The Biology of Microalgae - Biotechnological Applications of Microalgae - page 8

bacteria, based on the nature of the cell wall, cell structure, and capacity to fix atmo-
spheric nitrogen; hence they are called cyanobacteria. However, they possess the pho-
tosynthetic system chlorophyll- a , accessory pigments, and thallus organization that
resembles other algae. Cyanophyceae members can be broadly divided into coccoid
forms and filamentous forms. The coccoid has various forms, from single cell to aggre-
gates of unicellular cells; regular or irregular colonies; and pseudofilamentous and
pseudoparenchymatous conditions. The filamentous forms exist as simple uniseriate
filaments to heterotrichous filaments, which may be differentiated into heterocysts and
akinetes. These are ubiquitous in nature, occurring in several habitats with extreme
conditions (i.e., temperature, light, pH, and nutritional resources). They are found abun-
dantly in a variety of natural and artificial aquatic ecosystems. Cyanophyceae members
can be easily identified within a mixture of other algae by their distinct blue-green color.
The Chlorophyceae constitute a major group of algae occurring in various habi-
tats. The cells are usually green in color due to the presence of pigments such as
chlorophyll- a and - b . The cells contain chloroplasts of various shapes that are located
differently in each group of organisms. In addition, the chloroplasts also contain
pyrenoids. The nucleus of this group may present either singly or in multiples but in
major organisms occurs singly although some genera are multinucleate. Flagellated
cells are common either in the vegetative or reproductive phase. There is at least one
group without any flagellated cells.
Euglenophyceae members are unicellular, motile, and usually contain one promi-
nent flagellum and in some cases two flagella. The anterior position of a cell has
a visible gullet, and many dissimilar chloroplasts are found in autotrophic forms and
are absent in other forms. Euglenoid cells are enclosed by a proteinacious pellicle
and help the organisms achieve pleomorphism. These are widely distributed in all
types of water bodies, particularly in organic-rich aquatic ecosystems.
Bacillariophyceae members are popularly known as diatoms. They are basically
unicellular, and also occur as pseudofilaments or aggregated in colonies. The cell
wall of a diatom is impregnated with silica and they have been well preserved as
microfossils. The diatom cell is also called a frustule, and the classification of dia-
toms is based on the pattern of ornamentation on their wall. The cells have either
radial or bilateral symmetry. The frustules consist of two halves (epitheca and
hypotheca) and connecting girdle bands. The surface of the valve has typical mark-
ings. Punctae are regularly or irregularly arranged to form striae. Areolae are pores
or chambers within the valve wall. Costae are elongated thickenings of the valve
wall due to heavy deposition of silica. The valves of some diatoms have an open-
ing or fissure along the apical axis called the raphe. The presence of the raphe or its
absence on the walls of the diatoms has been one of the features in the identification
of diatoms and distinguishing different genera. The radially symmetric forms are
grouped as Centracles and the bilaterally symmetric ones are Pennales.
2.2 MORPHOLOGICAL IDENTIFICATION
Understanding biodiversity is critical in ecological research because it unravels the
role of each single species in the ecosystem in mediating the environment for the
entire biological community. The microalgal biodiversity of a region has economic