Nitrogen Fixers

Nitrogen-fixing Bacteria

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:

  • Proteobacteria (includes the nitrogen-fixing alphaproteobacteria genera Rhizobium, Bradyrhizobium, Azospirillum, and Beijerinckia, and the nitrogen-fixing gammaproteobacteria genus Azotobacter)

  • Actinobacteria (includes the nitrogen-fixing genus Frankia)

  • Cyanobacteria (includes the nitrogen-fixing genera Anabaena, Nostoc, Trichodesmium, Calothrix, Phormidium, Scytonema, and Oscillatoria)

  • Endospora (includes the genus Clostridium, which contains some nitrogen-fixing species)

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:

Amorpha fruticosa (false indigo)
Amphicarpaea bracteata (hog-peanut)
Apios americana (groundnut/wild bean)
Arachis hypogaea (peanut)
Astragalus spp. (milk-vetch)
Baptisia spp. (Indigo)
Caragana arborescens (Pea-tree)
Cicer arietinum (chick-pea)
Cladrastis kentukea (yellowwood)
Colutea arborescens (bladder-senna)
Coronilla varia (crown-vetch)
Crotalaria sagittalis (rattlebox)
Cytisus spp. (broom)
Dalea leporina (foxtail prairie-clover)
Desmodium spp. (tick-trefoil)
Galega officinalis (goat’s rue)
Genista tinctoria (Dyer’s greenweed)
Glycine max (soybean)
Glycyrrhiza lepidota (licorice)
Laburnum anagyroides (golden chain tree)
Lathyrus spp. (pea/vetchling)
Lens culinaris (lentil)
Lespedeza spp. (bush-clover)
Lotus spp. (trefoil)
Lupinus spp. (lupine)
Medicago spp. (medick/alfalfa/bur-clover)
Melilotus spp. (sweet clover)
Phaseolus spp. (scarlet runner or bush bean)
Pisum sativum (pea)
Pueraria montana (kudzu)
Robinia spp. (locust)
Scorpiurus muricatus (scorpionpod)
Sesbania exaltata (Colorado river-hemp/indigo-weed)
Strophostyles helvula (beach or wild bean)
Tephrosia virginiana (goat’s rue)
Thermopsis villosa (hairy bush-pea)
Trifolium spp. (clover)
Trigonella spp. (fenugreek)
Ulex europaeus (gorse, furze)
Vicia spp. (vetch)
Wisteria spp. (wisteria)

white clover nodules white clover nodules
Nodules on the roots of (Trifolium repens)

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:

Alders (Alnus spp.) [Betulaceae/Birch Family]:
Alnus glutinosa (black or European alder)
Alnus incana (speckled alder)
Alnus serrulata (smooth alder)
Alnus viridis (mountain or green alder)

Elaeagnaceae (Oleaster Family) species:
Elaeagnus angustifolia (Russian olive)
Elaeagnus pungens (Japanese oleaster)
Elaeagnus umbellata (autumn olive)

Myricaceae (Bayberry Family) species:
Comptonia peregrina (sweet fern)
Myrica gale (sweet gale)
Myrica pensylvanica (bayberry)

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:

Azolla caroliniana (floating fern) – Anabaena occurs in cavities in the undersides of the leaves of this introduced fern.

Codium fragile (green fleece) – Nitrogen-fixing cyanobacterial symbionts of this introduced seaweed include Calothrix, Anabaena, and Phormidium.

Sphagnum spp. (Sphagnum moss) - Nostoc is usually attached to the exterior of the moss, and, on rare occasions may be found inside the plant cells.

Other plants with nitrogen-fixing cyanobacterial symbionts include certain species of liverworts and hornworts and other mosses, including Polytrichum.


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Gosner, K.L. 1978. A Field Guide to the Atlantic Seashore from the Bay of Fundy to Cape Hatteras. Houghton Mifflin Company: Boston.

Margulis, L. and K.V. Schwartz, 1998. Five Kingdoms. An Illustrated Guide to the Phyla of Life on Earth. Third Edition. W.H. Freeman and Company, New York. 520 pp.

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Sorrie, B.A. and Sommers, P. 1999. The Vascular Plants of Massachusetts: A County Checklist. Massachusetts Division of Fisheries and Wildlife Natural Heritage & Endangered Species Program.

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