The effects of short- and long-term freezing on Porphyra umbilicalis Kuetzing (Bangiales, Rhodophyta) blade viability

Seaweeds inhabiting the upper intertidal zone are subjected to temperature, light, and water stresses and vertical distribution has been linked to environmental tolerance. Previous studies have also attributed successful recovery from freezing stress in intertidal seaweeds to desiccation tolerance. Porphyra umbilicalis Kuetzing is an aseasonal red alga inhabiting the mid to upper intertidal zone in temperate and subarctic regions of the North Atlantic. It is a member of the economically important group of foliose Bangiales, and has been documented to only reproduce asexually via neutral spores in the Northwest Atlantic. The goal of this study was to assess the effects of freezing on the viability of small blades of P. umbilicalis. Cultured blades of P. umbilicalis (4.8 plus or minus 0.22mg) were air dried to 5% or 30% absolute water content (AWC) and frozen for 1, 3, 6, or 12months at -80 degree C or -20 degree C. Following freezing, blades were rehydrated and the growth rate of each blade was measured weekly for 4weeks. Photosynthetic efficiency of photosystem II (Fv/Fm) was assessed for each blade 3h and 4weeks post-rehydration. Overall, there was 100% blade survival and all blades continued to grow after rehydration. Although the conditions under which the blades were frozen did have statistically significant effects on post-rehydration growth rate and Fv/Fm, in general the differences were quite small. Post-rehydration growth rates ranged from 7.06 to 8.03 plus or minus 0.16% day-1. AWC had an effect on post-rehydration growth rates for blades frozen at -80 degree C, but not blades frozen at -20 degree C. The length of freezing had a somewhat greater effect on blades with 5% AWC than blades with 30% AWC. Growth rates peaked two weeks post-rehydration followed by a small decline in weeks 3 and 4. Fv/Fm values following freezing were generally similar to those recorded in previous studies on non-frozen blades; however, blades frozen for 6months performed better than blades frozen for 12months. Overall, these results indicate that short- and long-term freezing have little physiological effect on blades of P. umbilicalis. Therefore, freezing may be a viable method for preservation of P. umbilicalis for aquaculture.