Taxonomic implications of osmoacclimation in Cyanidium caldarium (Tilden) Geitler

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Abstract
The evolutionary and phylogenetic relationships of the photosynthetic unicell Cyanidium caldarium have been the subject of considerable debate since its original collection from thermal springs in Yellowstone National Park, Wyoming by Tilden in 1896. This is due partly to its unique natural habitat (it is the only photoautotrophic eukaryote able to grow at temperatures in excess of 50°C and at pH 1) and partly to the fact that no unequivocal data concerning its phylogenetic affiliations have been forthcoming. An investigation into the responses of Cyanidium to changes in external water status has revealed that fioridoside, one of the characteristic photo assimilatory products found in members of the Rhodophyta, is synthesized when this organism is subjected to salt stress. This osmoacclimative response is common to several red algae and confirms that Cyanidium is best regarded as a red alga.