Kingdom Monera includes three groups:

  1. Archaebacteria
  2. Eubacteria
  3. Cyanobacteria

Archaebacteria includes bacteria that live in unusual environments particularly at low levels of oxygen. Main types of Archaebacteria are:

  • Methanogenic bacteria that live in sewage and intestinal tracts of animals.
  • Thermoacidophilic bacteria that live in hot springs.
  • Halophilic bacteria which live in salty conditions where hot sun concentrates sea water.

Eubacteria include all other bacteria excluding cyanobacteria. All cyanobacteria (commonly called blue-green algae) are oxygenic photoautotrophs.

Since only bacteria are prokaryotic, Monera is the only prokaryotic kingdom. Most bacteria are single celled or unicellular.

Bacteria were the first cellular organisms to evolve on the planet earth after life originated around 3.5 billion years ago and were the only cellular organisms on earth for almost the next two billion years. Monerans are also the most numerous of all living cellular organisms.