Symbioses Between Cyanobacterial Communities and Feather Mosses in Boreal Forests and Consequences for Dinitrogen Fixation

Feather moss-cyanobacteria associations, although still poorly understood, are recognised as essential for the regulation of nitrogen (N) input into N-limited ecosystems such as boreal forests due to their ability to carry out N2-fixation. This thesis aimed to investigate the diversity and composition of feather mossassociated cyanobacterial communities, the mechanisms by which cyanobacterial colonisation on mosses occurs and the fate of the N that they fix, and how various combinations of biotic and abiotic conditions influence cyanobacterial N2-fixation activity. The results demonstrated that the cyanobacterial communities associated with these mosses are highly hostspecific and diverse, with moss species identity rather than environmental factors being the primary driver of their composition and diversity. Furthermore, cyanobacterial colonisation on feather mosses appears to be possible only if the plants are able to produce a chemo-attractant (to attract and guide cyanobacteria towards colonisation sites), the plant eventually gaining N from the cyanobacteria according to its needs. Moreover, the results reveal that both abiotic (light intensity, temperature) and biotic (host plant presence, cyanobacterial community composition, cyanobacterial density) factors must be considered when studying what causes variation in feather mosscyanobacteria N2-fixation rates over time and space. Overall, the results of this thesis help in developing our understanding of the drivers of cyanobacterial communities on boreal feather mosses, and highlight the important role that these mosses play in the N cycle in boreal forest ecosystems, by hosting cyanobacteria and acquiring the N2 that they fix. In addition, this work also contributes to a better understanding of the complex interactions regulating biological N2-fixation, a major driver of ecosystem processes.