Abstract
The Azolla-Anabaena symbiosis is an association between the water fern Azolla and the heterocystous, N 2-fixing cyanobacterium, A nabaena azollae. The A nabaena is associated with all stages of the fern's life cycle, and in the sporophyte the two organisms undergo a parallel pattern of differentiation
and development. Undifferentiated Anabaena filaments are associated with the apical regions of the fern. These filaments are partitioned into cavities formed in the dorsal leaf lobes where they rapidly differentiate heterocysts and synthesize nitrogenase. Filaments in the cavities of mature leaves exhibit a high heterocyst frequency and dinitrogen fixed by these filaments is transported to the apices. Dinitrogen fixed by the Anabaena can meet the total N requirement of the association and when the symbiosis is provided with an exogenous source of combined nitrogen, such as ammonium or nitrate, fixation of dinitrogen continues to account for about 50% of the total nitrogen input.
The association, the endophyte-free Azolla, and the Anabaena freshly isolated from the leaf cavities are photosynthetical competents. Rates of carbon dioxide fixation by the freshly isolated Anabaena, which are similar to those of free-living cyanobacteria, appear to be considerably greater than the rate of CO2 fixation by the Anabaena in the leaf cavities. Our data indicate that the A nabaena contributes very little to the total carbon fixation of the association and that the fern actually provides fixed carbon to the Anabaena.
and development. Undifferentiated Anabaena filaments are associated with the apical regions of the fern. These filaments are partitioned into cavities formed in the dorsal leaf lobes where they rapidly differentiate heterocysts and synthesize nitrogenase. Filaments in the cavities of mature leaves exhibit a high heterocyst frequency and dinitrogen fixed by these filaments is transported to the apices. Dinitrogen fixed by the Anabaena can meet the total N requirement of the association and when the symbiosis is provided with an exogenous source of combined nitrogen, such as ammonium or nitrate, fixation of dinitrogen continues to account for about 50% of the total nitrogen input.
The association, the endophyte-free Azolla, and the Anabaena freshly isolated from the leaf cavities are photosynthetical competents. Rates of carbon dioxide fixation by the freshly isolated Anabaena, which are similar to those of free-living cyanobacteria, appear to be considerably greater than the rate of CO2 fixation by the Anabaena in the leaf cavities. Our data indicate that the A nabaena contributes very little to the total carbon fixation of the association and that the fern actually provides fixed carbon to the Anabaena.
| Original language | English GB |
|---|---|
| Pages (from-to) | 53-67 |
| Journal | Symbiosis |
| Volume | 6 |
| Issue number | 1-2 |
| State | Published - 1988 |