As part of our work site's open house, we decided to show some microscopic views of everyday objects. Diatoms are always underfoot, even if nobody realizes it (except for the readers of photomacrography.net of course). The scopes are of course pretty heavily subscribed, so I don't get to do this very often, but for the open house we made an exception. I walked out to a nearby pond and collected a few ml of bottom material. Not having a lot of time to formally process it, I tried the following quick sample prep method;
Spread goo onto an APTES-treated coverslip, fixed it in acetone, vacuum dried it then sputtered a thin (<1000 A typically) layer of Au/Pd over it and stuck the coverglass onto a "carbon dot" and put it into the SEM on a standard mounting "stub"...
The gold / palladium coating is to provide some conduction path from the glass (diatoms and the coverslip) to the grounded sample holder. If there's not some return path your sample charges up, and starts repelling the electrons, wiping out the image of your sample.
The SEM cognoscenti out there will realize that these images are not nearly as good as I could have gotten, you'll note that my working distance is huge, while the accelerating voltage is quite low. That's because I didn't want to charge up my pond scum and have it fly all over the inside of the SEM chamber; it would have been VERY obvious who had filthed up the SEM (not too many other people look at samples from ponds in my lab).
One thing I was struck by, not being a diatomist, is just how much variation exists in the pore size and morphology. Additionally, in the 2nd and 3rd images, you can see an interior set of smaller (approximately 20 nm or so) openings inside each pore. The interior "holes" show up faintly, since I was keeping the sample very far away from the bottom lens and using pretty weak accelerating voltage lowering the SEM's ultimate resolution. I think these could be pores in the diatom's cell wall (or a processing artifact).
I have guessed at the genus for the centric diatom in the first 3 images. The ID of the larger diatom (fragment) in the next 3 images is still a mystery to me. Note the elaborate interior details of the pores. As always, please correct my taxonomic guesswork, I'm a p-chemist, not a diatomist...
David
