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.
Phylum Cyanobacteria – blue-green algae
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.
Chroobacteria include single cells, aggregates, and simple filaments of disk- or barrel-shaped cells. A few have special dispersal phases but otherwise the cells are all similar. They are ancestral to the more specialized Hormogoneae.
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.
Greek arthron, joint, speira, coil
Arthrospira are narrow helical filaments, with regular coils like a stretched spring. The
walls between cells are generally thick enough to be seen with a light microscope.
Hormogoneae are either simple chains of cells, tapering threads, or filaments with true branches formed by divisions at angles to the main series. In most some cells specialize to fix molecular nitrogen, becoming pale larger heterocysts, and some also have resting spores called akinetes.
Perhaps Old English nosþyrl, nostril
Nostoc form unbranched strings of round cells, occurring in globular masses enclosed by a firm sheath of common mucilage. The very largest may reach several centimetres across.
Phylum Proteobacteria – purple bacteria & allies
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.