One of my favorite, and in my opinion one of the most important, topics in chemistry is isotopes.  Isotopes are atoms of the same element with different masses.  This means that the atoms have the same number of protons (which is what determines the element’s identity) but different numbers of neutrons – thereby giving a different atomic mass because

atomic mass = # protons + # neutrons

Every element has naturally-occurring isotopes.  For example, Carbon exists as 3 isotopes: Carbon-12 (6 protons + 6 neutrons and by far the most abundant of the C isotopes), Carbon-13 (6 protons + 7 neutrons) and Carbon-14 (6 protons + 8 neutrons) while Nitrogen’s two main isotopes are Nitrogen-14 (7 protons + 7 neutrons) and Nitrogen-15 (7 protons + 8 neutrons). Additionally, some isotopes, like C-14, are radioactive, whereas others like N-15 are stable.

What’s really interesting about isotopes, and how we’re using them in the research, is that they can be used to determine the rates and amounts of different biological processes.  Two examples of this that will be conducted during the expedition are uptake tracer experiments and natural abundance nitrogen isotope ratios.  Today’s post will discuss the uptake tracer experiment; I will talk about natural abundance in a later post.

Uptake Tracer Experiments

uptake tracer experiments.  the labeled N-15 nutrients that are taken up by the phytoplankton and analyzed by a mass spectrometer

uptake tracer experiments. the labeled N-15 nutrients that are taken up by the phytoplankton and analyzed by a mass spectrometer

Ammonium (15NH4+) and nitrate (15NO3) stocks containing N-15 instead of the vastly more abundant N-14 are injected into our seawater samples.  The phytoplankton take in these nutrients to build up their biomass and are left to grow in incubators (water temperature remains constant to allow phytoplankton to continue to grow) that are on deck.  After some time has elapsed, the phytoplankton and seawater are put through a very small filter to remove all the water; therefore, only phytoplankton are left on the filter.  A machine called a mass spectrometer then determines the amount of N-15 on the filter.  A mass spec combusts and ionizes a sample and separates the ions based on mass.  This allows us to see the mass profile of the phytoplankton and, consequently, how much 15NH4and 15NO3–  the phytoplankton consumed.

Because we know how much time elapsed during the incubation, and how much ammonium and nitrate the phytoplankton took up, we can use the final amounts to determine the rate of nutrient incorporation into the phytoplankton biomass.

We don’t have a mass spec onboard the R/V Endeavor.  In order to fully perform these analyses, the phytoplankton filters are immediately frozen after the filtration to preserve them.  The researchers studying them will run the mass spec analysis once we get back to Princeton.

Until next time,

Ms. B.


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