`star` Nitrogen Cycle
`star` Biological Nitrogen Fixation
`star` Symbiotic Biological Nitrogen Fixation


● Apart from carbon, hydrogen and oxygen, `color{Brown}"nitrogen"` is the most `color{violet}"prevalent element"` in living organisms.

● Nitrogen is a constituent of `color{violet}"amino acids, proteins"`, `color{violet}"hormones, chlorophylls"` and many of the vitamins.

● Plants `color{violet}"compete with microbes"` for the `color{violet}"limited nitrogen"` that is available in soil.

● Thus, nitrogen is a `color{Brown}"limiting nutrient"` for both `color{violet}"natural and agricultural"` eco-systems.

● Nitrogen exists as `color{violet}"two nitrogen atoms"` joined by a very strong `color{violet}"triple covalent bond"` (N ≡ N).

● The process of `color{violet}"conversion of nitrogen"` (`N_2` ) to `color{violet}"ammonia"` is termed as `color{Brown}"nitrogen fixation"`.

● In nature, `color{violet}"lightning and ultraviolet radiation"` provide enough energy to convert `color{Brown}"nitrogen to nitrogen oxides"` (`NO`, `NO_2` , `N_2O`).

● `color{violet}"Industrial combustions"`, `color{violet}"forest fires, automobile exhausts"` and `color{violet}"power-generating stations"` are also sources of atmospheric nitrogen oxides.

● `color{violet}"Decomposition"` of `color{violet}"organic nitrogen"` of dead plants and animals into `color{violet}"ammonia"` is called `color{Brown}"ammonification"`.

● Some of this `color{violet}"ammonia volatilises"` and `color{violet}"re-enters the atmosphere"` but most of it is converted into `color{violet}"nitrate by soil bacteria"` in the following steps:

● Ammonia is `color{violet}"first oxidised to nitrite"` by the bacteria `color{Brown}"𝘕𝘪𝘵𝘳𝘰𝘴𝘰𝘮𝘰𝘯𝘢𝘴 and/or 𝘕𝘪𝘵𝘳𝘰𝘤𝘰𝘤𝘤𝘶𝘴"`.

● The nitrite is further `color{violet}"oxidised to nitrate"` with the help of the bacterium `color{Brown}"𝘕𝘪𝘵𝘳𝘰𝘣𝘢𝘤𝘵𝘦𝘳."`

● These steps are called `color{Brown}"nitrification"`.

● These nitrifying bacteria are `color{violet}"chemoautotrophs"`.

● The nitrate thus formed is `color{violet}"absorbed by plants"` and is `color{violet}"transported to the leaves."`

● In leaves, it is `color{violet}"reduced to form ammonia"` that finally forms the `color{violet}"amine group of amino acids"`.

● `color{violet}"Nitrate present in the soil"` is also reduced to `color{violet}"nitrogen"` by the process of `color{Brown}"denitrification"`.

● `color{violet}"Denitrification"` is carried by bacteria `color{Brown}"𝘗𝘴𝘦𝘶𝘥𝘰𝘮𝘰𝘯𝘢𝘴 and 𝘛𝘩𝘪𝘰𝘣𝘢𝘤𝘪𝘭𝘭𝘶𝘴"`.


● Very `color{violet}"few living organisms"` can utilise the `color{violet}"nitrogen"` in the form `N_2`, available `color{violet}"abundantly in the air."`

● Only certain `color{violet}"prokaryotic species"` are capable of `color{violet}"fixing nitrogen"`.

● `color{violet}"Reduction"` of `color{violet}"nitrogen to ammonia"` by `color{violet}"living organisms"` is called `color{Brown"biological nitrogen fixation"`.

● The enzyme, `color{violet}"nitrogenase"` which is capable of `color{violet}"nitrogen reduction"` is present `color{violet}"exclusively in prokaryotes"`.

● Such microbes are called `color{violet}(N_2)` `color{violet}(- fixers)`.

● The nitrogen-fixing microbes could be `color{violet}"free-living or symbiotic"`.

● Examples of `color{Brown}"free-living nitrogen-fixing"` `color{violet}"aerobic microbes"` are `color{Brown}"𝘈𝘻𝘰𝘵𝘰𝘣𝘢𝘤𝘵𝘦𝘳 and 𝘉𝘦𝘪𝘫𝘦𝘳𝘯𝘪𝘤𝘬𝘪𝘢"` while `color{brown}"𝘙𝘩𝘰𝘥𝘰𝘴𝘱𝘪𝘳𝘪𝘭𝘭𝘶𝘮"` is anaerobic and `color{violet}"𝘉𝘢𝘤𝘪𝘭𝘭𝘶𝘴"` free-living.

● In addition, a number of `color{violet}"cyanobacteria"` such as `color{Brown}"𝘈𝘯𝘢𝘣𝘢𝘦𝘯𝘢 and 𝘕𝘰𝘴𝘵𝘰𝘤"` are also `color{violet}"free-living nitrogen-fixers"`.


● Several types of `color{violet}"symbiotic biological nitrogen fixing"` associations are known.

● The most prominent among them is the `color{Brown}"legume-bacteria relationship"`.

● Species of `color{violet}"rod-shaped 𝘙𝘩𝘪𝘻𝘰𝘣𝘪𝘶𝘮"` has such relationship with the roots of `color{violet}"several legumes"` such as alfalfa, sweet clover, sweet pea, lentils, garden pea, broad bean, clover beans, etc.

● The most common `color{violet}"association on roots"` is as `color{Brown}"nodules"`.

● These nodules are `color{violet}"small outgrowths"` on the roots.

● The microbe, `color{Brown}"𝘍𝘳𝘢𝘯𝘬𝘪𝘢"`, also produces `color{violet}"nitrogen-fixing nodules"` on the roots of `color{violet}"non leguminous plants"` (e.g., `color{Brown}"𝘈𝘭𝘯𝘶𝘴"`).

● Both `color{Brown}"𝘙𝘩𝘪𝘻𝘰𝘣𝘪𝘶𝘮 and 𝘍𝘳𝘢𝘯𝘬𝘪𝘢"` are `color{violet}"free living"` in soil, but as `color{violet}"symbionts,"` can fix atmospheric nitrogen.

● Uproot any `color{violet}"one plant"` of a `color{violet}"common pulse"`, just before flowering.

● You will see `color{violet}"near-spherical outgrowths"` on the roots. These are `color{violet}"nodules"`.

● If you cut through them you will notice that the `color{violet}"central portion is red or pink"`.

● This is due to the presence of `color{violet}"leguminous haemoglobin"` or `color{violet}"leg-haemoglobin"`.