EFFECTS OF CULTURE AGE ON CRYOPRESERVATION OF MARINE MICROALGAE

The effects of culture age (3, 7 and 14 days) on viability after cryop reservation of five marine microalgae were studied. Higher viability l evels were generally found in older cultures, particularly when condit ions of cryoprotectant concentration and salinity were not optimal. Th ese differences decreased or disappeared when the algae were cryoprese rved under optimal conditions, except for Rhodomonas baltica. Its cryo preservation using 15% dimethylsulphoxide in medium of 20 parts per th ousand salinity (optimal conditions) showed 45.1 +/- 1.3%, 42.4 +/- 13 .3% and 35.0 +/- 13.7% viability respectively for cells from 3-, 7- an d 14-day-old cultures. For the same culture ages, the viability of Cha etoceros gracilis was 25.7 +/- 7.7%, 28.1 +/- 4.9% and 36.0 +/- 4.6%. This diatom could be recovered from -196 degrees C without the use of any cryoprotectant only when cells were collected from 14-day-old cult ures (mean viability of 3.1 +/- 3.7%). The cryopreservation of Tetrase lmis chuii was relatively unaffected by culture age and was very close to 100% under optimum cryoprotectant concentration and salinity. Only in the absence of cryoprotectant were cells from the exponential phas e (3 days) found to be more sensitive to cryopreservation (average via bility 22.2 +/- 5.5%) than were cells from older cultures (37.2 +/- 14 .9% and 42 +/- 12.6% for cells from 7- and 14-day-old cultures respect ively). A lower resistance to cryopreservation was found in 3-day-old cultures of Nannochloris atomus (83.0 +/- 11.0%) in comparison with ol der cultures (100 +/- 7.8% and 94.9 +/- 8.9% for 7- and 14-day-old cul tures). The main increase in the viability of this alga with age was f ound in the absence of cryoprotectant and using a suboptimum salinity of 20 parts per thousand. Three-day-old cultures of Nannochloropsis ga ditana completely failed to recover after freezing under all condition s tested. Cells from 7- and 14-day-old cultures achieved mean viabilit ies of c. 67%.