STUDIES WITH C Y A N I D I U M CALDARIUM I. The Fine Structure and Systematic Position of the Organism

The fine structure of Cyanidium caldarium, as seen in thin sections of KMnO4-fixed cells examined with the electron microscope, is described. This organism, whose taxonomic position among algae is undetermined, contains a single well defined chloroplast, a nucleus, and mitochondria. Studies, with the electron microscope, of Chlorella pyrenoidosa and Nostoc are also reported. Structural differences within ceils of Cyanidium, chlorella, and Nostoc are discussed. It is concluded that if Nostoc can be taken as a typical Cyanophyte and Chlorella as a representative Chlorophyte and if the items of fine structure examined are diagnostic, then Cyanidium is certainly not a Cyanophyte and, while it has numerous features in common with Chlorella, is not a green alga similar to Chlorella. Comparisons are also made between Cyanidium and other algae whose fine structure has been described by others. The systematic position of Cyanidium caldarium has been questioned for many years. It has been classified variously as a blue-green alga, a green alga, a red alga, a coccoid cryptomonad, or a symbiotic association between a blue-green alga and a colorless chlorophyte (1-6). The purpose of the present report is to compare the fine structure of the normal wild-type strain with that of Chlorella and Nostoc, as determined by electron microscopy, and to provide information which may be useful in establishing the systematic position of this organism. A brief description of some aspects of the fine structure of C. caldarium was published recently by Rosen and Siegesmund (6). All descriptions refer to wild-type cells of a strain kindly provided by M. B. Allen of the Kaiser Foundation, and a mutant III-D-2 which resembles the wild type but contains more chlorophyll and phycocyanin, some of which can be formed in darkness (7). The cells were grown in a liquid medium (5) with 1 per cent glucose at 43°C -42°C under fluorescent illumination of t50 to 500 ft-c. Growth was vigorous, and the cells were harvested after 4 or 5 days. For fixation the cells were centrifuged, and the pellet was resuspended in 2 per cent buffered KMnO4 fixative pH 7.2 (veronal acetate, calcium and magnesium chloride 0.001 M, respectively). After 30 minutes the cells were washed briefly in water before being dehydrated in an ethanol series: 40, 70, 100 per cent. The cells were then embedded in methacrylate (75 per cent butyl, 25 per cent methyl, 0.05 per cent benzol peroxide, polymerized at 70°C). Sections were prepared with a Porter-Blum microtome using a diamond knife, 393 on Jne 7, 2017 D ow nladed fom Published June 1, 1962