Abstract: A system and method for its use for efficiently culturing algae such as Spirulina are provided. The system comprises a culture pool for exposing a culture fluid containing the algae to sunlight, a culture tank having a larger depth than the culture pool, which is disposed adjacent to the culture pool, a supply unit for supplying the culture fluid from the culture pool to the culture tank, and at least one filter for removing grown algae from the culture fluid overflowing from the culture tank to the culture pool, while allowing to return a filtrate containing immature algae to the culture pool. It is preferred that this system further comprises a unit for mixing a gas containing carbon dioxide in the culture fluid to be supplied in the culture tank, and a lighting unit disposed in the culture tank to provide an artificial light to the culture fluid. In this case, it is possible to carry out the photosynthesis of the algae night and day.

Abstract: A system for purifying a polluted air by using algae such as Spirulina is capable of reducing carbon dioxide (CO.sub.2), nitrogen oxide (NO.sub.x) and/or sulfur oxide (SO.sub.x) in the polluted air and generating oxygen. That is, this system comprises a culture tank filled with a culture fluid including the algae, an air supply unit for forcing the polluted air into the culture fluid to dissolve carbon dioxide and nitrogen oxide and/or sulfur oxide in the culture fluid, and a lighting unit for radiating a light to the culture fluid. By radiating the light to the culture fluid in the presence of carbon dioxide, photosynthesis of the algae is promoted to convert carbon dioxide into oxygen. In addition, the algae use the nitrogen oxide and/or sulfur oxide as a nutrient during the photosynthesis. As a result, the present system can efficiently purify the polluted air to generate a purified air, which is rich in oxygen.