Genetic Background, Range Shifts and Associated Microbial Responses of Canopy Algae under Changing Environment

Canopy-forming macroalgae are experiencing a general global decline, mostly driven by human pressures on coastal ecosystems and global changes. In contrast to their high ecological relevance, little is known about their resilience and vulnerability to changing environmental conditions. This thesis aimed to understand genetic diversity, dispersal and connectivity of canopy-forming algae and on how these forest formers can be affected by global changes with consequences for population resilience. I focused on intertidal brown macroalgae of the genus Cystoseira, one of the leading canopy-forming seaweed genera along European coasts, coupling molecular and ecological approaches. First I investigated the projected ditribution of of the closely related C. amentacea, C. tamariscifolia, and C. compressa under predicted climatic sccenario and the interplay of distribution shifts with genetic strcture and connectivity. We found that considerable diversity could be lost in the future, particularly whithin Mediterranean area. I then modeled the effect of habitat fragmentation on species dispersal and connectivity by simulating stepping-stone dispersal dynamics for the speceis that showed to be more vulerable to climate change. Results helped describing and predicting the potential impacts of habitat fragmentation and loss. Finally, because the potential role of associated bacteria in macroalgal adaptation and resilience, I explored the association and functional role of the microbial community with Cystoseira spp. I showed that speceis-specific bacterial communitiy could be found and that the structure of Cystoseira associated bacterial communities followed a geographical isolation-by-distance pattern, clearly distinguishable among basins (Atlantic, Mediterranean and Adriatic) possibly driven by population genetic structure of the host. Also, a consistent set of shared bacteria formed the core community of C. compressa across its distribution and was characterized by key ecological functions for the holobiont system. Overall the knowledge adquired in this doctoral thesis can contribute improving the conservation and management of Cystoseira populations.