Cycles
Unlike energy, which is gained through sunlight and lost as heat, nutrients do not enter or leave an ecosystem but are endlessly cycled. In most cases nutrients are taken up by producers, passed to consumers when they feed, and ultimately returned to the environment, for example by saprobiotic decomposing microorganisms.
The Nitrogen Cycle
Plants and animals need nitrogen to synthesise non-essential amino acids (to make proteins) and nucleotides (to make DNA, RNA and ATP). The main source of nitrogen in a food chain is the atmosphere, which is 78% nitrogen (N₂). However, producers cannot use it in that form, and use bacteria to convert it to useable nitrogen compounds first. Nitrogen fixation processes break up the nitrogen atoms from their diatomic form. For an insight into the nitrogen cycle within the taiga biome, see Case Study 1.
1)Nitrogen Fixation
2) Ammonification
The Phosphorus Cycle
Phosphorus is equally important for organisms: it is a component of ATP, nucleic acid and phospholipids. Unlike in the nitrogen and carbon cycles, the main source of phosphorus is inorganic.
The Nitrogen Cycle
Plants and animals need nitrogen to synthesise non-essential amino acids (to make proteins) and nucleotides (to make DNA, RNA and ATP). The main source of nitrogen in a food chain is the atmosphere, which is 78% nitrogen (N₂). However, producers cannot use it in that form, and use bacteria to convert it to useable nitrogen compounds first. Nitrogen fixation processes break up the nitrogen atoms from their diatomic form. For an insight into the nitrogen cycle within the taiga biome, see Case Study 1.
1)Nitrogen Fixation
- occurs under anaerobic conditions
- bacteria in root nodules of legumes can fix nitrogen into ammonia, or NH₃ (this can later be converted to other nitrogenous compounds that will be used by the plant)
- this is because they possess the enzyme nitrogenase, which can produce two moles of ammonia from one mole of nitrogen gas
- in return, the leguminous plant supplies the bacteria with glucose (the relationship is symbiotic)
- nitrogen-fixing bacteria can also be free-living
- nitrogen fixation may also occur when lightning passes through the atmosphere as its powerful electrical energy suffices to break the strong bonds of the nitrogen molecule in the atmosphere, resulting in the formation of nitrogen dioxide which dissolves in water (rain), creating nitric acid which forms nitrates (NO₃)
- nitrogen fixation can also be conducted on an industrial scale: the Haber process is used to create ammonia for fertilisers, freeing nitrogen atoms from their diatomic form using an iron-based catalyst, and very high pressures and temperatures
2) Ammonification
- this process can occur in aerobic or anaerobic conditions but is more rapid in oxygen-rich conditions
- nitrogen compounds from dead organisms or animal waste are turned into ammonia by saprobionts
- when dissolved in water, the ammonia forms ammonium ions
- this must occur in aerobic conditions
- it constitutes the oxidation of ammonium ions into nitrate ions by nitrifying bacteria, which can then be used by plants
- firstly: certain species of nitrifying bacteria synthesise nitrite (NO₂-) ions from ammonia
- secondly: other species of nitrifying bacteria convert the nitrite into nitrate (NO₃-)
- as this process requires oxygen it is most efficient in soil with lots of air spaces, so that is well ploughed and drained
- occurs under anaerobic conditions, especially in waterlogged soil
- nitrates in soil may be converted back into nitrogen gas by denitrifying bacteria as opposed to being taken up by plants
- these bacteria use the nitrates to carry out respiration
- the nitrogen gas is rereleased back into the atmosphere; the cycle starts anew!
The Phosphorus Cycle
Phosphorus is equally important for organisms: it is a component of ATP, nucleic acid and phospholipids. Unlike in the nitrogen and carbon cycles, the main source of phosphorus is inorganic.
- for example: phosphate ions may be found in sedimentary rock
- they may become soluble in water (such as rain) and therefore available for uptake by plants / primary producers (such as by entering soil through runoff)
- consumers gain the phosphate when they eat the plants and excess ions are excreted or deposited in bones or shells
- these may later be found in sedimentary rock
- decomposition of dead animals or plants by saprobionts also releases phosphate ions that are immediately available for uptake by plants.
