Nitrogen Fixation Process

Study Notes

Nitrogen fixation is a biochemical process that transforms atmospheric nitrogen (N2) into ammonia (NH3), ammonium (NH4+), or other bioavailable forms of nitrogen for eukaryotes.
Prokaryotes, such as bacteria and archaea, are capable of nitrogen fixation using the enzyme nitrogenase, consuming 16 ATP molecules in the process.

Nitrogen fixation is the first step in the nitrogen cycle, which cycles atmospheric nitrogen to the living biosphere.
The nitrogen cycle includes: biological nitrogen fixation, nitrification, ammonification, assimilation, and denitrification.
Nitrogen-fixing bacteria develop symbiotic relationships with plants, providing bioavailable nitrogen in exchange for sugar and nutrients.

Nitrogen fixation is vital for ecosystems, as plants and animals rely on bioavailable nitrogen for metabolic and biological functions.
Plants require nitrogen for photosynthesis, chlorophyll production, and other cellular processes.
Humans indirectly depend on the nitrogen cycle through dietary sources, obtaining nitrogen from plants and animals that have incorporated nitrogen in their bodies.

Terrestrial examples include mutualistic bacteria like Rhizobium, which fix nitrogen for legumes like beans and lentils.
Marine examples include free-floating diazotrophic microorganisms like cyanobacteria, which fix atmospheric nitrogen into bioavailable forms for primary producers like Prochlorococcus.

Biological nitrogen fixation: Atmospheric nitrogen is transformed into ammonia by free-living prokaryotic organisms in the soil or mutualistic bacteria in plant roots.
Nitrification: Ammonia is converted into nitrates (NO3-) by prokaryotic nitrifying bacteria, and nitrates are further converted into nitrites (NO2-).
Ammonification: Nitrogen-containing compounds are returned to the soil via animal waste or decomposition, converted to ammonia by soil bacteria.
Assimilation: Bioavailable nitrogen is absorbed by eukaryotic organisms in the soil, like plants.
Denitrification: Bioavailable nitrogen is transformed back into nitrogen gas, returned to the atmosphere.