An ancient and pervasive group, cyanobacteria are found in soils around the globe in many diverse habitats. Cyanobacteria are
producing their own fixed carbon by oxigenic photosynthesis, or by true anoxygenic photosynthesis. They are also capable of chemoheterotrophy. In addition, many cyanobacteria fix nitrogen via nitrogenase. Self-sufficiency and flexibility are the key to this group's ability to colonize inhospitable, widely fluctuating environments. These characteristics are also factors in the propensity of cyanobacteria to enter into a wide range of symbiotic relationships. Cyanobacteria are found in symbiosis with a limited number of species from all the major plant groups; algae, fungi, bryophytes, pteridophytes, gymnosperms and angiosperms
(Carr and Whitton, 1982).
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Cyanobacteria inhabiting cycad coralloid roots are
containing heterocysts and vegetative cells. They are aligned in short linear or undulating fragments, or grouped into clusters. Vegetative cells and heterocysts are the predominant cell types in symbiosis, hormogonia and akinetes being rare. Akinetes are most often found in degenerate coralloids
(Grilli Caiola; 1975a,
Upon entering into symbiosis with cycads, cyanobacteria undergo structural-functional changes.
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Though it appears that nearly all cycad symbionts are from the genus Nostoc, cyanobiont populations have been shown to be species diverse between different cycad genera, and even within a single cycad individual. Several studies have shown a mixture of Nostoc strains can inhabit the root system of a single plant. In contrast, populations contained in individual coralloids have been shown to be
(Lindblad et al., 1989).
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