Colony formation in Asterionella formosa
Diatoms of the planktonic species Asterionella formosa were found in the river Neckar (Germany 49°04'41.8 N 9°09'17.9 E). Since planktonic organisms typically do not occur in flowing waters, they probably originated by
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upstream barrages.As the name Asterionella indicates, they form star-shaped colonies. On the left side the picture gallery from the introduction is shown again.
For phytoplankton, it is usually important to have a sinking speed as low as possible. Because of the small size of organisms and colonies, the friction force is well described by Stoke's law (small Reynolds numbers). Thus, the sinking speed is proportional to the density difference of phytoplankton and surrounding water and reciprocally proportional to the form resistance, which depends on the geometry of the sinking body (Ostwald 1912). Star-shaped structures have a high form resistance compared to a sphere of the same volume. Judit Padisák et al. (*) have experimentally shown by experiments with differently shaped models that the shape resistance of Asterionella increases with the number of diatoms in the star up to 6 diatoms and remains approximately constant thereafter. Deviations from symmetry, i.e. unequal angles, decrease the shape resistance.
Stellate and zig-zag shaped colonies of different species may differ with respect to the concatenation of adjacent diatoms (connecting points), the angles between adjacent diatoms, and the lengths of the chains. The rules leading to pattern formation or morphogenesis at the colony level are correspondingly different. Asterionella formosa probably exhibits the simplest possible formation processes. This is due to the fact that the connection points of neighboring diatoms are always located at the same apex. Therefore, there can also be only one possibility of separation of neighboring diatoms. Star-shaped colonies always transform into star-shaped colonies when they divide. A synchronicity of the divisions is not necessary for this (no transitional forms). The following composite photograph illustrates this exemplarily. In the star-shaped colony on the left, a pair of diatoms is about to split into a V-shaped structure. If no external forces act, as in the case presented, this opening occurs in a short time compared to the generation time. In the middle image, the splitting has occurred (angle α > 0) and thus the angle β between the two diatoms at the ends of the chain decreases. In the following time, the angle α increases and β diminishes correspondingly.
For simplicity, it is assumed that no further division occurs during the process shown and that the other angles remain unchanged. The two halves then rotate against each other as a rigid entity. In the case shown, contact of the diatoms at the ends of the chain occurs at the apices in the center of the star (marked by short arrow). Unless the ends of the chain slide over each other, which is often observed, each further increase of α and each further division leads to mechanical stresses in the chain. This can cause the angles in the colony to change and eventually break the chain, creating two new colonies. Even if the ends slide over each other, further divisions will eventually cause diatoms to become hooked and the chain to break. Rotations of the partial chains in space can also be observed. Thus, the colonies do not always lie in the plane of the substrate. The size of the colonies is limited by the fractures that occur sooner or later. The breakage of colonies seems to be the result of external tensions as a rule, but spontaneous breakage at older joints cannot be excluded either.
From a considered diatom, the doubling of the number of diatoms per generation quickly results in longer sequences that break into pieces. Since the splittings are approximately synchronous over some generations, one often observes that after a longer phase without splittings (about one generation time) the splittings happen in rapid succession.
The asymptotically reached opening angles vary strongly, so that also smaller regular colonies with similar angles, as well as formations with more than 8 diatoms are found. Star-shaped colonies with 8 diatoms are frequently observed and are often shown in the literature.
Each split begins with a sudden opening of the neighboring diatoms to a V-shaped structure. Apparently, a mechanical tension built up in the EPS pads at the hinges, which exceeds the adhesion forces between newly formed diatoms. Thereafter, the increase in opening angle is uniform, with a decrease in angular velocity.
Without collision between parts of the same chain or with other objects, the final angle is reached asymptotically. Such behavior was also seen in all observed Diatoma species (D. tenuis, D. eherenbergii, D. vulgaris). In the video below, on the left, 1500-fold time-lapse views of the splitting of a chain initially consisting of four diatoms are shown. To the right, the opening angles of the four diatoms are shown as a function of time. The moment of splitting was set to the origin of the time axis in each case. There are clearly recognizable deviations between the functions..
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The angles taken at sudden opening are 17°, 23°, 17° and 19° in the order given.
A series of other observations of divisions can be seen in the following time-lapse videos (time lapse factors 1500 and 1600). The upper video on the left shows a larger part of a culture that is not yet very dense. The individual separation processes are better visible at a higher magnification.
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In summary, it should be noted that in A. formosa the formation of the star-shaped structure, which is advantageous for a low sinking speed, is achieved by a simple splitting process and a fracture mechanism due to mechanical stresses. From this point of view, A. formosa has a model character and is a good starting point for studies on other star-shaped and zigzag shaped colonies.
(*) Padisák, J., Soróczki-Pintér, É., and Rezner, Z. (2003) Sinking properties of some phytoplankton shapes and the relation of form resistance to morphological diversity of plankton—an experimental study. Hydrobiologia 500(1), 243-257.
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