Domain Bacteria

Bacteria are prokaryotic organisms whose cells lack a defined, membrane-bound nucleus. Their genetic material exists as a circular chromosome not associated with histone proteins, and bacterial cells generally have no membrane-bound organelles apart from ribosomes, which are of the 70S type. Bacterial cell walls characteristically contain peptidoglycan, a structural molecule absent from plant, fungal and animal cell walls.

Ecologically, bacteria play sharply contrasting roles: many act as decomposers recycling nutrients, others are photosynthesisers, and cyanobacteria in particular are photosynthetic blue-green bacteria that produce oxygen and are thought to have driven the historical shift of Earth's atmosphere from anaerobic to aerobic conditions. …