Nitzschia sigmoidea on the water surface

In 2015 we cultivated Nitzschia sigmoidea from the Aichstruter Stausee (reservoir) and the creek Lein, its outflow. In autumn 2016 I was able to cultivate very large specimens (360 μm) from a pond near Stuttgart-Hohenheim.

It turned out that densely populated cultures had many diatoms that floated on the water surface. The movement patterns of these diatoms and the observable properties appeared so remarkable that they are the real reason for the post about floating diatoms.

The floating Nitzschia sigmoidea are visible to the naked eye due to their size. The observation of the movement can therefore be performed well with a stereomicroscope. For details an inverse microscope with phase contrast or DIC is appropriate. Observation is often hindered by convection in the petri dish which is caused by the illumination. It is very helpful to insert a cylindrical ring to reduce the convection. On the top left is a time-lapse video recording of floating diatoms from the Aichstruter Stausee which was recorded in dark field with a stereomicroscope. For comparison the movement on substrate is shown in the top right. For readers with a closer interest in the movement sequences, some more videos are compiled on a separate page.

In a newly prepared culture the phenomenon of floating diatoms is evident after about two to three weeks, the first floating isolated diatoms often being found after about a week. In a fully developed culture, contiguous structures can be formed that cover a large part of the water surface. There are cultures in which the number of diatoms on the water surface is several times as high as that on the substrate.

While in the case of a vertical view onto the substrate this species occasionally can be seen in valvar view, it is never found on the surface of the water in this position, but only in the belt view. This is for a floating Nitzschia sigmoidea the position of equilibrium. On the surface of the water a possibility to rotate around the apical axis does not seem to exist.

The same questions arise as have already been discussed in the context of the floating Cymatopleura solea.

How do the diatoms come to the water surface?

Apparently, even small water currents lead to the detachment of Nitzschia sigmoidea from the substrate. There is some evidence that carrying the culture to the microscope is essential for the transport of the diatoms to the surface. As the diatoms obviously do not sink again, they accumulate on the surface of the water. In order to check whether the phenomenon is sufficiently explained, one would have to make experiments with stationary cultures.

Can diatoms survive on the water surface for a long time?

Nitzschia sigmoidea can apparently survive many days on the surface of the water and even reproduce asexually. The sometimes small number of benthic diatoms would not explain the rapid increase of diatoms on the surface of the water. In addition, a large proportion of diatoms in the process of cell division can be seen on some video recordings. I would not like to claim that the surface of the water is actually a habitat in nature.

Why does diatoms of species Nitzschia sigmoidea do not sink to the bottom?

In the phase contrast pronounced brightening is visible on most apices.

Significant changes in brightness also occur in differential interference contrast (DIC) which can be seen in the picture on the left (click to enlarge). This indicates phase objects which are based on different optical path lengths. Under the stereomicroscope one can see with an oblique view of the water surface that the surface is arched around the apices. There is a more or less pronounced convex meniscus which explains the appearance in phase contrast and DIC. The diatom lies deep in the water. This is not easy to be photographed. With some effort pictures are taken with the help of a mirror, which is inserted into the petri dish of the culture. Its inclination is adjusted so that the water surface is observed at a small angle. The equipment is shown on the left. Below you can see a picture which has been taken with the equipment.

In the following figure a diatom from a horizontal view is sketched (valvar plane). The ends of the diatoms obviously have hydrophobic properties which lead to the deformation of the water surface and give it a buoyancy, as is known from the water strider.

Wang Y et. al. (2012) have found that valves of Coscinodiscus sp. float on water surfaces. However, the ability of living diatoms to float is not reported. 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. The structures of the valves differ. Moreover, living diatoms have an EPS cell coating which could prevent the hydrophobic effect of sieve pores. I think it is quite possible that EPS, which is secreted at the apical pores, has hydrophobic properties. I cannot answer the question of the nature of the hydrophobia with the tools available to me. I would appreciate if this question would be taken up by visitors of the site.

In this context it should be noted that after a few months the cultures of Nitzschia sigmoidea lost the ability to float.  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 side by side but more and more frequently crossed, and occasionally they hung with only one end on the surface of the water. Later, the proportion of diatoms on the surface of the water diminished. At the moment I can only speculate about the cause.

I am just watching this weakening hydrophobia again. The video on the left (4x time lapse) shows a culture in which the phenomenon is already clearly visible.

Attractive forces between diatoms on the water surface

When looking at the patterns that form floating Nitzschia sigmoidea, one can see that the apices of the diatoms very often lie next to each other. This leads often to parallel diatoms, but also to star-shaped, linearly concatenated and polygonal patterns. Even dead diatoms can be part of these structures. Also Wang Y et al. (2012) report on the formation of regular structures in Coscinodiscus. This is a consequence of the hydrophobic properties. If floating hydrophobic bodies accumulate, energy is released to the environment. The system prefers the state of minimal energy. This phenomenon of spreading has been known for a long time. Wang Y et al. (2012) call this effect ‘self-assembly’.

One can observe such a self-assembling well by scattering Lycopodium powder on a surface of water. You can see this in the video on the left (4x time lapse). It is remarkable that sometimes existing contacts between particles are broken. In total, energy is reduced in the multi-particle system, for example by establishment of other connections at different locations. To explain the processes one must consider the many-particle system.

Between hydrophobic bodies there is an attractive interaction at the water surface. Specifically, in this case there is an attractive force between the ends of the diatoms. A similar behavior would result if the ends of the diatoms were hydrophilic, because there is also attraction between hydrophilic floating structures. However, there is a repulsive force between hydrophobic and hydrophilic bodies. The following drawing shows two diatoms in the valvar plane with the resulting water surface.

The water surface has a lower energy than in the case of two separate diatoms.

Regarding the form of the diatoms an analytical representation of the law of force can certainly not be given. 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. Unfortunately, even in this approximation no analytical description of force is known to me.

For the experimental determination of the attractive interaction one can examine the movement of diatoms, which move toward each other or the movement of a hydrophobic particle like spores of Lycopodium in the vicinity of a diatom. Under the plausible assumption that the inertia force can be neglected in the equation of motion, and Stokes' law holds, the velocity is proportional to the force. I have not systematically carried this out. In the picture on the left the velocity of a diatom which is proportional to the force is plotted against the distance of two approaching diatoms. This is an exemplary result in which it is not ensured that the water surface is sufficiently calm. In addition, the temporal resolution in the vicinity of the collision is not sufficient. The picture is only intended to illustrate the principle of the procedure.

As mentioned at the beginning, 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 also systematic differences between the strength of the hydrophobicity of the diatoms from different localities. This affects the accuracy of the patterns of movement.

Wang Y, Pan J, Cai J, Zhang D (2012), Floating assembly of diatom Coscinodiscus sp. microshells. Biochem Biophys Res Commun 420(1):1–5

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