Culturing Algae

Algae culture begins with a pure stock or starter culture of the algal species desired. These can be obtained from a number of sources. Commercial biological supply houses often sell algal cultures, but the species may not be suitable for raising rainbowfish larvae. Aquacultural suppliers are perhaps the best method of obtaining algae cultures.

Once obtained, starter cultures (usually transported in test tubes) are used to inoculate several new cultures. Some of these are kept as stocks for when an old culture dies, is harvested, or otherwise lost and must be restarted. The rest are used to inoculate progressively larger vessels until there is enough culture to start mass production tanks. The culturist must supply light, aeration, relatively stable temperature control, and sterile water with nutrients (media) to produce successful algal cultures. By avoiding major contamination from unwanted algal species and microscopic predators, a continuous, dependable supply of high quality algae will be available. However, pure cultures can become contamination by unwanted algae, bacteria, and predatory protozoans, and is a problem that cannot always be controlled successfully. Cultures are easily contaminated from non-sterile containers or splashed water from buckets, hoses, or hands.

Production of pure algae cultures is accomplished by providing a favourable environment for the species being cultured. As with all plants, microalgae photosynthesise, i.e. they assimilate inorganic carbon for conversion into organic matter. Light is the source of energy which drives this reaction and in this regard intensity, spectral quality and photoperiod need to be considered. Light intensity plays an important role, but the requirements vary greatly with the culture depth and the density of the algal culture: at higher depths and cell concentrations the light intensity must be increased to penetrate through the culture (e.g. 1,000 lux is suitable for Erlenmeyer flasks; 5,000–10,000 is required for larger volumes).

Light may be natural or supplied by fluorescent tubes. Too high light intensity (e.g. direct sunlight, small container close to artificial light) may result in photo-inhibition. Also, overheating due to both natural and artificial illumination should be avoided.

Fluorescent tubes emitting either in the blue or the red light spectrum should be preferred as these are the most active portions of the light spectrum for photosynthesis. The duration of artificial illumination should be minimum 18 hours of light per day, although cultivated phytoplankton develops normally under constant illumination.

The optimal temperature for microalgae cultures is generally between 20 and 24°C, although this may vary with the composition of the culture medium, the species and strain cultured. Most commonly cultured species of microalgae tolerate temperatures between 16 and 27°C. Temperatures lower than 16°C will slow down growth, whereas those higher than 35°C are lethal for a number of species. If necessary, algal cultures can be cooled by a flow of cold water over the surface of the culture vessel or by controlling the air temperature with refrigerated airconditioning units.