SEM stands for scanning electron microscope, which is powerful technique for taking very clear, detailed close up images of any solid object. An electron microscope is superior to a light microscope in that it can reach much large magnifications with a high resolution. Reason for this is that a light microscope is limited to a maximium magnification of around 1000 times or so. This is the absolute limit due the wavelength of light and its properties. To get to larger magnifications you need to move to something that has a shorter wavelength, which is where electrons come in. The limit to an electron microscope is in the region of 200,000 times magnification!
In order for an image to be taken of an object by an electron microscope, firstly it has to be stuck to a metallic disk then coated by a very thin (10 nanometers, 10 x 10-9 m) layer of gold. The gold is required so that the sample conducts electricity. The images are created by an electron beam being directed at the sample, directed using magnetic lenses. As the electron beam hits each spot on the sample, secondary electrons are knocked out and a detector counts these electrons.
Part of my work involves the use of an electron microscope (Hitachi S-570 scanning electron microscope), taking images of powdered drug formulations. The stage takes 6 samples, so the extra space was taken up by two extra samples that are marine aquarium related, providing an interesting look what most reefers don't even think about.
Foraminferans
Out the back of our house was a bucket of some extra sand, which had been out of water for around 2 years, and out of that was taken a number of small particles. These are the sort of things that most people wouldn't even notice of being something else other than a sand particle. But these are star shaped and are a representative of a very unusual and overlooked group of organisms, foraminferans. Foraminferans are placed in the kingdom Protista and tend to be free living substrate or reef structure, but some are also planktonic or fixed to the substrate. They build a calcareous skeleton, which is called a frustule or teste. This is what is left after they die and can make up a substantial amount of a sand sample. They can vary signficantly in size, with the ones shown below being approximately 1 mm across up to over 10 cm in diameter. The shape of their frustule is the manner in which the species is identified and they can produce some very elaborate and specatular ones.
So, on to the images .....
30X magnification showing all four of them surface.
One single foramiferan at 60X magnification.
Close up of the surface, 2500X magnification.
Acropora
Sitting out the back as well, was the skeleton of the green Acropora sp. that can now be found in a large number of reef tanks throughout Australia. This was the original colony that came from Mentone Aquarium, which didn't survive the temperature spike back in May 2004. It had sat in the tank for a number of weeks after it died, before it was removed, and then outside undercover since then. This is an important thing to note, as will be seen a bit further along in this article.
The tip of one branch was snipped off and the axial corallite was cut off and stuck down pointing upwards. The rest of the branch was placed so that one of the radial corallites was pointing upwards to allow it to be images.
40X magnification showing radial corallite.
200X magnification showing the region just to the left of the axial corallite. Note the coverage by diatoms.
600X magnification, the center of the last image. The surface is covered by diatoms.
And finally an extreme closeup of the diatoms on the surface, at 2500X magnfication.
Presented here is roughly half of the images taken, the other images can be viewed in the SEM Images section of the Gallery.
Hopefully in the not too distant future I will be back running some more samples on the SEM, so should be able to get some more interesting images. The plan is to get some actual samples from within the aquarium, I just have to be a bit more organised and find/work out how to mount such samples. Stay tuned.
