Nuclear and Cell Morphological Changes during the Cell Cycle and Growth of the Toxic Dinoflagellate Alexandrium minutum

Elucidation of the cell cycle of dinoflagellates is essential to understand the processes leading to their massive proliferations, known as harmful algal blooms. In this study, we used imaging flow cytometry (IFC) to monitor the changes in DNA content and nuclear and cell morphology that occur during clonal growth of the toxic species Alexandrium minutum Halim. Our results indicate that the population was in S phase (C→2C DNA content) during the light period, whereas haploid cells with a C DNA content peaked only during a short interval of the dark period. The timing of the phases, identified based on the nuclear morphology and cytoplasmic-to-nuclear (CNR) ratio of the cells, suggests that the length of the G2/M phase is regulated by nutrient levels whereas the beginning of S phase is clock controlled. In addition we found that up to 7% of individual cells achieved a DNA content higher than 2C, indicative of either zygote formation and replication (homothallism), or of double-haploid cells able to divide (polyploid forms). Cells belonging to different cell cycle phases (G1-S-G2/M) could be readily discriminated based on nuclear size. Our study provides evidence of cell-cycle plasticity during clonal growth and unambiguously characterizes the cell-cycle phases of this dinoflagellate species.