Читать книгу Diatom Gliding Motility - Группа авторов - Страница 17

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 we will focus here. Influences by inhomogeneous illumination, chemical gradients, thermal gradients, flow, unevenness or other anisotropy of the substrate, such as inclination to the horizontal, cannot be completely excluded in the experiment, but were avoided where possible. Such controlled conditions can also be achieved over a longer observation period by the use of diatoms cultures. All observations described below were performed either directly using batch cultures in Petri dishes or after transfer of the diatoms from the culture into a suitable test vessel.

Figure 1.1 Drawing of a pennate diatom with two raphe branches on its valve.

The analysis of the trajectories of diatoms has long been of qualitative nature. This has changed with the use of video technology, manual or automatic tracking of motion and the use of computers to analyze movement data. Edgar [1.10] determined speeds and accelerations of diatoms and the speed of particles transported along the dorsal raphe of Nitzschia sigmoidea. A statistical analysis of the trajectory of Navicula sp. was published by Murase et al. [1.23] whereby the movement of the diatoms was confined by a micro chamber. Murguía et al. [1.24] performed a time series analysis of the Hurst exponent. The above-mentioned experiments on chemotaxis [1.5] [1.6] are based on the statistical evaluation of frames of video recordings.