Cymatopleura solea on the water surface

On the water surface of densely populated cultures occasionally float diatoms, which are in mutual mechanical contact and show a relative movement. Surprisingly, there also is the case that diatoms can move on or near the water surface without having such a contact. In the 4x time-lapse video on the left you see Cymatopleura solea with a length of approximately 180 ┬Ám drifting with the water which moves relative to the observer. In addition to this drifting movement of the water, locomotion of the diatoms relative to the surrounding water can be recognized. This also applies to diatoms which are not in mechanical contact with adjacent diatoms. These questions are particularly relevant:

  • How do the diatoms come to the water surface?
  • Why do the diatoms not immediately sink again, but remain for a long time on the water surface?
  • Can diatoms survive on the water surface for a long time?
  • Why can they move near and on the water surface?
  • What is the difference between the motion on the water surface and the motion on the substrate?
  • Do diatoms have an advantage from the ability to float and to move on the water surface?

This post about Cymatopleura solea on the water surface is very speculative, because I cannot answer most of these questions. I will make some assumptions for discussion. In the subsequent contribution to Nitzschia sigmoidea the situation is clearer.

How do the diatoms come to the water surface?

Diatoms have a higher density than water. They can therefore not reach the water surface by buoyancy. An active lifting from the substrate and directed swimming through the water body is not possible.

If a diatom is not attached to the substrate with strong adhesive forces, a slight water flow is sufficient for some species (e.g., Nitzschia sigmoidea) to detach them from the ground. Then they remain in the water body (tychoplankton) for a period of time and can also be transported to the surface. Obviously, it is sometimes sufficient to carry the petri dish to the microscope. One can check this by swirling the water in the culture with a rod. After the flow has decreased additional floating diatoms often occur. In Cymatopleura solea, however, the success of the procedure was not convincing. Only occasionally some diatoms were found on the surface.

In densely populated cultures groups or flakes of diatoms adhering to each other by extracellular polymeric substances (EPS) can often be observed. Occasionally gas bubbles are included, which may be composed of oxygen. Such flakes may have sufficient buoyancy to bring the flake to the water surface. This situation can be seen in the video on the left (4x time lapse), but with the example of Nitzschia spec. In the case of the video of the Cymatopleura solea neither EPS nor gas bubbles can be recognized.

The question therefore arises whether these two approaches are sufficient.

Why do the diatoms not immediately sink again, but remain for a long time on the water surface?

Staying on the water surface can take a very different time. Some diatoms of the species Cymatopleura solea sink relative quickly, others can be found on the surface even after a few days. In the case of floating Nitzschia sigmoidea a permanent stay on the water surface is possible.

As diatoms have a greater specific gravity than water, a longer presence on the surface requires an explanation. Extracellular polymeric substances with included gas have already been mentioned as an option for transportation to the surface. This would also explain whereabouts. In the case of the video of Cymatopleura solea this possibility is obviously to be excluded.

It is striking that diatoms on the surface of the water often accumulate. If this is caused by hydrophobic properties, it would explain swimming. This can be proved for Nitzschia sigmoidea.

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

Even Diatoms of the species Cymatopleura solea that float flatly on the water surface seem to be surrounded by water to survive for several days. After a few days, however, the proportion of dead diatoms is significantly higher than any of the diatoms on substrate. The water surface is not a habitat of Cymatopleura solea.

Why can diatoms move near and on the water surface?

As our cultures are not axenic one observes on the water surface bacteria when using DIC or phase contrast. Mostly, they are scattered or form small contiguous colonies. With their low mass compared to the diatoms and their small size, they have no noticeable influence on the movement behavior. However, if they form a closed layer, they could form a kind of substrate on the water surface. In principle, I think this is quite possible, but not in the case of clean cultures like the culture shown in the video of Cymatopleura solea.

A movement within a floating flake of EPS is possible, as one can see in the above video of the Nitzschia spec. This corresponds to the discussed movement within a biofilm. In the video Cymatopleura solea seems to be mobile without this EPS.

It would also be conceivable that the individual diatoms carry lumps of EPS with a sufficiently large mass and volume on their valves and transport them along their raphes. Then one would expect, however that the diatoms cannot touch easily. In the video there are actively moving diatoms, which are colliding with other diatoms. There is no evidence of such lumps. Therefore, this does not seem to be a convincing explanation for the movement.

Finally, one should consider whether Diatoms of the species Cymatopleura solea have a sufficient interaction with water even without a substrate. A model which could explain this is not known to me. It may be important that Cymatopleura solea has a canal raphe.

What is the difference between the motion on the water surface and the motion on the substrate?

In the case of movement on the bottom, a substrate exists which is fixed from the observer's viewpoint. Except for back and forth movements caused by lumps of EPS, there is a contact between the raphe and the substrate. This gives the diatom a mechanical boundary condition and thus a reduction of the degrees of freedom. In addition, movement usually follows the orientation of the raphe.

These external forces, which limit the movement, do not exist in the situation of single floating diatoms. They show rotational movements in all spatial directions and short back and forth moves. Longer straight paths apparently do not occur. This speaks is in principle for the model of a transport of transparent EPS lumps along the raphe.

Do diatoms have an advantage from the ability to float and to move on the water surface?

I doubt this and consider drifting on the surface of the water as an artifact. At best, it would be helpful to spread the species by drifting (hydrochory). Without indications to the occurrence of the phenomenon in nature this is pure speculation.

Locations of diatoms in cultures

In the context of the previous page I would like to summarize the places where diatoms live in our cultures.

Locations of diatoms in cultures

Diatoms in the body of water can in small culture vessels only be observed for a short time after the water has been whirled up. At the small height of the petri dishes, sedimentation occurs rapidly. Occasionally diatoms that have been swirled up remain attached to the water surface for a long time. If diatoms form sufficient thick biofilms, a distinction between diatoms within or on the biofilm can be made.

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