~ Classification of micro-organisms ~
With some tips for further study

Most people know the difference between plants and animals. But many micro-organisms are neither plant nor animal. Some so-called plant-like organisms (because they make use of photosynthesis) move around like animals or have relatives that do not photosynthesise. It is thought that many protozoan flagellated forms became photosynthetic by obtaining primitive algae as endosymbionts (having the benefit of photosynthetic organisms living inside).

In the 'Pond Life Identification Kit' we have tried to keep it simple by using the old fashioned distinction between algae and protozoa. If possible we categorised the micro-organisms into their main groups (called phyla) or into some very common classes. In many cases we grouped them by their major features.

If you are more interested in the classification of life forms there are many publications that will give you a much more detailed overview. A very interesting book is 'Five Kingdoms' by Margulis and Schwartz. In this book life forms are divided into 5 kingdoms. 1.Monera (bacteria), 2.Protoctista (this phylum contains what we called algae and protozoa), 3.Fungi, 4.Plants and 5.Animals. Furthermore the book divides all these kingdoms into many phyla. Humans belong to the phylum Chordata. Rotifers that are also animals have their own phylum Rotifera.

More about this subject can be read in David Goldstein's Micscape article Classification of Living Things.

The Protoctista is a group that is in fact compiled of everything that is neither bacteria, fungi, plant or animal. Many are single-celled micro-organisms but some protoctists are huge (brown algae like kelp) Scientists decided to use the term Protoctist instead of Protist since protist was always used for single celled creatures. Several Protoctists are multi-celled or large.

In our identification kit we often use the term 'green algae'. This is in fact a phylum (Chlorophyta) containing Volvox, Pediastrum, Hydrodictyon, etc. But many organisms that look very similar to these green algae are put in other phyla, like desmids and the Spirogyra-like filamentous algae. These are both placed in the phylum Gamophyta, the 'conjugating green algae'.

To make it much more complicated there are many similar looking organisms (tiny oval creatures with flagella, or filamentous types) that are not related at all. We did not mention Phyla like Cryptophyta, Chrysophyta or Xanthophyta since they are difficult to keep apart. Scientists are still making new classifications as well. You have to study the reproductive stages, the type of chlorophyl or the types of flagella. It is a very interesting topic so if you are interested do try to go beyond our simple introduction. Below we give some suggestions for further reading.

Text by Wim van Egmond.



First Page




Insect stages


Further reading:

'Five Kingdoms' by Lynn Margulis and Karlene V. Schwartz. W. H. Freeman and Company, 1982

'Algae, an introduction to phycology' C. van den Hoek, D.G. Mann and H.M. Jahns, Cambridge University Press, 1995

From the Pictured Key Nature series: 'How to know the freshwater algae' by G.W. Prescott.
Wm. C. Brown Company Publishers, Dubuque, Iowa
in this series also avaliable:
'How to know the Protozoa' by Jahn, Bovee and Jahn

An Introduction to Microscopy

Comments to the compilers Wim van Egmond and Dave Walker are welcomed.

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