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SEM image of the week: Infusorial earth

Posted on Monday, January 2, 2012 by for SEM.

hand labelled jar of infusorial earth

Two weeks ago, I posted images of diatoms I found clinging to the outside of a blue mussel shell. We continue the theme this week with images of diatoms found by the billions in a jar labelled "infusorial earth". We begin with a review.

Diatoms are microscopic, unicellular algae with hard shells made of silica (2H2O·SiO2). Diatoms are one of the most successful classes of living creatures and can be found anywhere there is water — from the bottoms of glaciers to the tops of clouds. When diatoms die their soft interiors decay, but their hard exteriors persist. The floor of the world’s oceans and some lakes are carpeted with many meters of diatomaceous ooze. When the ocean floor is raised high and dry by geologic forces or when lakes are dried by climatic changes, this ooze becomes diatomaceous earth.

Diatomaceous earth is an important industrial compound. Typical uses are as a filtrant (especially swimming pool and aquarium filters), as an abrasive (you may have brushed your teeth with diatomaceous toothpaste), and as an absorbant (inert dry diatomaceous earth plus dangerously unstable liquid nitroglycerine equals stable and pliable dynamite).

I asked the earth science teachers if we had any diatomaceous earth in our collection of rocks and minerals. One of them found a jar full of yellow dust with a handwritten label that said "infusorial earth". It looked nearly as old as Midwood (which was founded in 1940). I had to consult a dictionary for this one. Infusoria is an obsolete term for Protista — a name given to microscopic organisms that are larger and more complex than bacteria, but not large enough or complex enough to be called plants, animals, or fungi.

Diatoms come in two basic types: pennate and centric. Pennate diatoms are left-right symmetric and vaguely resemble feathers, thus the name pennate. (Remember when feathers were used for pens? Me neither.) Pennate diatoms have a top half (epitheca) and a bottom half (hypotheca). Centric diatoms are rotationally symmetric and resemble cylinders. The have a top half and a bottom half, but each half is composed of two parts — a flat or slightly domed end cap called a valve and a cylindrical sidewall called a girdle band. The diatoms on the blue mussel were all pennate and probably all of the same species. They were all mostly intact (although some of them popped apart when they became charged by the electron beam). The diatoms in the jar of infusorial earth are mostly centric and came in a variety of types. None of the were intact. It was a mixed up pile of valves and girdle bands.

The images below were made by dipping an empty microscope platform (which is just a cylinder of solid stainless steel) into the jar of infusorial earth. Whatever stuck is what you see below. It was kind of like frosting a cupcake.

diatom valve diatom valve diatom valve
Valves (End caps)
diatom girdle band diatom girdle band diatom girdle band
Girdle Bands (Middle Segments)
diatom valve giant diatom valve candy cane
This valve is about the same size as the others in this set of images. This valve is unusually large. See the little circle to the right? That’s the valve shown in the previous image. A word or two about last week’s image. It was the broken end of a candy cane like this one.

Image credit: Glenn Elert

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