Cyanidiophyceae

An Italian scientist, Giuseppe Meneghini (1839) (Fig. 1) was the first to study and describe the thermo-acidophilic algae inhabiting the sulphur hot-spring of Acquasanta (Ascoli Piceno, Italy). He observed “small and very small globules” (0.2–2.0 µm) and he proposed the new species Coccochloris orsiniana (Cyanophyta) without any diagnosis.

In the following 100 years, these algae, always considered as one species, captured the interest of many phycologists who tried to clarify their systematic position. Because of their simple morphology and the absence of sophisticated means of investigation, these algae were variously identified as Palmella orsiniana (Chlorophyta) (Kützing, 1849), Chroococcus varius (Tilden, 1898), Protococcus botryoides f. caldarium (Tilden, 1898), Pleurococcus sulphurarius (Chlorophyta) (Galdieri, 1899), Pleurocapsa caldaria (Collins et al., 1901), Palmellococcus thermalis (West, 1904), Pluto caldarius (Copeland, 1936), Cyanidium caldarium (Geitler and Ruttner, 1936), Dermocarpa caldaria (Drouet, 1943) and Rhodococcus caldarius (Hirose, 1958). By using morphometric analyses of field samples, De Luca and Taddei (1970) identified for the first time, two thermo-acidophilic algae (Fig. 2). These were provisionally named Cyanidium caldarium forma A and Cyanidium caldarium forma B. In the early 1980s the Neapolitan school used morphological, physiological and ultrastructural data to invalidate the previous descriptions, and made a definitive revision of the taxonomy of these algae. Two species were formally recognized: Cyanidium caldarium (Tilden) Geitler (Fig. 3) and Galdieria sulphuraria (Galdieri) Merola (Merola et al., 1981) (Fig. 4).