Kingdom Monera

Kingdom Monera contains every prokaryotic organism -- that is, every organism whose cell lacks a true, membrane-bound nucleus and membrane-bound cell organelles. Bacteria, the sole members of this kingdom, are the most abundant and widely distributed micro-organisms on Earth: a mere handful of ordinary soil contains hundreds of millions of bacterial cells, and bacteria are also found thriving in extreme habitats such as hot springs, deep-sea vents, deserts and permanently frozen snow, where very few other organisms can survive.

Shape-based grouping. Structurally, bacteria are simple, and are commonly grouped into four categories purely on the basis of cell shape: the spherical coccus (plural cocci), the rod-shaped bacillus (plural bacilli), the comma-shaped vibrium, and the spirally coiled spirillum (plural spirilla).

Nutritional diversity. Despite their structural simplicity, bacteria as a group show the widest metabolic diversity of any kingdom. Some are autotrophic, synthesising their own food either by photosynthesis (photosynthetic autotrophs, using bacteriochlorophyll) or by oxidising simple inorganic substances such as ammonia, nitrite or sulphur compounds (chemosynthetic autotrophs). The great majority of bacteria, however, are heterotrophic, depending on other organisms or on dead and decaying organic matter for their nutrition; many of these heterotrophs are of direct significance to humans, whether as decomposers that recycle nutrients in the soil, as agents used industrially to make curd, antibiotics or biogas, or as pathogens that cause diseases such as cholera, typhoid and tuberculosis.

Reproduction. Bacteria reproduce mainly by simple binary fission. Under unfavourable conditions, many species form thick-walled resistant spores that can survive extreme heat, cold or desiccation and germinate once conditions improve. Bacteria also show a primitive form of sexual reproduction (conjugation), in which genetic material (DNA) is transferred directly from a donor cell to a recipient cell through a temporary cytoplasmic bridge.

Based chiefly on differences in cell-wall chemistry and the ability to survive in extreme environments, Kingdom Monera is divided into two broad groups:

Archaebacteria. These bacteria are distinguished by a cell wall that differs in chemical composition from that of all other bacteria, an adaptation that allows them to survive in some of the most hostile habitats on the planet. Three ecological groups are usually described: halophiles, which live in extremely salty environments such as salt pans and salt lakes; thermoacidophiles, which thrive in hot, acidic springs (some tolerating temperatures near boiling point); and methanogens, which are found in marshy areas where they produce marsh gas (methane) from organic matter, and which also live in the gut of ruminant animals such as cattle and buffaloes, where they contribute to the methane content of the animals' dung and belching. …