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Nitrogen Isotope Techniques

An isotope is a variant of an element with a different atomic weight. Two stable isotopes of nitrogen occur in nature: nitrogen-14 and nitrogen-15. The N-14 isotope is far more abundant, both in Earth's atmosphere and in plant and animal tissue. The comparative rarity of the N-15 isotope makes it valuable to biologists and other researchers as a tracer (i.e., a sort of flag that allows scientists to follow nitrogen through a biochemical or geochemical system).

Biological Macromolecules
- If cells in culture are supplied only with nutrients containing the N-15 isotope, their DNA will come to contain N-15 exclusively over the course of multiple generations. In a famous experiment, Matthew Meselson and Franklin Stahl grew E. coli in a culture medium containing heavy N-15 nutrients. They then transferred the bacteria to media containing the lighter N-14 isotope, and centrifuged DNA from these bacteria one generation and two generations later. The density of the DNA in the daughter cells was intermediate. Through this result, the Meselson-Stahl experiment demonstrated that DNA replication produces two copies, each containing an original strand that serves as a template for the new strand.
Pollution
- The ratio of N-15 to N-14 is often higher in sewage and agricultural waste than in ocean environments. As the distance from a sewage source increases, this ratio gradually trends toward the normal values for the marine environment. Scientists can use this difference to map sewage plumes, and to determine the origin of excess nitrogen in marine environments. These techniques are important because excess nitrogen contributes to harmful algal blooms in coastal waters.
Groundwater
- High concentrations of nitrates in groundwater can be harmful to humans, so tracking these contaminants is an issue of special concern. Depending on the path they take to reach local streams, nitrates may break down through the action of denitrifying bacteria, so scientists can use local concentrations of isotopes to track what happens to nitrates in groundwater (e.g., whether they end up in local streams, the route they take to reach those streams and so forth). As with seawater, they can also use isotope ratios to identify the origin of this contamination.
Ecology
- To understand ecosystems, ecologists need to know where organisms derive their nutrients, and the rate at which nutrients cycle through the ecosystem. Because nitrogen is an essential nutrient, and living organisms preferentially make use of the N-14 isotope, the N-14 to N-15 ratio differs in various parts of the food web. Ecologists can use this data to track nitrogen through an ecosystem.

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