Colonies in Petri dishes are growing on a flat surface. On leaf surfaces or stones with unevenness which is smaller than the size of the diatoms, similar conditions could exist. In three-dimensional fibrous nettings, heaps or spherical colonies would be more likely to form. Diatoms escaping from dense populations must then move along thin filaments. Such netting was found in a sample from a pond where nutrient solution was added. A migration from and to colonies over the fibers could be observed. In this topology, a much lower fluctuation and less exchange between colonies are to be expected, as the colonies only offer possibilities for moving in and out at a few points.
1.7 Conclusion
The present study consolidates observations on motile pennate diatoms in different environments. With respect to the movement on a smooth solid substrate, it is demonstrated that the inclusion of the orientation of the apical axis as a degree of freedom is useful in the analysis of trajectories, as it provides information about the point of the valve where the diatom touches the substrate. This point is subject to statistical fluctuations and is at different positions in Navicula sp. and Craticula cuspidata. Methodically independent observations confirm these results and allow investigating the question of the processes of motion reversal.
In and on biofilms, pennate diatoms move with significantly different motion patterns and speeds. Adhesion to the surface of the viscoelastic film is sufficient to enable movement similar to that on a solid substrate. When the activity of the raphe branches is opposite, distortions of a biofilm marked with particles can be observed. Diatoms inside the biofilm without contact to the substrate rotate around changing axes.
At the water-air interface, Pinnularia gentilis can perform active movements without coupling to a substrate, which is interpreted as the interaction between raphe activity and the surrounding water. Significantly different phenomena occur in Nitzschia sigmoidea. Hydrophobia of unknown origin at the apices gives this species buoyancy and leads to the formation of connected dynamic patterns. A deeper understanding of hydrophobia and dynamic pattern formation requires further investigation.
The formation of colonies in Cymbella lanceolata can be described on the basis of elementary processes and their light dependence. They enable the exchange of diatoms between colonies, whose frequency depends on the topology of the environment.
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