Light microscopy of the unicellular green alga Cylindocystis brebissoni during cooling at a rate of 10°C per minute in the cryoprotective additive dimethylsuphoxide. This image was taken at -40°C. The cells have partitioned into the freeze concentrated solution, which has further decreased in size. At this rate of cooling extensive cellular shrinkage is observed, the cell contents have become very dense but intracellular ice is not apparent.