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Giorgio Carboni, March 2000
Translation edited by Giselle Walker



Protists are microscopical, unicellular eukaryotes. They generally live in water. In some cases, protists form colonies of individuals, though the individuals are generally autonomous. Historically, researchers referred these minuscule living beings to the kingdoms of larger organisms. Then, according to the characteristics that a given protist showed, they tried to refer it to the plant kingdom (in the protophyta or algae), the animal kingdom (in the protozoa) or to the fungal kingdom. This led to a great deal of doubtful classification, for example "algae" deprived of chloroplasts and which fed on other microorganisms, "moulds" endowed with amoeboid movements like animals, or "protozoans" anchored to the substratum by a foot like they were a plant. Intuitively, it is possible to understand why protists often possess characteristics common to animals, plants and fungi: the larger organisms derive from protists. In order to avoid confusion in the classification of these microorganisms, a kingdom has been created for them, so that they need not be referred to the animals, plants or fungi. Of course, grouping things in kingdoms is arbitrary- in the history of life there has been no clear-cut division between protists and other eukaryotes.

Not all microorganisms are protists. Another "kingdom" contains unicellular microorganisms, the Monera. Bacteria belong to this kingdom. What difference is there between bacteria and protists? This distinction is founded on the complexity of a cell's organization. The cellular organization of bacteria is particularly simple -they do not have membranes binding their nuclear material- and for this reason they are also named prokaryotes ("before-nucleus"). The cellular organization of protists is more complex -they have a membrane-bound nucleus (and other organelles distinct from the cytoplasm)- and they are therefore called eukaryotes ("true-nucleus"). Animals, plants and fungi, being derived from protists, are also eukaryotes.


- - -
Monera (bacteria): archebacteria, eubacteria unicellular org. prokaryotes: simple cellular organization,
nucleus lacking membrane
Protista protozoa
brown, red, some green algae
phycomycetes, myxomycetes, yeasts
eukaryotes: complex cellular organization,
nucleus membrane-bound
Fungi ascomycetes, basidiomycetes, etc. multicellular org. motionless and not photosynthetic organisms
Plantae flowering plants, conifers, mosses, bryophytes, some green algae motionless and photosynthetic organisms
Animalia vertebrate, invertebrate motile and not photosynthetic organisms


Another important feature of protists is unicellularity, that is being formed by a single cell. However filamentous algae also exist, formed by cells placed side by side; protists also live in colonies. Unlike animals, plants and fungi, the cells of filamentous algae and colonial protists are not or very little differentiated and they can often live autonomously. Phycomycetes and myxomycetes have a coenocytic structure, that is they have large multinucleate cells. They possess amoeboid forms.

Here therefore we can return to the definition of these microorganisms: protists are unicellular eukaryotes.

A conundrum: where to put viruses? While their DNA or RNA is encapsulated in proteins and they lack sub-"cellular" membranes (like bacteria), they are not able to reproduce themselves without a host cell. They have been included among the Monerans. However it has been observed that viruses often have the DNA in common with the cells they infect. One solution to this is to provide every kingdom with an appendix containing its own viruses. A more common solution is to note the features all viruses have in common and put all them together in the category of Virus. They are usually considered separately from other organisms, both in terms of taxonomy and biology. The International Committee for the Taxonomy of Viruses has developed a system for naming them different from that of other life (see 5th and 6th ICTV reports). Viruses are not always considered to be living, since they cannot reproduce themselves autonomously.

BIBLIOGRAPHY   Prokaryotes, Eukaryotes, & Viruses Tutorial

1 - Jahn,T.L., Bovee, E.C. & Jahn, F.F. 1979 How to Know the Protozoa. 2nd ed. Wm. C. Brown Publishers, Div. of McGraw Hill, Dubuque, Iowa.
2 - Patterson, D.J. 1996. Free-Living Freshwater Protozoa: A Colour Guide. John Wiley & Sons, NY (A key for beginners with good photomicrographs)
3 - Streble H., Krauter D. 1988. Life in a waterdrop. Microscopic freshwater flora and fauna. An identification book. ISBN 3-440-05909 (1700 drawings very useful to identify protists)
4 - Rainis, K.G. and Russell, B.J. 1996. Guide to Microlife. 287pp, ISBN 0-531-11266-7, Franklin Watts, Danbury, CT, USA. (Comprehensive field guide to the microworld. Recommended for schools and amateurs).
5 - Anderson, R. and Drudger, M. 1997. Explore the World Using Protozoa. 240pp. ISBN 0-87355-159-1, National Science Teachers Association, 800-722-NSTA. (A collection of 28 investigations with microscope. How to do "live" biology class with protozoa).
6 - Lee, J.J., Hutner, S.H. & Bovee, E.C.eds.1985. An Illustrated Guide to the Protozoa. Soc. Protozoologists, Lawrence Kansas.
7 - Prescott, G.W. 1978. How to Know Freshwater Algae. 293pp. ISBN: 0697047547, McGraw-Hill Professional
8 - Canter-Lund H. 1995. Freshwater Algae: Their Microscopic World Explored. 360pp. ISBN: 0948737255, Balogh Scientific Books. (Absolutely beautiful pictures and well written. Recommended).

9 - Anderson, O.R. 1988.Comparative Protozoology. Springer-Verlag, NY (One of the best in general)
10 - Hausmann, K and Hulsmann, N. Protozoology, Thieme, 1996 (Very nicely illustrated)
11 - Van den Hoek, C.; Mann, D.G. and Jahns, H.M. (1995). Algae: an introduction to phycology. 627pp. ISBN: 0521316871. Cambridge University Press. (From microscopic unicellular algae to gigantic kelps. Very well made and widely used by professional protistologists. A German edition also exist).
12 - Lee, J.J. & Soldo, A.T. ed. 1992. Protocols in Protozoology. Society of Protozoology, Lawrence, KS (Techniques used in protozoology)
13 - Margulis, L., Corliss, J.O., Melkonian, M. & Chapman, D.J. 1990. Handbook of Protoctista, Jones & BArlett publishers, Boston, MA
14 - Farmer, J.N. 1980. The Protozoa. Introduction to Protozoology. C.V. Mosby Co., St.Louis, Mo
15 - Fenchel, T. 1987. Ecology of Protozoa. Springer-Verlag, NY
16 - Sleigh, M.A. 1989. Protozoa and Other Protists. Edward Arnold, Div. Hodder & Stoughton, London, GB

You can find further information on these books (i.e. price and often also a review) in the main bookstores present in Internet.

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