Kingdoms of Organisms:

This presentation has taken as standard the "three domain - six kingdom" model of organism diversity, based on 16S ribosomal RNA sequence comparisons, of Carl Woese and co-workers. This divides cellular organisms into:

• Bacteria (Eubacteria),
• Archaea (Archaebacteria) and
• Eukarya (Eukaryotes; further divided into Protista, Plantae, Animalia and Fungi).

Reference:
GJ Olsen and CR Woese (1993). FASEB Journal 7: 113-123.

However, Robert H Whittaker and Lyn Margulis have suggested that organisms should be divided into five kingdoms: that is,

• Monera or Procaryotae (prokaryotes, including Bacteria and Archaea)
• Protista (eukaryotes with single- or colonial cellular organisation, may be photosynthetic)
• Fungi (generally multinucleate eukaryotes with walled syncytia)
• Animalia (wall-less eukaryote cells organised into complex organisms)
• Plantae (generally autotrophic plants, mostly with photosynthetic plastids, simple multicellular to advanced tissue organisation)

This model includes some "plants" as protists, and makes no distinction between Archaea and Bacteria.

Reference:
RH Whittaker and L Margulis (1978). BioSystems 10: 3-18

To complicate matters, James Lake and co-workers have lately proposed a radical re-structuring of the "universal" phylogenetic tree, to split Archaea into Halobacteria, Methanogens and Eocytes. This would mean there are three major groupings of prokaryotes (Eubacteria + Halobacteria, Methanogens and Eocytes), which could all constitute kingdoms on their own, given each is as unrelated to the others as any are to Eukarya.

Reference:
JA Lake (1991). Trends in Biochemical Sciences 16: 46-50.

There is also a 2-empire / 8-kingdom scheme, with Archaeobacteria and Eubacteria as kingdoms in Empire Bacteria, and 6 kingdoms in Empire Eukaryota:

• Archezoa (primitive eukaryotes without chloroplasts or mitochondria)
• Protozoa (unicellular wall-less eukaryotes with mitochondria)
• Chromista (photosynthetic organisms with chloroplasts within the lumen of the rough ER; eg: diatoms, brown algae, cryptomonads, oomycetes)
• Plantae (photosynthetic organisms with chloroplasts within cytoplasm; all land plants, many green algae)
• Fungi
• Animalia

Reference:
Cavalier-Smith T (1993). Microbiological Reviews 57: 953-994

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