Certain bacteria are capable of fixing nitrogen. In this process, nitrogen gas (N2) is converted to ammonium (NH4+), a form of nitrogen that is biologically available to plants. The reaction is catalyzed by the enzyme nitrogenase. Because nitrogenase is inactivated by oxygen, the reaction must occur in a low oxygen environment.
Species capable of fixing atmospheric nitrogen generally belong to one of the following phyla of bacteria:
Because nitrogen-fixing genes may be carried on plasmids, which may be transferred from one bacterium to another of a different type, other bacteria, in addition to those listed above, may acquire this ability.
Some nitrogen-fixing bacterial species are free-living (e.g., Azotobacter and Beijerinckia) while others have symbiotic associations with plants. Symbiotic bacteria fix, by far, the greatest amounts of nitrogen. Nitrogen fixation cannot occur in plants without nitrogen-fixing bacterial symbionts. The association is mutually beneficial, with the bacteria providing the plant with a source of nitrogen (a macronutrient required by all living things to make proteins and other molecules) that may be otherwise lacking in the soil, and the plant providing the bacteria with a protective, low oxygen environment and a source of organic carbon.
Plant Species with Nitrogen-Fixing Symbionts
Nitrogen-fixing bacteria that form symbiotic associations with plants include Rhizobium, Bradyrhizobium, Frankia, Nostoc, and Anabaena. The associations are species specific, and in many cases highly evolved. Plant species occurring in Massachusetts that host nitrogen-fixing symbionts are listed below.
Plants with Rhizobium, Bradyrhizobium, or other Alphaproteobacteria symbionts
Alphaproteobacteria may be found in nodules on the roots of plants belonging to the family Fabaceae (Pea or Bean Family). Inside the nodules are molecules of an iron-containing protein called leghemoglobin, which is produced by the bacteria and the host plant together. Like the hemoglobin in our blood, leghemoglobin binds oxygen. This binding is crucial in order to prevent oxygen from interfering with the nitrogen fixation reaction. The following plant species found in Massachusetts host alphaproteobacteria symbionts:
Plants with Frankia symbionts
Like Rhizobium and Bradyrhizobium, Frankia spp. induce nodule formation in the roots of host plants. Nodules containing Frankia are referred to as actinorrhizal root nodules. The following plant species found in Massachusetts host Frankia symbionts:
Plants with Cyanobacterial symbionts
A number of plants host nitrogen-fixing cyanobacteria such as Nostoc or Anaebaena. In many cases, the cyanobacteria are attached to the outside of the plant. The following plants found in Massachusetts may have nitrogen-fixing cyanobacterial symbionts:
Other plants with nitrogen-fixing cyanobacterial symbionts include certain species of liverworts and hornworts and other mosses, including Polytrichum.
Dyer, B.D., 2003. A Field Guide to Bacteria. Comstock Publishing Associates, Ithaca, New York. 355 pp.