Prokaryotes

Prokaryotes are the most frequent and well-known organisms on earth, and classified because they have no defined membrane-bound nucleus. Prokaryotes comprise 2 separate but related groups:  the archaea (or archaebacteria) and the bacteria (or eubacteria).  These 2 distinct groups of prokaryotes early diverged in the history of life on Earth. The living world hence has 3 major divisions or domains:   archaea, bacteria and eukaryotes  . Commonly  the bacteria are the encountered  prokaryotes  in water , soil and living  in  or  on  larger  organisms,  and  include  Bacillus species and Escherichia coli, in addition to  the  cyanobacteria   (photosynthetic   blue-green  algae).  Mainly the archaea inhabit unusual environments   like hot acid springs, salt brines, and the ocean depths bog and include the sulfur bacteria and the methanogens, although some are found in less unfriendly environments.