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Cell of Xanthidium fasciculatum enveloped by a distinct mucilaginous layer. The outside of the envelope is marked by numerous bar-shaped micro-particles (presumably bacteria).
In many desmid species cells are surrounded by a thick, sharply outlined gelatinous layer. The mucous cell envelope usually encloses the complete cell body inclusive of its possible processes. The occurrence of sharply bordered cell envelopes is taxon-linked. We don’t meet it in, e.g., the genera Closterium and Micrasterias whereas it is a rather common phenomenon in, e.g., the genera Cosmarium, Staurastrum and Staurodesmus. Though, also within those latter genera there are species lacking a distinct envelope.

The function of the extracellular mucous envelope is not quite clear. Because density of the mucous is lower than that of the proper cell in planktic species it will reduce sinking velocity. As an envelope increases particle size considerably, it also could obstruct grazing by zooplankton. A third hypothesis is that it might help in trapping scarce, dissolved nutrients. Arguing for that latter hypothesis is that desmid species characteristic of eutrophic (nutrient-rich) habitats are in want of a distinct envelope (Coesel 1994). 

Reference:

Coesel, P.F.M., 1994. On the ecological significance of a cellular mucilaginous envelope in planktic desmids. — Algological Studies 73: 65-74.

Cells of Staurodesmus cuspidatus var. curvatus in Indian ink suspension (not penetrating into the mucous envelope).

 

Image © IBED

 

Cells of the oligo-mesotrophic taxon Cosmarium abbreviatum var. planctonicum (with envelope) and the eutrophic species Staurastrum chaetoceras (without envelope) in a mixed continuous flow culture showing that the occurrence of an extracellular envelope is genotypically, not phenotypically determined.

Image © IBED

 

Cell of Xanthidium antilopaeum (in phase contrast) showing a radiate, fibrillar structure of the envelope.

 


Image © Wim van Egmond