2.3.3. Isolating/obtaining and maintaining of cultures
Sterile cultures of micro-algae used for aquaculture purposes may be obtained from
specialized culture collections. A list of culture collections is provided by Vonshak (1986) and
Smith et al. (1993a). Alternatively, the isolation of endemic strains could be considered
because of their ability to grow under the local environmental conditions. Isolation of algal
species is not simple because of the small cell size and the association with other epiphytic
species. Several laboratory techniques are available for isolating individual cells, such as
serial dilution culture, successive plating on agar media (See Worksheet 2.1), and separation
using capillary pipettes. Bacteria can be eliminated from the phytoplankton culture by
washing or plating in the presence of antibiotics. The sterility of the culture can be checked
with a test tube containing sea water with 1 g.l-1 bactopeptone. After sterilization, a drop of
the culture to be tested is added and any residual bacteria will turn the bactopeptone solution
turbid.
The collection of algal strains should be carefully protected against contamination during
handling and poor temperature regulation. To reduce risks, two series of stocks are often
retained, one which supplies the starter cultures for the production system and the other
which is only subjected to the handling necessary for maintenance. Stock cultures are kept in
test tubes at a light intensity of about 1000 lux and a temperature of 16 to 19°C. Constant
illumination is suitable for the maintenance of flagellates, but may result in decreased cell
size in diatom stock cultures. Stock cultures are maintained for about a month and then
transferred to create a new culture line (Fig. 2.4.).