What type of bacteria is responsible for converting atmospheric nitrogen to ammonium in legumes?

The correct answer is Rhizobium bacteria because they play a critical role in the nitrogen-fixation process in leguminous plants. These bacteria form symbiotic relationships with the roots of legumes, such as beans and peas, where they reside in specialized structures called root nodules. Within these nodules, Rhizobium bacteria convert atmospheric nitrogen (N2) into ammonium (NH4+), a form of nitrogen that is readily taken up by plants and utilized for growth. This symbiotic relationship benefits both the plants and the bacteria: the legumes provide carbohydrates and a suitable environment for bacterial growth, while the bacteria supply the plants with essential nitrogen for their development.

In contrast, Actinobacteria typically are involved in the decomposition of organic matter and the breakdown of complex compounds in soil, and while they can contribute to soil health and nutrient cycling, they do not specifically convert atmospheric nitrogen into ammonium. Clostridium bacteria are known for nitrogen fixation as well, but they are primarily anaerobic and often found in soil rather than forming a symbiotic relationship with legumes. Bacillus bacteria are prominent in soil health and plant growth promotion but do not possess the nitrogen-fixing capabilities associated with the Rhizobium genus. Thus, Rhizob