Mention the role of cyanobacteria as biofertilizer.

Hint:The biofertilizers are the fertilizers produced by eco-friendly products or by some microorganisms during the process of nitrogen fixation. In this process ammonia is produced that works as a fertilizer for soil and it is a very useful mechanism of nature to reduce the use of synthetic fertilizers that has many adverse effects.

Complete answer:
To answer this question, we have to know details about nitrogen fixation. Nitrogen fixation is the process of converting nitrogen found in the atmosphere into ammonia. This process is maintained by organisms like Nostoc and Anabaena, they are considered as the nitrogen fixing bacteria. Both of these bacteria show a symbiotic association with some of the plants like mosquito fern. In Anabaena, when there is a decrease in the amount of nitrogen in the soil the vegetative cells differentiated into heterocyst, this cell is formed for the nitrogen fixation process. It increases the respiration process as the inner surface of the cell is micro-oxic thus involved in the inactivation of the oxygen production photosystem II and forms a thick outer cell wall. Nitrogenase, the enzyme required for the conversion of nitrogen to ammonia is found within the heterocyst. During the process of photosystem I the plant produces carbohydrates in the form of glucose, this moves inside the heterocysts and in return it maintains the nitrogen fixation and gives this back to the plant. Anabaena forms a symbiotic association with the fern Azolla and is named Anabaena Azollae. This involves the nitrogen fixation process and in return it provides the plant essential nutrients.

Now, Rhizobium, Nitrosomonas, Azotobacter, Nostoc, and cyanobacteria are the nitrogen fixing bacteria that involve the conversion of atmospheric nitrogen into usable form in association with the plants. Nitrosomonas are involved in the conversion of ammonia to nitrites and nitrates that are nitrification process and Nostoc are cyanobacteria involved in fixing atmospheric nitrogen. This ammonia functions as a biofertilizer to maintain the fertility of the soils and improve the crop yield.

Note:This process of nitrogen fixation is also used for the production of synthetic fertilizers, in which nitrogen in presence of nitrogenase is converted to ammonia. Beside this there is one more way in which nitrogen fixation takes place that is during lightning atmospheric nitrogen is converted to ammonia.