3.3 INTERMEDIATE-SCALE CULTURE

Most laboratories and hatcheries requiring small volumes of algae for food use spherical glass flasks or glass or clear plastic carboys of up to 25 l volume (Figure 18). These are generally operated as batch culture systems or semi-continuously. Batch culture involves the inoculation of the culture medium with the required species.

The culture is then grown rapidly until a further increase in cell density is inhibited by the failure of the light to adequately penetrate the culture, The culture is then completely harvested, the container washed and sterilized and started again with a new culture.

Figure 18: Two different approaches to the intermediate-scale culture of algae: A – 20 l volume round flasks; B – using equally as effective wine making carboys of 15 to 20 l volume.

The semi-continuous method involves starting the cultures in the same way but instead of completely harvesting them when they have grown, they are partially harvested before the light limiting stage is reached. The harvested volume is then replaced with freshly prepared culture medium and the process repeated 2 or 3 days later. In this way the life of a culture is extended. With some of the hardier species, e.g. Tetraselmis suecica, cultures will last for 3 months or more with harvests of 25 to 50% of the culture volume 3 times each week. Batch culture is generally used for delicate species and the rapidly growing diatoms. Semi-continuous culture is mainly used with hardier species of flagellates.