The atmosphere contains about 79% nitrogen gas. Lichens need a continual supply of nitrogen atoms and despite such as high concentration of nitrogen in the atmosphere, they cannot use it directly. The nitrogen cycle outlines the pathway nitrogen atoms take as they pass from the atmosphere and move through lichens. There are two gateways through which nitrogen enters lichens: (i) through bacteria and (ii) with the help of lightening.
Why is nitrogen important for lichens?
Protein molecules cannot be made without nitrogen as it is an important ingredient. Proteins are essential as they help in metabolism by acting as enzymes and hormones (chemical messengers). Nitrogen forms part of DNA and chlorophyll.
The nitrogen cycle – how it works
The lettering below matches the illustration.
The nitrogen cycle and cyanobacteria pathway
(a) The free nitrogen in the NO2 molecules is ‘fixed’ (‘fixing’ in chemistry results in hydrogen bonding with nitrogen) by cyanobacteria to form nitrate. The cyanobacteria that make the nitrates live in the lichen (mainly in the medulla). They are called nitrifying bacteria.
(b) Nitrates are absorbed by lichens and used to
(c) Built lichen protein.
(d) Lichens are eaten by herbivores and omnivores (primary consumers). The nitrogen in the lichen protein can then be used by animals.
(d) Dead lichens/plants and animals are decomposed by bacteria to produce ammonia (contains nitrogen). The ammonia is broken down by bacteria to form either nitrites (g) or nitrates (h).
(g) If nitrites are formed then they release nitrogen back to the atmosphere as NO2 by denitrifying bacteria.
(h) If nitrates are formed they are absorbed by lichens to make new proteins.
The nitrogen cycle and lightening pathway
Lightening burns the NO2 in the atmosphere and converts it into nitrate. The nitrate is washed into the soil from where it is absorbed into the lichens.