Nitrogen is one of the most important elements for biological systems. While we think of air as containing oxygen and maybe a little carbon dioxide, it’s mostly (80 %) nitrogen. Nitrogen is also one of the basic elements of life, along with carbon, hydrogen, phosphorus, and oxygen.
Although nitrogen is incredibly abundant in the air we breathe, it is often a limiting nutrient for the growth of living organisms. This is because the particular form of nitrogen found in air—nitrogen gas—cannot be assimilated by most organisms. Most plants, algae, and microbes require biologically available, or “fixed” forms of nitrogen, such as ammonia or ammonium. That’s why these compounds are a primary component of many plant fertilizers.
In the open ocean, as on land, fixed nitrogen is one of the most important growth-limiting nutrients for photosynthetic organisms (primary producers) such as algae and marine bacteria. Nitrogen can also serve as an energy source or as an oxidant for marine bacteria and archaea. (Archaea are single celled organisms that look similar to bacteria, but which are in an entirely separate biological domain).
The ocean absorbs nitrogen gas from the atmosphere. In open-ocean areas with low concentrations of nutrients (“oligotrophic” regions), some of this nitrogen is taken up by microbes and transformed into various chemical compounds. The key steps in this process are listed below and shown in the simplified diagram at right (click on it to see a larger version).
The open ocean is host to a wide variety of microbes, each adapted to a perform one or more steps in the nitrogen cycle. This enormous diversity of microbes has evolved to use every form of nitrogen in the ocean, either as a source of energy or as a nutrient. However, “reduced” nitrogen compounds, such as ammonium are easier for most microbes to use than “oxidized” nitrogen compounds such as nitrate.
Many types of microbes extract energy from nitrogen compounds by “oxidizing” these compounds (for example, converting ammonium to nitrite or nitrite to nitrate). However, relatively few microbes can “reduce” nitrogen (for example converting nitrogen gas to ammonium), because this requires extra energy, which they must get from other sources (“food” and/or sunlight).
For more information on some the different types of nitrogen-processing microbes we’ll be studying during the BioLINCS cruise, see the marine microbes page.
[ Top of Page ]