A primaeval alga bridging the blue-green and the red algae: further biochemical and ultrastructure studies of Cyanidium caldarium with special reference to the plastid membranes.

The storage polyglucan and isozyme distribution together with the fine structure of the chloroplasts have been investigated in Cyanidium caldarium. This alga is a unicellular eukaryote which thrives in acid hot habitats. Our observations propose C. caldarium as a primitive Rhodophytan which is the "bridge" alga linking the prokaryotic blue-green and the red algae. some features of the Cyanidium chloroplast e.g. the single membrane enveloping the plastid, or the cyanobacterial nature of its thylakoids and other cellular characteristics, point to the primaeval status of this early-evolved eukaryote which is considered a "living fossil". It seems to be the true transitional organism between the anucleated and nucleated algae and is unlikely to be an endosymbiontic association of two or more cells, as speculated elsewhere.