Bacteria are prokaryotes, lacking complex internal structures such as nuclei and microtubules. Most are small, usually only a few micrometres, and round or rod-shaped. They are ubiquitous and extremely diverse in the chemical reactions they perform, but very few kinds can be recognized by appearance.
Cyanobacteria are common photosynthetic bacteria, unique in using water as their hydrogen source and so producing oxygen as a by-product. They may be bluish-green or other colours, with the pigments appearing uniformly spread in each cell but actually associated with internal membranes.
Relationships are still being studied but broadly it looks like there are various smaller types, mostly single cells or narrow filaments rarely more than 3-4 µm wide, and then a diverse lineage that often have larger cells or more complex forms, for which the name Macrocyanobacteria has been offered.
These in turn split into mostly non-filamentous and filamentous groups. Except for a few smaller types the former includes most where cells divide in multiple planes, giving various sorts of colonies or sometimes special dispersal cells.
Greek merismos, division, pedion, plain
Merismopedia form sheet-like colonies embedded in mucilage. The cells are arranged in a regular grid, resulting from divisions in two perpendicular planes.
After Nikolai Voronikhin, 1882-1956
Woronichinia form mucilaginous colonies with cells in an outer layer. Each has a stalk, faint but about as wide as the cell itself and sometimes extending past it.
These are among various cyanobacteria that can produce gas vesicles, which aid in flotation and give the cells a darker brownish appearance.
The filamentous groups are divided into Oscillatoriales and Nostocales, with the first ancestral to the second. A number form sheaths or envelopes that may have yellowish, brown, or red pigments hiding the colour of the cells. Several toxins are also found in some strains here.
Latin oscillare, to swing
Oscillatoriales have all cells similar, usually shorter than wide. They may form single series of cells or sometimes bundles with several enclosed in a common sheath.
These are common in mats and films, often seen turning on their axis as they glide, a type of movement resulting from directed secretions.
Perhaps Old English nosþyrl, nostril
Nostocales typically have cells specialized for fixing nitrogen, becoming enlarged pale heterocysts, and some also have resting spores called akinetes.
Different types here take the form of simple chains of cells, tapering threads, or filaments with true branches. Some are further embedded in mucilaginous colonies.
Proteobacteria are a broad group united by genetic characteristics. Some conduct photosynthesis, called purple bacteria, but most obtain energy from organic nutrients or by oxidizing inorganic material. As in most bacterial groups, the cells have two membranes, and of these the majority with external flagella belong here.
Spirilla, found in several families, are helix-shaped and inflexible. Most occur in stagnant and putrid water, rotating as they move by means of inconspicuous flagella at the ends.
These are often more than 1 µm wide and may form chains over 100 µm long. In contrast other helical bacteria, the spirochaetes, are thinner and move by bending.