The effect of chemical fixation on some optical properties of phytoplankton

2To whom reprint requests should be addressed Abstract. Parafonnaldehyde fixation of phytoplankton cells for delayed flow cytometric analysis or for permeabilization of cells for immunofluorescent staining generally resulted in reduced chlorophyll fluorescence. The reduced fluorescence was a result both of decreased absorption by pigments and of decreased yield in the fluorescent re-emission of absorbed light. Although some phaeopigments were formed and some uncoupling of energy transfer from accessory pigments to chlorophyll a or phaeopigments did occur, most of the light energy absorbed by the pigments in fixed cells was transferred to chlorophyll a or phaeopigment. Despite the variable loss of chlorophyll fluorescence for eight species examined,>50 % of the fluorescence was retained within the first 24 h after fixation. Forward angle light scatter, a parameter related to cell diameter, also changed after fixation, initially increasing by as much as 54 % for Phaeodactylum tricornuXum, and then later decreasing. These results indicate that discrimination of groups or classes of phytoplankton, identified by characteristic forward scatter and chlorophyll fluorescence patterns, can be distin-guished in recently fixed samples and that information about the approximate concentration of chlorophyll per cell can also be obtained within a few days of paraformaldehyde fixation of cells for immunofluorescent analyses