The greatest issue in agriculture nowadays is the availability of chemical fertilizers at af­fordable costs. Nitrogen fixation has been acknowledged as the limiting factor in food pro­duction. The concept of using cyanobacteria to fix nitrogen is based on the ability of these microalgae to grow in soil.

The microalgae Nostoc, Anabaena, Oscillatoria, Cylindrospermun, and Mastigocladus Tolypothrix form heterocysts and can fix nitrogen aerobically. Nonheterocyst-forming fila­mentous microalgae, such as Oscillatoria and Phormidium, can fix nitrogen in the absence of oxygen and in the presence of nitrogen and carbon dioxide. Filamentous forms without heterocysts, such as Trichodesmium, may fix nitrogen aerobically (Richmond, 1990).

The heterocysts, which are specialized in aerobic nitrogen fixation, are the site of the en­zyme nitrogenase, which catalyzes the conversion of nitrogen into ammonia. Nitrogen-fixing cyanobacteria were isolated in soils from various cities in South Asia, India, and Africa. In that study, 33% of 2,213 soil samples collected in India contained cyanobacteria. Microalgae such as Nostoc, Anabaena, Calothrix, Aulosira, and Plectonema were found in soils in India, while Halosiphon, Scytonema and Cylindrospermum were observed in the other regions (Richmond, 1990).

1.2 CONCLUSION

Open ponds are the most widely used reactors in the world for large-scale microalgal cul­tures. This is due to the low construction cost, low power demand, appropriate scale-up, and their easy cleaning process compared to closed photobioreactors. The cultures that are grown in open ponds can be protected from adverse environmental conditions (rainfall, tempera­ture, and luminosity) through the use of a greenhouse. Microalgae that grow in extreme conditions, such as an alkaline medium and high salinity, should be adopted in order to achieve axenic cultures. The obtained microalgal biomass can be used in the production of food, drugs, biopigments, biopolymers, biofuels, and biofertilizers.