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

نویسنده

  • Dr. Bogorad
چکیده

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 O cber 0, 2017 jcb.rress.org D ow nladed fom and examined in a Siemens Elmiskop I at 80 kv. Electron micrographs were taken at 10,000 to 40,000 magnifications. Osmium tetroxide fixation was attempted but was unsuccessful; most ceils failed to embed after treatment with osmium tetroxide. Phase contrast examination of the living material established the growth stages of Cyanidium. The cells are spherical, 3 to 5 ~ in diameter, but little detail of internal structure can be detected. There is a single lobed chloroplast, which appears to lack a pyrenoid, filling much of the cell. An ill defined region with areas of different refractive densities occupy the rest of the cell, but a definite nucleus and other organelles cannot be distinguished. Multiplication occurs by endospore formation. First, the chloroplast becomes vague. Then, four daughter cells appear within the mother cell; they are released after the mother cell wall ruptures. This description of the growth stages and cell structure corresponds with those reported by previous authors. Severn electron micrographs of sections of adult cells are shown in Figs. 1 to 5. In contrast to its appearance under the light microscope, the cell appears highly differentiated: a cell wall plasmaiemma, ground matrix, endoplasmic reticulum, chloroplast, mitochondria, nucleus, and vacuoles may be identified. The cell wall appears as a homogeneous dense layer, 0.1 to 0.05 /~ in thickness in KMnO4-fixed cells (Figs. 1 to 5); it is of low density in OsO4. No evidence of stratification or layering has been observed, and it is not clear whether the wall has the fibrillar structure characteristic of plant cell walls in general. The protoplast is bounded externally by an indistinct plasma-membrane approximately 100 A in thickness, which appears as a double structure of two dense lines separated by a light intermediate layer in favorable sections (Fig. 5). Although the membrane follows the general curvature of the protoplast, it is not always in close contact with the cell wall. I t is frequently conspicuously indented and convoluted; the resulting space between the cell wall and the membrane contains a material of low density when fixed with KMnO4. Various organelles, vacuoles, and membranes are embedded in a granular ground cytoplasm of medium density which is not obviously different in texture from the matrix between the chloroplast lamellae or the mitochondrial matrix, as is to be expected in KMnO4-flxed tissue. Some of the organdies are obviously mitochondria. They have the typical mitochondrial structure consisting of a double membrane with cristae arising from the inner membrane, and contain a matrix of light density between the cristae (Fig. 4). From the shapes of the profiles, it appears that rods and filaments (0.2 # in diameter X 0.4 to 1.5/z in length) are the common forms of the mitochondria in Cyanidium. The cristae are not numerous. They are irregularly spaced and probably occur as infrequently branching tubes; circular and tubular profiles, occasionall,/ forked, are common. No pattern of distribution of the mitochondria within the protoplast has been found. They appear to occur at random. As endospore formation commences the mitochondria become more elongate, up to 2 /~ in length, the cristae become indistinct and the matrix much denser. Some of the dense rodshaped bodies become too indefinite to be identified with certainty as mitochondria (Fig. 3). The mature cell contains a single, much lobed chloroplast which occupies a large part of the

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تاریخ انتشار 2003