The novel blue-light excited fluorescent DNA dye YOYO-1® was tested on cultures of 8 eukaryotic phytoplankton species from the classes of Prymnesiophyceae, Bacillariophyceae, Coscinodiscophyceae, Prasinophyceae and Dinophyceae, and the coccoid cyanobacterium Synechococcus sp. In all tested species, YOYO staining of formalin fixed cells allowed to differentiate cell cycle phases of G1, S, and G2+M in the presence of chlorophyll autofluorescence in flow cytometric analyses. Diel cell cycle patterns of Emiliania huxleyi and Skeletonema costatum grown under light/dark cycle and continuous light could be established. Under light/dark cycle, DNA replication occurred during the night and cell division in the early morning in both species. S. costatum spent, however, a longer fraction of their cell cycle in S and G2+M phases as compared to E. huxleyi. Changes in cellular chlorophyll content and cell size could be observed in relation to cell cycle phasing. Cell division phasing was nearly lost under continuous light. Growth rates calculated from DNA cell cycle analyses agreed well with those established from cell counts. 

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