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Ecology : Introduction
The Nitrogen Cycle
Biological nitrogen fixation
An experiment carried out on the growth of legumes (peas) and non-legumes (oats) in the 19th century, showed the following results:
The difference is due to the present of mutualistic nitrogen fixing bacteria which live in the pea roots.
Only prokaryotes show nitrogen fixation
These organisms possess the nif gene complex which make the proteins, such as nitrogenase enzyme, used in nitrogen fixation.
Nitrogenase is a metalloprotein, protein subunits being combined with an iron, sulphur and molybdenum complex.
The reaction involves splitting nitrogen gas molecules and adding hydrogen to make ammonia.
This is extremely energy expensive requiring 16 ATP molecules for each nitrogen molecule fixed.
The microbes that can fix nitrogen need a good supply of energy.
Cyanobacteria are nitrogen fixers that also fix carbon (these are photosynthetic).
Rhizobium bacteria are mutualistic with certain plant species e.g. Legumes. They grow in root nodules. The plant provides shelter and sugars (through photosynthesis), the bacterium provides fixed nitrogen.
Azotobacter are bacteria associated with the rooting zone (the rhizosphere) of plants in grasslands. Once again the plants provide organic compounds via material which exudes from the roots.
Atmospheric nitrogen fixation
This occurs during electrical storms. Lightning provides sufficient energy to split the nitrogen atoms of nitrogen gas, forming oxides of nitrogen NOx and NO2.
This also happens inside the internal combustion engines of cars. The exhaust emissions of cars contribute a lot to atmospheric pollution in the form of NOx. These compounds form photochemical smogs and they are green house gases. They dissolve in rain to contribute to acid rain in the form of nitric acid. The rain falling on soil and running into rivers. They also contribute to the eutrophication of water bodies.
The estimated contributions of the different sources of fixed nitrogen
Nitrogen enters the soil through the decomposition of protein in dead organic matter.
This process liberates a lot of energy which can be used by the saprotrophic microbes
This involves two oxidation processes.
The ammonia produced by ammonification is an energy rich substrate for Nitrosomas bacteria. They oxidise it to nitrite:
This in turn provides a substrate for Nitrobacter bacteria oxidise the nitrite to nitrate:
This energy is the only source of energy for these prokaryotes. They are chemoautotrophs
Finally nitrates and nitrites can be used a source of oxygen for Pseudomonas bacteria living in cold waterlogged (anaerobic) soils.
The liberated oxygen is used as an electron acceptor in the processes that oxidise organic molecules, such as glucose. These microbes are, therefore, heterotrophs. Their source of reduced carbon comes from other organisms.
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