Open Systems

Several different types of open culture systems have been proposed (Fig. 2a-d). These open culture systems are the simplest method of algal cultivation and offer advantages in construction cost and ease of operation. Open culture systems require a large surface area and shallow depth (ca. 12-15 cm) to improve light utilization. Furthermore, mixing the culture prevents the cells from sinking to the bottom and distributes efficient cell growth with sunlight. The raceway pound proposed by Oswald (33) has been developed into various types, including raceway pounds employing a paddle wheel, which have most generally been used for outdoor production of microalgae (34,35). The raceway pounds for commercial production of microalgae require an area of 1,000-5,000 m2.

Contamination by different algal species and other organisms is a serious problem in open culture systems. Chlorella, Dunaliella, and Spirulina, which are tolerant to extreme conditions (high nutrients, high salinity, and high pH), are adequate strains for open culture systems. Von-shak et al. (36) demonstrated that contamination by Chlorella in outdoor Spirulina culture was prevented by maintaining the culture medium at a high bicarbonate concentration (0.2 M). Grazers sometimes found in Spirulina culture were arrested by addition of ammonia (2 mM).

The open culture system is easily affected by weather conditions. Rain dilutes salinity, causing contamination.

Table 2. Production of Useful Materials by Marine Microalgae

Product

Strain

Amount (mg or unit*/g dry wt)

Ref.

Coccolith

Emiliania huxleyi

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