Notes on flake formation and movement in flakes
The observations on Pinnularia in a biofilm have shown, among other things, that benthic diatoms move in a suitable environment in a three-dimensional space. However, the diatoms are often located on the surface of the
substrate or of the biofilm. Motile diatoms in cultures necessarily no longer move in one plane but within a more or less thick layer when the population density becomes so high that they cannot all be in contact with the substrate at the same time. The video on the left shows Cymatopleura solea in such a culture (4x time-lapse). The focus of the objective lies in the plane of the bottom of the Petri dish. The diatoms move in mutual contact with each other, but do not form a homogeneous spatial pattern. Hydrated extracellular polymeric substances (EPS) are probably the reason why the diatoms lump together locally.
In some genera the diatoms form flakes even at lower population densities. The diatoms are bound in a more or less pronounced matrix of mucilage. In cultures of a single species and low bacterial density, this matrix is formed by these diatom species alone.
At first we will consider Cymatopleura elliptica. The video below left shows a culture in 80x time-lapse in which many diatoms move independently. However, it can be recognized that the diatoms adhere to each other locally and temporarily in certain areas. To the right of it is a video with flakes of this species in different sizes (300x time-lapse).
The diatoms apparently stick together. However, an EPS film between the diatoms is so thin at the chosen magnifications that it is not visible. The possibilities of movement are strongly limited by the connection to neighbouring diatoms. Often one recognizes rotary movements around the contact points. The mechanical coupling of several diatoms leads to complex motion sequences.
Nitzschia sigmoidea also forms flakes, as shown in the video below left (dark field, 40x time-lapse). A small flake at higher magnification can be seen to the right (dark field, 4x time lapse).
In dark field, the connecting mucilage is clearly visible at this magnification. It completely fills the space between the diatoms. Therefore, the movement of the diatoms in an EPS environment without direct contact between the diatoms plays an important role.
Oxygen produced by photosynthesis can sometimes accumulate within the EPS matrix into macroscopic bubbles. Their buoyancy is able to transport the flake to the water surface, as can be seen in the video on the left (see also post on diatoms on the water surface). Such flakes represent a biofilm that is not or not generally connected to a substrate.
The tendency to the formation of such flakes increased in both species with the age of the culture strains. Often the diatoms do not move far apart after a vegetative division, but remain more or less loosely connected by EPS.