TUBULAR ELEMENTS-A NEW STRUCTURE IN BLUE-GREEN ALGAL CELLS

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1 J. Cell Sci. 38, (i977) 303 Printed in Great Britain Company of Biologists Limited TUBULAR ELEMENTS-A NEW STRUCTURE IN BLUE-GREEN ALGAL CELLS Z. N. TAHMIDA KHAN AND M. B. E. GODWARD Department of Plant Biology and Microbiology, Queen Mary College, London, Great Britain SUMMARY New cell inclusions, 'tubular elements' are described from the blue-green alga Calothrix marchica. These structures are found in about 20% of all cells observed. They are nm in diameter, apparently composed of sub-units of which the estimated number, as seen in transverse section, is INTRODUCTION Unusual bodies of unknown significance have frequently been found in the cells of blue-green algae. These include 'cylindrical bodies' (Pankratz & Bowen, 1963; Wildman & Bowen, 1974) and 'filamentous elements' (Jensen & Bowen, 1970). In the present study, tubular elements of Calothrix marchica have been investigated. These structures have much in common with the 'filamentous elements' in their general appearance when sectioned longitudinally but not when sectioned transversely. MATERIALS AND METHODS C. marchica was isolated from wet soil, near the margin of a pool in Dacca, Bangladesh. It was cultured in modified Fogg (1949) medium at 30 C and 3420 lux. Fixation was carried out using a slightly modified version of the method of Gantt & Conti (1969), in 4% glutaraldehyde + o-i M phosphate buffer (ph 6-8) for 1-5 h at room temperature. Postfixation was in 1 % OSOJ + O-I M phosphate buffer (ph 6-8) for 2 h at room temperature. After dehydration in grades of ethanol specimens were taken through propylene oxide and embedded in Taab embedding resin. Sections were stained with methanolic uranyl acetate (Wildman & Bowen, 1974). Zeiss EM9 and Siemens Elmiskop IA electron microscopes were used. RESULTS These bodies appear as bundles of cylindrical tubules lying side by side (Figs. 1,3). Usually an electron-transparent narrow zone, 5-5 nm wide, is present between the tubules but this width may be much less, probably depending on the angle of sectioning. The tubules are normally parallel and straight, although the angle at which they are lying may vary (Fig. 1). Their diameter ranges from 18 to 22 nm, while the length is more variable, ranging from 166 to 233 nm. On average this measurement is shorter than that recorded for the filamentous elements in other blue-green algae (Jensen &

2 3 4 Z. N. Tahmida Khan and M. B. E. Godward Fig. i. Calothrix marchica. Cell with tubular elements showing their distribution in the cell, x

3 Tubular elements in algal cells 305 *&r T-vs&ytr- Fig. 2. C. marchica. Stack of tubular elements in transverse section. Subunits are discernible (arrow), x Fig. 3. C. marchica. Stack of tubular elements in longitudinal section, x

4 No. of subunits in transverse 8-10 section Length, m-n up to Iooo Table I. Tubular structures of blue-green akae Organism... Calothrix Sy nechococcus Nostoc Nostoc Anabaena Reference... (present study) (Bailey-Watts et al. (Jensen & Bowen, 1970) (Bisalputra et al. 1975) Uensen & Ayah, 1976) 1968) Structure... Tubular elements Microtubules Filamentous elements Microtubules Microtubules Total diameter, 20 (mean) nm (range) Wall thickness, 4'0

