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The Kingdom of Monera in the Six Realms
Prokaryotes are single-celled organisms without nuclei. Their cells have no organelles with membranes, and there are no other organelles (such as mitochondria and chloroplasts) except ribosomes. Prokaryotes, including bacteria and cyanobacteria formerly known as "blue-green algae", are the simplest group of organisms that can divide and reproduce. Prokaryotes used to be the only life form on the earth, and they monopolized the earth for more than 2 billion years. Today, they are still very prosperous and play an important role in the nutrient cycle. The kingdom of monera includes at least 4000 species.

A large group of organisms with prokaryotic cell structure. Include bacteria (including actinomycetes), cyanophyta, protochlorophyta, rickettsia, mycoplasma and chlamydia. In the above categories, cyanophyta and protochlorophyta are green autotrophs, and there are also a few photoautotrophic or chemoautotrophic bacteria in the phylum Bacterium. But most bacteria and other kinds of bacteria are heterotrophic, saprophytic or parasitic. Rickettsia is a small intracellular parasite between virus and bacteria, which is spherical or rod-shaped. Mycoplasma, also known as Bacteroides, is an independent microorganism between Rickettsia and bacteria. It has no cell wall, is soft, changeable in shape, spherical or filamentous, and its branches are of different lengths. Chlamydia is an obligate parasite. American biologist R H Whittaker clearly pointed out in 1969 that all kinds of prokaryotes got together to form the kingdom of monera. At that time, he advocated dividing biology into five realms, which has been gradually accepted and endorsed by people.

In evolution, prokaryotes are the oldest; Prokaryotes are the largest group at present. From a biological point of view, their success is undoubtedly due to their rapid cell division and metabolic diversity. Prokaryotes can survive in many unbearable environments, such as Antarctic ice, deep ocean, and even hot springs near the boiling point. Some species can live in an environment lacking free oxygen and get energy through anaerobic respiration. Some organisms that live in extreme conditions and have special structures are also called archaea. Generally, they are classified as prokaryotes according to their inner structure, circular DNA structure, cell productivity, cell division, metabolism and other lifestyles (however, archaea is not only different from eubacteria in some aspects such as gene transcription, but some are even close to eukaryotes).

From the ecological point of view, prokaryotes generally play the role of decomposers, which can decompose the remains of animals and plants and release elements that can be used by plants. Prokaryotes also play an important role in nitrogen fixation. Although the atmosphere is rich in nitrogen, eukaryotes cannot directly use the nitrogen in the air. They must transform gaseous nitrogen into inorganic compounds such as ammonia or amine ions through the nitrogen fixation of some species in prokaryotes for use by eukaryotes (such as rhizobia symbiotic with leguminous plants). Some parasitic bacteria belong to consumers. There are also autotrophic prokaryotes, such as nitrifying bacteria specializing in synthesis, cyanobacteria performing oxygen-producing photosynthesis, and oxygen-producing photosynthetic bacteria performing oxygen-producing photosynthesis.