Change in the trajectories of Cymatopleura solea as a result of cell size reduction

As described in the introduction, a vegetative division produces an equally large and a smaller diatom (MacDonald-Pfitzer rule). If one begins the cultivation with a single cell and generates a sequence of batch cultures by taking a larger number of diatoms from a mother culture and inoculating the following culture, then one observes a gradual reduction of the cell sizes. Each culture is started with a sample from the mother culture. As a result of the vegetative divisions, the mean value of the size decreases in each culture.

With this reduction of the cells, particularly the distance between the apices decreases. The transapical size changes comparatively little. Therefore, the cells of different sizes are not similar to each other, but have different proportions.

In the previous contributions, the relationship between the paths of diatoms and the shape of raphes was discussed. As the proportions of the diatoms and thus the radii of curvature of the raphes change with successive reduction, it is to be expected that the properties of the trajectories will also change. This will be shown below with the example of Cymatopleura solea.

Diatoms of the species Cymatopleura solea were found in a sample of Lake Ebnisee (48°55'25.5"N 9°36'32.8"E) on 25 March 2017, isolated and kept in clonal culture. New cultures were inoculated approximately every two weeks, with a larger number of cells (in the order of 100) being transferred into a new nutrient solution every time. The video above shows a 30-times time-lapse view of such a culture on 16 May 2017. At that time, the diatoms still had an almost uniform length of about 160 µm. To the right is a superposition image of all frames. Apparently, the paths between the reversal points and collision locations are straight or slightly curved.

In February 2019, almost two years after the beginning of cultivation, the shape of diatoms had changed considerably. The characteristic violin-like shape in valve view could only be found among the longest diatoms. Below a certain length (approx. 50 µm) the diatom is convex in the valve view. The diatoms with the strongest shortening exhibit a striking similarity to Cymatopleura elliptica. In the picture on the left (click to enlarge) four valves with decreasing length (63 µm, 54 µm, 43 µm, 36 µm) are shown. Because of the used superposition of focal planes, the undulated valve face hardly becomes noticeable. It is probably less pronounced than in Cymatopleura elliptica.

The movement of the diatoms can be seen in the following video on the left in time lapse (20x at low magnification, 10x at all higher magnification levels).

The first sequence was recorded in a culture with a strong EPS deposition that influenced the movement. The following sequences were obtained with diatoms previously removed from a culture. In comparison to the original culture, one recognizes stronger curved paths. This is probably a consequence of the more curved raphes. In the case of the shortest diatoms, rotation is frequent. This movement is not accidentally reminiscent of the movement of Cymatopleura elliptica, as can be observed in the video on the right.

In conclusion, it can be said that the trajectories of motile diatoms also depend on the number of reduction steps passed through. This certainly is not only true for the species Cymatopleura solea considered here.

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