What to do with protists?

Abstract

The definition of ‘protists’ has changed over time, once including all living organisms which were neither plant nor animal, now including a multitude of organisms that cannot be assembled into a monophyletic group. Protists were once subdivided into algae, fungi and protozoa, based upon mode of nutrition, and further subdivided based upon the dominate life history stage (e.g. flagellate, amoeboid, coccoid), however, studies during the past 40 years have shown that these divisions are artificial. Electron microscopic studies, as well as a revival of the Endosymbiotic Theory for the origin of organelles, have caused a demolition of classical protistan taxonomy. Numerous new higher level taxa were described. Molecular studies, especially nucleotide sequence comparisons, have provided a new means for determining phylogenetic relationships. Although these molecular studies have not succeeded in providing an overall consensus classification for the protists, many advances have been made. It now appears that the protists are, at best, a grade, not a clade, and they do not form a monophyletic taxon (i.e. the Kingdom Protista cannot be recognised as a natural group). Despite this taxonomic limitation, there are advantages to maintaining the protists as an assemblage for ecological, biomedical or economic reasons. The biodiversity of protists is discussed along with remarks on their ecological and economic significance.