2. Eukaryotes and Prokaryotes


Prokaryotes include the domains, Eubacteria and Archaea. Examples of prokaryotes are bacteria, archaea, and cyanobacteria (blue-green algae).


Bacteria are microscopic, single-celled organisms that belong to Domain Eubacteria (true bacteria). Their cells lack lipid-bound subcellular compartments and their DNA is found in the cytoplasm (nucleoid region) rather than inside a nucleus. They reproduce by fission or by forming spores. They can inhabit all kinds of environments, such as in soil, acidic hot springs, radioactive waste, seawater, deep in the Earths crust, in the stratosphere, and even in the bodies of other organisms. Bacteria include the bacilli, the cocci, the spirochetes, and the vibrios.


Archaea belong to the Domain Archaea. They are unicellular microorganisms that are genetically distinct from bacteria and eukaryotes. Similar to prokaryotes, they lack a nucleus. However, the genes of archaebacterial are more similar to eukaryotes. Both of them produce certain enzymes that are used in transcription, translation, and other metabolic pathways. Many archaebacterial are found thriving in extreme habitats. Archaebacteria include the halophiles (those inhabiting extremely salty environments), the methanogens (archaea species producing methane), and the thermophiles (those that can thrive in extremely hot habitats).


A diagram of a typical cyanobacterial cell showing parts.
A diagram of a typical cyanobacterial cell showing parts.

Cyanobacteria, also called blue-green algae, are microorganisms that are formerly considered as protists for being single-celled and photosynthetic. However, they now belong to a group or phylum of photosynthetic bacteria that inhabit aquatic habitats and moist soils. Cyanobacteria are ecologically significant because a huge percentage of gaseous oxygen comes from their photosynthetic activity. They may lack a nucleus but they possess microcompartments (e.g. thylakoids and carboxysomes). They also have photosynthetic pigments (particularly, phycobiliproteins) that account for the bluish-green color of their cells. Cyanobacteria include Chroococcales, Pleurocapsales, Oscillatoriales, Nostocales, and

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