1.15 On the left (a) the superimposition of images of a Cymbella rotating around a point near the helictoglossa is shown, on the right (b) a sketch of the diatom with raphe.
Figure 1.16 Surirella biseriata in valve view.
Figure 1.17 Trajectory of a Surirella biseriata. The driving side changes at the reversal points.
Figure 1.18 Paths of Surirella biseriata in a culture. They were visualized by overlaying frames of a video.
1.4 Movement of Diatoms in and on Biofilms
Diatoms often live in biofilms and contribute to their formation by secreting EPS. Such biofilms, consisting of numerous cell types enmeshed within a secreted polysaccharide matrix [1.9], often form their own complex ecosystems and are important contributors to biofouling [1.22] [1.1]. While most descriptions of diatom motility have referred to movement on solid substrates, Consalvey et al. [1.8] suggest that diatoms move in a biofilm during the vertical migration in the mudflats.
The appearance of biofilms was observed in several batch cultures. In contrast to biofilms in nature, these biofilms are only colonized by one diatom species and bacteria (no axenic cultures). Such biofilms are not generally suitable for making statements about biocenosis. They allow, however, studying the possible movements of pennate diatoms in such a film. To accomplish this, observations were made on cultures in Petri dishes in which the focal plane was chosen between the plane of the substrate and the surface of the biofilm. With the focal plane fixed, time-lapse recordings were made and analyzed.
A biofilm of Pinnularia viridiformis (Figure 1.19) adhering stably to the substrate was used as a model system to investigate the movement. In the 30-day-old culture the thickness of the biofilm ranged from about 150 μm to 380 μm. Diatoms were found directly on the substrate, on the surface of the biofilm and occasionally in between (Figure 1.20). In most cases, the surface of the biofilm was more densely populated than the substrate. On both surfaces, the diatoms showed motility, whereby the diatoms, which are in contact with the substrate, have a significantly lower speed, often move jerkily, perform pivoting movements and frequently stand vertically, which they do not do in free water. On the substrate, the diatoms lie predominantly on their girdle bands. On the surface of the biofilm you can see the diatoms in valve view and girdle band view and you can find the movement patterns that are typical for motion on a substrate, but the movement occurs somewhat more erratically and slower. In girdle band view there are only back and forth movements [1.18]. Lumps of trail substance enable the transmission of force from the raphe to the biofilm. The relatively rare diatoms between the bottom of the Petri dish and the surface of the film often rotate around changing axes.
Figure 1.19 Pinnularia viridiformis with a length of approx. 90 μm.
Figure 1.20 Places within and on a biofilm where Pinnularia viridiformis can be found. The typical movement patterns are indicated by arrows. Shunting movements are marked with short arrows at both apices.
The biofilm exhibits viscoelasticity, i.e., it is neither purely viscous nor purely elastic, which is typical for biofilms [1.13]. The viscous property of the gel allows the movement of the diatoms at the bottom of the Petri dish and within the biofilm. Because of the elasticity the diatoms are able to stay on the surface without sinking to the bottom of the Petri dish. From the observations, no statement can be made as to whether diatoms from the interior of the biofilm can reach its surface. At slight vibrations of the culture, the gel gets into a horizontal oscillation, which can be recognized by the co-swinging diatoms. A description of the movement of Pinnularia within the biofilm must take into account the viscoelastic properties, because elastic forces occur which do not exist in Newtonian, purely viscous liquids. To describe movement in this environment, Edgar’s [1.11] analysis of movement in biofilms would have to be extended.
These observations do not provide any information about the structure of the biofilm. Lauterborn [1.21] already reported in 1896 that Pinnularia maior extrudes gelatinous filaments during movement. The elasticity of these structures, which often contract into lumps, can easily be seen in the ink coloring used by Lauterborn. Presumably these gelatinous formations stick together to form a biofilm that is smooth at the top.
The surface of the transparent biofilm is not visible under the light microscope. When small particles are placed from above, they remain on the gel and mark the surface of the biofilm to the water. This can be used to observe the activity of the raphes, because due to the small modulus of elasticity of the film, the diatoms cause easily observable elastic deformations. Mineral particles with a linear expansion of typically less than 3 μm were distributed on the gel layer. Diatoms with uniform gliding motion do not cause local tensions on the surface of the gel. Frequently, however, the movement of the diatoms stops and it can be seen that the gel around the diatom is compressed (negative strain) towards the proximal raphe ends. As long as the opposite activity of the raphe continues, this elastic deformation in the direction of the apical axis persists. Figure 1.21 shows a superimposition of the frames of a video during the standstill of the diatom. The deformation has a visible effect at a distance of several diatom lengths. After a while the raphes no longer work against each other and the movement is continued in the same or opposite direction. The biofilm returns to its equilibrium position within a short time, which is noticeable in the movement of the gel surface in some distance from the diatom. An elastic elongation (tensile strain) resulting from simultaneous activity towards the distal raphe ends is not observed. The question remains whether the opposing activity of the raphe branches provides an advantage or whether there is a lack of coordination.
Figure 1.21 Superimposed frames of a video during the standstill of a diatom. A tension has built up in the biofilm.
The investigation of the elastic deformations of the biofilm by marking can therefore serve the qualitative study of the relationship between movement and activity of the raphes, since both can be observed simultaneously. The method provides the possibility of quantitatively determining the forces exerted on the substrate by the raphe. For this purpose the elasticity module and the displacement of the particles have to be determined. Furthermore, a model is to be developed which
