Morphological variation in a small Thalassiosira species (Bacillariophyta) under different culture regimes

The structure of a small marine planktonic centric diatom belonging to the genus Thalassiosira was characterised using electron microscopy; and data on its ultrastructure, autecology and initial valve formation are provided here. Although our specimens shared some characteristics with Thalassiosira proschkinae sensu lato, they differed from the type in their permanent lack of areolae and in being constitutively weakly silicified, regardless of the silicate concentration in the medium. Valve areolation in our Thalassiosira isolate was less pronounced than in many other members of the genus and varied with salinity. The most significant effect of changing salinity was that the areola pattern becomes more diffuse, so that it was more difficult to observe areolae even at the margin of the valve. A decrease in salinity also induced changes in cell size, number of marginal fultoportulae and surface architecture. Cells in very low salinity (2) died after 24-48 h. Low silicate in the culture medium affected cell diameter (10 % increase in the presence of added silicate). Putative auxospores and initial cells were found in some of the cultures, and some interesting variations in the valve morphology of initial cells were observed.