Cultivation of algae in open ponds mimics the natural method of growing algae (Pearson, 1996; Chisti, 2007). Open ponds can be categorized into natural waters (lakes, lagoons, ponds, etc.) and artificial ponds or containers. The most commonly used systems include shallow ponds (large in size), raceway ponds, tanks, and circular ponds. Raceway ponds generally consist of an oval-shaped shallow pond lined with PVC, cement, or clay, having an area of 1-200 ha (Andersen, 2005). Ponds are divided by a series of baffles, and water is moved through the ponds in order to promote mixing of nutrients and uniform algae growth. These ponds are usually constructed in shallow dimensions as the algae need to be exposed to sunlight, and sunlight can only penetrate the water up to a certain limited depth (Chisti, 2007). The ponds are operated in a continuous mode, with CO2 and nutrients being constantly fed to the pond while the algae-containing water is removed at the other end. Large open-pond cultivation for mass algal production of single-cell protein, health food, and beta-carotene is one of the oldest industrial systems since the 1950s (Chisti, 2007; Perez-Garcia et al., 2011).

Cultivation of microalgae in open ponds presents relatively low construction and operat­ing costs, which invariably result in low production costs (Stephenson et al., 2010; Chen, 1996; Tredici, 2004). Large ponds can be constructed on degraded and nonagricultural lands that avoid the use of high-value lands and crop-producing areas (Chen, 1996; Tredici, 2004). On the contrary, open pond cultivation inherits some drawbacks such as poor light diffusion, losses due to evaporation, CO2 diffusion from the atmosphere, and the requirement of large areas of land (Harun et al., 2010; Perez-Garcia et al., 2011). Furthermore, contamination by predators and other fast-growing heterotrophs restricts the commercial production of algae in open-air pond/culture systems. Not-so-efficient mixing in open cultivation permits poor mass transfer rates, resulting in low biomass productivity (Pulz, 2001; Harun et al., 2010). Uncontrolled environments in and around the pond pose a multitude of problems that can directly or indirectly stunt algae growth (Mata et al., 2010). Uneven light intensity and distribution within the pond (Kazamia et al., 2012) and uncontrolled pond temperature also have a significant influence on the algal biomass productivity.