Question

5.



short Summarize the movement of nitrogen between the atmosphere and biosphere.



Describe the reactions or organisms that are involved in the movement of the nitrogen.

Answer 1

Short summary
- The nitrogen cycle moves nitrogen between the atmosphere (mostly inert N2) and the biosphere by converting N2 into biologically available forms (ammonium, nitrate), recycling organic nitrogen, and returning nitrogen to the atmosphere as N2 or N2O. Microbial processes drive most of these transformations; abiotic processes (lightning, industrial fixation) also contribute.

Key steps, reactions, and organisms
1. Nitrogen fixation
- Reaction: N2 + 8 H+ + 8 e− + 16 ATP → 2 NH3 + H2 + 16 ADP + 16 Pi (catalyzed by nitrogenase)
- Organisms: symbiotic rhizobia (in legume root nodules), free‑living diazotrophs (Azotobacter, Clostridium), cyanobacteria (Anabaena, Nostoc), some archaea.
- Abiotic: lightning or industrial Haber–Bosch process converts N2 → NOx or NH3.

2. Assimilation
- Reaction: NH4+ or NO3− → organic N (amino acids, nucleotides) via incorporation into glutamate/glutamine and biosynthetic pathways.
- Organisms: plants, algae, microbes take up NH4+ or NO3− and build organic nitrogen compounds.

3. Ammonification (mineralization)
- Reaction: Organic N (proteins, nucleic acids) → NH4+ (by microbial decomposition)
- Organisms: heterotrophic bacteria and fungi that decompose dead biomass and excretions.

4. Nitrification (aerobic, two-step)
- Step 1: NH3/NH4+ + O2 → NO2− + H+ + H2O (ammonia monooxygenase, ammonia‑oxidizing bacteria/archaea)
- Organisms: Nitrosomonas, Nitrosospira, and ammonia‑oxidizing archaea (Thaumarchaeota).
- Step 2: NO2− + ½ O2 → NO3− (nitrite oxidoreductase)
- Organisms: Nitrobacter, Nitrospira.

5. Denitrification (anaerobic)
- Reactions (sequence): NO3− → NO2− → NO → N2O → N2 (reductases)
- Organisms: facultative anaerobes such as Pseudomonas, Paracoccus, some Bacillus and other soil bacteria.
- Returns N to the atmosphere as N2 (or partly as N2O, a greenhouse gas).

6. Anammox (anaerobic ammonium oxidation)
- Reaction: NH4+ + NO2− → N2 + 2 H2O
- Organisms: planctomycete bacteria (e.g., Brocadia) in oxygen‑poor sediments/wastewater.

Notes
- The cycle is tightly coupled: fixation and assimilation add biologically available N; mineralization, nitrification, denitrification and anammox recycle it and release gaseous N back to the atmosphere.
- Human activities (fertilizers, fossil fuel burning) increase reactive nitrogen and N2O emissions, altering ecosystem function and climate.