Algae World: Sellaphora

Sellaphora obesa cell in valve view, immediately after cytokinesis

Here, a recently divided cell is shown in valve view, with the focus exactly in the plane of division. The two daughter nuclei are not visible, but the two volutin granules are obvious though just out of focus, one on either side of the focal plane. Three pairs of photographs are shown, representing digital exposures at 08.47, 09.11 and 09.30 h.

The cell was first seen at 08.45 h, when a single, straight longitudinal element was visible (red arrow); this probably represents the tubular silicon deposition vesicle. At this stage, it did not lie along the midline of the cell but slightly to one side. Later, it became central and by 09.30, regularly spaced lateral elements could just be detected along the edges of the longtitudinal element. No raphe slits could be seen at this stage.

Later stages in valve formation

The cell was examined at intervals for the next 4.5 hours. Initially, ribs seemed to be formed along the whole length of the longitudinal element (the raphe-sternum), including the central portion. By 10.00 h, however, the distinction between the central ribs seemed to be breaking down, and by 11.00 h, the plain central area of the mature valve was obvious. The ribs steadily grew outwards to the margins to form the transverse ribs of the valve and thickened, again from the centre outwards. The raphe slits were visible by 10.00 h and became more and more obvious during the next 4 hours. The polar bars of the mature valve were visible by 11.00 h, but not fully thickened until later (red arrow). The whole series of images from which these three were selected is available for view; the image is large and may take some time to load.

The development of Sellaphora valves has not yet been studied with electron microscopy. However, it is already clear from light microscopy that the main features agree with the classic account of valve formation in naviculoid diatoms by Chiappino & Volcani (1977).

The original digital photographs (taken using differential interference contrast optics on a Reichert Polyvar 2 photomicroscope) were globally enhanced to produce the colour images shown above, using the Levels, Brightness/Contrast and Unsharp Mask tools in Adobe Photoshop. In order to reveal detail of the forming valves, the original photographs were also reduced to grey-scale images and processed using a High Pass filter and global enhancement using the Auto Levels, Curves and Unsharp Mask tools.