USC Dana and David Dornsife College of Letters, Arts & Sciences

Now as you all know, one of the main things I do on these cruises is measure nitrogen (N) fixation.  I do this through the acetylene reduction assay where I add a substrate that tricks the enzymes involved in N fixation into thinking they are carrying out the process.  This process is something that I can measure easily.  They convert acetylene into ethylene and this change can be quantified on a gas chromatograph.  I think that’s enough detail.  As I have said earlier I take Tricho (if none of this seems familiar to any of you just look back at previous posts) and measure N fixation of these organisms alone.  They are very abundant in the open ocean and can actually be seen with the naked eye so it is easy to isolate them for these kinds of measurements.

To the right we can see a net tow that I just finished.    The net tow concentrates the things in the surface ocean and that is why it looks like this color.  There are a ton of zooplankton that are mainly copepods, which are tiny animals that feed on the plankton.  Below is a picture on one such copepod.  From here I pick out the Tricho.  Once I have them, I seal them in a bottle and the assay begins.

http://www.sfrc.ufl.edu/planktonweb/taxonomy.htm

Once the assay is setup, I periodically take a subsample of the gas in the bottle the Tricho are in and measure an increase of ethylene gas over time.  I use an equation to convert that into an amount of N fixation.  On each side you can see me taking one such subsample.  I am also decked out in some of my cruise gear.

Once the subsample is taken I inject the gas into the gas chromatograph and it is separate into each component of the gas.

I then sit back and wait about a minute for my results.

Finally I get a readout like this and I compare it to a standard.  The first big peak is the ethylene that was converted and the second huge peak is the total acetylene I added.  This can tell me how much, if any N is being fixed in real time.  I got a lot of good results this cruise!  Still need to process it all though.  Going to take a lot of time.