Nutrient cycles in the natural ecosystem

• a natural ecosystem is one that hasn't been changed by human activity

  in a natural ecosystem, nutrients such as nitrogen and phosphorous are recycled through food webs - human activity disrupts
• The role of microorganisms:

  • such as bacteria and fungi are an important part of food webs and ecosystems - many a saprobionts
  • saprobionts do 2 things:
• 1. they feed on the remains of dead plants and animals and on their waste products (faeces and urine), breaking them down - makes them a type of decomposer and it allows important chemical elements in the remains and waste to be recylced
  2. secrete enzymes and digest their food externally, then absorb the nutrients they need - extracellular digestion - during this process, organic molecules are broken down into inorganic ions
• obtaining nutrients from dead organic matter and animal waste using extracellular digestion is known as saprobiotic nutrition
• Mycorrhizae:

  • some fungi form symbiotic relationships with the roots of plants
  • these relationships are known as mycorrhizae
  • the fungi are made up of long thin stands called hyphae which connect to the plants roots
  • the hyphae greatly increase the surface area of the plant's root system, helping the plant to absorb ions from the soil that are usually scarce (e.g. phosphorus)
  • hyphae also increase the uptake of water by the plant
  • in turn, fungi obtain organic compounds such as glucose from plant
• e.g. the fungus G. intraradices can develop mycorrihzal relationships with crops e.g. wheat and has been shown to increase the crop's phosphorus intake
• the nitrogen cycle:

  • plants and animas need nitrogen to make proteins and nucleic acids (DNA/RNA)
  • the atmosphere is made up of about 78% N2 gas - plants and animals can not use it in that form
  • need bacteria to convert it into nitrogen-containing compounds first
• a symbiotic relationship is when 2 species live closely together and one or both species depends on the other for survival
• • the nitrogen cycle shows how nitrogen is converted into a useable form and then passed on between different living organisms and the non-living environ
  • includes food chains (nitrogen is passed on when organisms are eaten)
  • and 4 different processes that involve bacteria:
  • nitrogen fixation, ammonification, nitrification and denitrification
• nitrogen fixation:

  • nitrogen fixation is when nitrogen gas in the atmosphere is turned into nitrogen-containing compounds
  • biological nitrogen fixation is carried out by bacteria such as Rhizobium - turns nitrogen into ammonia which goes on to form ammonium ions in solution that can be used by plants
• • Rhizobium are found inside root nodules (growths on the roots) of leguminous plants (e.g. peas, beans and clover)
  • form a mutualistic relationship with the plants - they provide the plants with nitrogen compounds, the plant provides them wit carbohydrates - there are other nitrogen-fixating bacteria - found in soil
• ammonification:

  • when nitrogen compounds from dead organisms are turned into ammonia by saprobionts - goes on to form ammonium ions
  • animal waste (urine and faeces) also contains nitrogen compounds - these are also turned into ammonia by saprobionts and go on to form ammonium ions
• nitrification:

  • when ammonium ions in the soil are changed into nitrogen compounds that can then be used by plants (nitrates)
  • first nitrifying bacteria - Nitrosomonas - change ammonium ions into nitrites
  • then other nitrifying bacteria - Nitrobacter - change nitrites into nitrates
• denitrification:

  • when nitrates in the soil are converted into nitrogen gas by denitrifying bacteria - use nitrates in the soil to carry out respiration and produce nitrogen gas
  • this happens under anaerobic conditions - no oxygen e.g. in waterlogged soils
• other ways that nitrogen gets into an ecosystem are :

  • by lightning (fixes atmospheric nitrogen into nitrogen oxides)
  • artificial fertilisers (produced from atmospheric nitrogen on an industrial scale in the Haber process)
• a mutualistic relationship is a type of symbiotic relationship where both species benefit
• the phosphorus cycle:

  • plants and animals need phosphorous to make biological molecules such as phospholipids (make up cell membranes), DNA and ATP
  • found in rocks and dissolved in the oceans in the form of phosphate ions PO43-
  • phosphate ions dissolved in water in the soil can be assimilated (absorbed and then used to make more complex molecules) by plants and other producers
  • the phosphorous cycle shows show phosphorous is passed through an ecosystem
• 1. phosphate ions in rocks are released into the soil by weathering
  2. phosphate ions are taken into plants through roots - mycorrhizae greatly increase the rate at which phosphorous can be assimilated
  3. phosphate ions are transferred through the food chain as animals eat the plants and are in turn eaten by other animals
  4. phosphate ions are lost from the animal in waste products
• 5. when plants and animals die, saprobionts are involved in breaking down the organic compounds, releasing phosphate ions into the soil for assimilation by plants - these microorganisms also release the phosphate ions from urine and faeces
• 6. weathering of rocks also releases phosphate ions into seas, lakes and rivers - taken up by aquatic producers, such as algae, and passed along the food chain to birds

  7. the waste produced by sea birds is known as guano and contains a high proportion of phosphate ions. Guano returns a significant amount of phosphate ions to soils (particularly in coastal areas)

  often used a a natural fertiliser
• weathering is the breakdown of rocks by mechanical, chemical and biological processes