants. Release n/p slowly
  • Ploughing - introduces more oxygen into soil so increase the activity of aerobic bacteria such as saprobionts and nitrifying bacteria, speeding up the release of nitrates
• Eutrophication:

  1. Excess nitrates and phosphates enter water body due to leaching/run off
  2. Increase in nitrates/phosphates causing an increase in the number of algal cells/plant growth
  3. Algal blooms, blocking light so less/no photosynthesis can occur
  4. Saprobionts increase in number, decomposing dead matter