The main characteristics of microalgae are primarily simple morphological features that can easily be observed under a light microscope. Cyanophyceae consist of prokary­otic cells commonly called the blue-green algae. They are similar to Gram-negative bacteria, based on the nature of the cell wall, cell structure, and capacity to fix atmo­spheric nitrogen; hence they are called cyanobacteria. However, they possess the pho­tosynthetic system chlorophyll-а, 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 aggre­gates 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 abun­dantly 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 habi­tats. The cells are usually green in color due to the presence of pigments such as chlorophyll-а 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 promi­nent 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 dia­toms 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 mark­ings. 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 open­ing 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.