A longitudinal transect in the Atlantic sector of the Southern Ocean extending from the ice‐covered Weddell Sea across the Antarctic Circumpolar Current and into the Polar Frontal Zone was studied repeatedly during the austral spring. The centric diatom Corethron criophilum was found at most stations in variable numbers, but in the Polar Frontal Zone I encountered a large bloom undergoing a mass sexual phase. This event apparently triggered downward transport of empty diatom cell walls in numbers that suggest the phenomenon is significant for the vertical silica flux and the formation of monospecific layers of this important diatom species in Southern Ocean sediments. Comparison of the cell‐wall dimensions of such a monospecific layer with those in the plankton in the sexual phase reveals a characteristic signature that may indicate the provenance of these layers.
The structure of the theca of some 30 marine and freshwater members of the Dinophyceae has been examined by light and electron microscopy. The basic arrangement of membranes, an outer continuous plasma‐membrane beneath which lies a single layer of flattened vesicles, is the same in all flagellated forms. The periplast of coccoid and encysted forms is somewhat different. Thecae have been found to fall into eight reasonably distinct categories as follows: (1) Outer membrane underlain by irregular vesicles. (2) Membrane underlain by close‐packed vesicles. (3) As Group 2 but with plug‐like structures attached to the inner membrane of the vesicles. (4) As Group 2 but with thin plates within the vesicles and all plates of more or less similar shape. (5) As Group 4 but plates have slight overlap. (6) Similar to Group 5 but plates are reduced in number and can be individually recognized. (7) As Group 6 but plates have elaborate flanges and their surfaces are covered by reticulations. (8) Theca mainly consisting of only two plates.
It is suggested that the thecal types could from the basis for some revision of the taxonomy of the class.
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