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SUMMARY A new aerial alga, Stichococcus ampulliformis S. Handa sp. nov. (Trebouxiophyceae, Chlorophyta) is described based on a clone isolated from the bark of Cephalotaxus harringtonia (Knight ex Forbes) K. Koch collected from Taishaku‐kyo Gorge, Hiroshima Prefecture, south‐west Japan. This alga was examined by light microscopy and transmission electron microscopy and subjected to molecular phylogenetic analysis. Based on its morphological features and life‐cycle, especially short filament formation, the alga was assigned to the genus Stichococcus Nägeli. However, this alga differs from other described Stichococcus species in that it reproduces by a form of ‘budding’, producing two daughter cells of different sizes. The larger cell, enclosed within the mother cell wall, soon reaches the size of a normal vegetative cell. The smaller cell is extruded and takes longer to reach full vegetative size. A phylogenetic tree constructed using 18S rRNA sequences indicated that, within the Trebouxiophyceae, S. ampulliformis is closely related to S. bacillaris Nägeli and some species of Prasiola Meneghini.

Summary Stichococcus Nägeli is a unicellular green algal species that belongs to the Trebouxiophyceae. Stichococcus bacillaris Nägeli NIES-3639 propagates by binary fission, but can form a filament without separating into two daughter cells. Transmission electron microscopy showed that cytokinesis starts according to invagination of the plasma membrane at the plane of division. Synthesis of the daughter cell wall occurs on the surface of invaginating plasma membrane during the protoplast division phase. The mother cell wall may cleave after protoplast division in either the single-cell or filament state. Field emission scanning electron microscopy revealed scars of cleaved mother cell walls from the single-cell state, two-cell state after cell division, and fourcell state during filament formation. Because the mother cell wall was cleaved, the delay of the daughter cell separation may have caused filament formation. Since this cell division cycle repeats rapidly, it may begin before the separation of the daughter cells yielded by the former cell cycle.

SummaryThe unicellular green microalgal genus Stichococcus Nägeli propagates by binary fission. In the present study, we measured the DNA content of Stichococcus bacillaris Nägeli in propidium iodide stained nuclei using laser scanning cytometry (LSC). Using the nuclei of Ulva compressa Linnaeus and Saccharomyces cerevisiae Hansen as references, the nuclear genome size of S. bacillaris was calculated as 80 Mbp. This value is approximately 4 times larger than that of Nannochloris bacillaris (20.3 Mbp), another member of the Trebouxiophyceae that propagates by binary fission.

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