Microbiology

Microbiology Study Guide

Another major function in the nitrogen cycle is the fixation of nitrogen. The atmosphere is about 80 percent nitrogen, and it is through the chemical process of reduction, known as nitrogen fixation, that ammonia is produced using this atmospheric nitrogen gas. The main function in the fixation of nitrogen is performed by the free-living bacteria in the atmosphere, as well as the bacteria that form their habitat in leguminous plants, such as peas, beans, and soybeans. Some free-living bacteria that help in nitrogen fixation are from the species of Azotobacter, Azospirillum, Bacillus, Beijerinckia , and numerous species of cyanobacteria. In contrast, the bacteria that are attached to the legumes of plants are gram-negative, live in the roots of the leguminous plants, are majorly from the species of Rhizobium , and reside in a symbiotic relationship with plants. These bacteria derive the essential nutrients necessary for their survival and growth from the plant and penetrate through the root hairs of plants where they live, building a root nodule (a kind of thread of infection, which helps to fix the nitrogen present in the atmosphere). Once nitrogen is substituted in the form of ammonia, it is ready to use for many organic substances. It should also be noted that this nitrogen is recyclable through the process of ammonification when plants, animals, or other microorganisms die. Moreover, the excretion of animals is also a useful source of ammonia, which is produced when the urea-digesting bacteria break down the urea present in the excretion of animals. The next step in the nitrogen cycle is nitrification, which is performed mostly by the species Nitrosomonas and Nitrosococcus. In this process, the ammonia produced is converted to nitrite with the help of species Nitrosomonas, and then this nitrite is converted to nitrate by the species Nitrosococcus. The resultant nitrate provides a rich source of nutrients in the soil for plants. The process of nitrification is possible not only in soil, but also in marine and freshwater bodies. The last step in the nitrogen cycle is denitrification, through which the produced nitrate is released in the atmosphere in a gaseous form. This is performed by the nitrogen fixation bacteria, which helps to form nitrite to nitrous oxide and finally, atmospheric nitrogen is formed. The nitrogen cycle:

©2018

Achieve

Page 90

of 132

Made with FlippingBook flipbook maker