5.4 Nutrient cycles Flashcards
What three types of microorganisms play vital roles in recycling chemical elements such as phosphorus and nitrogen (names of individual species of bacteria are not required)?
Saprobionts play a vital role in in decomposition.
Mycorrhizae play a vital role in facilitating the uptake of water and inorganic ions by plants.
Bacteria play a vital role in the nitrogen cycle in the processes of saprobiotic nutrition, ammonification, nitrification, nitrogen fixation and denitrification.
Nitrogen fixation
Fixing: when inorganic compounds / elements become incorporated into biological systems.
Atmospheric N≡N is very unreactive. Only certain genera of Bacteria (and Archaea) can fix it.
Free living bacteria like Azotobacter (in soil) and the cyanobacteria (in freshwater) are responsible for most Nitrogen fixation, using nitrogenases (a class of enzyme) to react N2 with protons and electrons to form NH3 (ammonia) which they use to make amino acids and nucleotides.
Rhizobium is a genus of bacteria symbiotically associated with leguminous plants (eg clover). Rhizobium living in root nodules fix Nitrogen as NH3 to synthesise the amino acid glutamine, which the legume then uses to make other amino acids and nucleotides, without having to actively transport nitrogen compounds from soil. In return the legume gives the Rhizobium sucrose and other compounds.
Ammonification
When the free living bacteria, or the legumes, or the plants which have taken up nitrogenous compounds by active transport die (or drop leaves), or when any consumer which ate them dies, or defecates, or passes urine (or sheds skin cells, etc) then the amino acids and nucleotides become available for decomposition.
Decomposers break down amino acids and nucleotides into simpler inorganic compounds. Saprophytic Bacteria and Fungi are responsible for decomposition. Saprobionts secret their enzymes into the detritus (dead material) and absorb the products.
Decomposition releases NH4+ back into the environment. This is called AMMONIFICATION.
Nitrification
Nitrification is the conversion of ammonium into nitrate. This usually involves two genera of bacteria: Nitrosomonas and Nitrobacter.
Nitrosomonas converts ammonium into nitrite.
Nitrobacter converts nitrite into nitrate.
These bacteria are mutualistic - Nitrosomonas provides substrate for Nitrobacter, which prevents build up of product for Nitrosomonas.
Nitrate is the nutrient taken up by plant roots and mycorrhizae.
Denitrification
Bacteria in anaerobic soil conditions (waterlogged or non-aerated soil) have adapted to use nitrate as their final electron acceptor. They convert nitrate to nitrogen gas, so removing the nitrogen from the soil. Hence the name: denitrifying bacteria.
This process recycles nitrogen, returning it to the atmosphere, but is not great for farmers, who then have to replace it.
Why is phosphorous often a limiting nutrient in an ecosystem?
Phosphorous exists mostly as phosphate (Pi) , a soluble and highly charged (-3) mineral ion.
Pi binds strongly to sediments, meaning it gets incorporated into sedimentary rock. Only weathering of the rock will release it.
Why do farmers have to add fertilisers to their fields (when natural ecosystems just recycle nutrients)?
The use of natural and artificial fertilisers replaces the nitrates and phosphates lost by harvesting the plants and removing the livestock. When the plants are harvested and livestock removed they take nutrients form the field with them in their biomolecules.
Eutrophication
Farmers apply phosphate / nitrate fertilizers to their soils.
Rain washes this into rivers (streams, lakes, canals, the sea, whatever) - this is called leaching.
Algae absorb the nutrients and their population rapidly grows.
Algal bloom blocks out light for photosynthesising plants below, which die.
Algae finish all the leached nutrient in the water, and die.
Saprobiotic decomposers now break down all the dead plants and algae, using their molecules as respiratory substrate.
This respiration is aerobic, so oxygen levels fall.
Fish and other animals that rely on the oxygen die.
