The under-ice development of the meroplanktonic diatom Aulacoseira baicalensis was studied, using underwater video recording. This is an ice alga, which not only succeeds in vegetating under the ice, but also reproduces intensively in interstitial water-filled spaces inside the ice. This report yields first results of the underwater filming done during the under-ice period of 2000. Our studies have shown that the A. baicalensis aggregations on the ice bottom can be easily destroyed by water currents and drift away as large flakes up to tens of centimetres in size. These macro-aggregations are capable of free-floating in the water-column for some time, gradually sinking to the bottom. Cells in the aggregations continue to divide, and they cover objects on the bottom with peculiar mucous coats. These coats act as a major food resource for bottom invertebrates.

No abstract

The nuclei of trophozoites and digestive cysts as well as mitotic nuclei of several species of the vampyrellids Vampyrella, Gobiella, Hyalodiscus, Arachnula, and Leptophrys were investigated by electron microscopy. Except for some species of the genus Hyalodiscus, the vampyrellids are generally multinucleate. The nuclei of the trophozoite stage are in interphase. These nuclei are spherical, except for the genus Arachnula, which reveals elongated nuclei. In digestive cysts of all vampyrellids the nuclei enlarge and the pars granulosa of the nucleoli becomes prominent. Karyokineses take place synchronously in older digestive cysts, which transform into reproductive cysts. The nuclei divide by closed intranuclear orthomitosis. In telophase the old nuclear envelope disintegrates and a new one is rearranged. Only in the genus Leptophrys the nuclear envelope decomposes before telophase. Neither centrioles nor MTOC‐plaques have been found in any stage of mitosis. After karyokinesis the cell divides inside the cyst or when leaving the cyst.

No abstract