Cymbella colonies on jelly stalks

In the post on sexual reproduction, a Cymbella species (probably Cymbella cistula) was presented that dwell on mucilage stalks. Their lengths ranged from 54 µm to 120 µm. In the observations described below, their length was approximately 110 µm. These diatoms adhere to the substrate, form a stalk and then divide. In nature, these colonies are attached to aquatic plants or stones. This gives them an advantage in streaming waters. They can also be found in ponds and lakes.

Colony formation

The time-lapse video recorded with PlasDIC at the top left shows a two-week-old culture. You can easily recognize the mucilage stalks, many of which were abandoned by the diatoms. The free diatoms move between the mucilage formations until they settle and generate their own stalk.

In the video in the upper right corner you can see a young culture in phase contrast. It is recorded with a time-lapse factor of 1800. The observation time was 13.6 hours. On the right side you can see two diatoms on a stalk, both of which have almost divided and are now close to separation. These cell divisions take place simultaneously. Subsequently, diatoms become detached from the mucilage stalk. Towards the end of the video, another division occurs on the same stalk, which can be recognized because of the transparency of the diatoms.

In the lower part of the picture you can see a diatom that settles, excretes a pad of EPS and within about three hours develops a stalk on this basis. Such a stalk grows only for a few hours. The diatom probably uses its energy after the formation of the stalk primarily for division.

If the diatoms lie on the substrate, it is very easy to detach from the stalk. An active raphe can exert force on contact with the mucilage stalk or other diatoms may knock against them. In the video you can see how diatoms break off from the mucilage stalk shortly after separation. Here are two scenes from the video above, but with a time lapse factor of 125:

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In one case a diatom that lies on its girdle bands at the dorsal side has helped somehow. In the other case the diatom may have already been loosened by the other diatoms. It is therefore to be expected that older cultures with a high population density will be easily destroyed by freely moving diatoms.

In some cultures I have observed that after a cell division, one diatom often falls off the stalk while the other remains for a long time. I suspect that the larger cell through whose apical pores the EPS was secreted for the mucilage stalk is more firmly connected to it.

Form of colonies and spread

Cultures can develop very differently for reasons that I do not know. As you can see from the examples, there are cultures in which a diatom breaks off quickly after the division and moves away. Branching colonies do not develop. In other cultures, the diatoms remain on the stalks for a longer period of time on average, form their own stalks and divide. This creates a tree-like structure. Below you can see an example of such colonies in phase contrast (focus stacking) on the left. In the middle is an example of a colony in the DIC (again created from a stack of images), which has no branches. On the right you can see such a colony as it was drawn in the famous book "A History of Infusoria, including the Desmidiaceae and Diatomaceae" by Andrew Pritchards from 1861.

(All pictures can be enlarged by clicking on them.) Sometimes long stalks form, at the end of which there is often only one diatom, as can be seen in the lower left and lower center (both phase contrast). Quite often one comes across irregular branched structures, which are shown in PlasDIC at the bottom right.

(All pictures can be enlarged by clicking on them.) Below left a video of a culture of small diatoms and some initial cells is shown (60-x time-lapse, PlasDIC), in which no colonies have formed, because the diatoms apparently separated quickly from their stalks.

Different environmental conditions can be considered as a reason for the different forms of development in nature. In our cultures, however, the nutrient solution, temperature etc. are almost identical. There are differences in light intensity. It can be assumed that this plays a role in the stability of the tree-like structures, as it has a significant influence on the activity of the raphe. Incidentally, the largest structures were formed in my cultures in the first generations after sexual reproduction. The fast leaving of the jelly stalks could serve the search for a sexual partner in diatoms that are close to their minimal size.

As diatoms repeatedly break off from the tree, these diatoms are at least temporarily free living. At least in cultures the diatoms do not move far from their place of origin before they settle. On the smooth substrate the diatoms describe almost circular paths. This artifact makes it difficult to traverse long distances in cultures. Nevertheless, free moving diatoms in cultures enable a spatial spread in principle.

Structure of the mucilage stalks

During the formation of a stalk a thick gelatinous substance (EPS) is secreted through apical pores in the valves. If you look at the mucilage stalks with PlasDIC, you will find surface structures that run parallel to the direction of the stalks. The image at the bottom left was created by superimposing several images (focus stacking) and subsequently sharpening the picture (click to enlarge). In the phase contrast this can be recognized in some pictures, such as the branched small colony shown above.

This thick string is certainly not pressed through a single pore, but rather through an apical pore field (HUFFORD, T. L., COLLINS, G. B., 1972). The diameter of the pores and the distance between adjacent pores are far below the resolution of a light microscope. The structures could characterize the boundary of the pore field that contributes to EPS production. Similar structures in the stalk can be found, for example, in the genus Gomphonema.

Also remarkable are nodular thickenings which appear frequently in longer jelly stalks. They are obviously formed when the creation of a new section begins after a pause and a new pad is formed. This can be seen in the picture (phase contrast) on the right (click to enlarge).

Pritchard, A. (1861) A history of Infusoria, including the Desmidiaceae and Diatomaceae, Biritsh and foreign. Whittaker and Co., London, 968 pp.

HUFFORD, T. L., COLLINS, G. B., 1972: The stalk of the diatom Cymbella cistula: SEM observations. J. Phycol. 8, 208-210.

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