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Until a few years ago the term "bacteria" was synonymous with the term "prokaryote". Then technology revealed the existence of numerous forms of prokaryotes that possess distinctive differences when compared to those prokaryotes we refer to as "bacteria". Therefore, we now have two domains of prokaryotes: the Archaea (Archaeprokaryota; Archaebacteria) and the Prokaryota (bacteria, Eubacteria, or Euprokaryota).

Prokaryotes are ubiquitous, and vary in terms of their nutritional modes, reproductive styles, habitats and so on. Some are photosynthetic, some are heterotrophic, some are decomposers, and others are parasites. Some forms are unicellular, others are colonial.

Most modern bacteria have cell walls (except the mycoplasmas), and those walls are composed of peptidoglycan. Modern bacteria (except the mycoplasmas) exhibit these body shapes: bacillus, coccus, or some form of spiral (helical, spiral, or vibrio). Those bacteria with peptidoglycan cell walls respond to the Gram_stain as either Gram_positive (Gram+) or Gram_negative (Gram-).

Cyanobacteria are now classified (1971) as eubacteria (euprokaryota, or "bacteria"), although they were formerly thought to be a form of algae (the blue-green_algae).

Anabaena is a filamentous_cyanobacterium. Its vegetative_cells produce special cells called heterocysts when nitrogen is available. The heterocysts are then used for nitrogen_fixation.