We are a group of freshwater ecologists from the Biology Department at St. Catherine University in Saint Paul, Minnesota traveling to Iceland this summer to study the effect of temperature on nitrogen fixation rates in geothermally active streams in the Hengill region of Iceland. We are collaborating with a group of scientists from the U.S. and Iceland in our research.

Sunday, July 7, 2013

"The" Number

We found algae!
We finally had some decent weather after days of rain and wind, and we were able to go out and make our first measurements of nitrogen fixation on the tiles associated with the channel temperature experiment. Algae (and all photosynthetic organisms) typically acquire their source of nitrogen from the soil and water around them, but some species, called cyanobacteria, are able to acquire it from the atmosphere. The species that can do this have an enzyme called nitrogenase that can break apart the two nitrogen molecules in nitrogen gas (N2), the most abundant gas in the atmosphere. They then use the nitrogen to build amino acids and other nitrogen containing molecules for growth. 

This summer we are planning to measure nitrogen fixation with three different approaches as a comparison of the methods, as well as to use them as a check against each other.  For
Chemical conversion of nitrogen fixation (left) and
how it compares to acetylene reduction (right).
our first sampling day, we only used one method, which is called the Acetylene Reduction Assay, or ARA. The ARA method involves making acetylene gas, which we do in the field using calcium carbide and water in a flask, and then collect the gas in either a balloon or syringe. We then inject the acetylene gas into gas-tight chambers with the algal samples and then take an initial and final sample of the gas in the chamber. During the incubation the algae in the chamber are breaking the triple bond between the two carbon atoms in acetylene to produce ethylene (see diagram), which is similar to the chemical reaction in nitrogen fixation (see diagram), and we are able to measure production of ethylene gas and use this as a measure of how much nitrogen the algae are fixing. 

Working on getting "the" number.
Over the years, the ARA method has been modified and refined for better results while still trying to keep the procedure fairly simple. This summer, we are collaborating with another project and we are measuring nitrogen fixation on tiles that have been colonized with algae.  As part of this effort, we had to modify the ARA method again for use with custom chambers that were built for the project. This came with several challenges that involved some creative thinking and problem-solving skills, which are essential skills to have when conducting research. The physical act of research is easy; going out in the field, collecting data using technical instruments, putting samples of water and algae into vials, and following previously thought out methods.  However, it is naive to think that’s all there is to research. As Jill always says, “It’s easy to get 'a' number, but more challenging to get 'the' number”. What she means is that it’s easy to go out into the field, spends hours collecting data, and reading numbers off of instruments, but the hard part is deciding what that number tells us, and if we used correct and precise methods for obtaining the actual number that represents part of the answer to our question. For example, when we are trying to figure out how to make gas-tight chambers we have to think like a gas molecule and really channel our inner Sherlock Holmes to make sure we are getting good data, but at the same time not be biased.

So far, our research has been really frustrating, impossibly complicated, and infinitely
Excited after a day in the field of
hiking and looking at algae.
rewarding. I love a good challenge and this research is important in helping us understand how whole ecosystems work, which we can then use to help gauge how they are going to be affected by human activity. There is still so much we don’t know about our world and the ecosystem services we depend on for survival. I am starting to see how easy it is to build an entire career’s worth of work in research. I feel invested and driven by curiosity and I am really looking forward to seeing where it takes me.


  1. coolness! Question. . . you 'trick' the N-fixers into reducing acetylene instead of N2, but why are they tricked? Presumably they have a choice to fix N or 'tweak' acetylene. Why do they tweak acetylene?


  2. Well, the nitrogen fixers contain the enzyme nitrogenase, which is capable of breaking down both nitrogen gas and reducing acetylene into ethylene. It just so happens that acetylene inhibits nitrogen fixation, by having a higher affinity for the binding site on the enzyme than the nitrogen gas molecules. Nitrogen fixation is also energetically expensive and requires three times the amount of energy to break the triple bond in nitrogen gas than it does to break the triple bond in acetylene gas, because the nitrogen molecules bond together stronger than the carbon molecules of acetylene. It is not that the enzyme is not fixing nitrogen, but they are fixing more acetylene, so we use a theoretical ratio that says for every 3 moles of acetylene that are reduced, 1 mole of nitrogen gas is fixed (Stewart et al. 1967, Hardy et al. 1968), so we can use this ratio to correct for our final nitrogen fixation rates. Since this ratio is theoretical and based on chemical properties of the molecules, it is not always what is seen in real measurements of nitrogen fixation. Literature has shown ratios of acetylene reduced to nitrogen fixed to be anywhere from as low as 0.0216:1 (Liengen 1999) to as high as 94:1 (Seitzinger and Garber 1987). Because there has been such variation in these ratios, and discussion about what the right ratio is, we are interested in measuring what these ratios are for the Hengill watershed in Iceland, because they can really vary from site to site.