Protozoan Taxonomy and Systematics

The classification of protozoa and other microorganisms above the organizational level of the bacteria has always been dependent on microscopy because the body sizes involved generally range from only one micrometre to one or two millimetres in length. Any structures these species possess, useful in comparative studies of their morphology and thus their taxonomyand systematics, are at the cellular and subcellular levels, and invisible to the naked eye. The physiological properties of protozoa (and the neighbouring algae) have also played a role in the classification of these ubiquitous eukaryotic microorganisms; and of growing significance are the findings made possible by research using molecular biological approaches. Becausewe are continuing to learnmore andmore about such minute organisms, protozoan systematics – that is, the taxonomy (classification) and the evolutionary interrelationships of major groups of protozoa – remains a topic of debate and change, still today. Some of the rather large and unwieldy taxonomic groupings of past years are particularly subject to revision with expansion of and refinement in our knowledge about themembers of those – and related – assemblages. Paradoxically, the protozoa themselves are becoming more difficult to define with precision as our information about them and other microbial assemblages increases. Thus, presenting a single satisfactory circumscribed definition for them is not an easy task. It is attempted towards the end of this article; but, for sake of clarity, further background information is first supplied.