Which organisms help convert gaseous nitrogen into ammonia in the nitrogen cycle?

 In the nitrogen cycle, certain bacteria and archaea can convert gaseous nitrogen (N2) from the atmosphere into ammonia (NH3). This process is called nitrogen fixation. The main groups of organisms that facilitate this process are:

1. Symbiotic bacteria, such as Rhizobium and Frankia species. Rhizobium bacteria form symbiotic relationships with leguminous plants, such as beans and peas, by living inside nodules on the plant's roots. Frankia bacteria form similar symbioses with non-leguminous plants, like alders and bayberries.

2. Free-living soil bacteria, such as Azotobacter and Clostridium species, which are able to independently fix nitrogen from the air without the direct involvement of other organisms.

3. Cyanobacteria, also known as blue-green algae, which are capable of nitrogen fixation and are found in a variety of aquatic and terrestrial habitats. Some cyanobacteria form symbiotic relationships with fungi (forming lichens) or with plants (such as the aquatic fern Azolla).

4. Actinobacteria, which can also fix nitrogen and are common in soil.

These microorganisms contain an enzyme called nitrogenase, which is capable of breaking the strong triple bond of atmospheric nitrogen molecules (N2), allowing the nitrogen atoms to be combined with hydrogen to form ammonia (NH3). The ammonia can then be utilized by plants to synthesize proteins and other nitrogen-containing biomolecules.