Which organisms are involved in the fixation of nitrogen?

Nitrogen fixation is the process through which certain organisms convert atmospheric nitrogen (N2) into a more usable form for plants, such as ammonia (NH3). The organisms primarily involved in this essential process include symbiotic and free-living bacteria, as well as cyanobacteria.

Symbiotic bacteria, such as those found in the root nodules of legumes, form close relationships with plants, allowing them to efficiently convert nitrogen into a form that the host plant can utilize. Free-living bacteria, which can thrive in the soil or water without a host, also play a crucial role in nitrogen fixation by converting atmospheric nitrogen independently. Cyanobacteria, often found in aquatic environments and some terrestrial ecosystems, are another vital group that can fix nitrogen, contributing significantly to nitrogen availability in various ecological niches.

In contrast, other choices involve organisms that do not play a direct role in nitrogen fixation. For example, fungi, while they are important decomposers and nutrient cyclers, do not fix nitrogen. Insects and plants lack the necessary biochemical pathways for fixing nitrogen. Likewise, algae and protozoa, although they contribute to various aspects of nutrient cycling, are not known for nitrogen fixation in the same manner as the organisms listed in the correct answer. This