Cyanobacteria belong to the kingdom Monera. Cyanobacteria, also called blue-green algae, are organisms that belong to a large and heterogeneous group of species that are primarily autotrophic and produce their own food using photosynthesis.
Cyanobacteria appear quite similar to eukaryotic algae and share several key biological features, and these dominant prokaryotic features are what makes them classifiable as bacteria in the kingdom Monera.
As with other prokaryotes, cyanobacteria lack nuclei enclosed in membranes. They do not have Golgi apparatuses, chloroplasts, or endoplasmic reticulum structures. While eukaryotic organisms use these organelles to carry out their basic life functions, cyanobacteria and other prokaryotes rely on bacterial cell membranes to perform those tasks.
Some cyanobacteria, namely planktons and some types of kelp and seaweed, have gas chambers to increase their levels of buoyancy, which is another characteristic common to monerans. Cyanobacteria have other physiological, chemical and genetic characteristics that distinguish them as monerans. These organisms may be unicellular or have multiple cells and have filamentous structures. Many have strong sheath tissues that bind other cells or their filaments into structured colonies.
Cyanobacteria, like others in the kingdom Monera, contain just one type of chlorophyll, which forms a green pigment. Different species contain different types and quantities of carotenoids, such as blue pigments and red pigments, which give them slightly different colors.