1. Prokaryotes: These are single-celled organisms that lack a nucleus and other membrane-bound organelles. They are the primary nitrogen fixers, and include:

* Bacteria: A diverse group of bacteria, including cyanobacteria (blue-green algae) and nitrogen-fixing bacteria (like *Rhizobium* and *Azotobacter*). These bacteria have the enzyme nitrogenase, which allows them to convert atmospheric nitrogen into usable forms.

* Archaea: These are another type of single-celled organism that are similar to bacteria but have different biochemistry. Some archaea are also capable of nitrogen fixation.

2. Eukaryotes: These are organisms with cells that have a nucleus and other membrane-bound organelles. While eukaryotes cannot fix nitrogen directly, they can form symbiotic relationships with nitrogen-fixing prokaryotes:

* Plants: Many plants, especially legumes (like beans, peas, and lentils), have a symbiotic relationship with nitrogen-fixing bacteria (like *Rhizobium*). These bacteria live in nodules on the plant's roots, providing the plant with nitrogen in exchange for sugars.

* Fungi: Some fungi also form symbiotic relationships with nitrogen-fixing bacteria, allowing them to access nitrogen in environments where it is scarce.

It's important to remember that nitrogen fixation is primarily done by prokaryotes. Eukaryotes can utilize the fixed nitrogen but cannot do the initial conversion themselves.