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Nitrogen Cycle

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Nitrogen Cycle Global Nitrogen Budget Nitrogen Transformations Denitrifying Bacteria Nitrifying Bacteria Nitrogen Fixation Rhizobium-Legume Symbiosis – PowerPoint PPT presentation

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Title: Nitrogen Cycle


1
Nitrogen Cycle
  • Global Nitrogen Budget
  • Nitrogen Transformations
  • Denitrifying Bacteria
  • Nitrifying Bacteria
  • Nitrogen Fixation
  • Rhizobium-Legume Symbiosis

2
Nitrogen Facts
  • Nitrogen Oxidation States
  • Ammonia and organic amines -3
  • Nitrogen Gas (N2) 0
  • Nitrous Oxide (N2O) 1
  • Nitrite ion (NO2-) 3
  • Nitrate ion (NO3-) 5
  • Sources of Nitrogen on a Global Scale
  • Agriculture (fertilizers and livestock wastes)
  • Human waste
  • Combustion of fuels
  • Natural N2-fixation (biological and geothermal)

3
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
4
Ammonium is also assimilated by plants, bacteria
and fungi! Dissimilatory nitrate reduction is
performed by anaerobic respiring bacteria.
5
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 (a, ß, ?, ?)

6
a-Proteobacteria
7
ß-Proteobacteria
Nitrosomonadaceae
8
?-Proteobacteria
Nitrococcus
Nitrosococcus
9
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.

10
Anaerobic Ammonium OxidationNH4 NO2- ? N2O
H2O 2HPhylum Planctomycetes
11
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
  • (d-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
  • ?-Proteobacteria (Pseudomonas)
  • Firmicutes (Low GC Gram Positives) (Bacillus)

12
Sediment or Hypereutrophic LakeNitrogen Cycling
Profiles
13
N2-Fixation
  • Nitrogenase enzyme complex and 8 ATP needed for
    reducing N2 to ammonia.
  • Must have low oxygen level (lt 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.
14
Rhizobium-Legume Symbiosis
60 terrestrial nitrogen fixation
Bacteria gets protection and organic nutrients
legume gets a supply of nitrogen for growth.
15
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.
16
Root hair surface lectins specifically bind
rhicadhesin Nod factor proteins induce roothair
curling.
17
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.
18
Comparison with other related legume nodulating
bacteria
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