Cryopreservation of gametophytic thalli of Ulva prolifera (Ulvales, Chlorophyta) from Korea

Cryopreservation is needed to ensure long-term storage without any genetic drift and contamination. To conserve Ulva species, gametophytic thalli of Ulva prolifera (Chlorophyta) were first cryopreserved at -196 A degrees C using the two-step cooling method with cooling rate of 1 A degrees C min(-1) to -40 A degrees C. Three cryoprotectants (dimethyl sulfoxide (DMSO), glycerol, and proline) at five concentrations (5, 10, 15, 20, and 25 %) and mixed cryoprotectants were assessed to determine the optimal solution. The mixture of cryoprotectants with the highest viability was 10 % glycerol in combination with 5 % DMSO and 5 % proline, with a viability of 89.2 %. The cryoprotectant treated singly with the highest viability was 20 % glycerol with a viability of 91.6 %. However, there was no statistically significant difference between 20 % glycerol and the mixed treatment. An evaluation of long-term storage in liquid nitrogen showed very high viability of 91.2-92.1 % with no statistically significant effect from storage time up to 120 days. The specimens developed normally in culture after cryopreservation for 120 days with the rate of gametogenesis reaching 95.7 % on the fourth day of culture. The released gametes developed normally into gametothalli. In conclusion, the use of glycerol as a cryoprotectant was very effective for cryopreservation of the gametothalli of U. prolifera. With 20 % glycerol, viability was very high as 91.6 %. The results suggest that gametothalli of U. prolifera can be cryopreserved completely by a treatment with 15-25 % glycerol singly as well as by a mixture of cryoprotectants with usual efficiency as a cryoprotectant.