Nitrogen Fixation

Part I. The range of organisms that can fix nitrogen

Section A: Everything but the legumes
Section B: The legumes

(Note: If you click on any of the pictures displayed on this page, you will be shown a larger, higher-quality version of the same image.)

Figure 1. A microbial mat at a hot spring in eastern Oregon. Some of the organisms in the mat are cyanobacteria that can fix nitrogen. Cyanobacteria are found in a variety of terrestrial and aquatic habitats. Many, but not all, are capable of nitrogen fixation.


Figure 2. Left: Free-living colonies ("balls") of Nostoc in a fresh water pond in southern Oregon. Each ball is about 0.5 to 1.0 cm in diameter. The tadpole is Pseudacris regilla, the Pacific chorus frog. Right: Micrograph of Nostoc filaments with specialized nitrogen-fixing cells known as heterocysts.


Figure 3. The aquatic fern Azolla is the only fern that can fix nitrogen. It does so by virtue of a symbiotic association with a cyanobacterium ( Anabaena azollae ). Azolla is found worldwide and is sometimes used as a valuable source of nitrogen for agriculture. The plants shown here are each about 2 cm across. The pale yellow plant has been deprived of cobalt (essential for the cyanobacterial symbiont) and thus is showing typical signs of N deficiency.


Figure 4. This is another example of a cyanobacterial nitrogen-fixing association. In this case the cyanobacteria Nostoc (visible as small dark colonies in this photomicrograph) has taken up residence on leaves of a common leafy liverwort ( Porella navicularis ). The scale bar is 1 mm.

Figure 5. Porella navicularis as it appears to the naked eye (See Fig. 4). Porella is an abundant epiphyte in Pacific Northwest forests.

Figure 6a and b Nitrogen-fixing cyanobacteria can occasionally form associations with bryophytes (notably leafy liverworts) that are growing as "epiphylls" on the leaf surface of other plants. In this case the host plant is the palm Welfia regia , a common understory plant in the tropical rainforests around the La Selva field station which is operated by the Organization for Tropical Studies in Costa Rica. Some of the nitrogen fixed by the cyanobacteria is transferred directly to the host leaf.

Figure 7. Lobaria pulmonaria , a common N-fixing lichen in Pacific Northwest forests. The nitrogen-fixing symbiont is the cyanobacterium Nostoc which is to be found in pockets within the lichen referred to as cephalodia. Lichens such as this are a major source of N in old growth forests.

Figure 8. Cycads are gymnosperms ("naked-seed" plants) that are common in tropical climes, though a few do make it into Florida. They are have two unique features with regards to nitrogen fixation. Cycads are the only gymnosperms that fix and they are the only vascular plant that forms root nodules in which the prokaryotic partner is a cyanobacterium.


Figure 9. Shown here are the female cones of the cycad Ceratozamia mexicana . All cycads are dioecious(i.e. male and female parts are on different individual plants).


Figure 10. Cross section through coralloid root of Cycas revoluta showing the cyanobacterial zone.


Figures 11 & 12. Gunnera sp., an unusual angiosperm that contains nitrogen-fixing cyanobacteria in pockets at the base of petioles. This is often referred to as the only angiosperm that forms a nitrogen-fixing symbiosis with cyanobacteria, however, this is not strictly true since some tropical angiosperms have cyanobacterial films on their leaf surfaces. Gunnera has some of the largest leaves of any plant and is very common in parts of South and Central America where it is called "nalca." The young petioles are often peeled and eaten as demonstrated by Emily Ross (former Reed college student, at right) during her travels to Chile.  

Figure 13. Cross section through the stem of Gunnera with arrows indicating pockets of cyanobacteria ( Nostoc punctiforma ). This specimen is from the central mountains of Costa Rica


Figure 14. Young plants of red alder ( Alnus rubra ). Alder is a N-fixing plant that forms a symbiotic association with bacteria (more specifically an actinomycete) of the genus Frankia . There are about 21 genera of non-legumes that fix N. These plants are collectively called actinorhizal plants and are important contributors of N in ecosystems where fixed N is scarce. In this figure, all plants are the same age and are growing in sand. The plants on the left were inoculated with Frankia . The plants on the right were not inoculated and are displaying signs of extreme N deficiency. For more on the ecology of nitrogen fixation by alder go to the ecology section of this site.

Figure 15. Root system of red alder showing abundant root nodules.

Figures 16 & 17. Snowbrush ( Ceanothus velutinus ), a common nitrogen-fixing actinorhizal shrub throughout the western U.S  

Figure 18. Root nodules of snowbrush ( Ceanothus velutinus ).

Figure 19. Wax myrtle ( Myrica californica , on left), a common N-fixing actinorhizal shrub found in sandy coastal areas in the western U.S.

Figure 20. Mountain-mahogany ( Cercocarpus ledifolius ), a common nitrogen-fixing actinorhizal shrub in arid regions of the western U.S. It is often found on ridge tops in central and eastern Oregon.

Figures 21 & 22. Bitterbrush ( Purshia tridentata ), a nitrogen-fixing actinorhizal shrub in the rose family (Rosaceae). Bitterbrush is common in the understory of ponderosa pine forests in the Pacific Northwest. It is highly desirable forage for deer and livestock

Figure 23. Buffalo berry ( Shepherdia argentea ), an actinorhizal shrub from Arizona.

Figure 24. Beefwood ( Casuarina equisetifolia ), a common nitrogen-fixing actinorhizal tree native to the Old World tropics, but now widely introduced throughout tropical beaches worldwide including the Caribbean and Hawaii.

Figure 25. Excised root nodules of beefwood ( Casuarina equisetifolia ).

...and don't forget to visit Section B: The legumes

Photo Credits
Figures 3, 7,14, and 15 Harold Evans, Oregon State Univ.
Figures 16 , 17, and 23

Keith Karoly, Reed College

All other photos by David Dalton


Nitrogen Fixation Home Page

Part I. The range of organisms that can fix nitrogen

Part II. Physiology and anatomy of nitrogen fixation

Part III. Ecology of nitrogen fixation

This page was created for David Dalton, a faculty member in the Biology Department at Reed College.

Questions or comments? Send e-mail to:
Created 7/31/97. Last modified 9/17/97.
Copyright ©1997 David Dalton