Loading in 2 Seconds...
Loading in 2 Seconds...
Nitrogen cycle/nitrogen fixation Nitrogen is an important constituent of biological molecules. The availability of N can affect plant growth and thus primary Production. Microbes are intimately involved in this process. Nitrogen is a very stable and common molecule.
Nitrogen is an important constituent
of biological molecules.
The availability of N can affect plant
growth and thus primary Production.
Microbes are intimately involved in this process.
A lot of energy is required to break the NN bond.
Most organic nitrogen is recycled from the more
easily available forms nitrate and NH4+.
But N fixation is a critically important process
in the environment and in agriculture
Dissimilatory (anaerobic process), nitrate is used as an electron acceptor, producing N2. It is beneficial in waste water treatment, removing nitrate, thus reducing algal growth (blooms), eutrophication.
Note they will grow aerobically using O2 as an electron acceptor if it is available (THIS IS NOT FERMENTATION).
2NO3- + 10e- +2H+ N2 + 6H2O The electrons come from metabolism of carbohydrates etc.
NO3- NO2- NO N2O N2
Thought to be an enzyme for each stage. Nitric and nitrous oxide can be released into the atmosphere causing potential problems.
It is oxidation of ammonia to nitrate (via nitrite).
Occurs in well drained, aerated soils by two nitrifying bacteria,
Nitrosomas and Nitrobacter together (example of syntrophism).
Manure and sewage promote nitrification.
Nitrate is rapidly absorbed by plants but as it is very soluble
it is easily leached out by rain, so it is not always of benefit.
Ammonia at neutral and acid pH, is cationic and is absorbed by
Anhydrous ammonia is used as a fertiliser, chemicals are added
to inhibit nitrification. This (nitrapyrin) increases efficiency
of the fertiliser and reduces run off water pollution.
The process in energetically fairly inefficient,
generating few ATP molecules the bacteria grow slowly.
NH4 + 3/2O2- NO2- +2H+ + energy
NO2- + 3/2O2 – NO3- + energy
The energy generated is used to fix CO2
is is the conversion of nitrate (or NH3) to NH3
and then to nitrogenous compounds like amino acids.
This reaction is very important. N2 is very stable and there is a large a reservoir of N in the atmosphere.
It requires a lot of energy to break the triple bond, and only a small number of organisms can do it, all prokaryotes. Both free living and in symbiotic associations.
Free living aerobes Azotobacter, Azomonas, Cyanobacteria (some)
Free living anaerobes Closridium, Rhodobacter etc
Symbiotic Rhizobium and Bradyrhizobium with legumes
(Soya, peas, clover etc)
Frankia with woody shrubs
Anabaena with azolla (fern) in paddy fields
Nitrogen has a triple bond and this requires a lot of energy to break it.
940 kj compared to 493kj for O2.
6 electrons are required to reduce N2 to 2NH3.
This reduction process is catalysed by Nitrogenase.
Made up of two proteins – dinitrogenase and dinitrogen reductase.
Both contain Fe and DR also has Mo.
In DR the Fe and Mo are contained in a cofactor
FeMo-co and the reduction of N2 occurs here.
The formula is MoFe7S9.
Two molecules of FeMo-co per molecule. FeS forms a cage.
Nitrogen fixation is inhibited by O2.
As it is a highly reducing process.
Both enzymes are rapidly and irreversibly inactivated by O2.
In aerobic bacteria, nitrogenase is protected from O2
either by rapid respiration, production O2 retarding slime
or production of special cells (heterocysts),
or O2 is removed by special chemicals.
For every molecule of N2 fixed. 16-24 molecules ATP are used.
Ferredoxin, flavodoxin or low potential iron-sulphur protein
are the electron donors.
They transfer electrons to dinitrogen reductase.
For each cycle of e- transfer, dintrogen reductase binds two ATP,
which is then able to interact with dinitrogenase and transfer
electrons to it. ATP is hydrolysed and the two proteins disassociate
to begin another cycle of reduction. Only 6 electrons used in the
useful reduction, another two are wasted to make H2, which can
back react withN2H2.
These bacteria are unable to fix, N2 alone, they need the plant.
In the nodule O2 levels are controlled by leghaemolobin.
The bacteria and plant form this iron containing compound
which binds O2.
Bound: free O2 is 10,000:1
secretions. Rhicadhesin on the surface of bacterium may bind
calcium complexes on the root hair surface.
Invasion of the root hair is via the tip as a result of the action of
bacterial encoded nod factors, inducing formation of a cellulosic
tube, the infection thread.
Root cells adjacent to this thread also become infected.
Nod factors stimulate plant cell division.
Bacteria multiply rapidly in the root.
Bacterial cells become swollen into bacteroids, become
surrounded singly, or in groups by plant cell membrane to
become symbiosome. Only then does nit fix take place.
When the plant dies, nodule deteriorate, bacteroid cannot divide, but
some dormant rods always there which proliferate on the
products released from the dying nodules. The fixed N is released to the soil
Soils get leached because of high microbial activity.