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1
Some Observations of Movements of Pennate Diatoms in Cultures and Their Possible Interpretation
Thomas Harbich
Independent Researcher, Am Brüdenrain, Weissach im Tal, Germany
Email: [email protected] Thomas Harbich: https://www.researchgate.net/profile/Thomas_Harbich, https://diatoms.de/en/
Abstract
This chapter presents observations on the motility of pennate diatoms with interpretations and hypotheses. They refer to movement in different environments, on a solid substrate, in a biofilm, on the water surface and in the context of colonies.
The trajectories of certain species on solid substrate are analyzed including the orientation of the apical axis and two hypotheses are made which establish a relationship between motion and the location of the contact point on the diatom to the substrate. The obtained location is validated by observing the movement with a viewing direction parallel to the substrate. Using this method, the author also investigates the change in location of the contact point during reversal of motion. It is furthermore shown that the analysis of the path curvature of certain species allows conclusions to be drawn about the location of the propulsion.
Observations of the movement of Pinnularia viridiformis in and on a biofilm confirm its viscoelasticity. It is shown that elastic deformations of the biofilm, made visible by marking with particles, reveal details of the activity of the raphe branches.
Investigations were carried out on cultivated diatoms which are found on the surface of the water. There, they show movements that partly deviate considerably from the movement patterns on substrate. In Nitzschia sigmoidea, hydrophobicity is assumed to be responsible for buoyancy and its role in structure formation is examined.
Finally, the movement patterns in connection with colony formation in Cymbella lanceolata are studied. Qualitative and quantitative methods are used to show how typical colonies develop on flat substrates due to changing movement activity during the day-night cycle and to the influence of the adhesion force of the gelatinous excretions.
As all observations are based on diatom cultures in a laboratory environment, the possibilities of occurrence of the observed phenomena in nature are discussed in the context of the respective observations.
Keywords: Movements, kinematics, contact point, biofilm, floatability, hydrophobicity, pattern formation, colony formation
1.1 Introduction
Diatoms form an ecologically important class of single-celled photosynthetic algae found in freshwater and seawater. They are characterized by a solid skeleton (frustule) consisting of amorphous hydrated silicon dioxide and organic substance. It is composed of two halves, the epitheca and the hypotheca. The structure is similar to a Petri dish, with the epitheca overlapping the hypotheca. Each theca consists of a more or less arched valve and the cingulum, a number of associated siliceous bands (girdle bands). According to their shape, diatoms are divided into centric diatoms that are radially symmetric and pennate diatoms that are bilaterally symmetric. Among the pennate diatoms there are many species which are able to glide over a substratum. An essential morphological feature of these motile diatoms is one or two distinct slits in the valve called raphe (Figure 1.1). Through the raphe, strands of mucilage are secreted, which enable the cells to adhere to the substrate and are involved in motility. The various theories on motility will not be discussed here. Reference is made to the paper by Edgar and Pickett-Heaps [1.12] as well as to a short compilation by Häder and Hoiczyk [1.17].
The paths of pennate diatoms having a raphe system on a solid smooth substrate like a microscopic slide are often almost circular, spiral, straight and occasionally sigmoid. These forms of movement are related to the form of the raphe system (Nultsch [1.25], Cohn [1.7], overview in Round et al. [1.31]). The gliding movement is interrupted by sudden back and forth movement and in many species also by complex motion sequences such as horizontal rotation around a point of the cell, vertical pivoting or pirouettes. A systematic survey of the movement patterns of 135 raphid diatom species from 35 genera was given by Bertrand [1.2] in 1992.
Movement can be advantageous for diatoms in many ways. Some diatoms, which inhabit sand deposits in intertidal zones, show a vertical migration [1.14]. As a consequence of wind, tides and currents, sediment is being constantly deposited, so that the diatoms have to migrate to the top of the sediment continuously to photosynthesize (review article in [1.20]). Diatoms use phototaxis for this purpose [1.19] and allegedly geotaxis [1.29] [1.30]. In addition, under certain lighting conditions diatoms show a phobotaxis, a kind of shock reaction to local light exposure with a high intensity gradient [1.27]. Diatoms reverse when they are partially illuminated, for example, when crossing a light-dark boundary. There were early indications of chemotaxis [1.25]. Recent studies show a movement towards a silicate source [1.5] and a pheromone-directed movement [1.15] [1.6]. Pennate diatoms, possessing a raphe system, also show motility under isotropic conditions, on which
