Final answer:
Bacteria are the best organisms at fixing nitrogen through Biological Nitrogen Fixation (BNF), especially the rhizobia that form symbiotic relationships with legumes and free-living bacteria like Azotobacter and Clostridium.
Step-by-step explanation:
The best type of organism at fixing nitrogen is bacteria. Biological nitrogen fixation (BNF) is a crucial process by which atmospheric nitrogen (N₂) is converted into ammonia (NH₃), and this process is carried out exclusively by prokaryotes such as soil bacteria, cyanobacteria, and certain bacteria within the Frankia genus. Among these, the most significant nitrogen-fixing agents are the rhizobia bacteria that live in symbiotic relationships with the roots of legumes, forming specialized structures called nodules where they can fix nitrogen away from oxygen, which would otherwise inhibit the nitrogenase enzyme responsible for the fixation process.
Legume-associated rhizobia and other free-living nitrogen-fixing bacteria like Azotobacter and Clostridium dramatically contribute to the availability of usable nitrogen in ecosystems, providing the essential element to various life forms including plants and, indirectly, the animals that consume them. Besides the terrestrial bacteria, cyanobacteria play a significant role as nitrogen fixers in aquatic environments.