Biological Nitrogen Fixation | Learn Science At Scitable - Nature

The reduction of atmospheric nitrogen is a complex process that requires a large input of energy to proceed (Postgate 1982). The nitrogen molecule is composed of two nitrogen atoms joined by a triple covalent bond, thus making the molecule highly inert and nonreactive. Nitrogenase catalyzes the breaking of this bond and the addition of three hydrogen atoms to each nitrogen atom. Microorganisms that fix nitrogen require 16 moles of adenosine triphosphate (ATP) to reduce each mole of nitrogen (Hubbell & Kidder, 2009). These organisms obtain this energy by oxidizing organic molecules. Non-photosynthetic free-living microorganisms must obtain these molecules from other organisms, while photosynthetic microorganisms, such as cyanobacteria, use sugars produced by photosynthesis. Associative and symbiotic nitrogen-fixing microorganisms obtain these compounds from their host plants’ rhizospheres (National Research Council 1994, Hubbell & Kidder 2009). Industries use the Haber-Bosch process to reduce nitrogen essentially in the same way. Conventional agriculture has depended upon this process to produce the commercial fertilizer needed to grow most of the world’s hybrid crops. But this approach comes with many consequences, including using fossil fuels for the energy needed to produce this fertilizer, the resulting carbon dioxide emissions and pollution from burning these fuels, and adverse affects on human health (Vitousek 1997). Overuse of these chemical fertilizers has led to an upset in the nitrogen cycle and consequently to surface water as well as groundwater pollution. Increased loads of nitrogen fertilizer to freshwater, as well as marine ecosystems, has caused eutrophication, the process whereby these systems have a proliferation of microorganisms, especially algae. This “greening” of the water column has caused decreased levels of dissolved oxygen (DO) in bottom waters as planktonic algae die and fuel microbial respiration. These depleted DO levels result in massive mortality of aquatic organisms and create so-called dead zones, areas where little or no aquatic life can be found (Figure 2). Since the 1960’s, dead zones have increased exponentially worldwide, and have now been documented from over 400 systems, affecting more than 245,000 square kilometers of coastal regions (Diaz & Rosenberg 2008, Figure 3). This phenomenon is now deemed the key stressor on marine ecosystems.Figure 2True-color image of Mississippi River sediment deposition into the Gulf of Mexico.Courtesy of NASA.

Figure 3Aquatic dead zones across the world.Courtesy of NASA.