Page 141 - Algae Anatomy, Biochemistry, and Biotechnology

124 Algae: Anatomy, Biochemistry, and Biotechnology

mitosis is open, because the nuclear envelope breaks down during metaphase; endoplasmic reticu-
lum vesicles are present among the spindle microtubules. The chromosomes align in a distinct
metaphase plate but the chromosomal microtubules do not attach to defined kinetochores. At
early telophase daughter nuclei become separated through elongation of the spindle, which is per-
sistent during telophase and holds the nuclei far apart.
By the end of the telophase, new microtubules proliferate and surround the spindle microtu-
bules, forming the phragmoplast, which includes also actin filaments. Golgi-derived vesicles
appear within the phragmoplast, guided by the microtubules and the filaments to the future
plane of division, where they become arranged to form a cell plate. The vesicles contain cell
wall material and their coalescence produces a transverse septum consisting of two cell membranes
with the new transverse wall between them. As the coalescence is not complete, connections
leading to plasmodesmata are left between the daughter cells. This type of mitosis and cytokinesis
is rare in the green algae, but is the common mode of division in plants such as bryophytes and
tracheophytes.

EJECTILE ORGANELLES AND FEEDING APPARATA
In this section we will describe organelles that upon stimulation by contact, heat, or chemicals
discharge a structure such as a thread or a tube from the surface of the cell. These organelles
may serve for defense purpose or as feeding adjuvant.

Heterokontophyta

Many species of Raphidophyceae have extrusome organelles that explode on strong stimulation
throwing out up to 200 mm long slime threads. The material produced may surround a motile
individual with mucilage so that it becomes palmelloid.

Haptophyta

During its motile stage, Phaeocystis can eject ribbon-like filaments a few tens of nanometers wide
and several tens of micrometers long. Their interconnections form a five-branch star-like configur-
ation (Figure 2.82). The winding of the filaments inside vesicles, as well as their axial twist, are
probably the consequences of their biosynthesis within a confined space. As a result, the filaments
behave like spring-coils whose stored energy is released once the vesicles are broken and the fila-
ments ejected. Using electron diffraction techniques, the filaments have been unambiguously
characterized as being made of a-chitin crystals, the polymer chain axis lying along the filament
direction. These chains are arranged antiparallel (allomorph) and this arrangement has never
been reported before in the algal world.

Cryptophyta
These algae possess large refractile ejectosomes, lining the oral groove, and small ejectosomes scat-
tered around the cell surface at the anterior corners of the periplast plates. An undischarged ejecto-
some is a tightly coiled, tapered ribbon that is wound with the wider end towards the outside; a
smaller coil is attached to it and lies in the depression of the larger one. Prior to release, ejectosomes
are enclosed within vesicles. The large ejectosomes are explosive organelles. If cryptophyte cells
are irritated by mechanical or chemical stress, they escape the potentially lethal influences by
discharging the ejectosomes: the cells jump backwards in fast zig-zag movements through the
water. When discharged, the ribbon unfurls, with the shorter segment forming a beaklike tip on
the longer. The edges of the ribbon tend to curl inwards, producing circular and e-shaped profiles
in cross-section (Figure 2.83).