How can cyanobacteria, in contrast to saprotrophic bacteria, live in an environment that lacks organic nutrients?

Cyanobacteria, in contrast to saprotrophic bacteria, are able live in an environment that lacks organic nutrients by utilizing nitrogen from the atmosphere. Also, they are able to serve since they are able to make their own food by the process of photosynthesis. They are also known as the blue-green algae. These bacteria live in water and contain chlorophyll which makes them photosynthetic. They are considered to be the only prokaryotes that has the ability to produce oxygen. They are so small and unicellular so that they would grow in colonies which can are large that can be seen by the naked eye. 

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