Cytological evidence of cell-nuclear genome size of a new ultra-small unicellular freshwater green alga, "Medakamo hakoo" strain M-hakoo 311 I. Comparison with Cyanidioschyzon merolae and Ostreococcus tauri

Ultra-small unicellular algae provide information on the basic cellular mechanisms and essential genes that support the lives of photosynthetic eukaryotes, including higher plants. We have discovered the smallest free-living photosynthetic picoeukaryote to date, a green alga, "Medakamo hakoo" (provisional name) strain M-hakoo 311, which was isolated from freshwater. Based on its pigment composition, M. hakoo belongs to the Chlorophyta. Its cell size and nuclear genome size were compared with those of the primitive red alga Cyanidioschyzon merolae, which lives in freshwater, and with the green alga Ostreococcus tauri, which lives in seawater. The minor and major axes of M. hakoo, C. merolae and O. tauri in the G₁ phase were 0.73 and 0.98 μm, 1.2 and 2.37 μm, and 0.76 and 0.96 μm, respectively. The cell size of M. hakoo was thus very similar to that of O. tauri. The nuclear genome sizes of the three algae in the G₁ phase were examined using the video-intensified photon-counting system (VIMPCS). The nuclear genome size of C. merolae has previously been determined as 16.5 Mbp by sequencing. Using that value as a standard, the genome sizes of M. hakoo and O. tauri were determined as 9.2 Mbp and 20 Mbp, respectively.