Identification And Preparation

To identify intact (whole) diatoms we first of all need to know which side the cell is viewed: valve, broad or narrow girdle view, or from an angle. Intact single cells with a short pervalvar axis tend to lie valve side up under the coverslip (Coscinodiscus radiatus and Pleurosigma). Diatoms with a pervalvar axis longer than the cell diameter or the apical axis turn girdle side upwards (Corethron and Rhizosolenia). Some colony types are normally seen in girdle view in a water mount (Chaetoceros, Fragilariopsis, and Thalassiosira); others may show either valve or girdle side (Pseudo-nitzschia, Asterionellopsis, and Thalassionema).

Cylindrical and discoid diatoms are readily recognized by the general circular outlines in valve view and rectangular outlines in girdle view. The distinction between the two views of the more or less spindle-shaped cells (e.g. Pseudo-nitzschia, Thalassionema, and Thalassiothrix) is more problematic. The shape of cell ends will usually help, being straight cut off or pointed in girdle view and more obtuse in valve view.

Once the orientation of the cell is ascertained, the next step is to look for outstanding features like setae (Chaetocerotaceae), shape of linking processes (Skeletonema), and in unpreserved material, organic threads from the valve (Thalassiosiraceae) or gliding movements indicating the presence of a raphe.

Frustular elements cleaned of organic material may also be oriented in various ways in a permanent mount. Flattened valves with a low mantle will usually be seen in valve view (some Coscinodiscus spp., most Navicula spp.), while valves with a high mantle and/or protuberances may appear in girdle view (Eucampia, and Rhizosolenia). Lightly silicified bands shaped as those in Rhizosolenia and Stephanopyxis often lie with girdle side up. The more heavily silicified bands, especially those with a septum, tend to be seen as ellipses or circles (Rhabdonema and Thalassiosira).

Valves and bands are often seen in strewn permanent mounts in a view that does not provide the necessary information for identification. The single frustular element may be brought into the desirable position under the microscope by using a micropipette before drying. Alternatively, they may be turned by means of a needle when dried to the coverslip. The single element may also be turned after the resin is added if utilizing one that does not need to be heated (e.g., Pleurax).

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