3.2 MAINTENANCE OF STOCK AND STARTER CULTURES

Stock cultures, otherwise known as master cultures, of the preferred species are the basic foundation of culture. They are normally supplied as monospecific (uni-algal) cultures from reputable culture collections maintained by national institutions or research laboratories. Since they are valuable, they are normally kept in specialized maintenance media, for example, Erdschreiber, or alternatively in F/2 media, or on nutrient enriched agar plates or slopes, under closely controlled conditions of temperature and illumination.

A special area or room off the algal culture room is usually allocated to this purpose.
Stock cultures are used only to provide lines of starter cultures (also known as inocula) when required. Every effort should be made to minimize the risk of contaminating the stock and starter cultures with competing microorganisms. The sterile procedures described below should be followed to ensure that contamination does not occur.
Stock cultures are kept in small, transparent, autoclavable containers. For example, 500 ml borosilicate glass, flat-bottomed boiling or conical flasks fitted with a cotton wool plug at the neck, suitable for containing 250 ml of sterile, autoclaved medium, are ideal.

The composition and preparation of Erdschreiber medium is given in Table 2. Alternate media suitable for the purpose are Guillard’s F/2 (see Table 3) and HESAW (see Table 4). Proprietary algal culture enrichment products for addition to suitably treated seawater can also be used according to the manufacturer’s instructions. Stock cultures are also often maintained in seawater agar medium impregnated with suitable nutrients in Petri dishes or on slopes in test tubes.

Figure 15: Illuminated, temperature controlled incubators for the maintenance of small algal cultures.
Stock cultures are best kept in a cooled incubator at 4 to 12°C (according to preference), illuminated by two or more 8-watt (W) fluorescent lamps that provide a light intensity of 450 lux measured at the culture surface (Figure 15). Alternatively they can be kept in cool conditions close to a north-facing window (out of direct sunlight), or in a cool room illuminated by fluorescent lamps. The objective is not to allow rapid growth, but to maintain the cultures in good condition. The cultures are not aerated, nor is carbon dioxide introduced.