Download The nitrogen cycle

The nitrogen cycle
 Animals can not fix N2. They get their nitrogen
by eating plants or by eating something that eats
plants.
 Nitrogen Fixation is very expensive process
 In the biosphere, the nitrogen cycle is a vast
collection of metabolic processes of
different species function
interdependently to salvage and reuse
biologically available nitrogen.
Key terms of The Nitrogen Cycle
 Nitrogen Fixation: Conversion of N2 to
ammonia (NH3)
 By any bacteria in soil/water having the nitrogenase
complex, e.g. Rhizobiumin root nodules of legumes.
 Nitrification: Conversion of ammonia to
nitrite (NO2-) and then nitrate (NO3-).
 Both reactions carried out by bacteria
 Assimilation: Conversion of NH3, NO2-,, NO3-(inorganic)
into organic compounds (proteins, DNA, & other
forms)
 All living cells (plants, animals, & bacteria).
 Ammonification: Conversion of the amine groups of
organic compounds into simpler compounds (often,
ammonia NH3).
 Mostly via decay processes carried out by decomposer bacteria
 Denitrification: Conversion of NH3, NO2-,, NO3-to
N2
 Mostly by anaerobic bacteria in water logged soil, bottom
sediments of lakes, swamps, bogs and oceans.
Overview of the N-cycle
 The first product of biological fixation is ammonia (NH3or
+NH4).
 In principle: this ammonia can be used by most living
organisms,However, soil bacteria and plant are in fierce
competition for NH3
 Bacteria are more abundant and active, but plants have their ways.
 In either case, Nitrification proceeds:
 NH3
-NO2
-NO3
 Plants and many bacteria can also reduce nitrate and nitrite
ammonia (reductases).
-NO3
NO2
NH3
 The new ammonia is incorporated into organic molecules by
plants& bacteria. (Assimilation).
 When organisms die, microbial degradation of their proteins
returns ammonia to restart the cycle.
 Some bacteria can convert nitrate to N2 under anaerobic
conditions (denitrification)
Nitrification
 Nitrification is the biological oxidation of ammonia
with oxygen into nitrite followed by the oxidation
of these nitrites into nitrates.
 Degradation of ammonia to nitrite is usually the
rate limiting step of nitrification. Nitrification is an
important step in the nitrogen cycle in soil.
Nitrifying bacteria
 Nitrifiers: (heterotrophs& autotroph)
are delicate organisms and extremely susceptible to
a variety of inhibitors.
 They are extremely slow growing
 Nitrifying bacteria need a relatively clean
environment with a continuous supply of ammonia
and oxygen.
 Two bacterial species are required for nitrification:
1) Ammonia-Oxidizing Bacteria: Nitrosomonas
Present in large numbers
They require ammonia and CO2 , and found in a
great variety of soils, oceans, rivers, lakes, and
sewage disposal systems
2) Nitrite Oxidation Microorganism: Nitrobacter
Aerobic, but occasionally also anaerobic
They are widely distributed in soils, fresh water,
seawater, mud layers, sewage disposal systems, and
inside stones of buildings, rocks, and inside
concrete surfaces
Nitrate Assimilation
 NO3
NO2
NH4+
amino acids
 Requires large input of energy
 Forms toxic intermediates
 Mediated by enzymes (Reductases) that are closely
regulated
 Nitrate levels, light intensity, and concentration of
carbohydrates all influence the activity of nitrate
reductases at the transcription and translation levels
 These factors stimulate a protein, phosphatase, that
dephosphorylates several serine residues on the nitrate
reductase protein thereby activating the enzyme
Ammonium Assimilation
 Ammonium is highly toxic, yet essential to both
animals and plants.
 Animal & Plant cells rapidly assimilate into amino
acids.
 In plants: this requires the action of two enzymes:
Glutamine synthetase and Glutamate synthase
Denitrification
 Denitrification converts nitrates (NO3)in the soil to
atmospheric nitrogen (N2)
 Denitrifying bacteria live deep in soil and in
aquatic sediments where conditions make it
difficult for them to get oxygen.
 The denitrifying bacteria use Nitrates as an
alternative to oxygen, leaving free nitrogen gas as
a byproduct.They close the nitrogen cycle!