Those species within the genus Pseudo-nitzschia are pennate diatoms with a raphe and therefore experience motility that araphid pennate diatoms may not. Centric diatoms experience rotational movement, and the flagellated sperm gametes of centric diatoms also have motile capacity. The movement within raphid diatoms has been hypothesized to occur through a variety of mechanisms: cytoplasmic streaming, protrusions of flagella through the raphe slits, and movement via secretions from the raphe (described below).
The Path of Movement
Pseudo-nitzschia moves smoothly for short amounts of time with reversals and complete halts punctuating its path. By using high speed cinematography, one can garner a greater appreciation for the movement of these diatoms. They are abrupt accelerations, decelerations, halts, and periods of constant velocity - definitely not what I'd call a "smooth ride"! Amongst raphid diatoms, there is a certain directionality that exists corresponding to the shape of the raphe. Since Pseudo-nitzschia has a curved raphe, these diatoms often follow a curved path (seen in third frame of the figure below). It should also be noted that since diatoms are so small, viscous forces play a large role in the movement of the diatom.
Figure: Straight Raphe, Sigmoid Raphe, & Curved Raphe
Mechanism of Movement
Movement is possible on a solid substratum for the raphid Pseudo-nitzschia diatoms. There seems to be a secretion of polysaccharide into the raphe along the entire slit, often leaving a trail in their wake. But how exactly could this mechanism of movement work? It is hypothesized that the force that is able to propel these diatoms can be derived from interactions among actin filaments and transmembrane structures (proteins). These transmembrane structures are thought to contribute to movement because their extracellular portion associates with a substratum while their intracellular portion has freedom of lateral motion within the cell (the raphe). More specifically, there is thought to be a transmembrane protein that is associated to a network of actin within the cell and to filaments of sticky mucopolysaccharide on the outside of the cell. The actin proteins serve to tug the proteins along the raphe, while the sticky mucopolysaccharides and motors (ATPases) pull in the opposite direction. As the proteins travel along the raphe and reach the central nodule, it is hypothesized that they detach from the mucopolysaccharides, allowing the mucopolysaccharides to attach to anther protein. When the mucopolysaccharides reach the end of the raphe, they detach from the transmembrane protein and leave a sticky trail on the substratum. The mucopolysaccharide material is found within cell vesicles called crystalloid bodies, but it is not known from where these polysaccharides are secreted into the raphe. It is thought that a single vesicle may secrete enough polysaccharide to travel half a cell length.
Figure: Movement along substratum (blue proteins, green mucopolysaccharides, black striated lines are the actin filament network, black circles indicate the central nodule)
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copyright Jennifer Shin 1999.