to the order Rhizobiales. Actinobacteria provided the other option and we call their
descendants Frankia, a genus that was named in honor of Bernhard Frank. The
Frankia are members of the phylum Actinobacteria, order Frankiales.
26.6.1 Cyanobacteria as Symbionts that Provide Fixed
Nitrogen for Their Host Plants
Azolla is a genus of water ferns, also called mosquito ferns, that grow floating on the
surface of freshwater. As shown in Fig. 26.21, these plants contain leaf pockets in
which are housed symbiotic cyanobacteria of the genus Anabeana. The Anabeana
provide fixed nitrogen to the plant. In warm climates where Azolla can overwinter, it
has a long history of being used as a natural fertilizer in rice paddies. The density of
Azolla plants also can block light which prevents competition from plants other than
the rice. Rice plants are transferred to the paddies after growing tall enough to
emerge above the Azolla. Cyanobacteria have a similar asociation with Lemna and
Spirodela, both of which are floating freshwater plants commonly called duckweed.
Cycads (class Cycadopsida) are an ancient group of terrestrial plants which
produce lateral roots that contain coralloid structures, meaning that those structures
are coral-like in appearance. Figure 26.22 shows the difference between coralloid
roots and more typical plant roots of cycads. Coralloid roots can be visible on the soil
surface and they can extend to approximately 30 cm beneath the surface. Figure 26.23 shows a large mass of coralloid roots growing on a larger cycad. There
is a diverse group of microorganisms associated with these coralloid roots. Importantly, cyanobacteria enter the coralloid roots of cycads and there the cyanobacteria
become endosymbiotic. These symbiotic microbes typically are identified as
Anabaena cycadae, also recognized as Nostoc cycadae. The coralloid roots similarly
may contain members of the cyanobacterial genus Calothrix. The role of these
symbiotic cyanobacteria includes fixing nitrogen and additionally producing beneficial amino acids among which are asparagine and citrulline. Figure 26.24 shows the
ring shaped band of cyanobacteria that is located inside of a cycad coralloid root.
I want to thank Prashant Singh for very kindly helping me to understand the state
of knowledge and questions that still exist regarding the taxonomy of those
Fig. 26.20 (continued) Light microscopy, differential interference contrast (DIC)”. The lower right
image is titled “Mikrofoto.de-Stentor-1.jpg” by author Frank Fox, it is being used under a Creative
Commons Attribution-Share Alike 3.0 Germany license, the image has been adjusted and cropped.
Stentor polymorphus, the species shown in this image, typically can be 500–1500 μm in length. The
lower left image is titled “Myrionecta rubra.jpg” by author Minami Himemiya, it is being used
under a Creative Commons Attribution-Share Alike 3.0 Unported license and has been cropped.
Myrionecta rubra can be up to 100 μm in length and 75 μm in width. Myrionecta rubra feed upon
algae of the class cryptophyceae and form blooms that are called Red Tide. All of these images have
been rotated 90 degrees to the right so that the combined figure fits better on a printed page
570
C. J. Hurst
descendants Frankia, a genus that was named in honor of Bernhard Frank. The
Frankia are members of the phylum Actinobacteria, order Frankiales.
26.6.1 Cyanobacteria as Symbionts that Provide Fixed
Nitrogen for Their Host Plants
Azolla is a genus of water ferns, also called mosquito ferns, that grow floating on the
surface of freshwater. As shown in Fig. 26.21, these plants contain leaf pockets in
which are housed symbiotic cyanobacteria of the genus Anabeana. The Anabeana
provide fixed nitrogen to the plant. In warm climates where Azolla can overwinter, it
has a long history of being used as a natural fertilizer in rice paddies. The density of
Azolla plants also can block light which prevents competition from plants other than
the rice. Rice plants are transferred to the paddies after growing tall enough to
emerge above the Azolla. Cyanobacteria have a similar asociation with Lemna and
Spirodela, both of which are floating freshwater plants commonly called duckweed.
Cycads (class Cycadopsida) are an ancient group of terrestrial plants which
produce lateral roots that contain coralloid structures, meaning that those structures
are coral-like in appearance. Figure 26.22 shows the difference between coralloid
roots and more typical plant roots of cycads. Coralloid roots can be visible on the soil
surface and they can extend to approximately 30 cm beneath the surface. Figure 26.23 shows a large mass of coralloid roots growing on a larger cycad. There
is a diverse group of microorganisms associated with these coralloid roots. Importantly, cyanobacteria enter the coralloid roots of cycads and there the cyanobacteria
become endosymbiotic. These symbiotic microbes typically are identified as
Anabaena cycadae, also recognized as Nostoc cycadae. The coralloid roots similarly
may contain members of the cyanobacterial genus Calothrix. The role of these
symbiotic cyanobacteria includes fixing nitrogen and additionally producing beneficial amino acids among which are asparagine and citrulline. Figure 26.24 shows the
ring shaped band of cyanobacteria that is located inside of a cycad coralloid root.
I want to thank Prashant Singh for very kindly helping me to understand the state
of knowledge and questions that still exist regarding the taxonomy of those
Fig. 26.20 (continued) Light microscopy, differential interference contrast (DIC)”. The lower right
image is titled “Mikrofoto.de-Stentor-1.jpg” by author Frank Fox, it is being used under a Creative
Commons Attribution-Share Alike 3.0 Germany license, the image has been adjusted and cropped.
Stentor polymorphus, the species shown in this image, typically can be 500–1500 μm in length. The
lower left image is titled “Myrionecta rubra.jpg” by author Minami Himemiya, it is being used
under a Creative Commons Attribution-Share Alike 3.0 Unported license and has been cropped.
Myrionecta rubra can be up to 100 μm in length and 75 μm in width. Myrionecta rubra feed upon
algae of the class cryptophyceae and form blooms that are called Red Tide. All of these images have
been rotated 90 degrees to the right so that the combined figure fits better on a printed page
570
C. J. Hurst