Solar-powered N₂fixation in ferns: the<i>Azolla-Anabaena</i>symbioses

Synopsis The heterosporous aquatic ferns in the genus Azolla contain a heterocystous cyanobacterium, Anabaena azollae , as a symbiont. The Anabaena occupies cavities formed in the aerial dorsal leaf lobes of the ferns and can provide the symbiotic associations with their total N requirement via the fixation of atmospheric nitrogen. The photosynthetic pigments of the fern and cyanobacterium are complementary. Photosynthesis is of course the source of energy for growth and the ultimate source of the ATP and reductant required for N 2 fixation in the light or dark. However, nitrogen fixation is maximal in the light and the phycobili-proteins of the Anabaena are as effective as its chlorophyll in driving this photosystem I-linked process. The partners exhibit a coordinated pattern of development with the Azolla exerting a control over the Anabaena , affecting both its metabolism and differentiation. Anabaena filaments associated with the fern apices lack heterocysts. As cavities are formed and occupied by the Anabaena , it differentiates a high proportion of heterocysts and exhibits nitrogenase activity. In mature cavities, the Anabaena receives fixed carbon from the Azolla and releases fixed N 2 as ammonium. The ammonium is assimilated and/or transported by the Azolla toward its stem apices. Special epidermal cavity trichomes, which are intimately associated with the Anabaena at all stages in the ontogeny of the association, may facilitate metabolite exchange between the fern and cyanobacterium.