Cyanobacteria are a phylum of bacteria that obtain their energy through photosynthesis, and are the only photosynthetic prokaryotes able to produce oxygen. The name "cyanobacteria" comes from the color of the bacteria. Cyanobacteria, which are prokaryotes, are also called "blue-green algae", though the term "algae" in modern usage is restricted to eukaryotes. ... By producing and releasing oxygen (as a byproduct of photosynthesis), cyanobacteria are thought to have converted the early oxygen-poor, reducing atmosphere into an oxidizing one, causing the Great Oxygenation Event and the "rusting of the Earth, which dramatically changed the composition of the Earth's life forms and led to the near-extinction of anaerobic organisms.
The Earth is surrounded by a layer of gases which we call the atmosphere. It is the product of photosynthesis, of algae working for millions of years, converting light energy from the sun into air. Evolution’s answer to the atmosphere was the lung. Thus the atmosphere is essential for most living organisms, including people. GLOBALE: Infosphere, 
Since last thuesday we got some samples of different cyanobacteria in the lab. Which are Syctonema, Nodularia, Anabaea, Calothrix, Plantothrix and Synechococcus.
Nodularia is a genus of filamentous nitrogen-fixing cyanobacteria, or blue-green algae. They occur mainly in brackish or salinic waters, such as ... the Baltic Sea. Nodularia cells occasionally form heavy algal blooms. (Nodularia, wikipedia)
Scytonema grows in filaments that form dark mats. Many species are aquatic and are either free-floating or grow attached to a submerged substrate, while others species grow on terrestrial rocks, wood, soil, or plants. Scytonema is a nitrogen fixer, and can provide fixed nitrogen to the leaves of plants on which it is growing. Some species of Scytonema form a symbiotic relationship with fungi to produce a lichen. (Scytonema, wikipedia)
P. agardhii and P. rubescens are commonly observed in lakes of the Northern Hemisphere where they are known producers of potent hepatotoxins called microcystins. (Planktothrix, Wikipedia)
Anabaena are known for nitrogen-fixing abilities, and they form symbiotic relationships with certain plants, such as the mosquito fern. They are one of four genera of cyanobacteria that produce neurotoxins, which are harmful to local wildlife, as well as farm animals and pets. Production of these neurotoxins is assumed to be an input into its symbiotic relationships, protecting the plant from grazing pressure. (Anabaena, wikipedia)
Synechococcus is very widespread in the marine environment. The photosynthetic coccoid cells are preferentially found in well–lit surface waters where it can be very abundant (generally 1,000 to 200,000 cells per ml). Many freshwater species of Synechococcus have also been described. (Synechococcus, wikipedia)
Calothrix are generally found in freshwater.