The phylum Cyanobacteria (which means blue-green bacteria due to the bluish-green color they often have) is one of the major phyla of bacteria. They are organisms with a worldwide distribution, highly abundant in a wide range of ecosystems, such as deserts, where they form crusts on the soil, or in the open sea, where millions of tiny cells are part of the vast masses of water. They inhabit saline or hypersaline water and freshwater, humid environments, and extreme cold and hot deserts. Their main characteristic is their ability to perform photosynthesis using chlorophyll, releasing oxygen in this process. Photosynthesis can be carried out using water or hydrogen sulfide, so they are also abundant organisms in sulfuric hot springs.

Cyanobacteria were the creators of chlorophyll, one of the essential molecules for photosynthesis. All photosynthetic plants and eukaryotic algae live in symbiosis with a cyanobacterium with reduced physiology, called plastid, which is located inside plant cells. Symbiosis occurred during the early stages of the evolution of the Eukaryota domain, when an ancestral unicellular Eukaryota was able to live with a cyanobacterium inside its cellular cytoplasm. With evolution, its descendants would eventually give rise to plants and algae, in which this cyanobacterium, now called a plastid, is passed down from generation to generation through seeds and spores. Thus, photosynthesis based on chlorophyll has only arisen once in evolutionary history, in the lineage of cyanobacteria, and this capacity has been transmitted to other lineages through symbiosis of cyanobacteria with other organisms or through gene transfer in the case of some bacteria.

Cyanobacteria are microorganisms whose cells measure only a few micrometers in diameter but are larger than most other bacteria. The cytoplasm usually presents recognizable structures such as carboxysomes (organelles containing the enzyme that fixes CO2), glycogen granules, cyanophycin granules, polyphosphate granules, gas vesicles, and sometimes thylakoids, flattened vesicles formed by invagination of the plasma membrane where the molecular apparatus of photosynthesis resides. In the absence of thylakoids, this machinery is located in the cell membrane. The cell surface is composed, as in all gram-negative bacteria, of a plasma membrane and an outer membrane, with a murein wall (peptidoglycan) located between them.

The most common cyanobacteria are unicellular, spheroidal, sometimes aggregated in a mucilaginous capsule, or forming simple filaments. The filaments may appear aggregated in bundles, enveloped by mucilage, or sometimes are falsely branched by filament fragmentation. There are also cyanobacteria that form filaments with true branching. Between the cells of a filament, there is intimate communication in the form of micropores, and there is also a degree of functional specialization. The most notable difference is provided by heterocysts, special cells that are only present in one lineage of cyanobacteria. Heterocysts appear as larger cells with thickened walls interspersed in the filaments. Their walls contain cellulose, the most abundant polymer in plant cell walls. Heterocysts contain the nitrogen-fixing machinery, a process that is relatively incompatible with photosynthesis and essential for protein formation in nitrogen-poor environments. Another type of specialized cells are akinetes, cells that become larger, with a thicker wall than vegetative cells, sometimes with small protrusions, and with a granular cytoplasm due to the accumulation of a large amount of cyanophycin, a reserve substance. They secrete a new fibrous layer between the wall and the mucilaginous layers, and have a reduced metabolism, acting as resistance propagules.
Cyanobacteria are generally photosynthetic organisms, but some live heterotrophically, as decomposers, and some have a mixed metabolism. Cyanobacteria were the first to perform a variant of photosynthesis that has become predominant, and that has determined the evolution of the terrestrial biosphere. This is oxygenic photosynthesis. Photosynthesis requires a reductant molecule (an electron source), which in this case is the water molecule. When taking the hydrogen from water, oxygen is released. The evolutionary and ecological explosion of cyanobacteria billions of years ago produced the current atmosphere with oxigen, laying the groundwork for the emergence of aerobic metabolism and the radiation of organisms in the Eukaryota Domain. Thus, cyanobacteria were responsible for changing the destiny and characteristics of the entire Earth's crust.

Cyanobacteria share with various bacteria the ability to take nitrogen from the air and reduce it to ammonium (NH4+), a form of nitrogen that all cells can utilize. Autotrophs that cannot fix nitrogen must absorb nitrate (NO3-), which is a scarce and limiting substance. The enzymes that perform nitrogen fixation are the nitrogenases, which are inhibited by oxygen, making them incompatible with photosynthesis. Therefore, in many cyanobacteria, the two processes are separated in time, with photosynthesis occurring during daylight hours and nitrogen fixation only at night. Some species have solved the problem by using heterocysts, larger cells with a thickened cellulose wall that are responsible for nitrogen fixation. Heterocysts lack photosystem II, so there is no oxygen release, and nitrogenase can act without inhibition. Some cyanobacteria are symbionts of plants such as ferns of the genus Azolla, or plants of the genus Cycas or Gunnera, to which they supply nitrogen. Due to their abundance in different environments, cyanobacteria are important for nutrient cycling, incorporating nitrogen into the food chain, where they participate as primary producers or decomposers.

Some cyanobacteria produce toxins and can poison animals inhabiting the same environment or drinking the water where they inhabit. The phenomenon becomes significant only when there is a bloom (a demographic explosion), which sometimes occurs in fresh or brackish waters if temperature conditions are favorable and nutrients, especially phosphorus, aboundant. The most frequently implicated genera in blooms are Microcystis, Anabaena, and Aphanizomenon. The physiological mechanisms of intoxication are varied, with toxins being both cytotoxic (attacking cells), hepatotoxic (attacking the liver), or neurotoxic (attacking the nervous system).

Cyanobacteria were the primary producers of the biosphere for at least 1.5 billion years, although for the past 300 million years various groups of eukaryotic algae (diatoms, dinoflagellates, and haptophytes or coccolithophorids) have gained importance. Cyanobacteria, through oxygenic photosynthesis, flooded the atmosphere with oxygen about 2.5 billion years ago. The ability to use water as an electron donor in photosynthesis evolved only once in the common ancestor of all cyanobacteria. Geological data indicate that this crucial event took place early in Earth's history, at least 2.45-2.32 billion years ago, and probably much earlier. There is evidence that life existed 3.5 billion years ago, but the question of when oxygenic photosynthesis evolved remains a subject of debate and research. Clear evidence suggests that about 2 billion years ago, there was already a diverse biota of cyanobacteria, which were the primary producers during the Proterozoic eon (2.5-0.543 billion years ago), in part because the redox structure of the oceans favored photoautotrophs and nitrogen fixation. At the end of the Proterozoic, they were joined by green algae (phylum Chlorophyta), but it was not until the Mesozoic era (251-65 million years ago) that the radiation of dinoflagellates, coccolithophores, and diatoms reduced the prominence of cyanobacteria. Today, cyanobacteria are still crucial in marine ecosystems as primary producers and nitrogen fixers.

The following cladogram shows the evolutionary relationships of the 4 major lineages of cyanobacteria: