Exam 3 prep - N cycle!

General forms of N

inorganic and organic N

Forms of inorganic N

• dinitrogen gas (N2)

• Nitric acid (HNO3)

• Nitrate ion (NO3-)

Forms of organic N

• ammonia (NH3)

• ammonium (NH4+)

• organic N

• Nitrous oxide (N2O)

• nitric oxide (NO)

• nitrite (NO2-)

oxidation reactions

have increasing oxidation number

reduction reactions

have decreasing oxidation number

Microbes driving the N cycle - N2 in atmosphere

• fixation by bacteria and archaea

• to form organic compounds with amino (NH2) groups that are used by plants and animals (decomposition of proteins and nucleic acids)

Microbes driving the N cycle - ammonia

• nitrification by bacteria into nitrite (NO2-)

Microbes driving the N cycle - nitrite

nitrification by bacteria to nitrate (NO3-)

Microbes driving the N cycle - nitrate (NO3-)

denitrification by bacteria and archaea to N2 in atmosphere

N Reservoir

• most in the atmosphere — but it is NOT biologically available

• lots in sediments and rocks too but not in available organic pools

• plants and soils are the next largest pool

Dinitrogen gas (N2)

• relatively inert and inactive

• most stable form

• ~78% of the atmosphere and is the largest reservoir of N

• SLOWLY used

• microbes and humans use it

  • via N fixation

Name the steps of the N cycle

1. N fixation

2. N assimilation

3. Ammonification

4. Nitrification

5. Denitrification

6. Sedimentation

Nitrogen fixation

• the conversion of free N of atmosphere into the biologically acceptable form or into nitrogenous compounds

• 3 ways to convert N2 into more chemically reactive forms via:
  1) biological N fixation
  2) physiochemical N fixation
  3) industrial N fixation

Nitrogen fixation - physiochemical N fixation

• atmosphere N combines with oxygen as ozone during lightning or electrical discharges in the clouds to produce different oxides

Nitrogen fixation - industrial N fixation

• man-made process that converts N from the atmosphere into a form that can be used to make fertilizer for plants

  • via Haber-Bosch method

Haber-Bosch process

• produces fertilizer by combining hydrogen and nitrogen under high pressure and temperature.

• Fritz Haber (1918) and Carl Bosch (1931) = Nobel prizes

• “Most influential persons of the 20th century” (Nature, 1999)

• Combine N (air) with H at high temps (>400C) and pressure (>200 atm) to get NH3

• VERY energy intensive

3 CH4 + 6 H2O → 3 CO2 + 12 H2

4 N2 + 12 H2 → 8 NH3

• “detonated the population explosion” through the “Green Revolution”

Nitrogen fixation - Biological N fixation

• process by which certain bacteria convert inorganic atmospheric nitrogen into organic N, making it accessible to plants.
  – N2 + 6H+ + 6e- → 2NH3

  – Requires energy (ΔG = 630 kJ/mole)

• discovered by Beijernick (bacteria had already been doing this for a while, he just described the process)

• performed by symbiotic bacteria, symbiotic blue-green algae, and free-living bacteria

• many bacteria and archaea

  • diazotrophs

  • cyanobacteria, rhizobia, frankia

• nitrogenase enzyme

  • very O2 sensitive

Strategies for protecting nitrogenase

• avoid (escape) - move to lower O2

• protect - make proteins to protect nitrogenase under O2 exposure

• O2 scavenging - increase O2 update by increasing respiration

• separation in space - form special cells to perform N fixation

  • heterocysts in cyanobacteria

  • nodules in plants

• separation in time - fix at night when O2 is lower due to decreased photosynthesis

Types of N fixers - Symbionts

• rhizobia and legumes

• Actinomycetes (aka Frankia) and woody plants

Types of N fixers - Associative

• Azotobacter

• live in rhizosphere

Types of N fixers - Free-living

• cyanobacteria

• also in soils