3.4.2 Internally illuminated cultures

Internally illuminated culture vessels are costly to construct but inexpensive to operate. By mounting the lamps inside a glass or clear plastic cylinder, as in Figure 23, the effective distance for the light to penetrate the culture is greatly reduced. In the example shown, the culture vessel is 150 cm high by 40 cm diameter. The internally mounted light cylinder is 15 cm diameter, therefore, the light energy emitted by 6, 80 W, 150 cm length fluorescent lamps travels only about 14 cm to the perimeter of the culture. In a later development, this distance was further reduced in smaller 80 l culture vessels and yet the same total productivity was achieved as in 200 l cultures.

Productivity (or yield) is determined as the total number of algal cells harvested from a culture each day. Internally lit cultures have a long life, some lasting for more than 100 days with the hardier species. When a culture is finished, the vessel is sterilized by filling with 20 to 50 mg per l (free-chlorine) hypochlorite solution, allowing it to stand for at least an hour, before thorough rinsing with culture quality, filtered seawater, after which it is drained and re-started.

Basic conditions of culture are essentially the same as previously described. The main difference is in the treatment of water to be used as the culture medium. Autoclaving or sub-micron filtration is too expensive for the large volumes necessary. Seawater filtered to 1 or 2 ?m particle size by cartridge filtration is acceptable for some of the larger celled species, e.g. Tetraselmis and Skeletonema. Otherwise, pasteurization or chemical sterilization is recommended. Control of salinity and pH is required and, to obtain maximum productivity, the intensity of illumination must be carefully calculated for the diameter of the culture vessel.