Br. phycol. J. 8:197-201 30 June 1973

F U R T H E R O B S E R V A T I O N S O N THE G E N U S D I D Y M O S P H A E N I A M. S C H M I D T - -

D. S I B I R I C A ( G R U N . ) M. SCHM.

By PENELOPE A. DAWSON Department of Botany, University of Bristol, Bristol BS8 lUG

Maintenance of Didymosphaenia M. Schmidt as a distinct diatom genus is supported by an examination of D. sibirica (Grun.) M. Schm. with the scanning electron microscope. This investigation of D. sibirica reveals features similar to those of D. geminata (Lyngb.) M. Schm. and substantiates the existence of the genus separate from Gomphonema. D. sibirica is, how- ever, a quite distinct species.

In 1899, Mart in Schmidt illustrated a new taxon Didymosphaenia in A d o l f Schmidt 's Atlas der Diatomaceenkunde. For this new taxon, which he regarded as a subgenus o f Gomphonema C. Ag., he illustrated three species, D. curvi- rostrum (Temp. et Brun) M. Schm., D. sibirica (Grun.) M. Schm. and D. gemi- nata (Lyngb.) M. Schm., and var. stricta of the latter. These three species and one variety were formerly regarded as varieties o f the species Gomphonema geminatum (Lyngb.) C. Ag.

D. geminata and the variety stricta have already been described (Dawson, 1973). In this paper, a brief description o f D. sibirica is given to substantiate the evidence for the existence o f the genus, Didymosphaenia.

The species D. sibirica, formerly described as G. geminatum vat. sibericum by Grunow (1878), was established for shortened forms having only slightly capi- tate apices. The apical spines o f D. geminata were not apparent and only one isolated punc tum was found in the central hyaline area. However, the light microscopical features o f an arcuate raphe, slightly curved valve and thick transverse striae were sufficient to warrant its separation f rom other 'gom- phonemoid ' forms and place it firmly in the new taxon Didymosphaenia.

M A T E R I A L A N D M E T H O D S

D. sibirica was collected by Mr R. Ross from a wooden landing stage in Lake Baikal, Listuyanka, lrkutsk, U.S.S.R. on 30 August 1971. It was found growing epiphytically with other 'gomphonemoid' diatoms, including D. geminata var. stricta, 5 cm below the water level. The material, cleaned by the sulphuric acid/potassium chlorate method, was air dried on to specimen stubs, coated with gold/palladium and examined with a Mark IIa Cambridge Instrument Company Stereoscan Microscope. Ilford Pan F and FP4 films were used. Light micrography was carried out on a Zeiss Photomicroscope using Ilford Pan F film. The carbon replica was prepared and examined by the method previously described (Dawson, 1972).

O B S E R V A T I O N S Light microscopy

Light microscopical examinat ion reveals a valve 70-85/~m long and 35-40 /~m wide, with only slightly capitate apices (Fig. 1). The basal pole is consider- ably smaller and more at tenuated than the apical pole. The slightly arcuate

197

198 PENELOPE A. DAWSON

FI6s 1-6. Light micrograph (Fig. 1) and scanning electron micrographs (Figs 2-6) of Didymosphaenia sibirica. Fig. 1. Frustule in valve view. x 640. Fig. 2. External valve view. x 900. Fig. 3. External details of the central endings of the raphe canal, isolated punctum and loculi (arrow). x 7000. Fig. 4. The external apical polar region showing the curved extension of the terminal fissure, x 4200. Fig. 5. The apical pole in valve and girdle views showing the arrangement of the girdle bands, x 2000. Fig. 6. The basal pole with rows of 'mucilage pores', x 4500.

Morphology of Didymosphaenia sibirica 199

raphe is surrounded by a wide hyaline area which bears a unilateral isolated punctum in the enlarged central area. The transverse striae appear strongly granular, radiate, convergent in the middle of the valve and divergent towards the poles. Ten to twelve striae occur in 10/zm. These striae are absent at the basal pole.

Scanning electron microscopy In valve view, a typical 'gomphonemoid' form is revealed (Fig. 2). At the

centre of the valve face, a few striae alternate as long and short rows. Each stria is resolved into a series of loculi (arrow, Fig. 3). Each loculus has a series of marginal spines projecting into the lumen (Fig. 11). The inner aperture of the loculus bears dendritic projections (Figs 8, 9, 12).

Externally, the hyaline area around the slightly arcuate raphe is smooth (Figs 3-6) and no ridge is found at the junction between the valve face and valve mantle (Figs 4-6). In the central area a single isolated punctum is found, which appears as a round pore (Fig. 3). The central ends of the raphe canal curve slightly towards the isolated punctum (Fig. 3), terminating in large central pores. At the apical pole (Figs 4, 5), the terminal fissure curves in a wide angle over the valve face, on to the mantle, towards the side opposite that bearing the isolated punctum. At the basal pole (Fig. 6) the terminal fissure of the raphe curves at right angles, also in an opposite direction to the isolated punctum. Longitudinal rows of minute, simple, round pores are found below the terminal fissure. These are presumably the 'mucilage secreting pores' which produce the stalk for attachment of the cell to the substratum.

