Movements Other Than Swimming

In some algae movement cannot occur unless the cells are in contact with a solid substratum. This kind of movement, in some cases termed gliding, is present in cyanobacteria, in the red alga Porphyridium (Rhodophyta), in diatoms, and in some desmids (Chlorophyta).

The most efficient gliders among the cyanobacteria are found in the filamentous forms such as Oscillatoria, Spirulina, Phormidium, and Anabaena, which can travel at up to 10 µm sec^-1. Some species, such as Phormidium uncinatum and Oscillatoria, rotate about their long axis while gliding; while others, such as Anabaena variabilis translate laterally. Other unicellular coccoid cyanobacteria, such as Synechocystis, move by “twitching,” a flagella-independent form of translocation over moist surfaces. This type of motility is analogous to social gliding motility (S-motility) in myxobacteria, which involves coordinated movements of cells close to each other (cell–cell interactions) and requires both Type IV pili operating in a manner similar to a grappling hook and fibrils (extracellular matrix material consisting of polysaccharides and protein).

While moving, cyanobacterial gliders secrete mucilage, or slime, which plays an active role in gliding. Mucilage is extruded from rows of fine pores clustered circumferentially around the septa. These pores are part of a larger structure called the junctional pore complex (JPC), which span the entire cell wall, peptoglycan layer, and outer membrane. The channels formed by the JPCs are inclined relative to the cell axis, this angle providing directionality to the extruded slime, and are oppositely directed on either side of the septum. Propulsion of the filament results from the adherence of the slime to both the filament surface and the substratum, combined with its extrusion from a row of JPCs on one side of each septum. Switching slime extrusion to the JPCs on the other side of the septum would result in a reversal of the direction of gliding. In P. uncinatum the pores are aligned in a single row, whereas in A. variabilis several rows of pores line both sides of the septum. The outer surface of gliding cyanobacteria consists of parallelly arranged fibrils of a glycoprotein known as oscillin, a Ca-binding protein required for motility. The surface striations formed by these fibrils would act as channels for the extruded slime to flow along. Therefore, if the fibrils are helically arranged, the cell will rotate as it glides; if the fibrils are aligned radially, the cell will not rotate. In all species studied to date, this correlation is consistent, and provides a structural explanation for why some species rotate as they glide while others do not.

In diatoms, motility is restricted to pennate species possessing a raphe. These diatoms display a characteristic jerky movement forward or backward, with specie-specific path patterns. The general velocity of their movement is 1–25 µm sec^-1, but they can accelerate up to 100–200 µm sec^-1. Raphid diatoms possess an actin-based cytoskeletal system located just beneath the plasma membrane at the raphe. Transmembrane components with an adhesive extracellular domain are connected to these actin bundles, and their interaction is somehow involved in both adhesion and motility mechanisms. Microtubules are also present in this region; in addition secretory vesicles containing polysaccharides often appear near the actin filaments at the raphe, providing the mucilage strands that project from the raphe and adhere to the substratum during the gliding process.

At least two models exist which provide reasonable explanation for diatom locomotion. In the first model, a force applied to the transmembrane protein-actin connectors, parallel to the actin bundles, would result in the movement of trasmembrane proteins through the cell and subsequent movement of the cell in the opposite direction to the force.

In the second model, the energy required for motility would be generated by a conformational change of the adhesive mucilage on hydration that occurs when it is secreted from the raphe. In this model, the actin bundles restrict the secretion of mucilage to one end of the raphe, which generates a net force moving the cell over the site of secretion. In both models, the secreted mucilage plays a central role either by providing traction to translate the force into cell movement or by generating the energy through conformational changes on hydration. A slow gliding movement over solid substrata has been observed in Porphyridium sp. (Rhodophyta) and in some desmids (Chlorophyta). In Porphyridium, the mucilage produced in mucilage sacs located inside the cell is excreted through the membrane. In desmids mucilage is excreted through the cell wall by flask-shaped pores. As they move, these gliding cells leave behind a fibrillar mucilaginous trail, whose swelling by water pushes the cells forward.

Table 2.1 presents swimming and gliding speeds of some planktonic algae.