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The cyanids have been classified in the past as Cyanobacteria [Cyanophyceae at that time] (Tilden 1896, Setchell 1901, Geitler 1936, Copeland 1936), Chlorophyceae (Tilden 1898, West 1904, Allen 1959) and Rhodophyceae (Hirose 1958, Chapman 1974). Seckbach (1994) considered them to be a transitional [eukaryotic] group bridging the Cyanobacteria and the unicellular Rhodophyceae. Recent molecular evidence places them as an order of extremotherms within the class Rhodophyceae (Ciniglia et al. 2004). A more recent genetic comparison with the Rhodophyceae has suggested that they are a separate class or phylum (Saunders and Hommersand 2004). Similar to Cyanobacteria and Rhodophyceae, Cyanidium contains the light absorbing pigments Chl. a and phycocyanin. The cyanids are extreme acidophilic thermophiles (Chapman 1974) with the smallest known genome among photosynthetic eukaryotes (Muravenko et al. (2001), and appear in hot springs throughout the world lining pools with a brilliant cyan color (Allen 1959).
References:
Allen, M.B. 1959. Studies with Cyanidium caldarium, an anomalously pigmented chlorophyte. Archiv fur Mikrobiologie 32:270-277. Chapman, D.J. 1974. Taxonomic status of Cyanidium caldarium, the Porphyridiales, and Goniotrichales. Nova Hedwigia 25:673-682. Ciniglia, C., H.S. Yoon, A. Pollio, G. Pinto, and D. Bhattacharya 2004. Hidden biodiversity of the extremophilic Cyanidiales red algae. Molecular Ecology 13:1827-1838. Muravenko OV, Selyakh I.O., N.V. Kononenk, and I.N. Stadnichuk 2001. Chromosome numbers and nuclear DNA contents in the red microalgae Cyanidium caldarium and three Galdieria species. European Journal of Phycology 36: 227–232 Hirose, H. 1958. Rearrangement of the systematic position of a thermal alga Cyanidium caldarium. Botanical Magazine of Tokyo 71:347-352. Kononenko, and I.N. Stadnichuk 2001. Chromosome numbers and nuclear DNA contents in the red microalgae Cyanidium caldarium and three Galdieria species. European Journal ofPhycology 36:227-232. Saunders, G.W., and M.H. Hommersand 2004. Assessing red algal supraordinal diversity and taxonomy in the context of contemporary systematic data. American Journal of Botany 91:1494-1507. Seckbach, J. 1994 The first eukaryotic cells – acid hot-spring algae. Journal of Biological. Physics 20:335-345.