Download Nitrogen Cycle

Nitrogen Cycle
• Global Nitrogen Budget
• Nitrogen Transformations
• Denitrifying Bacteria
• Nitrifying Bacteria
• Nitrogen Fixation
• Rhizobium-Legume Symbiosis
Nitrogen Facts
• Nitrogen Oxidation States:
–
–
–
–
–
Ammonia and organic amines =
Nitrogen Gas (N2) =
Nitrous Oxide (N2O) =
Nitrite ion (NO2-) =
Nitrate ion (NO3-) =
-3
0
+1
+3
+5
• Sources of Nitrogen on a Global Scale:
–
–
–
–
Agriculture (fertilizers and livestock wastes)
Human waste
Combustion of fuels
Natural N2-fixation (biological and geothermal)
Agriculture
Inputs
7.1 mmol N m-2 y-1 for
each kg N ha-1 y-1.
Scale for agriculture is
7 x that for fossil fuel.
Fossil Fuel
Combustion
Inputs
Ammonium is also assimilated by plants, bacteria and fungi!
Dissimilatory nitrate reduction is performed by anaerobic respiring bacteria.
Nitrification
• One of two types of chemolithotrophs
– Ammonium Oxidizing Bacteria (AOB): form nitrite.
– Nitrite Oxidizing Bacteria (NOB): form nitrate
• All require oxygen as terminal electron acceptor;
mostly autotrophic
• Often AOB and NOB found together in aerobic soils
and aquatic habitats to collectively oxidize
ammonium to nitrate.
• Phylum: Proteobacteria (α, β, γ, γ)
α-Proteobacteria
β-Proteobacteria
Nitrosomonadaceae
γ-Proteobacteria
Nitrococcus
Nitrosococcus
Why care about
nitrifying bacteria?
• Loss of N-fertilizers and
groundwater contamination.
• Most mineral fertilizers are
added as ammonium.
• Presence of oxygen and
neutral pH promotes thriving
nitrifying bacterial community.
• Soil particles are negative
charged! “Likes repel”.
• Rain causes leaching.
• Oxygen depletion causes
denitrification and the
anammox process.
Anaerobic Ammonium Oxidation
NH4+ + NO2- → N2O + H2O +2H+
Phylum: Planctomycetes
Denitrification
• Type of dissimilatory nitrate reduction where nitrous
oxide or nitrogen gas is released as an end product of
anaerobic respiration.
– Compare with that of E. coli; nitrite end product.
– Compare to Clostridium (Firmicutes), and Desulfovibrio
(δ-Proteobacteria); ammonium end product.
• Found in anaerobic habitats, such as sediments and
saturated soils, yet in close proximity to nitrifying bacteria
that supply nitrate for their respiration.
• Phyla for denitrifying bacteria:
– γ-Proteobacteria (Pseudomonas)
– Firmicutes (Low G+C Gram Positives) (Bacillus)
Sediment or Hypereutrophic Lake
Nitrogen Cycling Profiles
N2-Fixation
• Nitrogenase enzyme complex
and 8 ATP needed for reducing
N2 to ammonia.
• Must have low oxygen level
(< 10% atmospheric).
• Different strategies for
reducing O2 in aerobic habitats.
• Extremely diverse
phylogenetic distribution in
Prokaryotes (never Eukarya).
• Example of horizontal gene
transfer throughout evolution.
Trichodesmium spp.
Rhizobium-Legume Symbiosis
Bacteria gets protection and
organic nutrients; legume gets a
supply of nitrogen for growth.
60 % terrestrial
nitrogen fixation
Chemotaxis to the root epidermis
cells which exude polyphenolic
flavonoid inducers of Rhizobium
nod genes.
Legume (pod plants), only release
the flavonoids when under
nitrogen stressed soil conditions.
A result of nod gene induction is
expression of rhicadhesin at the
outer membrane surface.
Root hair surface
lectins specifically
bind rhicadhesin;
Nod factor proteins
induce roothair
curling.
Rhizobium stimulates the formation of an infection
thread that the bacterium moves into and grows within.
Ultimately infection is within plant root cortex cells,
where Rhizobium transforms to a non-motile bacteroid
state; covered in host membrane (aka peribacteroid
membrane). Transformation continues to form the
sybiosome where in the bacterium is now pleomorphic
and begins fixing N2.
Leghemoglobin controls oxygen supply to bacteroid.
Active nodule as
indicated by pink.
Comparison with other related
legume nodulating bacteria