Collection and Study
Where Are They?
As mentioned in the "General Information" section, diatoms are globally distributed in both marine and fresh water. Their habitats can be further classified as either benthic (on the seafloor) or planktonic (within the water column). Diatoms may also be found in microhabitats like on macroalgae, feathers of birds, and even on sand grains!
A plankton tow involves using a plankton net that has a collection bottle at the end of it. The mesh of the net is very fine, usually with a measurement of 25-35 micrometers between the threads. These nets are drawn through the body of water and usually yield a good amount of plankton (including diatoms, dinoflagellates, invertebrates). Though plankton collections are useful in determining what species of diatoms are in a population, it is difficult to determine the actual numbers of species in an area. It should also be noted that plankton samples should do not have a very long "shelf life" and should be studied not long after they were collected.
Studying the Frustules
In order to study the frustules of the diatoms, the protoplasm of the cell must be separated out. Acids are usually used in order to break open the frustule into its individual valves, and the protoplasm is effectively removed. The acid also breaks down the organic casing that covers the surface of the valves and hinders the amount of detail that can be seen with microscopes.
In order to examine the morphology of diatoms, both transmission and scanning electron microscopes are able to provide a much more detailed image than light microscopes. These microscopes were necessary for taxonomical purposes, with the distinctions between species being so minute at times.
Transmission Electron Microscopy (TEM): This type of microscopy is best able to see the more fine, delicate details of the diatom frustule (even if the frustule is not heavily silicified).
Scanning Electron Microscopy (SEM): SEM is best suited for visualizing the entire diatom frustule. It is a tool that can aid in viewing the gross morphology of a diatom (both internal and external parts).
To see some great pictures of Pseudo-nitzschia taken using TEM and SEM, see the web page for the Institute for Marine Biosciences.
Back to the Introduction to Diatoms page...
copyright Jennifer Shin 1999.