A procedure for axenic isolation of the marine microalga Isochrysis galbana from heavily contaminated mass cultures

Isochrysis galbana, one of the most widely used marine microalgae in the rearing of finfish and shellfish larvae, is mass cultured frequently in outdoor tanks. Under prolonged and repeated culture, severe contamination occurs. Axenic isolation of I. galbana from such cultures was best achieved by using a ternary procedure involving percoll-gradient centrifugation, treatment with antibiotics, and growth on agar medium. Protozoa and other algae were removed most effectively by isolation of I. galbana at the 30-40% density layer on a percoll-gradient. Removal of bacteria was accomplished using a mixture of 5 antibiotics (250 mu g mL super(-1) ampicillin, 50 mu g mL super(-1) gentamycin, 100 mu g mL super(-1) kanamycin, 500 mu g mL super(-1) neomycin, 50 mu g mL super(-1) streptomycin). Axenic colonies were isolated from a solid medium prepared from 1% purified agar. The ternary procedure is considered applicable to the isolation of other axenic single-celled microalgae from heavily contaminated cultures.