Thursday, January 15, 2009

Nitrogen Fixation

Nitrogen is the most common element in the atmosphere and the air we breathe (78%), and it is a necessary element for life. It is an important element in proteins and enzymes, including the enzyme that allows plants to photosynthesize. However, the form of nitrogen that is in the air is not usable by plants. Plants use nitrogen compounds in the soil, not the nitrogen gas that is so abundant in the air. So how does all of that nitrogen from the air get into the soil for plants to use?

There are some organisms called cyanobacteria that can use the nitrogen gas in the air. They have special cells (called heterocysts) that can can grab nitrogen gas from the air and change it into a usable compound. This is a process called "nitrogen fixation". The cyanobacteria use that new form of nitrogen to grow. Then, when the cyanobacteria dies and decays, that usable form of nitrogen is released in the soil for plants and animals to use.

We see a lot of cyanobacteria near the streams and lakes in the dry valleys. It will grow in thick mats, like you see here. All of that black stuff is a bunch of cyanobacteria matted together, making a layer on top of the soil. One of the things we want to know is how much nitrogen the cyanobacteria are fixing. That way we can estimate how much nitrogen they are adding to the soil for the mosses and other soil animals to use. Unfortunately, it is not very easy to measure nitrogen fixation. But, we can measure a similar chemical process called "acetylene reduction" that helps us calculate how much nitrogen is being taken from the air. To measure acetylene reduction, we place small amounts of cyanobacteria (and mosses with cyanobacteria growing on it) into air-tight chambers, like the clear plastic ones in this picture. We then add a gas called acetylene to the chamber. The cyanobacteria will change the acetylene to ethylene in a process similar to how they change nitrogen gas. We leave the pieces of cyanobacteria and moss in the chambers for 1-5 hours. Then, we take a sample of the air inside the chamber and put it in a special vial. We then measure the amount of acetylene that has been changed to ethylene, and we can then calculate how much nitrogen would have been fixed from the air.

Here's Elizabeth and Ross measuring acetylene reduction at one of our field sites. Elizabeth is using a big syringe to pull new acetylene from a bag to inject into the plastic chambers. Ross is using another syringe to remove a gas sample from a chamber that contains some cyanobacteria.
We will take the vials of gas samples back to Dartmouth and measure the concentration of ethylene on one of the machines in our lab at home. So, we're doing the work now, but won't see the results for another month!

1 comment:

  1. Some more questions from my friends at Thetford Elementary, and the answers:

    Q: We loved reading about all those other organisms. It was fun to watch them dance. Our favorites were the "funky rotifer" and the "groovy water bear." How did you make them move like that?
    A: We didn't do anything to make the rotifer and tardigrade move. That's just what they naturally do! They walk around trying to find a mate or something to eat. So they kept moving when we put them in a dish to look at them through the microscope.

    Q: How long does it take for a nematode to completely unravel when you put water on it?
    A: It can take a nematode anywhere from one hour to one day to uncoil when you add water. That video was sped up.

    Q: Why do you have to wait until you get back to Dartmouth to get your results from those vials of gas samples that Ross and Elizabeth were getting from the soil?
    A: We can't process the gas samples until we get to Dartmouth because it requires a very expensive piece of machinery that we don't have down here in Antarctica. The machine is called a gas chromatograph, and can read the concentration of gases. So we have to send the samples back home to get processed, where the machine is.

    Q: How's your knitting going? What are you making?
    A: My knitting is going well. I finished the ear-warmers I was making in that photo from Lake Fryxell (when we were watching the movie). I also made a pair of baby booties for my cousin (who is having a baby girl in March). Now I am making a pillow. I also taught Katie how to knit! She's made a hat, and is working on a scarf.

    Q: Do you have the weekends off to do what you want or are you working each day?
    A: We work pretty much every day of the week. When we're in town, we take Sunday mornings off to relax and sleep in, but are back at work after brunch. In the field, we work all day, every day. Usually, we only have time in the evenings to do what we want. We work very hard to get all of our work done in the two months we're here!

    Q: Here is a math problem from Owen and Alden:
    Becky found 395 tardigrades. Each tardigrade carried 3 rotifers on its back. How many rotifers are there?
    A: Ooh, sounds like there's a lot of rotifers in Owen and Alden's math problem! 1185.