Page 110 - Algae Anatomy, Biochemistry, and Biotechnology

Anatomy 93

density of the cell sap in the vacuoles is less than that of seawater and the cells can therefore be
positively buoyant and float.
When buoyancy control is not possible by these mechanisms, algae can keep afloat and regulate
their orientation and depth through adaptations reducing sinking rates. The rate at which a small
object sinks in water varies with the amount by which its weight exceeds that of the water it dis-
places, and inversely with the viscous forces between the surface of the object and the water.
The viscous forces opposing the motion are approximately proportional to the surface area, and
therefore, other things being equal, the greater the surface area, the slower the sinking rate.
There are a number of structural features of planktonic organisms which increase their surface
area and must certainly assist in keeping them afloat. The majority of planktonts are of small size,
and therefore have a large surface to volume ratio. In many cases, modifications of the body surface
increase its area with very little increase in weight. These modifications generally take two forms:
a flattening of the body, or an expansion of the body surface into spines, bristles, knobs, wings,
or fins.
A great range of flattened or elaborately ornamented shapes occurs in diatoms such as Chaeto-
ceros sp. In dinoflagellates also, the cell wall in some cases is prolonged into spines (Ceratium)or
wings (Dinophysis). Among the Chlorophyceae, the wall of the peripheral cells of Pediastrum colo-
nies may bear clusters of very long and delicate chitinous bristles regarded as buoyancy devices. In
Scenedesmus also the cells are clothed by a large number of bristles with a complex structure, which
seem to help keep the cells in suspension.
Reduction of the sinking rate is also obtained by an increase in lipid content, which has a
density of about 0.86 g cm 23 . Oil droplets are common inclusions in the cytoplasm of algae;
lipids stored in this form are present in the Chrysophyceae and Phaeophyceae (Heterokontophyta),
and in the Haptophyta, Cryptophyta, and Dinophyta. The thermal expansion of these compounds
may be of some significance in affecting diurnal depth changes, through reduction of cell
density, but without producing neutral buoyancy.

How a Flagellum Is Built: The Intraflagellar Transport (IFT)
The mechanisms that determine and preserve the size and function of cellular organelles represent a
fundamental question in cell biology up to now only partially understood, and flagella has provided
a handy model system to investigate organelles’ size-control analysis. It was discovered that fla-
gella are dynamic structures and that flagellar length is regulated by a process called intraflagellar
transport. IFT is a motile process within flagella in which large protein complexes move from one
end of the flagellum to the other, and flagellar length is regulated by a balance between continuous
assembly of tubulin at the tip of the flagellum, counterbalanced by continuous disassembly.
According to Iomini et al. (2001), the IFT cycle consists of four phases. In Phase I, which takes
place in the basal body region of the flagellum, anterograde particles are assembled from retrograde
particles by remodeling or exchange of subunits with the cell body cytoplasm, with a concurrent
decrease in number. In this phase, the precursors of the flagellar structures that make up the
cargos are also loaded onto the particles. In Phase II, the particles are transported from the base
to the distal end of the flagellum by a heterometric kinesin II with a velocity of about
2 mm sec 21 . In Phase III, which occurs at the distal end of the flagellum, anterograde particles
are remodeled into retrograde particles with a concurrent increase in number, probably upon or
after unloading their cargo. Finally, in Phase IV, retrograde particles are transported by a cyto-
plasmic flagellar dynein from the distal end back to the basal body region of the flagellum, with
a velocity of about 3 mm sec 21 , higher than that of anterograde particles.

How a Flagellar Motor Works
Movement can arise by shape change of permanently linked elements, by reversible interactions
causing movement of elements relative to each other, by reversible assembly and disassembly,