5 Tubular elements in algal cells 307 Bowen, 1970). Their location inside the cell seems to be indefinite - near the transverse septa, the side walls, or even in the central region. Possibly they are located around the nucleoid. The tubule walls appear to be composed of subunits (Fig. 2, arrow), although the clarity is not sufficient for a certain count to be made; approximately 8-10 subunits seem to be present in transverse section (Table 1). DISCUSSION When the tubular elements of C. marchica are compared with other similar structures, it is found that, although there are certain resemblances, there are also differences. The first situation to be clarified is that of Nostoc pruniformae, where the authors (Jensen & Bowen, 1970) described filamentous inclusions associated with a 'plate-like membranous structure' parallel with the transverse septum. These filaments were termed 'plate filament arrays'. They were in fact tubular with a narrow lumen. What were undoubtedly the same structures were later described in Nostoc sphaericum and Nostoc sp. (Bisalputra, Oakley, Walker 8c Shields, 1975) as'microtubular elements' and in Stigonema hormoides (Butler & Allsop, 1972) as 'striated bodies'. Similar structuresthe 'microplate-microtubule array' were also described in Anabaena sp. (Jensen & Ayala, 1976). All these 'microtubular elements' or 'microtubules' in Nostoc spp. and Anabaena spp. were associated with either a 'dark amorphous plate' (Bisalputra et al. 1975) or a 'membranous plate' (Butler & Allsopp, 1972; Jensen & Ayala, 1976; Jensen & Bowen, 1970). No membranous or dark amorphous plates have been found in C. marchica. The tubular elements of this species are not arranged in any special position relative to the transverse septum, although there are marked plasmodesmata in C. marchica, as also in Stigonema hormoides, where they were associated, together with striated bodies, with transport of material across the septum (Butler & Allsopp, 1972). Microtubules not associated with any membranous plate were found at the ends of the cells, in Synechococcus sp. (Bailey-Watts, Bindloss & Belcher, 1968). Bisalputra et al. (1976) have compared the tubular elements to the microtubules of the mitotic spindle and the 'dark, thin amorphous plate' with the ' microtubuleorganizing centre' (M.T.O.C., Pickett-Heaps, 1969). There are no observations regarding the tubular elements of C. marchica which could justify this; in Synechococcus the only supporting observation seems to be the position of the microtubules near the septa. Even in Anabaena spp. (Jensen & Ayala, 1976), there appear to be several 'plates' in a cell, sometimes oriented at right angles to the septa; in addition the microtubules are sometimes associated also with polyhedral bodies. The comparison with microtubules and microtubule-organizing centres seems premature. There are also size differences between the tubular elements of C. marchica and the previously mentioned microtubular elements of the other authors. The tubular elements of C. marchica are notably wider (18-22 nm); the others range from 10 to 17-5 nm. The number of wall subunits estimated in cross-section is however similar, 8-10, to that of Nostoc spp. (Bisalputra et al. 1975), the only other species in which an estimate has been attempted.

6 308 Z. N. Tahmida Khan and M. B. E. Godward There seems no doubt that structures of a tubular nature, stacked in parallel arrays of the same short lengths are of fairly common occurrence in the blue-green algae; radial and cylindrical arrangements also occur. As yet, information about them is insufficient for any assessment of function, or value as taxonomic criteria. This paper forms part of a thesis (Z.N.T.K.) for the Ph.D. degree of the University of London. Dndon. REFERENCES BAILEY-WATTS, A. E., BINDLOSS, M. E. & BELCHER, J. H. (1968). Freshwater primary production by a blue-green alga of bacterial size. Nature, Land. 220, BISALPUTRA, T., OAKLEY, B. R., WALKER, D. C. & SHIELDS, C. M. (1975). Microtubular complexes in blue-green algae. Protoplasma 86, BUTLER, R. D. & ALLSOPP, A. (1972). Ultrastructural investigations in the Stigonemataceae (Cyanophyta). Arch. Mikrobiol. 82, FOGG, G. E. (1949). Growth and heterocyst production in Anabaena cylindrica Lemm. II. In relation to carbon and nitrogen metabolism. Ann. Bot., N.S. 13, GANTT, E. & CONTI, S. F. (1969). Ultrastructure of blue-green algae. J. Bad. 97, JENSEN, J. E. & AYALA, R. P. (1976). The fine structure of a microplate-microtubule array, microfilaments and polyhedral body associated microtubules in several species of Anabaena. Arch. Mikrobiol. 111, 1-6. JENSEN, J. E. & BOWEN, C. C. (1970). Cytology of blue-green algae. II. Unusual inclusions in the cytoplasm. Cytologia 35, PANKRATZ, H. S. & BOWEN, C. C. (1963). Cytology of blue-green algae. I. The cells of Symploca muscorum. Am. J. Bot. 50, PICKETT-HEAPS, J. D. (1969). The evolution of the mitotic apparatus: an attempt at comparative ultrastructural cytology in dividing plant cells. Cytobios 3, WILDMAN, R. B. & BOWEN, C. C. (1974). Phycobilisomes in blue-green algae. J. Bad. 117, {Received 7 March 1977)