Cell death in phytoplankton: correlation between changes in membrane permeability, photosynthetic activity, pigmentation and growth

Different stages of the automortality in phytoplankton have been studied ap plying flow cytometry. These stages are, in order of expression: (1) compro mised cell membranes, (2) degradation of the photosynthetic pigments and re duction of the photosynthetic activity, (3) fragmentation of the genomic DN A. The integrity test of the cell membranes is based on the inability of th e DNA-specific stain SYTOX Green to pass into cells with intact plasma memb ranes. The reduction in photosynthetic activity was examined by sorting C-1 4-labelled phytoplankton cells differing in viability. Finally, DNA fragmen tation was traced by measuring changes in genomic DNA. The different phytop lankton species tested showed a great variety in response when grown under the same conditions, but there was also considerable intraspecific variatio n. Unstained cells, fully stained cells (equivalent to full staining of gen omic DNA in fixed cells) and cells with intermediate fluorescence signal oc curred together within the same culture. The photosynthetic activity in cel ls with a reduced viability dropped by as much as 60 % relative to that of the viable cells. In the subsequent stage, when photosynthetic pigments wer e fully degraded, this value dropped further to around 10 %. Cells in this stage also showed subdiploidy as a result of genome fragmentation. Field te sts using samples of phytoplankton collected in the North Atlantic Ocean (4 0 degrees N, 23 degrees W) during spring showed staining properties similar to those found in cultures grown at suboptimal growth conditions. The perc entage of non-viable cells varied considerably (ranging from 5 % to 60 %) b etween the various phytoplankton groups present. The lowest value was obser ved for Synechococcus, but some pico-eukaryotes showed percentages as high as 60%. Moreover, the viability varied with depth (light level) and over a light-dark cycle. The present findings suggest the existence of a (genetica lly based) uniform process of automortality in phytoplankton. Non-viable ce lls are a substantial component of the oceanic phytoplankton, affecting the food-web structure and species succession.