Monera
The Monera is the largest kingdom of prokaryotic microorganisms. Prokaryotes are single-celled organisms that lack an organized nucleus, i.e., one that is separated from the protoplasm by a membrane-like envelope. (Organisms in all other kingdoms have a cellular nucleus and are termed eukaryotic.) Monerans have their genetic material organized as a single strand of DNA, which occurs throughout their cytoplasm. They have no subcellular organelles, such as chloroplasts, mitochondria, or flagella. Monerans were the first organisms to evolve, about 3.5 million years ago (the first eukaryotes evolved 2 billion years afterwards).
Bacteria and blue-green bacteria (or cyanobacteria) are the major groups in the Monera. They have rigid or semi-rigid cell walls, propagate by binary division of the cell, and do not undergo mitosis or meiosis. The cyanobacteria are photosynthetic, using chlorophyll dispersed within the cytoplasm as their primary light-capturing pigment.
Biologists have named about five thousand species of monerans. There are, however, large numbers of additional species that have not yet been described by microbiologists. Among the great diversity of monerans are species capable of exploiting a phenomenally wide range of ecological and metabolic opportunities. Some species are photosynthetic, utilizing the energy of sunlight to fix carbon dioxide and water into simple sugars. Others are chemosynthetic, being capable of using energy released by the oxidation of sulfide minerals to manufacture their organic nutrition. Yet other monerans can utilize virtually any organic materials as a substrate in their heterotrophic nutrition, either in the presence of oxygen or under anaerobic conditions. Some species of monerans can tolerate remarkably extreme environments, for example, living in hot springs at temperatures as high as 172° F (78°C), while others are active at sub-zero temperatures as deep as 1,300 ft (400 m) in glacial ice.
In addition to free-living monerans, many species of bacteria live in mutualisms (i.e., mutually beneficial symbioses) with more-complex organisms. For example, many bacterial species live in the rumens of cows and sheep, and others live in the human gut, in both cases aiding in the digestion of complex organic foods. Other bacteria, known as Rhizobium, live in a mutualism with the roots of leguminous plants (such as peas and clovers), fixing nitrogen gas into a form (ammonia) that plants can utilize as a nutrient.
Many species of bacteria are parasites of other organisms, causing various diseases. For example, Bacillus thuringiensis is a pathogen of moths, butterflies, and blackflies, and is used as a biological insecticide against certain pests in agriculture and forestry. Other bacteria cause diseases of humans, including various infections, bacterial pneumonia, cholera, diphtheria, gonorrhoea, Legionnaire's disease, leprosy, scarlet fever, syphilis, tetanus, tooth decay, tuberculosis, whooping cough, most kinds of food poisoning, and the "flesh-eating disease" caused by a virulent strain of Streptococcus.
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