Taxonomy and Phylogeny of Unicellular Eukaryotes

Abstract

The notion of ‘microbes’ encompasses Bacteria (unicellular and multicellular) and Archaea, together with unicellular eukaryotes. In addition, microbiologists have traditionally included within their study field a number of eukaryotes named ‘fungi’ (customary meaning), e.g. Fungi (modern meaning), which are generally multicellular, and Oobionta, which are constituted by a giant multinucleate cell. Unicellular eukaryotes only represent ~10 % of all eukaryotic species; however, if only kingdoms, sub-kingdoms and phyla (‘phyletic diversity’) are taken into consideration, most eukaryotes are unicellular. Taking into consideration the huge phyletic diversity of unicellular eukaryotes and of affiliate taxa, which nearly fits the whole diversity of eukaryotes, it is impossible to present here a comprehensive description of the whole of these taxa. The choice was therefore to select a part of high-level taxa, likely to illustrate the amazing diversity of eukaryotes. For each selected taxon, some traits are recurrently tackled topics, e.g. the chloroplast structure and the photosynthetic pigments, the kinetic apparatus and the cell wall. Some derived characters, more or less specific to a taxon and prone to constitute a biomarker (genetic, biochemical, cytological and/or biological), are also emphasized. The biological life cycle of at least one species belonging to the taxon is illustrated in a standardized way. Finally, the role of the taxon in the functioning of the biosphere is described. Eukaryotes, one of the three domains of Life (together with Bacteria and Archaea), encompass a dozen or so high-level taxa (here kingdoms). Most of these kingdoms include taxa traditionally considered as belonging to the former polyphyletic plant kingdom together with taxa belonging to the former animal kingdom. Similarly, most of these kingdoms encompass ensembles formerly referred to as ‘algae’, ‘fungi’ (customary meaning) and protozoa, which modern phylogenies proved to be highly polyphyletic and therefore artificial. Here, eukaryotic taxa are placed within putatively monophyletic ensembles (kingdoms): Archaeplastida (=Plantae), Rhizaria, Alveolata, Stramenopiles (=Heterokonta), Haptobionta (=Haptophytes), Discicristates, Excavates, Opisthokonta (including Metazoa and Fungi, modern meaning), Amoebobionta (=Amoebozoa) and Cryptobionta.