Which microorganisms carry out nitrification (ammonia → nitrite → nitrate)?
Nitrosomonas (NH₄⁺ → NO₂⁻); Nitrosospira or Nitrosopumilus (archaea); Nitrobacter and Nitrospira (NO₂⁻ → NO₃⁻). Mnemonic: ‘Nitroso starts, Nitro finishes.’
What type of metabolism do nitrifying bacteria use, and under what condition?
Chemolithoautotrophs — use inorganic compounds (NH₃, NO₂⁻) as energy source. Aerobic process (requires O₂).
Which bacteria perform denitrification (NO₃⁻ → N₂)?
Pseudomonas; Paracoccus; Bacillus (facultative anaerobes). Mnemonic: ‘PPB → Please Pass Back (N to air).’
Name the main groups of nitrogen-fixing microorganisms.
Free-living: Azotobacter, Clostridium, Cyanobacteria. Symbiotic: Rhizobium, Bradyrhizobium, Frankia. Mnemonic: ‘Fixers are FAS – Free, Anaerobic, Symbiotic.’
Which enzyme is responsible for biological nitrogen fixation, and what limits it?
Nitrogenase enzyme; inactivated by oxygen — functions best under anaerobic or microaerophilic conditions.
What microorganisms carry out ammonification (organic-N → NH₄⁺)?
Many heterotrophic bacteria and fungi. Examples: Bacillus, Clostridium, soil actinomycetes. Mnemonic: ‘Dead stuff → Ammonia.’
Which microbes perform anammox (NO₂⁻ + NH₄⁺ → N₂)?
Brocadia and other Planctomycetes. Mnemonic: ‘ANaerobic AMMonium OXidation = ANAMMOX.’
What is the overall ecological role of these microbes in the nitrogen cycle?
They convert nitrogen between gaseous, inorganic, and organic forms, maintaining ecosystem nitrogen balance and supporting plant growth.
What is nitrification?
Aerobic oxidation of ammonia (NH₃ / NH₄⁺) to nitrate (NO₃⁻) through nitrite (NO₂⁻), performed by nitrifying bacteria and archaea.
What is anammox?
Anaerobic oxidation of ammonium (NH₄⁺) using nitrite (NO₂⁻) as the electron acceptor, producing nitrogen gas (N₂).
Which environment favors nitrification?
Well-drained, oxygen-rich (oxic) soils at neutral pH.
Which environment favors anammox?
Anoxic, oxygen-free zones such as marine sediments, sewage digesters, and oxygen-depleted waters.
Electron donor and acceptor in nitrification
Donor = NH₃ / NH₄⁺, Acceptor = O₂.
Electron donor and acceptor in anammox
Donor = NH₄⁺, Acceptor = NO₂⁻.
End product of nitrification
NO₃⁻ (nitrate).
End product of anammox
N₂ gas (released to the atmosphere).
Key microorganisms in nitrification
Bacteria: Nitrosomonas, Nitrobacter, Nitrospira (comammox). Archaea: Nitrosopumilus.
Key microorganisms in anammox
Candidatus Brocadia, Kuenenia, Scalindua, Anammoxoglobus, and Jettenia (Planctomycetes family Brocadiaceae).
Type of respiration in nitrification
Aerobic.
Type of respiration in anammox
Anaerobic.
Ecological role of nitrification
Produces nitrate for plant uptake but can cause nitrate leaching and denitrification losses in wet soils.
Ecological role of anammox
Removes reactive nitrogen by converting NH₄⁺ and NO₂⁻ to N₂, reducing nitrogen loads in anoxic waters and wastewater.
Mnemonic to remember the difference
Nitrification Needs O₂ → NO₃⁻; Anammox Avoids O₂ → N₂.
What is nitrogen fixation and why is it important to the nitrogen cycle?
Nitrogen fixation is the biological conversion of atmospheric nitrogen gas (N₂) into ammonia (NH₃), a form organisms can assimilate into amino acids and nucleic acids. It is carried out by bacteria and archaea (e.g., Rhizobium, Azotobacter, Anabaena) using nitrogenase enzyme. Importance: introduces new usable nitrogen into ecosystems, supporting plant growth and maintaining the nitrogen cycle.