The extreme mantle edge, abutting on to the girdle bands, is non-perforate and is faintly sculptured. The girdle bands, which alternately oppose each other, lie inside the parent mantles (Fig. 5). One edge of each band is serrated and the whole band is poroid, with double rows of simple pores which terminate close to the open ends of each band.

On the interior of the valve (Fig. 7), the radial costae or ridges are very prominent. Between these costae are the dendritic openings occluding the inter- nal apertures of the loculi (Figs 8, 9, 12). The central fissures of the raphe end in a slight depression (Fig. 7). The isolated punctum appears as 'raised convolu- tions' on the inside (arrow, Fig. 9) and is separated from the central ends of the raphe by a small depression. On the inside, the raphe fissures end abruptly in terminal nodules and there is no curved extension as is seen on the exterior. Around the apical terminal nodule, the striae radiate (Fig. 8). At the basal pole (Fig. 10) the terminal nodule is found on a T-shaped ridge. The 'mucilage pores" terminate in a furrow behind this ridge.

DISCUSSION

The morphological features of the frustule of this organism are sufficient to warrant its separation from the genus Gomphonema and its inclusion within the taxon Didymosphaenia. However, there are also many features by which it differs from the previously described D. geminata (Dawson, 1973) sufficient to suggest that it is a distinct entity.

The complex locular structure is quite unlike that of the typical reniform

v

/

FIGS 7-12. Scanning electron micrographs (Figs 7-11) and carbon replica (Fig. 12) o f Didymosphaenia sibirica. Fig. 7. Internal valve view. × 850. Fig. 8. Internal apical pole showing the terminal nodule and the radiating striae, x 6500. Fig. 9. Internal central region showing the 'raised convolutions' of the isolated puncture (arrow) and the central ends of the raphe canal, x 6500. Fig. 10. Interior of the basal pole showing the terminal nodule on the T-shaped ridge and the furrow into which the 'mucilage pores' pass. x 6500. Fig. 11. Detail of the Ioculi and smooth axial area. x 12,000. Fig. 12. Carbon

replica o f the interior of the cell to show the dendritic openings of the loculi. × 15,000.

Morphology of Didymosphaenia sibirica 201

poroidal structure of species of Gomphonema but resembles that of D. geminaat (Dawson, 1972, 1973), Also, the striae are thicker in D. sibirica, exactly as in D. geminata. These features alone suggest that it should not be included within the genus Gomphonema. Further, the sculptured extreme mantle edge is not a feature of Gomphonema; neither are the wide angles of curvature of the exten- sions of the raphe fissures. Interiorly, the 'raised convolutions' are not regarded as features of the genus Gomphonema (Dawson, 1973). The T-shaped ridge and furrow, from which the 'mucilage pores' radiate, also have not been recorded in the genus Gomphonema. All these characters by which it differs from Gompho- nema are also found in D. geminata.

D. sibirica differs from D. geminata (Dawson, 1973) in the following features. Only one isolated punctum is present, whereas 3-5 are found in D. geminata. The ornamented, hexagonal to square ridges are absent around the external openings of the loculi and the hyaline area around the raphe canal is apparently smooth. The ridge at the junction of the valve face and valve mantle is not present, nor are the apical spines which extend from the ridge. The degree of curvature of the external apical terminal fissure of the raphe is slightly less in this taxon than in D. geminata. Thus the species D. sibirica, erected by M. Schmidt from light microscopical observations, is validated from these scanning electron microscopical observations.

A C K N O W L E D G E M E N T S

I would like to express my thanks to Dr F. E. Round for his encouragement and discussion throughout this work; to Mr R. Ross for the material and for his comments and suggestions; and to Mrs E. Parsons and Miss B. Haggett of the Long Ashton Research Station for operating the scanning electron microscope.

REFERENCES

DAWSON, P. A., 1972. Observations on the structure of some forms of Gomphonema parvulum Ki~tz. I. Morphology based on light microscopy, and transmission and scanning electron microscopy. Br. phycoL J., 7: 255-271.

DAWSON, P.A., 1973. The morphology of the siliceous components of Didymosphaeniageminata (Lyngb.) M. Schmidt. Br. phycol. J., g: 65-78.

GRUNOW, A., 1878. Algen und diatomaceen aus dem Kaspischen Meere. Seperat aus den Sitzungs-Bericht der Natur. Abh. zooL-bot. Ges. Wein, 10: 98-132.

SCHM~DT, M., 1899. In A. Schmidt's Atlas der Diatomaceenkunde. Tafel 214. Reisland, Leipzig.