Microalgae are microscopic photosynthetic organisms that are found in both marine and freshwater environments. Their photosynthetic mechanism is similar to that of land-based plants, but due to a simple cellular structure and submerged in an aqueous environment where they have efficient access to water, CO2, and other nutrients, they are generally more efficient in converting solar energy into biomass. These organisms constitute a polyphyletic and highly diverse group of prokaryotic (two divisions) and eukaryotic (nine divisions) or­ganisms. The classification into divisions is based on various properties such as pigmentation, the chemical nature of the photosynthetic storage product, the organization of photosynthetic membranes, and other morphological features. The most frequently used microalgae are Cyanophyceae (blue-green algae), Chlorophyceae (green algae), Bacillariophyceae (including the diatoms), and Chrysophyceae (including golden algae). Many microalgae species are able to switch from phototrophic to heterotrophic growth. As heterotrophs, the algae rely on glucose or other utilizable carbon sources for carbon metabolism and energy. Some algae can also grow mixotrophically (Carlsson et al., 2007).

Microalgae have the following advantages over crops as a source of biomass. They are more effective biological systems for converting sun power into organic compounds; microalgae, like bryophytes, have no complex reproductive system; it is possible to induce in many microalgae species generation of valuable proteins, hydrocarbons, lipids, and pig­ments in extremely high concentrations; they are organisms that have a simple cycle of cell pressure; and they can be grown in various water areas (Vonshak, 1990).