100
Algae
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 proteins). 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 spans 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 species-specific path patterns. The general
velocity of their movement is 1–25 μm s −1 , but they can accelerate up to 100–200 μm s −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 that 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 movement of the transmembrane proteins through the cell and the subsequent movement of the cell in the direction opposite 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.
Buoyancy Control
The alternative to swimming is to float by means of some types of buoyancy device. In some of
the attached brown algae of the seashore (Fucus vesicolosus, Ascophyllum nodosum, Sargassum
sp., Ochrophyta), the fronds gain buoyancy from air bladders (pneumatocysts) within the thallus,
which stands erect when submerged. Oxygen and nitrogen, in roughly the same proportion as in air,
Algae
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 proteins). 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 spans 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 species-specific path patterns. The general
velocity of their movement is 1–25 μm s −1 , but they can accelerate up to 100–200 μm s −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 that 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 movement of the transmembrane proteins through the cell and the subsequent movement of the cell in the direction opposite 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.
Buoyancy Control
The alternative to swimming is to float by means of some types of buoyancy device. In some of
the attached brown algae of the seashore (Fucus vesicolosus, Ascophyllum nodosum, Sargassum
sp., Ochrophyta), the fronds gain buoyancy from air bladders (pneumatocysts) within the thallus,
which stands erect when submerged. Oxygen and nitrogen, in roughly the same proportion as in air,