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

Another milestone is reached.  This cruise has definitely been a fruitful one.  On the first of the three ANACONDAS cruises (the one that I was not on), tons of DDAs were found.  That information, along with previous cruises to the ocean further north of here, lead to the hypothesis I discussed earlier.  That the DDAs were helping to draw down CO2 from the atmosphere making the Amazon River plume a net sink and therefore possibly affecting climate change.

Last cruise however we did not see many DDAs and the few times we did there were not many.  However we did see lots and lots of Tricho throughout the cruise and they were in areas that you would not normally expect to find them.  They tend to like the low nutrient, high salinity open ocean but we found them in the mesohaline (kinda middle of the road for salinity) plume where the nutrients can vary but are usually high.  I was able to measure nitrogen fixation and they appeared to be limited by phosphorous because when I added some to my incubations they would fix more compared to Tricho I did not add any phosphorous.  This combined with finding them in areas we did not expect was pretty interesting.  But not finding DDAs where we thought they would be was perplexing.

This cruise we found lots of Tricho in the open ocean areas and the plume again.  They were fixing but did not seem to be limited by phosphorus.  An interesting finding though is that they are fixing when there are other sources of nitrogen in the water.  Fixing nitrogen costs a lot of energy so it is thought that they would not waste the energy to fix if they did not have to.  Pretty cool.

Anyway, we had been looking for them all cruise again with no luck.  I was kinda thinking that they might not exist.  Then just to top off an incredible cruise we find them at the last station!  What luck!  They were pretty abundant as well.  We found two diatoms, Hemiaulus and Rhizosolenia, with the nitrogen fixing symbiot Richelia inside.  I was able to concentrate a water sample and measure a rate of nitrogen fixation.  This is the first time I have ever measured nitrogen fixation on DDAs!!!!  It was a great day.  Now hopefully my stable isotope probing incubations work as well and then I will win twice brother.  We will see. For now things just seem great.