How do the diatoms get to the water surface?
• Why do the diatoms not sink immediately and how long do the diatoms remain on the water surface?
• Can the diatoms survive on the water surface for a long time?
• Which movement patterns and interactions between the diatoms occur and how can they be explained?
• Do diatoms have an advantage from their ability to float and to move on the water surface?
It is anticipated that there will probably be no valid answers for all species, because the differences are considerable, especially with respect to their motility. The best observations to be interpreted are available in Nitzschia sigmoidea. For this reason, these are to be presented first. This is followed by comments on Pinnularia.
The cultivated Nitzschia sigmoidea came from various waters, a small reservoir (Aichstruter Stausee), its outflow (Lein) and a pond (in Stuttgart-Hohenheim). In a newly created culture, the phenomenon of floating diatoms is apparent after about 2 to 3 weeks, whereby the first isolated floating diatoms can often be found after only one week. In a fully developed culture, connected structures can form that cover a large part of the water surface. There are cultures in which the number of diatoms on the water surface is far greater than on the substrate. The diatoms are not located in a flake connected by EPS, which gets buoyancy by oxygen bubbles, something that occasionally occurs in this species at very high population density, but lie horizontally on the water surface. Whereas this species can occasionally be found in valve view when looking vertically at a solid substrate, on the water surface it is always found in girdle band view, apparently the equilibrium position. A possibility to rotate on the water surface around the apical axis does not seem to exist.
According to current knowledge, a buoyancy of the diatoms due to a lower specific weight than water can be excluded. The cultures were repeatedly carried a few meters to the microscopes for examination. In many cultures, one could observe that diatoms were whirled up and sedimented slowly. Apparently, even small water flows cause Nitzschia sigmoidea to detach from the substrate and to accumulate in the water. It can be assumed that diatoms also reach the water surface. As the diatoms obviously do not sink again, but show a remarkable floatability, they accumulate on the water surface. Further observations are required to substantiate this mechanism.
In the phase contrast, pronounced brightening is visible on most apices. Significant changes in brightness also occur in differential interference contrast (DIC) or differential interference contrast for plastic receptacles (PlasDIC). Figure 1.22 shows a typical appearance. Under the stereomicroscope you can see with an oblique view that the water surface is arched around the apices. There is a more or less distinct convex meniscus, which explains the appearance in phase contrast, DIC or PlasDIC. The diatom lies deep in the water (Figure 1.23).
The ends of the diatom have obviously hydrophobic properties, which lead to the deformation of the water surface and give it buoyancy due to the surface tension of the water, as is known from the water strider. In Figure 1.24 a diatom is sketched from a horizontal view. Wang et al. [1.33] have found that cleaned valves of Coscinodiscus sp. float on water surfaces. Here too, hydrophobicity is the cause. After examination of cleaned valves, the authors conclude that the hydrophobicity is based on the convex form and 40 nm sieve pores. I consider it an open question whether this explanation applies to floating living Nitzschia sigmoidea. Apart from the other structure of the valve, it can be assumed that living diatoms are surrounded by a layer of organic material. This could prevent the influence of the pores on one side and on the other side it could have hydrophobic properties itself. In this context, it should be mentioned that the cell lines of Nitzschia sigmoidea lost the ability to float after a few months and never regained it. At first the typical patterns of connected diatoms on the water surface became less regular and finally disappeared completely. The floating diatoms did not always lie in the same plane but frequently crossed, and occasionally they hung with only one end on the surface of the water. Later, the proportion of diatoms on the water surface decreased noticeably. An explanation of the origin of hydrophobia should also clarify this.
Figure 1.22 Nitzschia sigmoidea on the water surface viewed with PlasDIC.
Figure 1.23 Nitzschia sigmoidea with a stereomicroscope in oblique view.
Figure 1.24 Sketch of a Nitzschia sigmoidea on the water surface seen from the horizontal direction.
Nitzschia sigmoidea can survive many days on the water surface. This is probably made possible by the high proportion of wetted surface. The rapid increase of diatoms on the water surface and the sometimes high density of floating diatoms compared to benthic living diatoms suggest that they reproduce asexually on the water surface.
There is an attractive interaction between hydrophobic bodies on the water surface. When floating hydrophobic bodies move towards each other due to this force, energy is released into the environment. The system strives for a state of minimal energy. Lycopodium spores scattered on a water surface, for example, bond together and form two-dimensional structures with local order. In this process, restructuring takes place in which contacts are broken up and are closed at other locations until a local minimum of energy and thus a stable equilibrium is reached. The global minimum will only be reached in the case of very few particles. Wang et al. [1.33] report on the formation of regular structures at Coscinodiscus sp. on the surface of a water droplet. This self-assembled monolayer is a consequence of the short-range attractive interaction between the hydrophobic frustules.
In Nitzschia sigmoidea, the attractive force acts at the ends of the diatoms. Figure 1.25 shows two diatoms in the valvar plain with the resulting water surface. The water surface has a lower energy than with two separate diatoms according to Figure 1.24. When several diatoms drift with hydrophobic apices on the water surface, patterns are formed in which the ends preferably stick together. This results in locally star-shaped and polygonal structures. Not all diatoms have a pronounced hydrophobic apex. It is enough to stay on the water surface, but the attractive interaction is barely recognizable in the movement. There are systematic differences in the strength of the hydrophobicity in diatoms from different localities that affect the patterns. A low interaction leads to loose arrangements (Figure 1.22), a strong interaction to patterns where the apices are close together (Figure 1.26). It will probably not be possible to give an analytical representation of the attractive force. A simplified modeling consists in the replacement of the diatom by two rotationally symmetric hydrophobic particles, which are connected by a rod having the length of the diatom. For the experimental determination of the attractive interaction one can examine the motion of diatoms, which move toward each other, or the movement of a hydrophobic particle like spores of Lycopodium in the vicinity of a diatom.