Page 136 - Algae Anatomy, Biochemistry, and Biotechnology

Anatomy 119

over the pyrenoid itself. The outer pair of membranes, when four are present, is referred to as a type
of chloroplast endoplasmic reticulum; however, the inner pair is interpreted as the chloroplast
envelope. The thylakoids are often loosely stacked in three, with no girdle lamella. Each chloroplast
bears a central, pear-shaped pyrenoid projecting inward. Around the pyrenoid, often tightly associ-
ated with it, there are vesicles containing b-1,3-glucan, which is the principal storage carbohydrate.
A nucleomorph, that contains DNA and a nucleoulus-like body, is present between the second and
third envelope of each chloroplast, in a pocket located in the pyrenoid surface. Chloroplasts contain
chlorophylls a and b and xanthophylls.

Chlorophyta

The Chlorophyta are not uniform in the ultrastructure of chloroplast, still some generalization can
be made. The chloroplasts of these algae are enclosed only by the double membrane of the chlor-
oplast envelope; there is no additional envelope of endoplasmic reticulum or nuclear membrane. In
this respect they resemble Rhodophyta and Glaucophyta in the cell compartmentalization. The
chloroplasts vary greatly in shape and size. In unicellular forms it is often cup-shaped with a
thick base (Dunaliella); in filamentous forms it is often ring-like or net-like shaped and lies
against the cell wall (Oedogonium). More massive and elaborate plastids, lying along the longitudi-
nal axis of the cell are particularly characteristic of members of the Zygnematophyceae. Thylakoids
are arranged in stacks of two to six or more; their multilayered arrangement may take on the appear-
ance of grana with membrane interconnections, as in higher plants. Girdle lamellae are absent. One
to several pyrenoids occur in most of the algae of this division embedded within the chloroplast, and
are often penetrated by thylakoids. The DNA organized in small nucleoids is distributed throughout
the chloroplast matrix. Both chlorophylls a and b are present; accessory pigments include different
xanthophylls such as lutein, zeaxantin, and violaxantin; b-carotene is always present together with
other carotenoids.

NUCLEUS,NUCLEAR DIVISION, AND CYTOKINESIS

An organized nucleus is absent in both Cyanophyta and Prochlorophyta, where DNA molecules are
free in the cytoplasm. The central region of those algae features the naked (without histone pro-
teins) circular DNA genome, which is not contained within a double membrane, and consists of
a single unbranched molecule. Transcription and translation processes are accomplished by the
assistance of abundant 70S ribosomes located in the centroplasm. These ribosomes are smaller
than their counterparts in the eukaryotic cytoplasm, but are typical of all prokaryotes, mitocondria,
and chloroplasts. Cyanobacteria can only reproduce asexually, but appear to have some forms of
genetic recombination possible, which are divided into two categories: transformation and conju-
gation. Transformation occurs when DNA is shed by one cell into the environment and is taken up
by another cell and incorporated into its genome replacing homologous sections of DNA. The
ability to be transformed by external DNA is specie-specific and typically requires special environ-
mental conditions. Conjugation is a “parasexual” process in which one of the partners develops a
conjugation tube that connects to the recipient cell. Typically a plasmid (a small circle of DNA)
from the donor cell passes through the conjugation tube. It is thought that genes for gas vacuolation,
antibiotic resistance, and toxin production are carried on plasmids in cyanobacteria.
In eukaryotic algae genetic information in the form of DNA, together with the controlling ser-
vices for its selective expression, occurs in plastids (plastome), mitochondria (chondrome), and in
the cell nucleus (genome). In the previous section, we have described organization and behavior of
chloroplast DNA, which are similar to those of mitochondria. It is worthwhile to recall that plastid
and mithocondrial genes and the way in which they are expressed have much in common with the
system of gene expression of prokaryotes, and there are many plastid genes with extended introns
that are very rare in prokaryotes (genes are mosaics of introns and exons. Introns are the DNA