The cryopreservation of Chlorella. 1. Interactions of rate of cooling, protective additive and warming rate.

The cryoprotective additives glycerol and dimethylsulphoxide were found to be toxic to Chlorella cells at concentrations greater then 2.5% w/v. Polyvinylpyrrolidone, was not damaging up to a concentration of 15%w/v. Chlorella 211/7a had a recovery rate greater than 95% at all rates of cooling studied. With Chlorella 211/8h the survival was lower than 0.1% at all rates examined. The addition of dimethylsulphoxide (5% w/v) to Chlorella 211/8h increased the recovery, particularly at the faster rates of cooling; with polyvinylpyrrolidone (10% w/v) there was an optimum range of cooling rate. Cells of Chlorella 211/7a from the exponential phase of growth were found to be damaged both by a temperature reduction from 25 degrees C to 0 degrees C (thermal shock) and by freezing and thawing. In contrast cells from the stationary phase of growth were resistant to these stresses.