The features considered to be of greatest taxonomic importance at the rank of genus in the modern Chlorococcales are chromatophores, pyrenoids, and zoospores. None of these are preserved in fossils. Modifications due to environmental influences cannot always be discriminated from hereditary variability by microscopic analyses alone. In preserved modern material it is impossible to extract the various stages necessary for distinguishing genera and species. That is also a major difficulty in mixed cultures of living material—and of course in fossil material. Thus, cultivation is the essential method for taxonomic classification of many unicellular modern algae.
The fossil leiospheres lack all the characters mentioned above, which are necessary for their classification in modern algal taxa—even at the rank of class. It is evidently impossible also to discriminate between different morphological appearances due to ontogenetic variability and environmental modifications. Consequently the leiospheres must be arranged in an artificial system without biological implications, but as far as possible according to biological principles.
Cysts are metabolically dormant cells with a considerably higher resistance than vegetative cells to detrimental physical conditions, other than heat. Desiccation resistance may be a survival attribute in nature, and this property seems to be related to the outer cell wall.
The size is not a sole diagnostic character. Anyhow related modern taxa (genus, species) seem to have a limited size range so measurements may be important features for determination of modern algae. I have compared the size distribution in Leiosphaeridia to vegetative stages of modern unicellular Chlorococcales.
In the Cyanophyceae the normal reproductive cell, the akinete, is a spherical or cylindrical cell with a smooth or granulated surface. It is characterized by a considerable increase in size compared with the vegetative cell, and before maturation it will expand still more. The ultrastructure is similar to that of the vegetative cell, but with a new outer wall added. Thus the akinete can be regarded as an enlarged vegetative cell enclosed by a thick outer envelope.
Akinete-like cells with thickened walls develop also in the Chlorophyceae, but conditions associated with the production of this stage were not sufficiently investigated.
Akinete germination occurs in different ways. Frequently it generates the rupturing of the outer wall. The excystment mechanism is a true taxonomical feature. According to genetical control, some taxa will dehisce by an aperture (pylome), others by a rupture. However, this is recognizable only in certain stages of their life cycles.
Modern algae Trachelomonas
My laboratory investigations of the modern euglenophycean algae Trachelomonas volvocina showed that the lorica of these specimens is acid resistant and that after treatment in acids their flagellar pore is similar to the pylome of leiospheres. The living cells are spherical but they collapse during the acetolyzis process and form folds similar to those said to characterize the fossil genus Kildinella (=Leiosphaeridia).
Trachelomonas is characterized by a lorica that is often iron impregnated, with an apically located flagellar pore, and a very long flagellum. The species are discriminated by shape, ornamentation, processes, and flagellar pore features. A series of 447 illustrations (Conrad & van Meel 1952) of Trachelomonas species shows that shape, ornamentation, processes, and aperture characteristics are not significant taxonomic features at the rank of genus.
About 250 species of Trachelomonas are known. The cells show a considerable variability in size and shape, which makes the taxonomy of the group complex. This variability is also due to different stages of the life history of the organisms and to environmentally controlled morphogenesis. In acidic water some species may have a hyaline, smooth lorica, whereas the same species in alkaline water have thickened lorica impregnated with iron- and manganese-compounds. Exposed to unfavourable environmental conditions some species of Trachelomonas will develop into round, thick-walled resting cells that may be difficult to discriminate from specimens of other genera, even of different classes, e.g. the chlorophycean genus Chlamydomonas.
Specimens of Trachelomonas shown on the internet.
Morphology and taxonomy
Morphometry of modern and fossil algae
Distribution of Leiosphaeridia
- Conrad, W. & van Meel, L., 1952.
- Matériaux pour une monographie de Trachelomonas Ehrenberg, C., 1834, Strombomonas Deflandre, G., 1930 et Euglena Ehrenberg, C., 1832, Genres d’Euglénacées. Mém. Inst. R. Sci. Nat. Belg., 124: 1—176, pls. 1—13.
- Lindgren, S., 1981.
- Remarks on the taxonomy, botanical affinities, and distribution of leiospheres. (Summary in Russian) Stockholm Contrib. Geol., 38(1): 1—20. Stockholm. ISBN 91-22-00500-5. ISSN 0585-3532. — Buy at the lowest prices among books in Sweden.
- Lindgren, S., 1982.
- Taxonomic review of Leiosphaeridia from the Mesozoic and Tertiary. Stockholm Contrib. Geol., 38 (2): 21—33. Stockholm. ISBN 91-22-00502-1. ISSN 0585-3532. — Buy at the lowest prices among books in Sweden.
- Lindgren, S., 1982.
- Algal coenobia and leiospheres from the Upper Riphean of the Turukhansk region, eastern Siberia. Stockholm Contrib. Geol., 38 (3): 35—45. Stockholm. ISBN 91-22-00504-8. ISSN 0585-3532. — Buy at the lowest prices among books in Sweden.
- Lindgren, S., 1982.
- A new taxon of Leiosphaeridia (algae) from Upper Cretaceous clays, southern Sweden. Stockholm Contrib. Geol., 37 (11): 139—143. Stockholm. ISBN 91-22-00487-4. ISSN 0585-3532.
- Lindgren, S., 1984.
- A new taxon of Leiosphaeridia (algae) from Upper Cretaceous clays, southern Sweden. Stockholm Contrib. Geol., 39 (5): 139—144. Stockholm. ISBN 91-22-00517-x. ISSN 0585-3532. — Buy at the lowest prices among books in Sweden.