Rhizobia

Rhizobia are a group of soil bacteria that fix nitrogen after becoming established inside root nodules of legumes (Fabaceae). They are crucial for maintaining the nitrogen cycle in the environment.

Characteristics

Rhizobia are Gram-negative, motile, non-sporulating rods. Many species of rhizobia exist, each with the ability to infect different host plants. The plant provides the bacteria with nutrients and an environment for multiplication. In return, the rhizobia convert atmospheric nitrogen, which is inaccessible to the plant, into ammonia, a form that the plant can use.

Symbiosis

The symbiotic relationship between rhizobia and legumes is a well-studied example of mutualism. The bacteria enter the plant cells within the root where they differentiate into bacteroids and fix nitrogen in exchange for carbon compounds from the host. This process is facilitated by the formation of a specialized structure, the nodule.

Nitrogen Fixation

Rhizobia are capable of nitrogen fixation, a process that converts atmospheric nitrogen into ammonia. This is a significant contribution to the nitrogen cycle, as nitrogen is a limiting nutrient for many plants. The process is mediated by the enzyme nitrogenase, which is only active in the absence of oxygen.

Classification

Rhizobia are classified into several genera including Rhizobium, Sinorhizobium, Bradyrhizobium, and Azorhizobium. The classification is based on the host specificity, morphology, and biochemical characteristics.

Applications

Rhizobia have significant applications in agriculture due to their ability to enhance the soil fertility by nitrogen fixation. They are used in the production of biofertilizers, which are more sustainable and environmentally friendly compared to chemical fertilizers.

See also

• Nitrogen cycle
• Biofertilizer
• Legume

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