ABSTRACT
The Azolla-Anabaena symbiosis is a mutualistic association between the eukaryotic water fern Azolla and the prokaryotic, nitrogen-fixing cyanobacterium Anabaena. This association has gained attention, because it may represent a viable alternative to chemical nitrogen fertilizers. The growth and development of endosymbiotic Anabaena azollae are greatly influenced by the host fern. Although numerous physiological and biochemical studies on Azolla-Anabaena have been described, there is a paucity of information on ultrastructural characteristics of the cyanobacterial endosymbiont. Azolla can reproduce either sexually via sporocarp formation or asexually via vegetative fragmentation. Each megasporocarp contains one megasporangium (consisting of a single megaspore and the megaspore apparatus) and a distally located colony of Anabaena. Azolla microsporocarps are globular structures that are significantly larger than megasporocarps. The symbiotic association between Azolla and Anabaena is maintained throughout both sexual and asexual modes of reproduction. The cyanobacterial endophytes of all Azolla species are members of the Nostocaceae family and are collectively referred to as Anabaena azollae Strasburger.
