A type of bacteria that lives in water and obtains energy by photosynthesis. They are quite small and usually unicellular, although they often grow in colonies large enough to see. Many Proterozoic oil deposits are attributed to the activity of cyanobacteria. They are also important providers of nitrogen fertilizer in the cultivation of rice and beans. Cyanobacteria are often called "blue-green algae" due to their photosynthetic and aquatic nature.
They are prokaryotic microorganisms that evolved approximately 3.5 billion years ago. They are microscopic ranging from 1-80 m in cell diameter and have considerable morphological diversity. They belong to one of the eleven eubacteria phyla within the prokaryotes. They were earlier classified as blue green algae primarily due to their algal- and plant-like photosynthesis and their distinct pigmentation caused by phycobiliproteins (phycocyanin and phycoerythrin) and chlorophyll a (Chl a). Following the discovery that the cell wall is typical gram-negative and the subcellular arrangement (lack of membrane bound organelles) are typical prokaryotic, the group was reclassified as cyanobacteria.
Cyanobacteria belong to the most diverse prokaryotic group inhabit nearly all illuminated environments on Earth and play key roles in the carbon and nitrogen cycle of the biosphere. Generally, cyanobacteria are able to utilize a variety of inorganic and organic sources of combined nitrogen, like nitrate, nitrite, ammonium, urea or some amino acids. Several cyanobacterial strains are also capable of diazotrophic growth. Genome sequencing has provided a large amount of information on the genetic basis of nitrogen metabolism and its control in different cyanobacteria. Comparative genomics, together with functional studies, has led to a significant advance in this field over the past years. 2-oxoglutarate has turned out to be the central signalling molecule reflecting the carbon/nitrogen balance of cyanobacteria. Central players of nitrogen control are the global transcriptional factor NtcA, which controls the expression of many genes involved in nitrogen metabolism, as well as the PII signalling protein, which fine-tunes cellular activities in response to changing C/N conditions. These two proteins are sensors of the cellular 2-oxoglutarate level and have been conserved in all cyanobacteria. In contrast, the adaptation to nitrogen starvation involves heterogeneous responses in different strains.