CORALLOID ROOTS AND NITROGEN FIXATION
Juliana Medeiros and Dennis Stevenson
Cyanobacteria in Symbiosis with Cycads
The most noticeable difference between free living cyanobacteria and those in symbiosis with cycads is that heterocyst frequency in symbiosis is greatly increased. Heterocyst frequency increases moving from the tip to the base of the root. Heterocysts can be found in multiples within coralloid roots, with highest concentration of multiples nearer to the base of the root. Here they become increasingly degenerate, eventually being reduced to a mucilagenous mass. These may be destined to release their contents, including stores of fixed nitrogen
(Grilli Caiola, 1990).
Another difference between cycad symbionts and their free living counterparts is that cyanobacteria within coralloids are not photosynthetic but chemoheterotrophic, relying on fixed carbon supplied by the cycad. The mantle, an opaque periderm covering mature coralloids, prevents light from reaching the cyanobacterial zone. Even so, they retain all of the photosynthetic apparatus, carbon-fixing machinery and subcellular arrangement of autotrophic individuals. Like their free-living counterparts, cyanobacterial vegetative cells within coralloids contain
Phycobilisomes contain phycobiliproteins, water-soluble accessory pigments involved in the operation of
Phycobiliproteins absorb light in a broad spectral range between 500-650nm, complementary to chlorophyll a absorption. These may represent up to 50% of cyanobacterial total cellular protein. The wasteful nature of maintaining these structures under dark conditions is puzzling. The increase in differentiation of vegetative cells to heterocysts is in fact the only prominent modification of cyanobacteria present in coralloid roots. In contrast to other symbioses with cyanobacteria, those within coralloids remain largely unchanged in most aspects. Cellular and subcellular localization of nitrogenase and glutamine synthetase, glutamine synthetase activity levels, as well as cellular concentration of glutamine synthetase and
of cycad symbionts are all comparable to that of free-living cyanobacteria. This suggests an inability of cyanobacteria to regulate these activities
Lindblad and Bergman, 1988)..
The photosynthetic apparatus not only retains its morphology but also remains completely functional, as shown by the ability of cyanobacteria isolated from coralloids to regain photosynthetic ability about two weeks after isolation
(Joubert et al., 1989).
In addition to being light-free, the coralloid environment is microaerobic, in contrast to that of free living cyanobacteria. A study by
Lindblad et al., (1991)
indicated that increased frequency and oxygen permeability of heterocysts in coralloid roots is specific to a symbiotic, microaerobic, heterotrophic environment. They found that nitrogenase from symbiotic cyanobacteria was irreversibly inactivated by ambient oxygen pressure, whereas oxygen inhibition of nitrogenase isolated from free-living forms is completely reversible.