N₂-Fixation by Freshly Isolated Nostoc from Coralloid Roots of the Cycad Macrozamia riedlei (Fisch. ex Gaud.) Gardn.

Abstract

Nitrogenase (EC 1.7.99.2) activity (acetylene reduction) and nitrogen fixation (15N2 fixation) were measured in cyanobacteria freshly isolated from the coralloid roots of Macrozamia riedlei (Fisch. ex Gaud.) Gardn. Light and gas phase oxygen concentration had marked interactive effects on activity, with higher (up to 100-fold) rates of acetylene reduction and 15N2 fixation in light.The relationship between ethylene formation and N2-fixation varied in the freshly isolated cyanobacteria from 4 to 7 nanomoles of C2H4 per nanomole 15N2. Intact coralloid roots, incubated in darkness and ambient air, showed a value of 4.3. Maximum rates of nitrogenase activity occurred at about 0.6% 02 in light, while in darkness there was a broad optimum around 5 to 8% 02. Inhibition of nitrogenase, in light, by PO2 above 0.6% was irreversible. Measurements of light-dependent 02 evolution and 14CO2 fixation indicated negligible photosynthetic electron transport involving photosystem 11 and, on the basis of inhibitor studies, the stimulatory effect of light was attributed to cyclic photophosphorylation. Nitrogenase activity of free-living culture of an isolate from Macrozamia (Nostoc PCC 73102) was only slightly inhibited by 02 levels above 6% 02 and the inhibition was reversible. These cells showed rates of light-dependent 02 evolution and 14CO2 fixation which were 100-to 200-fold higher than those by the freshly isolated symbiont. Furthermore, nitrogenase activity was dependent on both photosynthetic electron transport and photophosphorylation. These data indicate that cyanobacteria within cycad coralloid roots are differentiated specifically for symbiotic functioning in a microaerobic environment. Specializations include a high heterocyst frequency, enhanced permeability to 02, and a direct dependence on the cycad for substrates to support nitrogenase activity.

Direct measurement of release and assimilation of ammonia in the Gunnera–Nostoc symbiosis

“…The symbiotic condition of Nostoc witbin host cells appears to provide a micro-aerobic environment not unlike that of the legutne nodule, in which the isolated bacterium is quite unprotected from oxygen. Tbis is in contrast to the cycad isolates (Lindblad, Atkins & Pate, 1991) wbicb appear to tolerate a low oxygen tension.…”

Tansley Review No. 116

“…However, the cyanobionts from cycads and Gunnera require microaerobic conditions in which to fix N # (Lindblad et al, 1991 ;Silvester et al, 1996). It is uncertain what changes have occurred in these heterocysts to make them oxygen-sensitive.…”

“…Although the cyanobionts develop heterocysts, these are not able to provide nitrogenase with protection from oxygen. Cyanobionts freshly isolated from roots or glands require microaerobic conditions to fix N # (Lindblad et al, 1991 ;Silvester et al, 1996). In symbiosis with diatoms, the cyanobiont Richelia does not seem to produce gas vesicles and might therefore decrease its energy costs by depending on the host for buoyancy.…”

Nodularin, a cyanobacterial toxin, is synthesized in planta by symbiotic Nostoc sp.

“…A total of 24 endosymbiont species, mostly Nostoc spp., were recently isolated from the coralloid roots of 74 cycad plants (Gehringer et al, 2010). It has been previously demonstrated that the symbiotic cyanobacteria in Macrozamia riedlei, while photosynthetically inactive, fixed nitrogen and transferred it to the host cycad plant in the form of citrulline and glutamine (Lindblad et al, 1991). Microcystin-producing Nostoc symbionts of lichens have also been identified (Oksanen et al, 2004;Kaasalainen et al, 2009), with both strains producing several variants of microcystin.…”