principles governing the classification of living beings

Previously, living organisms had been classified in two super kingdoms based on cell criteria (see also water, the medium for microbial life):

• prokaryotes (cells that do not have individual nuclei) that encompass the bacterial kingdom and blue-green algae or cyanophyceae, the latter having subsequently been included with bacteria under the new designation of cyanobacteria (Stanier et al., 1978);

• eukaryotes (chromosomes grouped in a nucleus that is isolated from the cytoplasm by a nuclear membrane) that include the plant kingdom (except blue-green algae) and the animal kingdom.

Molecular biology was then used to distinguish a further super-kingdom, that of the archaebacteria (1977): previously confused with bacteria, the ribosomal RNA of these single cell organisms includes completely different nucleotidic sequences and they now form a separate classification which means that we have three super-kingdoms.

• archaebacteria;
• "true" bacteria (bacteria kingdom);
• eukaryotes (plant and animal kingdoms).

Transition from one to the other of these gives a general picture of how life has developed. As far as the viruses are concerned, these are only large molecules of nucleic acid ( DNA or RNA ), unique or fragmented into several chromosomes (e.g. eight in the influenza virus) and combined with proteins (forming a shell known as a capsid round the nucleic acid); viruses can be regarded as forming a separate world, on the boundary between life and inert matter.

We provide below a simplified table for each kingdom (tables 1 to 5) that only shows certain subdivisions (branch, class, order, family or genus) and where we have only included groups that are of interest because they are found in the aquatic environment. At order, family and genus level, we have only provided a few isolated examples for illustration purposes; more extensive information can be found in the sub-sections on bacteriology and the study of plankton.