SpiruIina is a filamentous cyanobacterium recognized mainly by its multicellular cylindri­cal arrangement of trichomes in an open helix along the entire length (Vonshak, 1997). Under the microscope, it appears as blue-green filaments of unbranched cylindrical cells, in helical trichomes. The filaments are movable and move freely around its axis, and they are not heterocystic. They are up to 1 mm in length; the cell diameter ranges from 1-3 gm in small species and 3-12 gm in the larger species (Richmond, 1990).

This microalga inhabits various media such as soil, sand, swamps, alkaline lakes and brackish, marine, and fresh water. Through photosynthesis, it converts nutrients into cellular matter and releases oxygen. The components needed for cell growth are water, a carbon source, nitrogen, phosphorus, potassium, magnesium, iron, and other micronutrients.

In natural lakes, the limited supply of nutrients may regulate the growth cycles, and the cell density increases rapidly, reaches a maximum concentration, and retreats when nutrients are depleted. The release of nutrients from dead cells or the supply of nutrients initiates a new cycle (Henrikson, 1994).

There are many controversies in the morphology and taxonomy of cyanobacteria of the genera Spirulina and Arthospira. Many studies have described the properties of Spirulina max­ima and Spirulina platensis, and both species are considered to be of the genus Arthospira and not Spirulina. The differences between the genera have been based on the G + C content of DNA and lipid profile (Romano et al., 2000).

The helical shape is only maintained in liquid medium; in solid medium the filaments take a spiral shape, and the transition from the helical shape to the spiral shape is slow, whereas the opposite takes place instantaneously. Most species of Spirulina present a granular cyto­plasm containing gas vacuoles and septa that are easily visible. Electron microscopy reveals that the cell wall of Spirulina platensis is probably composed of four layers.

The life cycle of Spirulina begins when a trichome (filament consisting of cells) elongates, and this is followed by an increase in the number of cells as a result of repeated interspersed cell divisions. The microalga cell fragmented into several parts by the formation of special­ized, lysis-promoting necridic cells, which give rise to small chains (two or four cells) called hormogonia, which develop into new trichomes. The number of cells in the hormogonium increases by cellular fission, while the cytoplasm becomes granulated and the cells take on a bright bluish-green color. Due to this process, trichomes increase in length and take their typical helical shape (Richmond, 1990).