What is biological fixation? How is it different from nitrification? Give examples of organisms involved in each of these.

Biological fixation and nitrification are both important processes in the nitrogen cycle, but they occur at different stages and involve different microorganisms.

Biological fixation: This process involves the conversion of atmospheric nitrogen gas (N2) into forms that plants can use, such as ammonia (NH3) or nitrate (NO3-). Biological fixation can occur through two main mechanisms: symbiotic fixation and non-symbiotic fixation.

Symbiotic fixation: This occurs in the root nodules of certain plants, particularly legumes like soybeans, peas, and clover, where symbiotic bacteria called rhizobia reside. These bacteria have the enzyme nitrogenase, which allows them to convert atmospheric nitrogen into ammonia, which the plant can utilize for its growth.
Example organism: Rhizobium spp. (e.g., Rhizobium leguminosarum)

Non-symbiotic fixation: This process is carried out by free-living soil bacteria and cyanobacteria. These organisms also possess nitrogenase enzymes that allow them to convert atmospheric nitrogen into ammonia.
Example organisms: Azotobacter spp., Clostridium spp., Cyanobacteria (e.g., Anabaena)

Nitrification: Nitrification is the biological oxidation of ammonia (NH3) or ammonium (NH4+) to nitrite (NO2-) and then to nitrate (NO3-). This process is critical for making nitrogen available to plants in a form they can readily use.

Ammonia oxidation: Ammonia-oxidizing bacteria (AOB) and archaea (AOA) are responsible for oxidizing ammonia to nitrite in the first step of nitrification.
Example organisms: Nitrosomonas spp. (bacteria), Nitrososphaera spp. (archaea)

Nitrite oxidation: Nitrite-oxidizing bacteria (NOB) convert nitrite to nitrate in the second step of nitrification.
Example organisms: Nitrobacter spp., Nitrospira spp.

In summary, biological fixation involves the conversion of atmospheric nitrogen into forms usable by plants, while nitrification involves the conversion of ammonia or ammonium into nitrate, another form of nitrogen that plants can assimilate. The key difference lies in the starting material and the end products of each process, as well as the microorganisms involved.