Frustulia vulgaris - Details on the Formation and Expansion of Colonies

Start and end of the expansion work at one end of the tube

Activity at one end of a tube in the colony is temporary and typically lasts a few hours. The expansion begins when one or two diatoms encounter one end as they move. They are then mostly reflected and, after a spontaneous reversal of motion, return to the tip of the tube. This can happen several times. Eventually, however, diatoms begin to stay longer at the end. Other diatoms join them, and more and more diatoms gather in the vicinity. Their functions become differentiated, as previously described. The expansion begins some time after the diatoms have gathered. Video I shows this process in 500x time-lapse.

The expansion can come to an end very quickly. The diatoms migrate away from the tip in rapid succession. Video II illustrates this very well. Almost as if on command, the diatoms leave the area behind the tip, and only a single diatom remains.”

The Formation of a Colony Starting with a Single Diatom

When individual diatoms of the species F. vulgaris are removed from a colony and placed on a substrate, they initially move freely. Video 3 (100x time-lapse) shows such a diatom in the right half of the scene. It appears to be capable of standing on its apex as well. As with motile diatoms that do not form tubes, activity of the raphes acts on the substrate, making it possible to observe the diatom’s movement relative to the substrate.

As it moves back and forth, it continuously secretes EPS. This forms a cocoon-like envelop around the diatom, which adheres to the substrate. You can see it in Video 4 (1,000x time-lapse). The diatom moves inside the tube and can no longer move freely on the substrate but is restricted in its movement by the tube. By moving back and forth, it reinforces the initial tube by excreting mucilage, as described by Houpt (*). The length grows slowly. The elasticity of the short tube is evident because after a reversal of motion the tube shortens at the end, following the diatom. Then the first vegetative division occurs. At this point, the usable range of motion is barely longer than the diatom itself.

The further development of colonies is not uniform. Often there are subsequent divisions with successive expansion, in which the diatoms move towards each other in a disorganised manner. In a few colonies with only two diatoms, however, a form of coordinated expansion begins, in which the diatoms no longer act independently. The two diatoms lie one behind the other in the tube and together they exert force towards the end of the tube. Their forces add up. The result is a continuous process in which the tube is stretched by the two diatoms and strengthened by the secretion of mucilage. Video 5 shows several colonies undergoing this process in a 3,000x time-lapse. In total, the video shows 14 hours of the colonies’ development. The speed of expansion is apparently very low, at only 0.025 µm/s, which is about two orders of magnitude slower than the speed of freely moving diatoms. As mentioned earlier, for about 7 diatoms in Section I, the speed of expansion is typically 0.15 µm/s.

The number of diatoms in the colonies increases, and after some time, a situation develops as shown in Video 6 (3000x time-lapse). Some colonies are growing rapidly.

Finally, a 5,000x time-lapse video (Video 7) will be shown, depicting the development of a culture in a Petri dish as viewed under a stereo microscope in dark-field mode. Images were captured at fixed intervals. These images were then superimposed up to the moment of presentation by determining the maximum values across all frames. With each new image, the overlay extends further. Consequently, the movements of the ends, which indicate Section I, become visible as white tufts.

(*) Houpt, P.M. (1987) Observations on the marine tube-dwelling diatom Navicula pseudocomoides in culture. Diatom research 2(1), 47-53

More articles on F. vulgaris are planned.