SOME ASPECTS ON THE CRYOPRESERVATION OF MICROALGAE USED AS FOOD FOR MARINE SPECIES

The response of marine microalgae to different cryopreservation method s was described. Of the six species evaluated, only Chaetoceros gracil is depended on faster cooling rates to increase its postthaw viability (7.2% at 0.25 degrees C min(-1), 29.3% at 4 degrees C min(-1)) when u sing 15% dimethyl sulphoxide in 36 p.p.t. salinity seawater. Tetraselm is chuii, Nannochloris atomus and Nannochloropsis gaditana were the mo st tolerant species to biological freezing, achieving mean viabilities of 97.9%, 80.5% and 61.6% respectively. Rhodomonas baltica and Isochr ysis galbana, T-ISO strain, showed the lowest viability (means of 7.3% and 15.1% respectively) after cryopreservation under the same conditi ons of salinity, cryoprotectant concentration and cooling rates. Avoid ance of undercooling by inducing ice nucleation when reaching the free zing point did not change viability in comparison to all the procedure s that did not include seeding in any of the tested species. Five spec ies showed similar viabilities when a single controlled cooling step t o -50 degrees C procedure was compared to a two-step cooling process, in which algae were plunged into liquid nitrogen (LN) after the first step. Isochrysis galbana represented an exception. A mean viability of 25.8% was achieved when cooled to -50 degrees C, whereas viability de creased to 4.4% when the second cooling step to -196 degrees C was use d. Replacing the special biological freezing equipment by a -20 degree s C freezer to perform the first cooling step resulted in a steady coo ling rate after the commencement of ice formation. This was due to the fact that samples reached that temperature in a liquid state. Solidif ication occurred spontaneously at variable times once -20 degrees C wa s reached. A cooling rate of -14 degrees C min(-1) during the change f rom liquid to solid state was achieved when a -80 degrees C freezer wa s used to perform the first cooling step. The performance of the first cooling step in both type of freezers resulted in similar viabilities after thawing from liquid nitrogen, in comparison to the use of speci al equipment for controlling cooling rates in T. chuii (90.8% for -20 degrees C and 89.7% for -80 degrees C), N. gaditana (44.6% for -20 deg rees C and 42.6% for - 80 degrees C) and N. atomus (85.9% for -20 degr ees C and 85.6% for -80 degrees C). Lower viabilities were recorded fo r R. baltica and Ch. gracilis cooled to -20 degrees C and in LN (2.2% and 2.9% respectively) but no difference were found with respect to th e first technique, when both species were cooled to -80 degrees C and in LN (6.3% and 19.9% respectively). I. galbana showed no viability wh en cooled to -80 degrees C.