Fixation on Ice: "I Love the Smell of Calcium Carbide in the Morning!"

Getting ready for chamber
incubations in the field

We successfully measured nitrogen fixation rates in the field last week using the acetylene reduction method. It was great to be able to see our efforts come together and to be able to make actual measurements in the streams. One of our methods utilizes calcium carbide to generate acetylene gas, which we collect in a balloon that we then pop to release the gas, after it has been sealed within our gas-tight chambers. This technique allows us to measure nitrogen fixation rates because the of similar structure of atmospheric dinitrogen (N2) gas and acetylene, which "tricks" the nitrogenase enzyme into converting acetylene into ethylene. After releasing the acetylene gas into the chambers, we collect gas samples over time and then measure their ethylene content on a gas chromatograph. We then use the rate of ethylene production to estimate nitrogen fixation rates. We had great fun generating acetylene gas in a side arm flask and filling a balloon with it. Unfortunately, it was a little windy and rainy, making it challenging at times – something to get used to with the capricious weather in the Hengill watershed. The calcium carbide has a unique smell (like old, sweaty gym socks, yuck!), which Bayley enjoyed, prompting the title to this blog entry. After generating the acetylene, I got some practice collecting gas samples from the chambers, and more practice working with the gas chromatograph when we got back to the lab. We also processed our algal samples from the chambers, and we will have some precise nitrogen fixation values to compare shortly. The story behind each of the unique streams we are working with is just beginning to unfold. The algal diversity between streams is striking and it’s clear at first glance that there are some distinct changes in species composition along the temperature gradient. It seems like we have some powerful nitrogen fixers inhabiting the streams! It has been great to get our field measurements going, as working with these methods out in the streams is different from doing them in a lab, and we are still a little clumsy, especially with the challenges presented by the constantly changing weather conditions. It is very fulfilling to know that we are beginning to put all of our preparations into full use, and that the time we are spending troubleshooting will pay off in a tangible way very shortly.