A comparative study of different preservation methods for phytoplankton cell analysis by flow cytometry

Ten methods to preserve phytoplankton populations for flow cytometric analyses were tested. These methods were differentiated by the rate of freezing and thawing, and the use or non-use of cryoprotectants (DMSO and/or glycerol) and chemical fixation. After freezing, the samples were stored in liquid nitrogen. These methods were tested on 3 freshwater and marine algal species. Different intensity parameters and 2 properties were considered. Firstly the number of cells lost, which was more significant with rapid freezing and chemical fixation, and less significant with the addition of cryoprotectants. Secondly, the preservation of both light scattering and fluorescence, which was better with slow freezing than with cryoprotectants. Slow freezing followed by chemical fixation appeared to be the best protocol studied and even if glycerol addition without chemical fixation seemed to be overall the best method, implying the use of cryoprotectant, all these techniques had to be tested on a case by case basis, particularly when phycocyanin and chlorophyll fluorescence were studied.