May 28, 2006

Shiny, Happy Lichens

And now, for something completely geeky: I've been virtually sequestered for the last couple weeks writing a soil science report, but on Friday I let myself out of the house-food source-computer lab matrix to enjoy the campus' scanning electron microscope (SEM) for a couple of hours. I went through the workshops & passed the hands-on test, so they now let me in the room with it completely unattended. Can I get a 'Wahooo!'?

The inside of the SEM where you place the sample has to be under vacuum conditions, to prevent electrons bouncing off air molecules before they hit your sample. Now it doesn't help this if you put a wet sample in that will outgas as soon as the beam hits it, plus you may find yourself watching in shock and awe as your sample's potentially interesting features vaporize before you on the screen.

So biological samples are generally fixed (made, for the purposes of microscopy, inert and stable) after their cellular water has been replaced stepwise by organic solvents to preserve more of their structure then is possible with an air dried sample, and then the solvent is volatilized under pressure.* The sample may then be either sectioned or mounted whole before coating with a very fine layer of metal in a process called sputtercoating to allow better charge dispersal, thus better pictures. Another upshot of this is that SEM is most useful with the typical biological sample for determining surface structures, so even if the material being examined is transparent, you don't see through it like you might in a light microscope. Still, the detail and degree of resolution possible, as well as the depth of field, certainly provide significant advantages.

On short notice (the preparation process can take days to weeks, depending), the only thing I really had to try to see something with (you can't give someone a license to scan and expect them to just let it gather dust) was a dried, fruticose lichen that's been sitting unwatered on a shelf in my kitchen. Now you know why I don't keep houseplants. It was pretty dessicated, so I took one piece to mount topside up and another to mount underside up, then sputtercoated both of them.

The only decent pictures I got were from the underside, which on this particular specimen is black instead of light green like the top, but the picture itself makes it obvious as to why I didn't see a lot. A lichen is a symbiotic relationship between non-reproductive fungal threads (hyphae or mycelium) and photosynthetic cyanobacteria or algae. So it's made up of these fungal threads wrapped around photosynthetic cells that feed them sugar, which sounds like a pretty cool thing to look at. If you're into that sort of thing. Anyway.

As you can see from the picture, the outside is covered in a relatively thick film covering the inner workings. The underside (which the color leads me to believe was scarce in photosynthesizing cells) of this fragment was the only part of the sample with a significant enough crack in the outer shell to see any of the stuffing, which in this case appears to solely be composed of hyphae. Would this have looked very different if not air-dried? Don't know. But next time (the strands of this variety are hollow) I think I'll try cutting it open and looking at the insides, something I can try w/o having to do more in terms of preparation than I did this last time and still maybe get something interesting. It takes a lot of practice to get good at taking pictures on the SEM, which can take you down to magnifications of 10s of 1000s of times, so I might as well work on it while getting ready for more directed projects. So here's the image at 500x, while below the jump are the 1200x and 3300x pictures:

Scanning electron microscope image of a crack on the underside of a fruticose lichen, 500x magnification. Taken at the Evergreen State College, 5-26-2006 by natasha.

Scanning electron microscope image of a crack on the underside of a fruticose lichen, 1200x magnification. Taken at the Evergreen State College, 5-26-2006 by natasha.

Scanning electron microscope image of a crack on the underside of a fruticose lichen, 3300x magnification. Taken at the Evergreen State College, 5-26-2006 by natasha.

That's all for now, folks ;)

* Properly called critical point drying, fwiw: If all the liquid in a biological sample is replaced with a uniform solvent of known properties, the sample can be suddenly heated under the right pressure conditions to the solvent's critical point, the temperature and pressure in which a liquid instantly becomes a gas, in the prosaically named critical point dryer. If you do it right, the sample dries without changing shape much. This can give beautiful pictures of delicate tissue structures and is something I hope to get to try within the coming year. I will definitely have to RTFM.

Posted by natasha at May 28, 2006 02:17 AM | Shiny Things | Technorati links |

Wow, those are great pictures! I look forward to seeing the result of your adventures in SEM.

Posted by: Scorpio at May 28, 2006 12:43 PM

Absolutely captivating, the small world! I remember seeing pictures of ants and human mites that looked so grotesque under the electron microscope. Fascinating work you do!

Posted by: Jim Pankey, USN (Ret.) at May 30, 2006 06:39 AM