The systematics of Zoochlorella revisited employing an integrative approach.

Symbiosis of green algae with protozoa and invertebrates has been studied for more than 100 years. Endosymbiotic green algae are widely distributed in ciliates (e.g. Paramecium, Stentor, Climacostomum, Coleps, Euplotes), heliozoa (e.g. Acanthocystis) and invertebrates (e.g. Hydra, Spongilla), and have traditionally been identified as named or unnamed species of Chlorella Beij. or Zoochlorella K. Brandt or referred to as Chlorella-like algae or zoochlorellae. We studied 17 strains of endosymbionts isolated from various hosts and geographical localities using an integrative approach (nuclear encoded small subunit and internal transcribed spacer regions of rRNA gene sequences including their secondary structures, morphology, physiology and virus sensitivity). Phylogenetic analyses have revealed them to be polyphyletic. The strains examined belong to five independent clades within the Trebouxiophyceae (Choricystis-, Elliptochloris-, Auxenochlorella- and Chlorella-clades) and Chlorophyceae (Scenedesmus-clade). The most studied host organism, Paramecium bursaria, harbours endosymbionts representing at least five different species. On the basis of our results, we propose a taxonomic revision of endosymbiotic 'Chlorella'-like green algae. Zoochlorella conductrix K. Brandt is transferred to Micractinium Fresen. and Zoochlorella parasitica K. Brandt to Choricystis (Skuja) Fott. It was shown that Choricystis minor (Skuja) Fott, the generitype, is a later heterotypic synonym of Choricystis parasitica (K. Brandt) comb. nov. A new species, Chlorella heliozoae, is proposed to accommodate the endosymbiont of Acanthocystis turfacea.

Historical review of attempts to decrease subjectivity in species identification, with particular regard to algae.

The goal of a taxonomist should be to contribute to the construction of a framework that expresses interrelationships among taxa and provides pegs to which information from all possible sources may be attached. It is essential that this information be attached to the correct peg. Throughout the history of taxonomy, attempts have been made to reduce the subjectivity involved in determining the correct peg. Illustrations, which initially were relied on to produce accurate determinations, have maintained their importance, keeping pace with miraculous advances in the technologies of microscopy, electronics, graphics, and communication. The type method was introduced to provide an anchor for each name in a sea of ever-changing circumscriptions. The physical nature of types has kept pace with advances in taxonomic methodology and now includes the possibility of designating a living culture as type if it is preserved in a metabolically inactive state. The number of characters associated with a name has been greatly increased by studying organisms in culture, by using transmission and scanning electron microscopy, and by nucleotide sequence analysis. A marked increase in the number of discriminatory characters has resulted in greater assurance that the correct peg has been chosen on which to hang accumulated information. Integration of molecular and morphological data should theoretically strengthen the certainty of identification, but this certainty will always be tempered by some degree of subjectivity. Taxonomists form opinions on the basis of data that are reputedly objective, but that in fact are subject to varying interpretations. Genomic analysis is a very important taxonomic tool, but its application should not be assumed to be free of subjectivity.