Some are Symbiotic Cyanobacteria are involved in several symbiotic associations. CBs can fix atmospheric N which becomes available for its symbiotic partner . Cyanobacteria are frequently the Photosynthetic partners of Fungi that compose Lichens. Lichens are important pioneer organisms which inhabit extremely difficult sites that may be dry, hot, cold,windy or all of the above. Cyanobacteria which reside in mucilage filled chambers within the thallus.
Hornworts are colonizers and inhabit wet, unstable sites.
The ability to get N from the CBs is extremely advantageous because N is usually a limiting element in terrestrial environments. As they decay Hornworts release their N which becomes available for more complex plants. Species in the Anthocerophyta can be seen near eroded and trampled areas in the local mountains. These inhabit a cup-shaped area formed by the ventral leaves of the fern. The CBs fix nitrogen and release nitrogen-rich metabolites into the leaf cavity.