Page 107 - Algae Anatomy, Biochemistry, and Biotechnology

90 Algae: Anatomy, Biochemistry, and Biotechnology

FIGURE 2.64 Swimming pattern of isokont biflagellate algae (Dunaliella salina).

displacement. This is because a helical swimming path enables the detection of three-dimensional
component of a gradient, whereas the straight path allows detection of only one dimension.
Purcell (1977) summarized it by saying that the organism does not move like a cow that is
grazing on pasture, it moves to find greener pasture.
Only the species that swim very fast such as the dinoflagellates (about 500 mm sec 21 ) can over-
come the diffusion limitation. This high velocity should be related to the effective increase in the
probability to catch more preys and therefore to the heterotrophy metabolism of the algal species.

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 mm sec 21 .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 cyanobac-
teria, 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 myxo-
bacteria, 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 (extra-
cellular 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