 | Section A: Everything but the legumes |
| Section B: The legumes |
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 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
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
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 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). |
 |
Figures 10& 11. 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. |  |
 |
Figures 12. 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 13. 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
14. Root system of red alder showing abundant root nodules. |
 |
 Figures
15& 16. Snowbrush (Ceanothus velutinus),
a common nitrogen-fixing actinorhizal shrub throughout
the western U.S |
 |
Figure 17. Root
nodules of snowbrush (Ceanothus velutinus). |
 |
 |
Figure 18. Wax myrtle (Myrica californica, on left), a common N-fixing actinorhizal shrub found in sandy coastal areas in the western U.S. |
| Figure 19. 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 20& 21. 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
22. Buffalo berry (Shepherdia argentea), an actinorhizal
shrub from Arizona. |
 |
 |
Figure
23. 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
24. Excised root nodules of beefwood (Casuarina equisetifolia). |
 |
| Photo Credits | |
|---|---|
| Figures 3, 7,13, and 14 | Harold Evans, Oregon State Univ. |
| Figures 15 , 16, and 22 | Keith Karoly, Reed College |
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: david.dalton@reed.edu
Created 7/31/97. Last modified 9/17/97.
Copyright ©1997 David Dalton