Abstract
Cyanobacteria constitute one of the largest sub groups of Gram-negative photosynthetic prokary otes and are of great evolutionary antiquity, dating back to the Precambrian (2.8-3.5 billion years ago). They are found worldwide in highly diverse ecosystems, from aquatic (limnic and marine) habi tats to terrestrial systems and from polar to tropical regions of the globe. In terrestrial systems, nitro gen-fixing Nostoc is by far the most common genus and includes cyanobacteria capable of forming symbioses with a broad range of plants and other organisms. Hosts include fungi (Geosiphon species and lichenised fungi), bryophytes (liverworts and hornworts), aquatic ferns (genus Azolla), gym nosperms (cycads) and angiosperms (genus Gunnera) (Bergman et al. 1996).
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