Kingdom Monera consists of eubacteria, archaebacteria and mycoplasmas. Bacteria are the sole members of this kingdom. Bacteria can be divided into autotrophs and heterotrophs. Autotrophs can further be classified into photosynthetic autotrophs and chemosynthetic autotrophs. Heterotrophic bacteria are parasites or decomposers. Bacteria reproduce by fission and also sexually by the transfer of DNA from one bacterium to another through the pilus.
To tide over the unfavourable conditions, bacteria produce spores. Based on their shapes, bacteria are classified into four types: the spherical coccus, the rod-shaped bacillus, the coma shaped vibrium and the spiral spirillum. There are thousands of different eubacteria or ‘true bacteria’, characterised by the presence of a rigid cell wall and if motile, a flagellum.
Eubacteria include cyanobacteria, commonly known as blue-green algae. They are unicellular, filamentous algae that live in colonies. Cyanobacteria have chlorophyll-a, and are photosynthetic autotrophs. Even cyanobacteria such as Nostoc and Anabaena can fix atmospheric nitrogen in their specialised cells called heterocysts. Chemosynthetic autotrophs oxidise various inorganic substances such as nitrates or ammonia and use the energy released for ATP production. They help in recycling nutrients like nitrogen, sulphur and phosphorous.
Archaebacteria are special bacteria that live in extreme conditions. For example, halophiles live in extremely salty areas, thermoacidophiles in hot springs, while methanogens in marshy areas and also in the gut of ruminant animals. All archaebacteria have a different cell wall structure compared to other bacteria, which helps them survive in extreme conditions.
Mycoplasmas are the bacteria that do not have a cell wall. They are also the smallest living beings which can survive in anaerobic conditions and are pathogenic in animals and plants.