Cosmarium benedictum Ducellier

Geest, Alfred Van & Coesel, Peter, 2019, Some new and interesting desmids (Streptophyta, Desmidiales) from ephemeral puddles in the urban and industrial areas of Amsterdam (Netherlands), Phytotaxa 387 (2), pp. 119-128 : 121-123

Cosmarium benedictum Ducellier (Figs. 2, 9, 11, 20)

In puddles at locations 3 and 6 a desmid was found that at first glance somewhat looked like a reduced form of C. rostafinskii Gutwiński (1890) , but the more specimens we found, the more the faith rose that it was a separate taxon.

After consulting the extensive desmid literature, we came across an article by Ducellier (1919), who described a new species, C. benedictum , that he found in a holy water font at a church in Switzerland. His diagnoses and drawings agreed very well with our specimens. He also mentioned C. rostafinskii as a species to compare with.

However, a critical comparison of cell morphology and relative cell dimensions in our populations studied, revealed some relevant differences. Whereas cell length to breadth ratio in C. rostafinskii was always lower than 1.5, that in C. benedictum varied from 1.6 to 1.8, resulting in more elongate, less pyramidate semicells. Next to that, there was a marked difference in cell isthmus to cell breadth ratio, viz. 0.5 to 0.6 in C. rostafinskii versus 0.7 to 0.8 in C. benedictum , rendering that latter species a remarkably shallow, usually slightly open sinus.

Because even some of the scarcely found empty cells in the samples did not give a definite answer regarding the nature of the frontal ornamentation under the light microscope, SEM-images were made. Fig. 20 shows that, contrary to C. rostafinskii , there is hardly any supraisthmial ornamentation. Altogether, the above-described differences make us believe that two different species are at issue, also because at location 3, where both species were found in high numbers, no specimens with an intermediate cell form were observed.

Ducellier (1919) found semicells of C. benedictum to hold a variable number of pyrenoids. Whereas the majority of semicells contained one large pyrenoid, he found also semicells with two or three smaller pyrenoids. In our populations studied, however, all semicells were provided with a single, large pyrenoid. At location 3 several zygospores were found, to be characterized as globose and furnished with short, nozzle-shaped protuberances arising from a conical base ( fig. 11 ).

We could not trace any more records of C. benedictum in desmid literature but possibly in a number of cases it was confused with C. notabile sensu De Bary (1858) , see below.

Figures

FIGURES 8–11. 8. Cosmarium rostafinskii. 9. C. benedictum. 10. C. amstelodamense. 11. C. benedictum, zygote. Scalebar = 10μm.