Bacterial population is found together with the cyanobacterium Anabaena azollae in the leaf cavities and the sporocarps of Azolla. Different Arthrobacter species have been invariably isolated from the leaf cavities and sporocarps of several fern species. About the role of the bacteria in the association, it is probable that the bacteria affect the carbon and nitrogen metabolism of the cavity by competing with the Anabaena for the nutrients and in the meantime it is likely that their high respiratory activity favours the establishment of a microaerobic environment and consequently the nitrogenase activity. Furthermore Arthrobacter may contribute to the production of the mucilage present in the algal packets by excreting extracellular polysaccharides, which components can be involved in lectin binding with Azolla and Anabaena. Finally the IAA formation by Arthrobacter may influence the hormonal balance during the establishment of the association.
