5.2.1: Nitrogen fixation

Nitrogen-fixation occurs when microorganisms convert dinitrogen \(\left(\text{N}_{2}\right)\) to ammonia \(\left(\text{NH}_{3}\right)\) as illustrated in the following half reaction: \[\text{N}_{2} + 8 \ \text{H}^{+} + 8 \ e^{-} \longleftrightarrow 2 \ \text{NH}_{3} + \text{H}_{2}\]

Electrons necessary to reduce dinitrogen are provided by an electron carrier molecule, reduced ferrodoxin. The reaction is catalyzed by the nitrogenase complex, which consists of two proteins: a reductase and a nitrogenase. During the reaction, the electrons are deposited on the reductase, an iron protein, and then transferred to nitrogenase, a molybdenum-iron protein, where nitrogen is reduced to ammonia (Berg et al., 2002).

Only some bacteria and archaea are able to catalyze nitrogen fixation. The reaction does not provide them with energy. Instead, they have to use energy to drive the reaction forward (16 mol ATP per mol of \(\text{N}_{2}\)). Why would they do it?

Ammonia as well as nitrate are important sources of nitrogen for plants and microorganisms and organisms need quite a lot of nitrogen. In fact, nitrogen is the fourth most abundant element in biomass (see Chapter 7). Thus, by fixing nitrogen, the microorganisms provide themselves and other organisms with a nutrient that is critical for synthesizing biomass.

Some nitrogen fixers live symbiotically with plants. Examples of symbiotic bacteria include Rhizobium, Frankia, and some species of Azospirillum. These species enter the roots of host plants, where they multiply and stimulate formation of root nodules, which provide a shelter for the microorganisms. The plants benefit from interactions with the nitrogen fixers because they obtain usable nitrogen. The microorganisms benefit because they receive sugars produced by the plant via photosynthesis.

In addition to symbiotic nitrogen fixers, some nitrogen fixers are free-living. They live in close proximity to plant roots and also consume some of the sugar produced by plants. As such, free-living nitrogen fixation may be considered a subcategory of symbiotic nitrogen fixation, although the species involved are distinct and more diverse than plant-hosted nitrogen fixers (Smercina et al., 2019). Bacterial phyla that are known to contain some free-living nitrogen fixers include Alphaproteobacteria, Betaproteobacteria, Deltaproteobacteria, Gammaproteobacteria, Firmicutes, Cyanobacteria, and green-sulfur bacteria (Gaby and Buckley, 2015).

The symbiotic relationship between nitrogen fixers and some plants provides the basis for crop rotations. By this approach, plants hosting nitrogen fixers are grown periodically to increase the quantity of biologically available nitrogen in the crop soil. For example, farmers in the Midwestern US often rotate soybeans and corn. Soybean plants form root nodules that host nitrogen fixers. As such, they increase soil nitrogen content, which helps offsets the nitrogen demand of corn the following year(s).

Farmers have used crop rotations for thousands of years and continue to do so. However, early in the twentieth century, the Haber-Bosch process was developed, which is a chemical means of fixing nitrogen. Widespread use of nitrogen fertilizer produced by this process has greatly boosted food production worldwide but also greatly altered nitrogen cycling. Fluxes of nitrogen from Haber-Bosch as well as fossil fuel combustion have caused a doubling of the turnover rate for the global nitrogen cycle (Gruber and Galloway, 2008).