Flat colonies and loosely bound colonies

In the previous post colony-forming Cymbella were described which dwell on jelly stalks. Other Cymbella species do not form stalks. The diatoms are attached to the substrate at one end with the aid of a thick gelatinous substance. EPS excreted at apical pores causes a high adhesion. As shown on this page, this leads to the formation of local accumulations, which have a limited stability and are somewhat simplified referred to as colonies.

First of all it is reported on Cymbella lanceolata (Ehr.), which is about 190 µm long. Its valve is shown on the left (click to enlarge). A short video with the movement was presented in a post about the curvature of the trajectories (center of curvature is always on the ventral side).

The adhesive EPS excretions are clearly visible in phase contrast or DIC. Below you can see two pictures (click to enlarge) from the bottom of the Petri dish taken with PlasDIC. In the right picture the diatoms adhere to a fiber that was present in the nutrient solution. Apparently, there is a preference for attachment to such fine structures.

Among my Cymbella cultures there were different species that formed such irregular EPS excretions.

Processes

If you observe such accumulations for a longer period of time, you can recognize these processes:

1. Diatoms detach from a colony. This is typically done at the edge region of the colony.
2. Diatoms move in the space between the colonies.
3. Diatoms meet an existing colony and remain in this cluster.
4. Diatoms stop their movement and attach themselves to the substrate.
5. Diatoms reproduce asexually inside and outside colonies.

The collision between individual diatoms and contact with colonies without a long stay are not listed here, as they are transient and not of great importance for the structure formation.

Diatoms that attach to colonies usually remain on the edge of the colony. As they themselves secrete EPS, the area formed by EPS deposits is continuously increasing.

If individual diatoms come to a standstill, the cause may also be existing EPS from previous attachments. This effect, which is not immediately apparent, will be discussed later.

Events 1,2 and 3 do not allow the formation of new colonies. A colony can develop from individual adherent diatoms according to 4 by following cell divisions and attachment of diatoms.

In the time-lapse video on the left (20x time lapse) you can see examples of the detachment and connection of diatoms to a colony.

This change between colonies was rarely observed in Cymbella on gelatinous stalks. Here too a diatom may collide with a colony and come to rest there, but it always remains outside of existing mucilage trees.

Influence of light intensity

The movement activity of diatoms requires sufficient light intensity. With increasing intensity, the raphes become active in the observed Cymbella, regardless of whether they move freely or are in a colony. This leads to the fact that more and more diatoms are detached from the colony in bright light.

The driving force then exceeds the adhesion to the substrate produced by EPS. In the video on the left (600x time lapse), a culture was irradiated with several thousand lux using microscope illumination. One recognizes a considerable reduction of two small clusters.

The removal of diatoms from a colony and migration requires sufficient light intensity. On the other hand, the activity of the movement decreases when the intensity becomes low. If diatoms encounter existing colonies or deposits of EPS on the substrate at low brightness, they adhere to the colonies because they cannot overcome the adhesion of the polysaccharides (process 3).

At very low light intensity or darkness, the free movement between the colonies of the diatoms comes to rest. The diatoms then excrete an EPS pad, which they use to adhere to the substrate. As will be explained later, EPS deposits abandoned by diatoms can also lead to the attachment of freely moving diatoms.

Day-night cycle

In the last video it was demonstrated how very intense lighting can lead to the disappearance of existing Cymbella colonies. At lower light intensities, as we use them for cultivation, the effect is not so dramatic, but it is clearly visible. You can expect this to happen in a normal day-night rhythm in a body of water.

An exemplary time-lapse video, which was recorded over a day-night cycle with a time-lapse factor of 6000, can be seen on the left.

The image was taken with a macroscope (objective with 50 mm focal length). The observed culture is located between other cultures without additional lighting. To enable pictures to be taken in the dark phase, the culture was illuminated with low intensity from below by a diffusing screen with a white LED. In the light phase there was about 200 lux (incident light), in the dark phase (transmitted light) 15 lux. Probably because of this remaining brightness the movement never comes to rest completely.

Development of a culture

Various long-term observations were carried out with different Cymbella species. Subsequently, the longest observation on a very similar looking species will be described in more detail, which however was only slightly longer than 100 µm. The observation lasted for 24 days, with one image being taken every 10 seconds. For most analyses such a short time interval is not necessary. However, it is useful in connection with the evaluation of the activity of diatoms moving between colonies.

Also in this observation there was about 200 lux in the light phase and 15 lux in the dark phase. In addition, there was a window with fluctuating incidence of light. The light phase lasted 12.5 hours. The area visible in the pictures is 8.27 mm x 6.21 mm. As the culture was in a 50 mm diameter petri dish, only about 2.6% of the cultivated area was observed.

Particularly in the first few days after inoculation, the relative number of diatoms in the observation area fluctuates strongly, as the leaving and entering of individual diatoms has a significant effect. With the increasing number of moving diatoms and the formation of colonies, which started after about 5 days, the relative fluctuations decreased.

The video on the left shows a sequence of images of the culture in which it was recorded at a daily interval from the third day onwards. The status at the end of the observation is intentionally displayed longer. The images were converted into greyscale images and the differences in brightness due to varying incidence of light were corrected. The pictures were taken at 8 p. m., shortly before the dark phase. With this species and a light intensity of only 200 lux in the light phase, the activity of the diatoms between the colonies already decreased considerably before the beginning of the dark phase. Therefore, only a few diatoms are found outside the colonies.

A video with good temporal resolution and low compression cannot be played here because of its size. The following video was recorded with a time lapse factor of 40,000 and high compression.

Colony formation

A new colony can develop when a single diatom attaches itself to the substrate. It creates an adhesive area where other diatoms can get stuck. In addition, a colony typically grows by asexual reproduction. As has already been mentioned, diatoms attached to the substrate leave behind hardly soluble EPS, which can bind diatoms again. This can be seen by following the development of a colony, starting with observation before a stable colony is established. There is usually a small area in which diatoms are repeatedly attaching and releasing.

The video on the left shows in detail the formation of a colony. Pay particular attention to what is happening inside the white circle. Repeatedly diatoms attach themselves, divide or leave the area in light phase. Diatoms which get onto the already once populated areas adhere there preferably. After a few days, the population is so large that a permanent colony is created.

Not every abandoned EPS deposit is used for buildup again. If no diatom meets the stain for several days, its adhesive power decreases. I do not know whether it dissolves or is degraded by bacteria. In some cases EPS can be seen as a grey spot in the dark phase.

Natural environment

The described observations were carried out in a petri dish. Here, the diatoms inevitably form their colonies in a plane. Under favourable conditions, they can probably also develop flat colonies in a natural habitat such as a leaf surface or a rock. In a three-dimensional fibrous netting, on the other hand, heaps or spherical

accumulations would be more likely to form. Diatoms escaping from dense populations must then move along thin filaments. The question arises as to whether the movement and exchange of diatoms between colonies is possible in such epiphytic situation.

In a raw culture with all kinds of fibres, such an accumulation developed by chance. As it was in the middle of a deeper mesh, the recording was difficult. The disturbance of the water surface also made it more difficult to take pictures. On the left you can see the colony with some diatoms in the near surroundings, which move more or less ‘skilfully’. There were several colonies in the braid, but at such a distance from each other that an exchange of diatoms seemed unlikely.

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