When UV light bounces off chlorophyll, the frequency is shifted towards red (fluorescence), which can be safely picked up and observed through a UV filter. What is awesome about UV work is that you can view it simultaneously with our own spectrum, since we don't notice when UV light is filtered away. You can't do the same with substances whose excitation frequencies (the incoming colour that later changes) lie within our visible spectrum, since you have to filter out a band of colour - making the filtration pretty obvious. Thus, you can't simultaneously view GFP (green fluorescent protein) and normal light, since you'll have to filter out everything but green, which is the emission frequency (the resulting colour).
Fluorescent microscopy is a powerful tool in cell biology, as it enables one to "colour" certain proteins, and find them via fluorescence. You can do live cell imaging with that, and observe real cellular processes in vivo, as well as creating time-lapse (movies) and stacks (3-D reconstructions).
I wish people were taught about real cells in school, as opposed to that hideous textbook thing (which does NOT exist -- there is no 'typical' cell!) Cells are so alive and dynamic and exciting...all that gets thoroughly lost in those cartoon diagrams. Can't we provide highschool educators with our sexy movies of real cellular phenomena in action? Should be cheaper than textbooks themselves! And much more informative, not to mention memorable!
Now back to our diatom. Since UV light is converted to red upon hitting chlorophyll, what does the image stack tell us? Well, you can see certain compartments emit this red light, indicating the presence of chlorophyll. You may or may not have heard that diatoms are phytoplankton, meaning plant-like plankton. They are actually not particularly close to plants at all -- diatoms are brown algae, the ancestor of which has engulfed a green alga at one point, and incorporated it as an organelle. As a result, the chloroplasts in diatoms have two extra membranes around them, coming from the original green algal host. This is called secondary endosymbiosis -- a host engulfed a host of a cyanobacterial descendant (chloroplast)!
There's also tertiary endosymbiosis -- a host engulfing and host of a host of a chloroplast. Some of things have an extreme number of membranes layering each chloroplast.
I'll discuss endosymbiosis in more detail at a later day, but back to our diatom. I'm a bit dense, so it took me a while to remember that diatoms are phytoplankton, for they don't particularly look like plants or green algae -- they're not particularly vibrantly green like the green algae. But playing around with our UV lamp after a whole day of DAPI imaging (to see nuclei in plant cells), I threw on some seawater samples on the slide, and got views like the following:
And that little thing with a tail near the top may be a dinoflagellate of some sort, perhaps. Catching those things with a camera is nearly impossible...they like to move!
Anyway...most diatoms you find tend to be empty shells, as opposed to the live organism itself. So you often forget that they are in fact photosynthetic. I think the autofluorescence drives that point home quite well. Now we remember. Also, you could possibly identify plastids based on their autofluorescence emission, but I'm not sure how it's done yet... that would be topic for another day.
Another cool thing about diatoms -- when they divide asexually, the top shell separates and forms a bottom, which is smaller than the top. The bottom shell becomes...the top, and forms and even smaller bottom. Consequently, several generations later you end with a population of very tiny diatoms, so they have to somehow get bigger again. So they enter the sexual cycle, fuse and dissolve their old shells entirely, forming a new, large, pair upon separation. That's why you find a gradient of sizes within the same species.