Page 129 - Algae Anatomy, Biochemistry, and Biotechnology

112 Algae: Anatomy, Biochemistry, and Biotechnology

autonomous from the nuclear genome. The plastid genes are transcribed and translated within the
plastids. The machinery of protein synthesis is, in any case, partially composed of imported nuclear
gene products. In this respect, as in others, the chloroplast is no longer independent. Nevertheless,
about half of the plastid genome consists of genes that contribute to the machine of gene expression,
for example, genes for rRNA, tRNAs, RNA polymerase subunits, and ribosomal proteins. Plastids
code for, and synthesize, some proteins that are components for photosystems and particular sub-
units of photosynthetic enzymes. The missing subunits of these complexes are coded in the nucleus
and must be imported from the cytoplasm.
Chloroplast development and division (self-replication) may be coordinated with that of the cell
or may proceed independently. They divide in mother cells and they are inherited by daughter cells
during vegetative division and usually only from the maternal side in sexual reproduction. The
shape and number of chloroplasts are extremely variable from the single cup-shaped chloroplast
of Dunaliella salina, the ribbon-like chloroplast of Spirogyra or stellate one of Zygnema to the
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numerous (about 10 ) ellipsoidal chloroplasts of Acetabularia giant cell.
The fact that algae of different divisions have different colors, due to the presence of a variety of
pigments in the photosynthetic membrane system, might lead to the supposition that the photosyn-
thetic membrane structures are variable. This is true because the features of photosynthetic mem-
brane system represent a diagnostic important element at the class level. In this chapter we will
consider the structure, composition and location of the photosynthetic membrane system in each
algal division.

Cyanophyta and Prochlorophyta

The photosynthetic apparatus of these algae is localized on intracytoplasmic membranes termed
thylakoids. The thylakoids membranes show considerable variations in structure and arrangements
depending on the species. Also the amount of thylakoid membranes per cell is variable due to
growth conditions and taxonomic specificity.
In most species the thylakoids are arranged peripherally in three to six layers running parallel to
the cell membrane, forming an anastomosing network of concentric shells. This peripheral region
has been called the chromatoplast, which is separated from the inner nucleoplasmic region called
the centroplasm. This type of thylakoid arrangement is characteristic of many unicellular and
filamentous cyanobacteria, such as Synechococcus planctibus and Anabaena sp. In some other
organisms such as Oscillatoria and Arthrospira the thylakoids are orientated perpendicular to
the longitudinal cell wall. Radial arrangement is present in Phormidium retzi. Thylakoids are
not always restricted to the cell periphery but can be found scattered throughout the cell as in
Gloeotrichia sp. However, the arrangement of thylakoids can change from cell to cell and cell
types (vegetative cells, heterocyst, akinete) within the same culture from parallel to a convoluted
appearance. Thylakoids can fuse with each other and form an anastomosing network; unlike the
chloroplasts of eukaryotic algae, the thylakoids of cyanobacteria form stacking regions only to a
very limited extent. During cell division the thylakoids have to be separated and divided into the
daughter cells; generally this division is an active process. Cell division starts by a centripetal
growth of cytoplasmic membrane and peptidoglycan layer, to form the septum; the thylakoid cylin-
der is narrowed at the level of the cross wall by invagination before the septum is formed. Proteins,
lipids, carotenoids, and chlorophyll a are major components of thylakoids. On the outer surface of
the thylakoids regular rows of electron dense granular structures are closely attached. These gran-
ules termed phycobilisomes contain the light harvesting phycobiliproteins, that is, allophycocyanin,
phycocyanin, and phycoerythrin (Figure 2.75). The phycobilisome structure consists of a three-
cylinder core of four stacked molecules of allophycocyanin, closest to the thylakoid membrane,
on which converge rod-shaped assemblies of coaxially stacked hexameric molecules of only phy-
cocyanin or both phycocyanin and phycoerythrin (Figure 2.76). Phycobiliproteins are accessory
pigments for the operation of photosystem II also in Glaucophyta, Cryptophyta, and Rhodophyta.